A kelp forest consisting mostly of giant kelp (Macrocystis pyrifera) at La Jolla, photographed by the author.
Introduction to the Laminariales
Kelps are large brown algae that grow nearly worldwide in temperate and cold temperate waters. Kelps are the world's largest types of algae; most are all members of the Brown Algae (Phaeophyceae). While most famous for the largest species that produce kelp forests, most species of kelp are quite small, with many "medium sized" species growing to only a few meters, and even more numerous species growing to lengths less than a meter. The most important thing that defines kelp is not necessarily its size, or its structure, but the way it grows: it grows from the base - like grass blades - rather than at the tips, like trees do on land. Later on I'll discuss why this adaptation is considered to be a key innovation and the foundation of some modern marine ecosystems. Most kelp species are within the Laminariales, including the most famous forms such as giant kelp (Macrocystis) and bull kelp (Nereocystis) - but there is a separate group of brown algae, the Fucales, that include some forms that are also called kelp, such as the southern bull kelp (Durvillaea spp.). I'll be focusing on the Laminariales, with occasional mentions of Fucales.
Kelp Phylogeny
Kelps, or the order/clade Laminariales, are a group within the Brown Algae or the Phaeophyceae; there are about 2,000 species of brown algae, and species of kelp are quite limited in diversity - about 120 species in 30 genera. The earliest diverging species are quite small and filament-like (Chorda spp.). Most of the more obviously kelp-looking species with large blades that diverge after these filamentous forms have large, single blades that might branch somewhat - the genera Alaria, Pleurophycus, and Laminaria (the kelp that gives the group its name). Also in this group is the Western North Pacific Undaria, known in Japan as Wakame - edible seaweed. Closely related to Laminaria are the southern kelps in the genus Lessonia, which looks a bit like giant kelp without bladders or a long stipe. Feather boa kelp (Egregia menziesii) diverges next - which forms our shallow water kelp forests here in California, and can be surprisingly long, up to 15 meters.
The next diverging clade is the genus Ecklonia - these kelps look similar to Lessonia and Laminaria and consist of a short stipe and a bunch of long blades, forming dense but relatively shallow kelp forests only a couple of meters tall; these kelps live in Japan, the eastern Atlantic, and throughout the southern Ocean. There are loads of videos of giant cuttlefish filmed along the south coast of Australia - above 'forests' of Ecklonia.
The next group is perhaps the most important, worldwide, and also because of the regional focus of this blog - a tight clade formed by the sea palm (Postelsia palmaefloris), bull kelp (Nereocystis luetkeana), elk kelp (Pelagophycus porra), and of course, giant kelp (Macrocystis pyrifera). With the exception of sea palms (which colonize the most highly wave-battered rocks in the intertidal zones and are true gluttons for punishment), these kelps form the backbone of North Pacific kelp forests, along with feather boa kelp (Egregia), and can grow to incredible heights (45 meters in Macrocystis; 35 meters in Nereocystis; 30 meters in Pelagophycus). For simplicity's sake, I'll refer to these species as the "California kelp clade" since most of these species overlap in California.
After these giants is the most recently evolved clade, a closely related grouping of two dozen species mostly belonging to the genus Saccharina (though this might be taxonomically split up), mostly in the western North Pacific and Aleutians. As the name suggests, these are edible; S. japonicus is known in Japan as kombu. While economically important, these kelps do not get very large (1-2 meters). Here along the Pacific coast we have Saccharina dentigera.
Giant kelp (Macrocystis pyrifera) backlit by the sun, just off La Jolla, California. For this shot I had to swim down about 20 feet and aim upwards. Photo by the author.
My best macro shot of giant kelp (Macrocystis pyrifera) showing the apicalmost pneumatocysts and fronds. Laguna Beach, California. Photo by the author.
The North Pacific kelps - Nereocystis, Macrocystis, Pelagophycus, and Postelsia
The four kelp species native to the eastern North Pacific are the world's most famous and immediately distinctive kelp species. Giant kelp, Macrocystis pyrifera, is first among these - though it has colonized the southern Ocean as well (I was surprised to see it in New Zealand, and just fully expected it to be a different species when I first saw it). This species is a popular target for underwater photographers, myself included, and I have spent a fair amount of time snorkeling amongst the giant kelp forests around La Jolla here in southern California. This species has many individual small gas-filled bladders (pneumatocysts) that suspend the entire stalk in the water column, and can grow to heights of 45-60 meters (150-200 feet); it tends to live in water of the same depth, reaching the surface. I've seen small giant kelp growing in tidepools, and small stands of relatively short giant kelp in water about 5-7 meters deep. This species inhabits waters below 21C/70F, and starts to get pretty ragged when the water here in San Diego exceeds 70F. As a result, along the Pacific coast, it doesn't live much further south than here, rarely growing south of Ensenada in Baja California (though there are some records from the Vizcaino Peninsula midway down the peninsula). This species has dispersed to the southern hemisphere, and lives extensively along the temperate Pacific coastline of South America from southern Peru to Tierra del Fuego, South Africa, southestern Australia (Victoria, South Australia, and Tasmania), New Zealand, and many islands throughout the Southern Ocean, including some off the Antarctic Peninsula. In the north, it doesn't extend any further north or west than southern Alaska; on Inaturalist, there are a couple of records from Kodiak Island, and that's it.
A dense forest and canopy of bull kelp (Nereocystis luetkeana). Just off northern Vancouver Island. Photo by Neil McDaniel via Inaturalist.
A closeup of the pneumatocysts and fronds of bull kelp (Nereocystis luetkeana). Just off northern Vancouver Island. Photo by Neil McDaniel via Inaturalist.
Bull kelp, Nereocystis luetkeana, is perhaps the second most recognizable kelp species, and is restricted to the North Pacific. It has a somewhat more contracted range, from southern California through the Aleutians and possibly as far west as the Commander Islands (though these are beach wrack records on Inaturalist and could have floated there from hundreds of miles away). Like giant kelp, bull kelp attaches chiefly to rocky bottoms but has a rather massive cylindrical stipe and a single gigantic pneumatocyst that in fully grown individuals is about the size of a softball. A number of fronds attach directly to this pneumatocyst. Bull kelp rarely forms dense canopies in central and southern California, and dense accumulations of this species are more typical in the Pacific Northwest. Not quite as large as giant kelp, this species can grow to about 36 meters (118 feet) and tends to live in waters up to that depth. I can only imagine that one common name of bull kelp, bullwhip kelp, came about owing to the narrowly tapering stipe; I can't count the number of people I've seen whipping this kelp at each other (and being surprised by how much it hurts). I learned my lesson when I was a kid (the whipper usually gets hurt more than the whip-ee). In 2010, I got to see fellow paleontologist Denver Fowler learn the same lesson!
A stunning shot of a relatively large elk kelp (Pelagophycus porra) with five fronds per side. Just off Ocean Beach, San Diego, California. Photo by Melissa Foo via Inaturalist.com.
A loose stand of elk kelp (Pelagophyca porra) deep down in the photic zone. Photo by Brett Seymour via National Park Service.
Elk kelp (Pelagophycus porra) is an unusual species with a restricted range only in southern California and northern Baja California, from Point Conception near Santa Barbara down to about the middle of Baja California. This species looks like a cross between giant kelp and elk kelp. Like the latter, it has a single gigantic pneumatocyst about the size of a softball or somewhat larger, but it has a few branching fronds that branch off the pneumatocyst in a pectinage pattern like giant kelp; head-on, the pneumatocyst looks like it has antlers. And, indeed, when washed up on shore with the fronds chewed off, it really looks like you've found a flexible decaying antler. This species inhabits deeper waters, as shallow as 20 meters deep (66 feet) down to 90 meters (300 feet) and to a length/height of 7-27 meters (~20-90 feet). Elk kelp forests are deep, well below the low tide line, and less densely forested than giant and bull kelp forests. It mostly lives in waters cooler than 60F (15C). Unlike giant and bull kelp, elk kelp frequently attaches its holdfast to soft sand and gravel or shelly bottoms - a critical adaptation for it being such a deep species given the less frequent presence of rocky substrates further from shore.
A cluster of sea palms, Postelsia palmaefloris, in between waves at Pigeon Point along the southern San Mateo County coastline in California. Despite being quite small, Postelsia is the sister taxon of bull kelp, Nereocystis luetkeana. I took this photo the safe way: from the top of a rocky headland through a telephoto lens. Pigeon Point, California. Photo by the author.
Lastly, the sea palm, Postelsia palmaefloris, is a bit of a dwarf in this group. It can form dense little forests, but it only grows to about a 50-70 cm high. It also grows completely within the intertidal zone - and specifically, the middle and upper intertidal zones. It competes for space with California mussels (Mytilus californicus) and often rapidly colonizes surfaces where mussels might get blasted off by powerful winter waves. They have a stiff stipe and short fronds only up to 20-30 cm in length - it doesn't have pneumatocysts. You rarely see a single sea palm - they usually grown in small stands (not unlike land palms). I've never seen a live sea palm close up, for one major reason: I'd have to have a suicide wish to get that close to them. Most of the sea palms along the Northern California coast are in really dangerous settings. Unless you can get out onto some rocky outcroppings studded with mussels that get absolutely blasted by waves at a minus tide, you won't get close to them. I have seen them the safe way: with a telephoto lens from fifty meters away, or washed up on a beach. Apparently, sea palms live for about a year, and the 'stands' are ephemeral - once the sea palms die naturally or are wrenched off the rocks by merciless waves, the substrate can be quickly overrun by mussels. What is quite curious is that this species is the sister taxon to bull kelp, and is fully nested within the 'California kelp clade' formed by bull, elk, and giant kelp - indicating that the ancestor to sea palms likely was a canopy-forming kelp with pneumatocysts that became an intertidal dwarf kelp and lost its pneumatocysts.
The apex of a giant kelp (Macrocystis pyrifera) frond photographed at night and backlit - one of my nicest kelp photos. You can clearly see the turnip shaped pneumatocysts that keep this species of kelp aloft off the sea floor. San Diego, California. Photo by the author.
The single pneumatocyst of a rather young bull kelp, Nereocystis lutkeana. If this had been loose and not attached to a rock, it would have been a lovely specimen to collect and try to make a pressing of. Photo by the author.
Adaptations of Kelp
There are three major adaptations of kelp that have resulted in rapid growth and enormous size. One of the major adaptations of kelp is that growth occurs at the meristem (junction between the stipe and 'blades') and, in the larger forms, the base of the stipe (stem) rather than at the distal ('upper') parts of the kelp. This is an adaptation against extreme grazing, as the canopy can be consumed at a rapid rate but the 'conveyor belt' of kelp growth keeps bringing new kelp fronds to the surface. If the distal fronds get eaten first, that would greatly decrease growth if that's the part of the alga that is growing. This is much the same way that grass and bamboo (a giant type of grass) grow, and a bit different from how trees and many flowering plants grow. Giant and bull kelp can grow at a rate of a half meter per day (slower than bamboo on land at up to a meter or so per day, but still objectively rapid). The second is that many large kelps - ones that are longer than a few meters - have some kind of gas-filled bladder, known as a pneumatocyst. In elk kelp (Pelagophycus) and bull kelp (Nereocystis), there is one huge bladder; in giant kelp (Macrocystis) and feather boa kelp (Egregia), there are numerous small pneumatocysts. As you might predict from convergent evolution, pneumatocysts in kelp are not all filled with the same type of gas - some are filled with a mixture of oxygen (O2), carbon dioxide (CO2), nitrogen (N2), and even carbon monoxide (CO). Oxygen is the most common. The ratio of gas chemistry by volume does not seem to change with growth, at least in bull kelp where it has been studied. Instead of pneumatocysts, the Fucalean southern bull kelp (Durvillaea) has a honeycomb-like internal structure with internal gas-filled spaces that provide buoyancy.
Some more feather boa kelp (Egregia menziesii) holdfasts emerging from shifting sands. La Jolla, California. Photo by the author.
Most species of kelp (and algae in general) attach directly to rocks. Unlike plants, these are not roots - kelp already lives in the water, so uptake of water and nutrients is done directly from the water column. As a result, the attachment structure is purely mechanical and called a holdfast. The stipe expands and branches into a series of short, stout, tangled branches that extend outwards and contact the rock. Sometimes this can be rather short and stocky, as in the sea palms (Postelsia), and in some early diverging kelps, is even just a simple non-branching or 'discoidal' holdfast (Aureophycus). Some studies (Kawai et al. 2013) even proposed that this discoidal holdfast is the primitive condition for kelps. In many of the larger species, however, it can be a surprisingly large tangle of branches; in cross-section, it will look like a series of nested cones - each set of branches extends radially off of the stipe and slopes downwards toward the rock. As the kelp grows, so does the holdfast - another outer layer of holdfast branches are grown, one at a time. Large holdfasts can attain nearly a meter in size in giant kelp, though most holdfasts you might come across on the beach will range about 10-30 cm in diameter. Kelp holdfasts have even recently been found as fossils (see below).
A macro shot of the stipe of feather boa kelp (Egregia menziesii). There are two pneumatocysts in this shot but they're behind fronds at the lower left and right. La Jolla, California; photo by the author.
As in other algae (and in plants), kelps undergo a sporophyte (spore-bearing) and gametophyte (gamete-bearing) phase. The sporophyte is what we usually see: this is the large form anchored to the seafloor. It produces spores (which are diploid - possessing two copies of each chromosome), which mature into (still microscopic) male and female gametophytes (which are haploid, or possessing one single copy of each chromosome). Once the male and female gametophytes contact and 'fuse', a new sporophyte starts to grow and settles somewhere.
A giant kelp (Macrocystis pyrifera) sporophyll I found while snorkeling near La Jolla, California. Photo by the author. This individual threw me through a loop because it really, really looks like Lessonia from the southern hemisphere!
Giant kelp (Macrocystis pyrifera) is further notable for two reasons. First, it produces spore-rich growths called sporophylls near the base of the stipe. I recently came across a giant kelp sporophyll but had no idea what kind of kelp it was - it had no bladders (pneumatocysts) and had a short stipe, and didn't match anything. Additionally, the main stipe had evidently been removed or lost at some point. So - some kelps produce a separate spore-bearing growth that looks a bit different from the mature sporophyte. Second, and perhaps more fundamentally, the extensive pneumatocysts in this species have permitted it to disperse to the southern hemisphere - it inhabits New Zealand, southern South America, Tasmania, and many of the islands of the southern ocean in the vicinity of New Zealand, Australia, and South America. Initially named as different species, molecular work has recently determined that all of these are the same species with a North Pacific origin, and drift kelp probably dispersed southward in the tropical eastern Pacific during the late Pleistocene (when, presumably, this species would have been growing even closer to the equator). Cooler waters would have permitted the drift kelp to survive the journey (kelp doesn't tolerate warm water very well - an ominous note for today's kelp forests) and spread spores into the southern ocean. The lack of giant kelp in South Africa is puzzling, given the west to east direction of the circumpolar current.
Close up shot of the fronds of southern sea palm, Eisenia arborea. La Jolla, California. Photo by the author.
Lastly, many kelps have phenolic compounds in their tissues that are thought to be a sort of chemical defense against herbivores, much like the chemical defenses in many modern plants. Some of these phenolic compounds, polyphloroglucinols, are present (and quite abundant by volume) in most if not all brown algae, and experimentally have been shown to deter feeding by herbivorous gastropods and echinoids. In the North Pacific, some phenolic rich kelp (Agarum) is grazed at a much lower rate than phenolic poor kelp (Laminaria). Curiously, southern hemisphere species of kelp have dramatically higher levels of phenolic compounds than northern hemisphere species, leading at least one major study to propose that sea otter predation on marine invertebrate herbivores (e.g. sea urchins) decreased the adaptive necessity for kelp species in the North Pacific to evolve chemical defenses (Estes and Steinberg 1988).
Large fronds of southern bull kelp (Durvillaea antarctica) attached to Miocene age basalts, Tairoa Head, Otago Peninsula, New Zealand. Photo by the author.
Southern Bull Kelp - Durvillaea
Southern bull kelp (Durvillaea spp.) is not a true kelp in the order Laminariales but is a clade within the Fucales - a closely related group of brown algae; Durvillaea is actually somewhat closely related to Sargassum - that floating yellowy algae we were used to seeing wash up on east coast beaches during the summer. Southern bull kelp includes about eight species in the genus Durvillaea, and in New Zealand, we saw Durvillaea antarctica. Southern bull kelp doesn't float in the same way - the fronds themselves are filled with air and have an internal honeycomb texture, so the fronds float. Each frond is attached to a short and rather robust stalk, many of which can share the same holdfast; the fronds themselves are long and spindly or can be quite wide, but diverge off of a palmate expansion, a bit like Laminaria. It grows to a length of 10 meters, and only grows on rocks that take quite a beating from the waves - and, as a result, tends to only grow shallowly with the holdfasts in the intertidal zone (most species, including D. antarctica) or just below it (D. willana lives at a maximum depth of 6 meters). So, while southern bull kelp is called kelp because it is quite large and floats, it doesn't really form kelp forests as much as it does form halos around rocks - in terms of its attachment and growth, it has far more in common with sea palms (Postelsia spp.) in the North Pacific, which are restricted to the intertidal zone on the most violently wavy coastlines.
Southern bull kelp, Durvillaea antarctica, at Cannibal Bay in the Catlins, New Zealand. Here is Sarah for scale (5'4") to show the absurd scale of this algae. At low tide like this it rather looks like lasagna. Photo by the author.
More southern bull kelp, Durvillaea antarctica, at Tairoa Head, Otago Peninsula, New Zealand. Here it rather resembles angel hair pasta instead.
We first saw this species during our first visit to St Clair beach on the south side of Dunedin, and were amazed with the size of the fronds - far larger than anything from California - but also the colossal stench. I'm used to stinky wracks of kelp, since I was a kid visiting Moss Beach in the early 90s. Beach wrack consisting of fresh kelp is odorous enough, but after a few weeks of decomposition this black sludge begins to form in the middle - and it is putrid for sure. But the smell of even slightly decayed Durvillaea is something else. Regardless, Durvillaea is quite beautiful and its tendency to float means that the individual fronds can spread out like the spiky bits of a palm frond. At high tide, the thicker bits of the algae float in a tangled mass that can only be described as resembling giant green lasagna.
Kelp Forests
In many of the temperate and cold temperate parts of the world, there are extensive kelp forests where kelps grow quite densely, and with different types growing together. Kelp forests make up less than 1/1000 of the surface area of the earth - yet are responsible for just about 1% of all productivity on the planet, especially along coasts with extreme upwelling.
The Pacific coast of North America - especially California - hosts what is arguably the world's most famous kelp forests, and the most complex. Giant kelp (Macrocystis) and Bull kelp (Nereocystis) form a canopy, and smaller kelps such as Laminaria (no common name), feather boa kelp (Egregia), southern sea palms (Eisenia), stalked kelp (Pterygophora), and many others grow closer to the seafloor. Below these can be found red algae and many sessile marine invertebrates.
A kelp isopod (Pentidotea wosnesenskii), one of many marine invertebrates that have adapted to live on or around kelp. La Jolla, California. Photo by the author.
Kelp species that grow sufficiently long may attach to rocky substrate, and be suspended vertically in the water column through buoyant gas filled bladders (pneumatocysts). The major examples of these are giant kelp (Macrocystis), bull kelp (Nereocystis), and in deeper offshore waters of southern California, elk kelp (Pelagophycus). Pneumatocysts permit the fronds to float higher in the water column, and thus better able to photosynthesize; this is presumably an adaptation for being able to colonize deeper settings that smaller kelp species cannot.
Kelp forests are further one of the highest productivity marine environments out there - chiefly by their rapid growth, and the near constant delivery of dead kelp bits - drift kelp - into shallow marine settings. Visit any beach in California, especially in summer and fall, and you'll probably find bits of kelp - either small fragments, or entire kelp stalks (e.g. after storms) - washed up on the beach. I often come across floating kelp strands while snorkeling (and use the opportunity to look for a couple of kelp forest dwelling sea slugs you can't find during tidepooling... but never seem to spot them). These bits of drift kelp feed herbivorous invertebrates - in fact, most purple urchins live in small, spherical borings and never leave, and catch little bits of drift kelp with their tube feet and pull it into the burrow. Drift kelp in turns feeds and causes population booms of microorganisms that filtered out of the water column by filtering invertebrates - and even microscopic bits of kelp detritus can form the primary food of some filtering invertebrates like mussels and barnacles. For these reasons, driven by a few growth adaptations, kelp forests are one of, if not the most productive marine environments on the planet. Though kelp can grow quite rapidly, most species do not live very long. Giant kelp lives for about three years, and bull kelp generally lives for one year - though some individuals live a second year. Young bull kelp and giant kelp can be found in winter, and they become giants by late spring; large adult kelp washes up in abundance during the fall.
A Norris' top snail (Norrisia norrisii) - this gastropod lives subtidally and often grazes algal and diatomaceous films off of kelp. La Jolla, California. Photo by the author.
Some really chewed up giant kelp (Macrocystis pyrifera). I'm not sure if these snails, gilded tegulas (Tegula aureocincta) are the root of this herbivory; this was photographed during a heat wave that triggered accelerated grazing (and decomposition, according to some) of the kelp near the surface. Indeed, it was about 73-74F when I shot this - no wetsuit required! La Jolla, California. Photo by the author.
In certain areas, kelp can become overgrazed by sea urchins - leading to rocky bottoms overcolonized by dense aggregations of purple urchins (Strongylocentrotus purpuratus). These are called urchin barrens. As described below, sea urchins typically live in small spherical borings and "catch" bits of drift kelp and other algae and feed on those. When this food source becomes too scarce, they leave their borings, and start to graze directly on live algae - rapidly consuming kelp and destroying the kelp forest. Sea otters and sea stars keep urchins in check today - and areas where sea otters have re-colonized since their near extinction in the late 19th centory have seen the return of kelp forests. Today, urchin barrens are common north of Santa Cruz, California, through Oregon, and Washington - few kelp forests exist in these regions. Additionally, even areas where sea otters are common, including around Monterey, have had worrying declines in kelp forests.
A rare kelp clingfish (Rimicola muscarum), one of two that I have been very privileged to find while tidepooling. Clingfish have a little suction cup underneath their head formed by modified pelvic fins. La Jolla, California. Photo by the author.
More grazing on kelp - southern sea palm (Eisenia arborea), with a large gilded tegula snail (Tegula aureocincta) and some limpets (Lottia) and their corresponding traces. La Jolla, California. Photo by the author.
The Origin of Kelp and Kelp Forests
The timing of divergence of modern kelp lineages has been analyzed through molecular clock dating - this is a method where mutation rates for particular genes are assumed to be constant through time. Under this assumption, the timing - in thousands, hundreds of thousands, or millions of years - can be calculated for individual branching events on a cladogram (family tree). In one recent 2019 study (Starko), the reconstructed radiation of the Laminariales falls in the Paleogene period - likely during the Oligocene epoch, but with considerable error bars - as early as the middle Eocene (~48 Mya) and possibly as late as the early Miocene (~21 Mya) but with a mean estimate of early Oligocene (32 Mya). This study used the Miocene fossil kelp Julescraneia grandicornis from the Monterey Formation to calibrate the molecular clock - a built in 'correction' that pulls part of the tree to a particular point in time. Additionally, this study found that there was an increase in speciation rate during the late Oligocene through early Miocene, from about 30-20 Mya.
The geographic center of origin for kelp is certainly to be found somewhere in the North Pacific. Earlier studies suggested an origin in the western North Pacific (e.g. Japan, Kamchatka). However, a recent study (Starko et al. 2019) found that the Laminariales probably originated in the eastern North Pacific, with most lineages found outside this region being 'exported' to other regions (e.g. western North Pacific, southern Ocean, North Atlantic) during the Miocene and Pliocene epochs. The diversification of the western North Pacific "Saccarhina" clade seems to have happened quite rapidly, mostly since the late Miocene. The 'California kelp' clade of kelps including sea palms (Postelsia), giant kelp (Macrocystis), bull kelp (Nereocystis), and elk kelp (Pelagophycus) have an origination in the late Miocene - about 13 Mya - with all four lineages being fully differentiated by the latest Miocene, ~6-7 Mya (though it is worth noting that more recent dates for the microfossil ages for the fossils used for this molecular clock date may indicate that the clade diversified even more recently than that, around 8 Ma).
A school of kelp bass (Paralabrax clathratus) underneath fronds of giant kelp (Macrocystis pyrifera), just off La Jolla. Photo by the author.
Kelp forests, on the other hand, likely have a slightly different history. Adaptations for upright growth - chiefly, the development of stiff stipes, pneumatocysts, and branching - have evolved several times, but apparently quite recently. It seems most likely that until the last few million years, perhaps the Pliocene - kelp forests consisted of relatively "short" species only a few meters in length, similar to the kelp forests of the southern Australian coastline. Perhaps tall canopy kelps did not evolve until the last few million years - and species of limpets that feed only on kelp stipes (with a distinctive aperture shape, and a narrow shell that conforms to the width of the stipe) only appear in marine rocks that are a maximum of 3 Mya in age (see below).
Another recent study, summarized below, reports relatively small kelp holdfasts from the early Oligocene Makah Formation of the Olympic Peninsula in Washington state. These holdfasts consist of a complex structure of the 'roots' - haptera - and are identifiable to the Laminariales or true kelps. These holdfasts are similar in complexity to giant kelp (Macrocystis) and are very similar to bull kelp (Nereocystis). Given that 1) these holdfasts pre-date the fossil record of the canopy forming kelps like Julescraneia (see below) by over 20 million years, 2) they likely represent a kelp form with a soft rather than stiff stipe, and 3) owing to their relatively small size, it is likely that these represent some other clade than those in the 'California kelp clade'. Further, it seems likely that these holdfasts, if they did not possess a stiff stipe, were not canopy formers. I imagine a South Australia kelp forest situation with Ecklonia-like kelp growing a few meters above the seafloor, or perhaps if these kelps had pnmeumatocysts, something similar to the shallow marine feather boa kelp 'forest' that tends to occur on the California coast inshore of the giant kelp forests.
Branching pattern of frond 'antlers' in elk kelp (Pelagophycus), bull kelp (Nereocystis), and the extinct intermediate form Julescraneia grandicornis. From Parker and Dawson (1966).
The fossil record of kelp
Kelp Fronds: There is really only one known 'body' fossil of kelp - and that is Julescraneia grandicornis from the upper Miocene Monterey Formation of Los Angeles County. This fossil was described by Bruce Parker and E. Yale Dawson in 1966 from LACM collections (later transferred to UCMP collections at Berkeley). It's large, and shares features with both Nereocystis (bull kelp) and Pelagophycus (elk kelp). The specimen preserves a large pneumatocyst likely 16 cm in diameter when alive, but crushed and flattened so it is preserved now as 22 cm. The basal 'antlers' leading to the fronds are preserved, but none of the fronds themselves, and it's likely to be a piece of pretty rotten drift kelp that sank to the sea floor. You can actually make out some of the branches terminating. Kelp pneumatocysts can stay inflated for very long periods of time - isolated Macrocystis pneumatocysts with little stubs of the fronds are frequent finds. Given the thick walls of the pneumatocyst in Nereocystis and Pelagophycus, it's a wonder that we have any of these branches at all for Julescraneia. Regardless, the pattern of branching resembles Nereocystis, though the branches are much, much longer, like Pelagophycus. In this regard it has morphological similarities with both of these large-pneumatocyst kelps. Even as far back as 1966, these authors suggested that Julescraneia could potentially have given rise to both elk and bull kelp. Interestingly, despite their large pneumatocyst, these two modern kelps are NOT sister taxa - if you recall from above, bull kelp (Nereocystis) is the sister taxon of sea palms (Postelsia) and elk kelp (Pelagophycus) is sister to giant kelp (Macrocystis). Is the single large pneumatocyst primitive for this clade? Perhaps - three out of four extant species have a single unbranched stipe, and I can easily envision size decrease and loss of a large pneumatocyst resulting in the evolution of sea palms (Postelsia; however, Postelsia has a hollow stipe, which could in theory be homologous to a large, cylindrical pneumatocyst; Starko et al. 2016), and branching of the stipe and addition of pneumatocysts evolving in giant kelp (Macrocystis). In fact, quasi-sympatric speciation into adjacent but slightly different environments likely led to the shrinking of Postelsia as it inhabits much of the same geographic range, but shallower waters along the same rocky shores that Nereocystis occurs just offshore from. Lastly, the age: Julescrania is from LACM locality 1267 in the Monterey Formation on the north side of the Santa Monica mountains in Sherman Oaks, actually quite close to the 405/101 junction just a few miles west of Burbank in Los Angeles County. According to Parker and Dawson, the Monterey Formation here has produced upper Mohnian age diatoms, now corresponding to an age of about 8 Ma.
Another fossil is a species purported to be closely related to Postelsia palmaefloris from the Monte Bolca lagerstatte of Italy, Postelsiopsis caput-medusae. However, Monte Bolca is Eocene. I don't think it looks particularly close to Postelsia to begin with, and it is now considered to be some other sort of brown algae.
A teeny little holdfast formerly anchoring a feather boa kelp (Egregia menziesii) stalk to the seafloor; the entire kelp was floating around like this. Photographed near La Jolla by the author.
A massive, 50-70cm wide giant kelp (Macrocystis pyrifera) holdfast on the Scripps Pier underwater webcam in October 2025; the kelp had been torn loose from the seafloor and became entangled on the pier pilings, allowing me to get this screenshot. The little spiky things at the upper right are spiny lobsters (Panulirus interruptus).
Holdfasts: There is a slightly larger fossil record of kelp holdfasts, though these were not described until the past couple of years by Kiel et al. (2024). These authors reported no fewer than twelve permineralized holdfasts of some sort of kelp preserved in the Jansen Creek Member of the Makah Formation from the Olympic Peninsula of Washington. Woody debris is not uncommon in this unit, and I've encountered all sorts of bits of wood with a coal-like preservation. These holdfasts greatly resemble those of modern bull and giant kelp, consisting of a tangled mound of branching 'roots' (haptera). In cross-section, the haptera emerge in successive layers and appear to be radially oriented. In other words, each "layer" of haptera is conical, consisting of spoke-like radially oriented haptera, and each new conical layer is larger than the last. These holdfasts are shockingly similar to extant bull kelp (Nereocystis) and giant kelp (Macrocystis) holdfasts, Kiel et al. (2024) interpret these holdfasts as belonging to true kelp in the order Laminariales. At the center of the holdfast, there is no remains of the stipe ("stem") - instead, there is a conical hole from where the stipe was torn away, likely by winter storms (in my opinion). The authors also suggest that, since it is missing like this, the stipe was probably soft, perhaps like feather boa kelp (Egregia) - and that these kelps may not have been 'upright' like canopy kelps (Macrocystis, Pelagophycus, Nereocystis). These holdfasts are not large - ranging up to about 5 cm in diameter, similar in size to the holdfasts of extant feather boa kelp (Egregia). The fossil holdfasts are mostly attached to shells, including barnacles and bivalves including mussels - suggesting shallow water attachment like in the intertidal zone; the modern upper intertidal, for example, is dominated by California mussels and volcano barnacles. Holdfasts also seem to have trapped a number of small fossils including bivalves, gastropods, foraminifera, rocks, and sediment - similar to the microfauna associated with modern kelp holdfasts, which offer some shelter from waves and daily temperature swings.
32 million year old kelp holdfasts from the Makah Formation of Washington - from Kiel et al. (2024).
The trapping of calcium carbonate mollusk fossils permitted some isotopic analyses. The first are Oxygen isotope ratios used to reconstruct the temperature of the water during the growth of the shells. A barnacle yielded a range of 15.6-16.8 * C - about 60-61.5 F, quite chilly, but still a bit warmer than the Pacific today off of Washington (7-13 C, or 44-50 *F). The second specimen, a bivalve, yielded a temperature range of 19.9-23*C, or 67-73 *F - considerably warmer than the ocean there today, and actually quite similar to summer water temperature here in San Diego. It's unclear if these estimates reflect analytical error or if the holdfasts are reworked - many of the fossils in the Jansen Creek Member have been reworked into turbidites and deep sea slumps from slightly older deposits on the continental shelf. So, it's possible that some of the dropstones originated from strata representing considerably warmer conditions. These same shells also yielded the first Strontium isotope ratios from the Makah Formation - a powerful indicator of geologic time. The ratio of Strontium 87 and 86 has changed through geologic time, with one becoming more abundant than the other at different points. The ratio is usually somewhere below 1% Strontium 86, and over the course of the Cenozoic, the amount of Strontium 87 has generally been increasing (slightly, and steadily). When the graph fluctuates, the ratio is not useful, but during most of the middle Cenozoic, it increases steadily and therefore is quite accurate. One bivalve sampled for Strontium resulted in a ratio corresponding to a date of 32.1 Mya - or, earliest Oligocene.
Laminaria kelp torn off the rocks during a storm with some large pebbles (5-6 cm diameter) stuck in its holdfast. Santa Cruz, California. Photo by the author.
Kelp Dropstones
Strong storms during the winter have the capacity to rip kelp off the rocks, and when this happens, the holdfast can rip chunks of rocks that the holdfast was attached to. In some places, this is one of the major drivers of hard rock erosion below the low tide line; southern bull kelp (Durvillaea), for example, bonds so strongly to the rocks that this connection is frequently tougher than the rock itself. Kelp will drift for quite a ways given that the tissue is quite strong and still buoyed up by the pneumatocysts, permitting long-distance transport of drift kelp. Drift kelp has been documented rafting dropstones up to 500 km away from the source; most continental shelves on earth are less than 100 km wide, so intertidal rocks can be dropped into offshore shelf settings, or even onto the abyssal plane - or, making studies of sedimentary petrology potentially confusing - parallel with the shoreline for long distances away from the source area.
Southern bull kelp (Durvillaea antarctica) with potential dropstones - large chunks of rock ripped from the intertidal zone - that ended up on the south coast of the Otago Peninsula. The Murihiku Sandstone is exposed in the Catlins, the southernmost part of the South Island - and the gneiss is from Fiordland, the southwestern corner of the island; this sample must have travelled several hundred kilometers and through the Foveaux Strait. From Craw and Waters (2018).
Geologic map of the southern part of the South Island of New Zealand, showing just how far some of these kelp-rafted rocks traveled to get to the Otago Peninsula. From Craw and Waters (2018).
Experiments in this study by Frey and Dashtgard (2012) indicate that kelp can not only pluck dropstones off the seafloor and float them, but that even small kelp fronds attached to small rocks can transport them simply by pulling them in a light current.
In other cases, even small fronds of small or young kelp attached to loose rocks don't even have to have pneumatocysts or float to transport rocks: water currents can pull these along, the attached kelp acting like a sail, slowly dragging the attached rock across the seafloor. Except in rare cases (see above), it's pretty unlikely that the holdfast itself will fossilize - but as the kelp and its holdfast decay, the rock is left behind. One study of small kelps attached to cobble sized rocks (up to 10 cm) found that these were being actively transported by attached small kelp fronds in the intertidal zone in Washington state in relatively slow tidal currents (0.2 meters per second) that were only able to move 'kelpless' rocks up to 4 cm in diameter (Frey et al. 2012). Other studies have found kelp-transported rocks weighing up to 100 kg being moved onshore during storms.
An isolated pebble (about 3 cm diameter) likely of Salinian granodiorite - the same rocks that make up the Monterey Peninsula - isolated in silty fine sandstone of the Purisima Formation near Half Moon bay, California, in a "middle shelf" setting. The only real way for this pebble to have gotten this far out, without being on an erosional surface, is to have been transported there as a dropstone. Photo by the author.
More dropstones in the same section of the Purisima Formation. Photos by the author.
Fascinatingly, this study predicted that transport of dropstones - either as kelp rafted debris or dragged along the seafloor - would show up in the rock record as isolated large pebbles and cobbles seemingly 'floating' in shelf sandstone deposits. These sorts of deposits, to my knowledge, have not yet been published - but I've been documenting such occurrences in the Purisima Formation of northern California. Unfortunately at present, I don't have much in the way of in situ photographs I've taken in the field, though I'll certainly remedy this in the future.
Kelp dropstones are also potentially quite frustrating as they likely hinder the study of ice rafted debris - a classic example of evidence used to infer glacial paleoclimates.
The fossil record of kelp-associated fauna
Kelp might have a terrible fossil record, but what about the fossil record of some of the groups that have been ecologically associated with kelp forests, past and present?
Abalone - Haliotis
Abalones are flat-shelled snails (gastropods) in the family Haliotidae. The interior of the shells have a luxurious mother of pearl nacre that varies in color from species to species, and are highly sought after (I have a decent collection of shells, for example). Abalone have lost their operculum, and there are a series of holes for respiratory pores (and reproductive structures). The foot of an abalone is lined with a bunch of short, radially oriented tentacles. Abalone meat is also highly sought after, and abalone divers here on the Pacific coast need sport fishing licenses and cannot use scuba - all collected abalone must be done so by free-diving (just like a sea otter!) and the shells are pried off with a chisel. The abalone season here has been closed since 2017 owing to declining populations. One house in Inverness, CA, has hundreds, if not thousands, of abalone shells mounted to its exterior and fences.
A fossil example of Haliotis - a relatively small one, some 7-9 cm long or so, from the ~7 myo Monterey Formation of southern California. On display at LACM. Photo by the author.
There are about 70 species today, and all are voracious herbivores. Most are small, from 2-10 cm in maximum shell length, and the red abalone gets to a shocking 30 cm long (my largest examples are about 20-25 cm). Most are cold water species and inhabit the North Pacific (Japan to Baja California), New Zealand, South Africa, and Australia; they are surprisingly absent from the Pacific coast of South America, the western North Atlantic, and, unsurprisingly, absent from the Arctic and Antarctica. Abalone have an extensive fossil record going back to the late Cretaceous, though they are always generally quite rare; a handful of specimens exist in the Pacific coast Neogene, as they tend to be shallow rocky reef inhabitants - environments that are characterized by erosion rather than deposition. Their fossil record does not reveal a center of origin, though they are most diverse in the Indo-Pacific. Tropical species are generally always small, and only cold water species attain large sizes. Large size in general has evolved twice - once in the Pacific clade of Haliotis (including New Zealand Paua, Tropical east Pacific species, and all of our eastern and western North Pacific species) as well as the Australia/South African clade (that's right: NZ Paua, Haliotis iris, is more closely related to eastern North Pacific species than to anything from Australia).
All large bodied species of Haliotis live within kelp forest ecosystems. The rapid growth of kelp has likely permitted these giant body sizes to evolve, both through feeding directly on kelp and also on drift kelp - along with the other algae from the understory in kelp forests. One study proposed that the evolution of kelp forests was a prerequisite for giant abalones to evolve. Further, the "quality" of the kelp matters. North Pacific species of Haliotis - especially red abalone (Haliotis rufescens), pink abalone (Haliotis corrugata) green abalone (Haliotis fulgens) and giant abalone (Haliotis gigantea - though, not as giant as the red abalone!) - are thought to get so much larger than southern hemisphere forms owing to the lower levels of phenolic compounds in kelp. Higher levels in southern hemisphere kelps like Ecklonia might prevent truly gigantic body sizes from evolving (e.g. all of these, even the large H. midae, are smaller than 20 cm). Additionally, the eastern North Pacific - with the largest and most complex kelp forests in the world - has the most diverse fauna of abalones, locally hosting eight species, including the largest (H. rufescens). Shockingly, even our smallest species on the Pacific coast - black (Haliotis cracherodii) and flat (Haliotis walallensis) - are still large species, maxing out at around 15-20 cm long, several times larger than tropical species. Lastly, gigantism likely evolved quite recently. Based on fossils, large body size appears quite suddenly at around the Miocene-Pliocene boundary; all fossil abalones exceeding 10 cm in shell length appear in the Late Miocene or in younger rocks.
Further, body size is quite flexible - a pair of small-bodied sister species in the tropics, Haliotis roberti (Galapagos and tropical east Pacific) and Haliotis pourtalesii (Caribbean and Gulf of Mexico) - evolved from North Pacific species, dispersed south to the tropics before being separated into east and west populations by the uplift of the isthmus of Panama. Despite evolving from large ancestors, they ended up in the tropics where there are no fast growing, gigantic fleshy algae like kelps, and underwent an evolutionary reversal to small body size! Neither species exceeds 5 cm in shell length.
I really do love abalones, and could (and probably should) write a whole blog post on their evolution and fossil record, but I need to keep it brief here. To conclude: gigantism (body length greatly exceeding 10 cm) has only evolved in abalones that live near kelp forests, and suddenly evolved around 5-10 million years ago - likely giving us a decent date for the origin of kelp forests.
A sea otter (Enhydra lutris) eating a clam in Morro Bay. Photo by the author.
Sea otters - Enhydra
Sea otters are today intimately linked with the highly productive giant and bull kelp forests of the North Pacific, formerly inhabiting coastlines from Japan to the Aleutians and southeast to Baja California - but now restricted to the Aleutians, the Alaskan panhandle, British Columbia, and central California (Santa Barbara to Santa Cruz). However, sea otters currently inhabit many sandy shore environments in parts of Alaska and have a bit more environmental flexibility than typically assumed. Though most famous for using tools to smash open sea urchins, crabs, and mollusks - only a subset of sea otters use tools. Tool use does not seem to be correlated with genetics - e.g. otters have a bit of a random chance of tool use in their lifespan - and individual otters will be highly specialized in their feeding habits, but diet is highly variable between individuals. This permits a rather high population density of otters, which can only dive to shallow depths of about 20-30 meters, and thus forage quite close to shore.
Sea otters have been proposed to represent "keystone" species, preying upon herbivorous invertebrates that in turn feed on kelp; without this predation pressure, sea urchin populations get out of control and destroy kelp forests, reducing them to urchin barrens. Where sea otters are absent - places like northern California, Oregon, and part of the Washington coastline - kelp forests have disappeared since the early to mid 19th century when sea otters were nearly hunted to extinction for their furs. Rocky subtidal habitats along the Pacific Northwest coast have been transformed into urchin barren 'deserts'; meanwhile, in California, in areas where sea otters returned, kelp forests proliferated - suggesting a strong link between sea otter population size and kelp forest health.
The oldest known sea otter (Enhydra sp.) from the Pacific Basin - a 620,000 year old femur from the Merced Formation near San Francisco, California. From Boessenecker (2018).
How long has this relationship existed? Sea otters have a surprisingly shallow fossil record in the North Pacific. The earliest sea otter fossil, Enhydra reevei, is based on isolated teeth from the North Sea dating to the early Pleistocene (~2.1 Ma). Slightly younger specimens of Enhydra appear in the basal parts of the Gubik Formation of Alaska (~1-2 Ma), and the oldest Pacific basin record of a sea otter is a complete femur collected by my friend Chris Pirrone from the Merced Formation near San Francisco - and is only about 620,000 years old. These fossils, and the absence of fossils of Enhydra from the very densely sampled upper Pliocene (2-4 myo) San Diego Formation of southern California, strongly suggest a middle Pleistocene immigration of sea otters from the Arctic to the North Pacific. In turn, this indicates that the sea otter > sea urchin > kelp food web is very, very young. Given that kelp has a much older origin than the fossil record of sea otters,
Additionally, I need to mention that there is a somewhat older giant otter, Enhydritherium terraenovae, known mostly from the late Miocene of Florida but also from a few scraps in the latest Miocene and early Pliocene of California. However, this otter has sharp cheek teeth and lacks many of the adaptations for cracking open hard prey seen in extant Enhydra, and ecologically, was much more like a giant river otter; additionally, it is mostly known from nonmarine deposits and is a poor candidate for being an ecological predecessor of Enhydra.
Dusignathine walruses - Dusignathus and Gomphotaria?
The Dusignathinae are an extinct clade of walruses from the eastern North Pacific, basically including only three species, all from California: Dusignathus santacruzensis, Dusignathus seftoni, and Gomphotaria pugnax; these three species are known from the late Miocene Purisima Formation (of Santa Cruz), the Pliocene San Diego Formation (of San Diego), and the late Miocene Capistrano Formation (of Orange County). Scraps have been found in other rock units in California and the Almejas Formation of Baja. All of these have upper and lower canines modified into short tusks, and D. seftoni and Gomphotaria have extensive tooth wear on the cheek teeth and tusks, suggesting ingestion of abrasive material into the oral cavity like sand and silt, which has been interpreted as evidence for a molluskivorous diet. Dusignathus santacruzensis, on the other hand, has an elephant seal or sea lion like dentition lacking evidence for benthic feeding, and was likely a fish eater instead. Prior to most of this evidence being published, the dusignathines were proposed as a possible predecessor of sea otters that could have kept the invertebrate herbivores that decimate kelp beds in check.
However, this is pretty speculative, and it's entirely possible that the tooth wear in these walruses has nothing to do with benthic feeding; I seem to recall that years ago, my colleague Morgan Churchill had some isotopic data suggesting a different mode of feeding ecology. Further, when proposed in 1989, the dusignathines were defined differently, and at the time included mostly fish-eating species like Imagotaria downsi that are now considered to be in different groups within the family. The truth is, we definitely do not know enough about dusignathines to propose them as the ecological progenitors of a modern keystone species. However, dusignathines do predate the fossil record of Enhydra, having a continuous fossil record in California from about 8 Mya to about 2 Mya or so. At least the timing works out - but the paleoecological interpretation requires actual data (e.g. dental microwear, enamel isotopic studies of dusignathines).
The holotype mandible of Kolponomos newportensis found by Doug Emlong in the Nye Mudstone. Smithsonian collections. Photo by the author.
The "Beach Bear" - Kolponomos
Kolponomos is an unusual marine carnivore known from the earliest Miocene Clallam Formation of Washington and the Nye Mudstone of Oregon - there are a few decent specimens, of which Kolponomos newportensis from Oregon is the best known. I've held and examined the holotype skull at the Smithsonian - it is a wondrous specimen. Similar to a bear skull in many ways but with wide zygomatic arches (and therefore powerful jaw closing muscles), it is also studded with expanded, wide, teeth with thick enamel - almost exactly what you would predict if a bear like carnivore were to adapt to a sea otter like diet. Unlike sea otters, the mandibles are fused at the chin, indicating a somewhat different mode of crushing prey items (likely using both left and right teeth at the same time, instead of one side as in otters). Kolponomos has been proposed to occupy the sea otter niche in the eastern North Pacific and could have fostered kelp forest health early on. However, there are really only a handful of specimens, suggesting only a short geologic range - just a couple million years in the early Miocene. It's possible, but not something I'd hang my hat on.
A giant kelp fish (Heterostichus rostratus) that is certainly kelp colored to match the kelp fronds. Monterey Bay Aquarium. Photo by the author.
A giant kelp fish (Heterostichus rostratus) really blending in well amongst the algae on the rocks near San Diego, California. Photo by the author.
A spotted kelpfish (Gibbonsia elegans), amongst the algae near San Diego, California. Photo by the author.
Kelpfish - Heterostichus and Gibbonsia
Kelpfish are a group of fish within the 'blennies' (Blenniiformes - a rather cute group of fish often with distinctive branched "eyebrow" cirri that inhabit burrows and crevices) that have evolved to favor living in and amongst kelp, including some species in the genus Gibbonsia and especially the giant kelpfish, Heterostichus. There is no fossil record for any of these, and, surprisingly, little research on their evolution (including molecular clock divergence) has been attempted. I suspect that otoliths (ear stones) of these fish might be found at some stage.
A rather large northern kelp crab (Pugettia productus) fumbling around in the seagrass at low tide.
Another type of kelp crab, one that is a sort of decorator crab - the graceful kelp crab, Pugettia gracilis. These don't get quite as large, and they are difficult to spot.
A juvenile northern kelp crab, Pugettia productus, which has stuck a bit of kelp to its rostrum. Apparently these bits are held in place rather than affixed to the exoskeleton and growing on it.
Kelp Crabs - Pugettia
There are a number of species of crabs in the familiy Majidae that have evolved to live in and feed on kelp and other algae - though they are not strictly kelp inhabitants - and in the winter, they tend to be more omnivorous to carnivorous, feeding on other invertebrates as the winter storms tend to rip some of the kelp away. Kelp crabs include two genera here in the eastern North Pacific - Pugettia, and Taliepus- the most common example of each are the northern kelp crab (Pugettia producta) and the southern kelp crab (Taliepus nuttallii). Only Pugettia has a fossil record - the northern kelp crab, Pugettia producta, and the cryptic kelp crab, Pugettia richii, are both reported in one of Mary Rathbun's monographs, from numerous Pleistocene localities in southern California, including the San Pedro Sand. Unfortunately, these fossils weren't figured, and in any event are geologically too young to inform this discussion.
Limpets - Discurria and Lottia
Limpets (Patellogastropoda) are surprisingly rare in the Pacific coast fossil record, though their low conical shells are very common on intertidal rocks here. All are herbivorous, and while most live on rocks and graze algae and diatomaceous films, others have evolved to live perpetually on seagrass blades and on kelp. Seagrass limpets have very narrow, long shells, whereas kelp limpets have slightly narrow, saddle shaped shells to conform to the kelp stipe. Both seagrass and kelp limpets have evolved multiple times in parallel, and in the eastern North Pacific, there are kelp limpets in both the genera Lottia (L. pelta) and Discurria (D. insessa). These species appear as fossils during the late Pliocene, about 3 Mya - and saddle-shaped limpets do not appear in the fossil record any earlier. This has been underlined by some studies that perhaps canopy kelps did not evolve until the Pliocene. However - we generally don't have much of a fossil record of rocky shore environments, at least along the Pacific coast, from rocks that are any older than the Pliocene, and most are known from the Pleistocene. Is this reliable, or an example of "pull of the recent"?
Purple urchins (Strongylocentrotus purpuratus) in their typical habitus: embedded in spherical borings in the rock, waiting to catch pieces of drift kelp with their tube feet. So long as they stay in their borings, kelp forests can prosper. The smooth rocks are actually 'captured' by the urchins and held in place by tube feet, a sort of layer of armor against predators. Fitzgerald Marine Preserve, California. Photo by the author.
Perhaps my only sighting of a red urchin - Mesocentrotus franciscanus - these tend to live deeper than purple urchins and are difficult to find while tidepooling. Being only subtidal, this one I spotted about a meter below 0 while snorkeling in San Diego. Photo by the author.
A purple urchin (Strongylocentrotus purpuratus) and its reflection in a San Diego tidepool. Photo by the author.
An absolutely incredible specimen - a gigantic test (shell) of a red urchin, Mesocentrotus franciscanus, spotted recently at the Charleston Marine Life Center in Oregon. Photo by the author.
Purple and Red Sea urchins - Strongylocentrotidae
The Strongylocentrotidae are a family of sea urchins including the infamous purple urchin (Strongylocentrotus purpuratus) of the eastern North Pacific, and the larger red urchin, Mesocentrotus franscicanus (also from the Pacific coast). These urchins are herbivorous and, along the Pacific coast, tend to be sessile and excavate spherical borings and catch bits of drift kelp with their tube feet - at least in areas where there is plenty of drift kelp. If kelp becomes rare, they will leave their borings and become more mobile, grazing directly on live algae and kelp and causing the proliferation of urchin barrens. The sheer majority of strongylocentrotid species live in or adjacent to kelp forest habitat (or, former kelp forests) and feed either on drift kelp and other drift algae, or directly on macroalgae. The modern clade of North Pacific forms evolved mostly in the past 5-6 million years, surprisingly similar to the origin of gigantism in abalones (Haliotis; see above) and gigantism and toothlessness in sea cows (Hydrodamalis; see below). I suspect that the origin of these urchins in the eastern North Pacific around the end of the Miocene indicates the timing of initial kelp forest expansion.
Sea cows - Hydrodamalis (and Dusisiren?)
The gigantic sea cows of the Hydrodamalinae are unique to the North Pacific, and include two genera: Dusisiren and Hydrodamalis, which likely form an ancestor-descendant relationship, and culminated in the recently extinct Steller's sea cow, Hydrodamalis gigas. This gigantic relative of the manatee and the dugong of the tropics attained lengths of 7-8 meters, was completely toothless, lived in herds, and grazed chiefly on algae including sieve kelp (Agarum), bull kelp (Nereocystis), cup-and-saucer algae (Constantinea), and winged kelp (Alaria). These are all either short algae that can be accessed in shallow waters, or floating forms that with pneumatocysts that reach the surface. Despite being extinct, these sea cows were observed in detail by G.W. Steller when the ship St. Peter of the Bering expedition was shipwrecked in the Komandorskiye Islands, southwest of the Aleutians and east of Kamchatka. Critically, Hydrodamalis gigas lacked teeth - instead, there were keratinous (horny) pads on the palate for crushing the relatively soft tissues of fleshy kelps and other macroalgae. (Domning, 1978, points out that these are the same horny pads in front of the cheek teeth in other sirenians, and that they simply expanded further posteriorly in Hydrodamalis gigas, rather than replacing the teeth, which is a common misconception).
The titanic skeleton of Hydrodamalis cuestae, the largest sea cow to ever evolve, on display at the San Diego Natural History Museum. This skeleton is known from the Pliocene San Diego Formation of California. Photo by the author.
Extinct species of hydrodamalines further illuminate the evolutionary heritage. There are several species of Dusisiren, the best known of which is the ~4 meter long Dusisiren jordani from the Santa Margarita Sandstone and Monterey Formation of California; this resembled a gigantic tuskless Metaxytherium from the Atlantic, and likely evolved from Metaxytherium - perhaps something like M. arctodites from the middle Miocene of southern California and Baja. Though lacking tusks, Dusisiren jordani retained a full complement of adult teeth; additionally, the rostrum is not deflected, which in concert with the lack of tusks, suggests feeding upon seagrasses (or algae) in the water column. Dusisiren dewana is the very first hydrodamaline to disperse westward across the North Pacific rim to Japan, and is known from the latest Miocene of California as well; this species gave rise to Hydrodamalis cuestae, the largest sirenian ever, and the presumed direct ancestor of Hydrodamalis gigas.
An enormous skull of Hydrodamalis cuestae from the 2-4 Ma (Pliocene) San Diego Formation of southern California - perhaps the largest individual sirenian ever. On display at the San Diego Natural History Museum. Photo by the author.
Hydrodamalis cuestae was originally reported from the Pismo Formation of central California, and subsequently from the San Diego (early to late Pliocene) and San Mateo Formation (late Miocene and early Pliocene) of San Diego County. I've even discovered some pieces of this animal in the Purisima Formation at Point Reyes. Hydrodamalis cuestae is quite interesting because it is nearly double the length of Dusisiren jordani despite being just a few million years younger - indicating quite the increase in body length between ~8-9 Mya and 3-5 Mya; the largest known skull of Hydrodamalis cuestae is the titanic specimen SDSNH 90767, perhaps reflecting a formidable beast measuring up to 10 meters in body length. This incredible skull is on display at the San Diego Natural History Museum. Perhaps more critical than its stupendous size is the fact that Hydrodamalis cuestae is the first toothless sea cow - juveniles retained vestigial cheek teeth, but adult specimens lack them entirely, just like the recently extinct Steller's sea cow. Kelp is quite soft and decidedly non-gritty, and does not require much chewing. In fact, Steller both wrote that the sea cows chewed their food, but also that large pieces of intact kelp were present in the guts of dead individuals - perhaps suggesting that they simply snagged pieces of kelp right off the stipe and swallowed it whole without much chewing.
By the time of discovery by European explorers, Hydrodamalis gigas only lived in the shallow waters around the uninhabited Komandorskiye Islands. Subfossil and some historical evidence indicates that Hydrodamalis inhabited parts of the Western Aleutians (Near Islands, Rat Islands, Andreanof Islands) into the past 2,000 years and overlapped for thousands of years with the first aboriginal humans to arrive on these islands. Pleistocene fossils indicate a near circum North Pacific distribution for Hydrodamalis gigas, from southern California to Japan - and by the early Holocene, the population was restricted to the northernmost Pacific. Ecological modeling suggests that, even if aboriginal hunters did not even target Hydrodamalis in the Aleutians, overhunting of sea otters would have led to the loss of kelp forests and could have driven the majority of range loss of Hydrodamalis all on its own.
Overall, it seems as though hydrodamalines evolved toothlessness and gigantism right around the Miocene-Pliocene boundary, similar in timing to the gigantism of abalones and the diversification of strongylocentrotid urchins.
A Desmostylus mandible on display at the Natural History Musuem of Los Angeles in the "LA Underwater" exhibit. Photo by the author.
Desmostylians - Desmostylus and Neoparadoxia
The bizarre desmostylians are a group of extinct four-legged, somewhat hippo-like herbivorous marine mammals that are either most likely related to elephants and sirenians in the Afrotheria/Tethytheria or possibly to rhinos in the Perissodactyla; their teeth are so highly derived, that it's nearly impossible to know at present (we need more ancestral forms). These strange marine mammals can get quite large - skulls up to, and even exceeding a meter - and have upper and lower tusks that protrude from the mouth, along with a series of bizarre premolars and molars that look like a bunch of cylinders bound together; the most stereotypical teeth look like six packs of beer cans (or like a package of sushi rolls). Their fossil record is best sampled in California and especially Japan, where there are even some cartoon desmostylian mascots; in fact, it's quite clear that desmostylians are quite possibly the most emblematic of the Japanese fossil record as they're found throughout the country and on display in most natural history museums. There are a few genera - Desmostylus, "Vanderhoofius", Ounalashkastylus, Paleoparadoxia, and Neoparadoxia from the early through late Miocene (Neoparadoxia is classically only middle to late Miocene), and Ashoroa, Behemotops, Seuku, Cornwallius, and Archaeoparadoxia from the late Oligocene and earliest Miocene. All of these have similar dentitions.
While desmostylians are clearly not carnivores, they have been proposed to have been molluskivorous in the past. Their strange teeth, and lack of commonality with tooth wear seen in land mammals, has stymied attempts to interpret their feeding ecology from dental microwear. Desmostylus hesperus has, however, been sampled for oxygen isotopes. These isotopic ratios indicate that it mostly fed in shallow waters, likely to be estuaries and embayments- but nearly exclusively ate aquatic vegetation, likely seagrasses, but also possibly some algae like sea lettuce (Ulva spp.). The paleoparadoxiids Paleoparadoxia and Neoparadoxia have not yet been studied isotopically, but in the Japanese rock record, are nearly always found in deeper water rocks than Desmostylus - perhaps indicating that Paleoparadoxia and Neoparadoxia would have been more likely to feed on kelp, which grows in much deeper waters than seagrasses do.
As for a specific diet beyond marine herbivory - the jury's still out regarding kelp or seagrass as the major food for desmostylians. Did they feed on kelp? Daryl Domning wrote in his earlier studies that because desmostylians immigrated eastward from the coast of Asia during the late Oligocene, and that hydrodamaline sea cows did not immigrate westwards from the east Pacific until the latest Miocene, that desmostylians must have had a higher tolerance for cold climates; no doubt their semiaquatic habitus must have helped. Did desmostylians and hydrodamalines compete? Certainly, at some level - but since they coexisted with sirenians for much of the Miocene, it seems unlikely that they were outcompeted by hydrodamalines. On the contrary, Domning considered that desmostylians may have driven the tusked sirenian Dioplotherium to extinction in the middle Miocene. Cooling of the Pacific in the late Miocene seems to be an unlikely cause for their extinction, as they traversed the North Pacific quite early.
However, the extinction of desmostylians does coincide with the evolution of gigantism in hydrodamalines. Could the later species of Dusisiren have outcompeted desmostylians for food? Depending upon how little area was colonized by kelp, it's possible. Domning notes that gigantism doesn't seem to evolve until after the last desmostylians become extinct - indeed, Dusisiren dewana is the hydrodamaline that co-exists with the geochronologically latest desmostylians. I do find it troubling, if desmostylians were eating kelp, that they never evolved toothlessness (though gigantic sizes were attained by some desmostylians which had shed their last teeth; Santos et al. 2016). This to me suggests a sustained contribution of seagrasses. Perhaps the middle to late Miocene cooling that resulted in a proliferation of kelp, and likely caused a contraction in the size of seagrass beds - could explain both the size increase and development of toothlessness in hydrodamalines, and the extinction of desmostylians?
Blacksmith damselfish (Chromis punctipinnis) amidst a tangle of kelp in a tank at the Monterey Bay Aquarium. Photo by the author.
Future Directions and Unanswered Questions
1) Who fed on marine invertebrates prior to the middle Pleistocene arrival of sea otters?
This is a major paradox in the otters as keystone species hypothesis for Pacific coast kelp forests. Otters very much seem to foster kelp forest health today - though their fossil record is quite shallow, only seeming to have been here for slightly longer than half a million years - less than one twentieth the time since the Macrocystis-Nereocystis clade diverged in the late Miocene. So, what species might have kept the voracious appetites of sea urchins and abalones at bay prior to the Pleistocene? A few studies have proposed that dusignathine walruses during the Late Miocene and Pliocene may have fulfilled that role, and Kolponomos in the earliest Miocene. This is admittedly speculative, as the diet of dusignathines has not been studied in detail, and the jury's out as to what they were feeding on. Further, there are rather long gaps in between the fossil records of these taxa. Strongylocentrotid urchins may not have even existed along the Pacific coast until the late Miocene - so it's possible that invoking Kolponomos is not necessary. Additionally, Enhydritherium is known from a couple of latest Miocene to early Pliocene scraps from California, but this species was likely not dentally equipped for feeding on kelp forest invertebrates.
Is it possible - and this is going to sound uncharaceristically anti-mammal for me for once - that we've been ignoring other invertebrate predators? Sea otters are not the only durophagous predators of marine invertebrates along our coast. Many of these predators are fish and some are even invertebrates.
A wolf eel (Anarrichthys ocellatus) in a tank at the Monterey Bay Aquarium. Photo by the author.
A) Wolf eels (Anarrichthys ocellatus) are quite large (2 meters, up to 15-20 kg), have conical teeth in front and blunt, molar-like teeth in the back of the jaw and feed on mollusks, sea urchins, and crustaceans as adults. They live exclusively on rocky bottoms from Alaska to northern California. These fish also have a longer fossil record in the North Pacific, going back to at least the early Pliocene. Interestingly, these fish evidently eat so many urchins that they can also get echinochrome staining in their teeth and bones, just like sea otters!
A California horn shark (Heterodontus francisci) hunting through the red algae for crustaceans and other invertebrates to eat. La Jolla, California. Photo by the author.
B) Horn sharks (Heterodontus francisci) live along the California coast today, from near the Oregon border down to Baja and nearly to Puerto Vallarta. These tubby, cute little sharks grow to about a meter and have a dentition that is functionally similar to wolf eels; juveniles prefer soft bottom, but adults prefer rocky bottoms including kelp forests. And, they have a nearly identical diet to wolf eels. The horn sharks are an ancient lineage as far as modern shark species are concerned, and molecular clock dating suggests that our species here in California waters diverged from other lineages during the Eocene, about 43 Mya.
A juvenile bat ray (Myliobatis californica) in the touch tank at the Monterey Bay Aquarium. Photo by the author.
C) The bat ray (Myliobatis californica) lives on sandy and rocky bottoms from Oregon to southern Baja California and females attain diameters of nearly two meters and weigh up to 90 kg. Like wolf eels and horn sharks, it mostly feeds on mollusks, urchins, and crustaceans - throughout its life span - using the flat table-like crushing tooth plates common in bat and cownose rays. Myliobatis has been on the California coast for tens of millions of years. Additionally, there are several other species of durophagous rays including some species of guitarfish and stingrays, though admittedly they have a somewhat higher degree of a piscivorous diet.
A terminal phase California sheepshead (Semicossyphus/Bodianus pulcher) in the kelp forest tank at the Monterey Bay Aquarium. Photo by the author.
D) The California sheepshead, Semicossyphus pulcher (or, more recently, in the genus Bodianus) - one of our most famous marine fish - is a giant wrasse that lives from Monterey to southern Baja, grows to about a meter and 15 kg, and feeds chiefly on sea urchins, mollusks, and crustaceans. The jaws are lined with some conical caniniform teeth and a bunch of low crowned onion shaped teeth, which are not much use in crushing - but instead, has some massive pharyngeal tooth plates constructed out of many little spheres of enamel - essentially, crushing plates that are in the fish's throat. The crushing plates are common in the Santa Margarita Sandstone and other late Miocene through Pleistocene fish bearing deposits in California.
A massive sunflower star, Pycnopodia helianthoides. Photo by Marco Mazza.
E) The sunflower star (Pycnopodia helianthoides) is the world's largest sea star, growing up to a meter in diameter, and weighing in at about 5 kg. These sea stars formerly lived from Alaska to southern California, but sea star wasting disease has reduced their range south of British Columbia. Nevertheless, these are voracious predators of sea urchins. They do not have a fossil record, and I don't know how old the lineage is based on molecular clock divergence.
An ochre star, Pisaster ochraceus, found on the beach in Half Moon Bay - very much still alive, so I gave it a gentle toss into the surf zone. Probably a bit far away from rocky substrate - who knows how it washed up here. Photo by the author.
F) The giant pacific sea stars (Pisaster spp.), including the ochre star (Pisaster ochraceus), giant sea star (Pisaster brevispinus), and the giant pink sea star (Pisaster brevispinus), all get quite large (up to 25 cm in the ochre star, 40 cm in the giant, and 60 cm in the giant pink sea star) and live from Alaska to southern California or Baja. All feed on mollusks, but the ochre star in particular lives on the most violently active rocky shores and is similarly viewed as a keystone species for preying upon sea urchins - though tidepoolers will most likely see these going after California mussels. The other two species, though larger, tend to avoid open coast settings and prefer more gentle embayments. Unlike the sunflower star, the giant Pacific sea stars of the genus Pisaster do have a fossil record, and are known from the early Pliocene and late Miocene parts of the Purisima Formation (though these fossils are undescribed).
My most active giant pacific octopus (Enteroctopus dofleini) sighting in an aquarium ever, at the Steinhart Aquarium, California Academy of Sciences in San Francisco, California. Photo by the author.
G) And lastly, how could we forget the world's largest octopus? The Giant pacific octopus (Enteroctopus dofleini) lives mostly on rocky bottoms from the low tide line to 2,000 meters, and has a circum-North Pacific distribution from the East China Sea north to Japan and the Aleutians, and south through Alaska, British Columbia down to California and northern Baja. These eat all sorts of mollusks, crustaceans, sea urchins, and fish. Unfortunately these animals are basically all soft tissue and we have no fossils of them - though perhaps gigantic sized octopus feeding traces of the ichnogenus Oichnus could be found and attributed to Enteroctopus; the Enteroctopus lineage (which includes other smaller species) diverged from other members of the same family during the earliest Miocene or Oligocene.
In sum, there are numerous other non-mammalian species that we known have existed along the Pacific coast for millions of years that have diets that overlap completely or considerably with sea otters - or, likely existed along our coastline owing to molecular clock analyses (for the gooeyest species lacking a fossil record, that is). How much of the sea urchin biomass these species are responsible for dispatching is an open question - though it is worth noting that urchin barrens have considerably expanded in the wake of sea star wasting disease, which most heavily affected the sunflower and ochre stars - suggesting that these species are also, indeed, keystone species like sea otters.
2) Is our fossil record of kelp-associated species good enough to say anything conclusive about the proliferation of kelp forests?
This is truly an existential question of the sort that keeps - or, should keep - invertebrate paleontologists and paleobiologists awake at night. It's an easy question for taphonomists to ask, but an incredibly difficult question to answer. In essence: kelp forests typically only occur on rocky bottoms, and rocky bottoms are characterized by erosion, rather than deposition. While fossils do often get concentrated in thin strata representing periods of time of mild erosion, severe erosion that might expose bedrock on the seafloor as well as form cliffs and sea stacks - like the sheer majority of the modern California coastline - tends to result in wave action that is so strong that shells and bones alike are typically abraded down to little pebbles. I'm going to highlight, and contrast, two relevant rock units.
The first is the Purisima Formation of northern California. The Purisima Formation was deposited in a series of two or three different and relatively small basins around proto-Monterey Bay, and though it starts off as an offshore deposit in the latest Miocene, by the early Pliocene parts of the unit it reflects considerable shallowing to inner shelf settings (under 15 meters deep). None of the basins could have been so vastly large that rocky shore settings - if they existed - would have been more than 10-15 kilometers away. The Pliocene coastline was probably considerably sandier than now, and may not have had any cliffs - instead resembling the sandy shoreline present closer to Watsonville and south to Fisherman's Wharf in Monterey. There are virtually no fossils of kelp forest inhabitants in the Purisima: no kelp crabs, no abalone - though there are some kelp forest-adjacent gastropods (Tegula) and clams (Saxidomus) - or rocky shore environments including ochre stars (Pisaster) and mussels (Mytilus). There are erosional surfaces in the Purisima that have hard bottom - but were likely too deep to support kelp beds, and in any event, all of the calcium carbonate is gone. Lastly, there is some evidence of kelp along the periphery of the basin in the form of possible kelp dropstones (see above). This is largely consistent with a growing body of taphonomic evidence that suggests that invertebrate fossil assemblages are, in a spatial sense, very reflective of the original ecology: simply put, mollusk shells don't move around too much, at least not out of their original depositional environment. Inner shelf sandy bottom mollusks tend to stay in that environment and not end up in kelp forests (or the outer shelf, for that matter). The Purisima Formation - reflecting a predominantly soft bottom environment, with occasional evidence of kelp forests nearby - probably reflects the majority of the rock and invertebrate fossil record, at least on the Pacific coast.
The second is the Moonstone Beach Formation of northernmost California. This unit is not widespread and is only preserved as small lenses between rocky headlands, and occasionally even as sedimentary fill within rocky subtidal crevasses - likely representing sandy deposition within surge channels between masses of Franciscan Complex bedrock. It is quite densely fossiliferous, and has produced a pretty phenomenal fossil assemblage for such a small outcrop, including tons of mussels (Mytilus), barnacles (Megabalanus), keyhole limpets (Cranopsis, Diodora), chitons (Mopalia, Tonicella, Ischnochiton, Lepidochiton, Lepidozona), gooseneck barnacles (Pollicipes), a snail associated with feather boa kelp (Collisella), intertidal snailes (Littorina), kelp crabs (Pugettia gracilis), a lithodid crab associated with calcareous algae (Oedignathus inermis), and three species of urchins including the purple (Strongylocentrotus purpuratus), red (Mesocentrotus franciscanus), and green urchin (Strongylocentrotus droebachiensis). And, of course, there is the extinct sea otter, Enhydra macrodonta, known from the unit as well. In addition to the geomorphologic evidence of the bedrock and crevice and channel infill style deposition of the Moonstone Beach Formation, there is simply copious evidence from the fauna that there was a rich kelp forest habitat right there - unlike the Purisima Formation.
There's only one problem with the Moonstone Beach Formation. It's very young - only middle Pleistocene in age, perhaps 500,000 years. The cliffs were likely eroded during the Marine Isotope Stage 11 marine highstand, and coastal uplift raised the "bathtub ring" deposits of the Moonstone Beach formation about 20-30 meters up, just far enough to prevent further erosion. The deposit itself is not very voluminous and the outcrops themselves are very small and difficult to access, let alone even find, now that many are becoming increasingly overgrown. Accessing one deposit is a bit like an adventure from The Goonies - unless you want to swim, you need to wait for a minus tide in the winter or spring, walk out along a sandbar that remains shallowly submerged (taking care not to fall into a surge channel as I did), walk to a sea stack, find a cave underneath, walk through and around to the top where the unit fills a 3-4 meter wide crevasse - all the while trying not to accidentally step on baby seabirds.
The point of all this is that if the Moonstone Beach Formation was any older, it would either likely be completely eroded away after a couple of million years, or be completely buried. There very well might be deposits of the Purisima Formation that are rocky shore settings - but those are along the margins of the basins where the Purisima sediments meet the ancient shoreline - and are probably still deeply buried. Once exposed by natural erosion, however, these deposits likely don't last very long. Rocky shore environments, at least on the Pacific coast, are probably quite ephemeral - virtually all of our rocky shore fossil assemblages, as a result, are all from the middle and late Pleistocene. What this means is - the sudden appearance of many kelp associated invertebrates in the Pliocene and Pleistocene might simply reflect the fact that most of our rocky shore deposits happen to be extremely young and therefore might be considerably biased (e.g. the limpets mentioned above).
3) Will we find more kelp holdfasts and fossil kelp?
I damn well hope so. I think further fieldwork is needed, and in units like the Purisima Formation - where fossil baleen has been preserved, and fossilized skate cartilage is common - there's probably a high capacity for preserving kelp holdfasts if we know where to look. I've never spotted something like a branching holdfast, but I never thought something like that could fossilize - so now I'll really be paying attention.
Why haven't there been any studies on Monterey Formation macroalgae since the 1960s? The collection reported by Parker and Dawson (1966) is quite large - surely, museums like LACM must have continued fieldwork in some of these localities. I suspect that there is probably more fossil kelp awaiting discovery, or that some might even be sitting in museum collections somewhere awaiting study.
4) Can we find more indirect evidence of kelps, such as dropstones?
I actually think this has a lot of promise. There are only so many ways to get large stones into middle and outer shelf deposits - kelp rafting/dragging, and gastroliths in some birds and pinnipeds. I'd wager that kelp is considerably more abundant than any gastrolith producing vertebrates. Even a field survey reporting on the stratigraphic pattern of kelp dropstones could be informative - do these stones appear suddenly during the late Miocene? A careful analysis could highlight such a pattern.
5) Do kelp holdfasts leave any traces on rocky or shelly substrate?
This is a question for an ichnologist to figure out. If there is some signature or trace fossil that kelp holdfasts might leave behind, this could further expand our search for evidence of kelp forests.
6) And lastly, what drove the evolution of rapid kelp growth?
This is admittedly a chicken and the egg question that is typically quite difficult to answer in paleobiology - but, is it possible that intense herbivory drove the rapid growth of kelp, and not the other way around? In other words - is it possible that things like desmostylians, hydrodamaline sea cows, strongylocentrotid sea urchins, and abalones could have triggered the unique growth pattern of kelps? The rather ancient (e.g. Eocene-Oligocene) origin of Laminariales suggests against this, but the truly rapid growth rates seen in the canopy kelps that have a more recent phylogenetic origin could have been influenced by intense herbivory during the Miocene.
A special thank you to "Piranha" on thefossilforum.com for providing me with a much-needed pdf of Parker and Dawson (1965).
References and Further Reading
Boessenecker 2018. A Middle Pleistocene Sea Otter from Northern California and the Antiquity of Enhydra in the Pacific Basin. Journal of Mammalian Evolution 25:27-35. https://link.springer.com/article/10.1007/s10914-016-9373-6
Clementz et al. 2003. A paleoecological paradox: the habitat and dietary preferences of the extinct tethythere Desmostylus, inferred from stable isotope analysis. Paleobiology, 29:506-519. https://www.cambridge.org/core/journals/paleobiology/article/abs/paleoecological-paradox-the-habitat-and-dietary-preferences-of-the-extinct-tethythere-desmostylus-inferred-from-stable-isotope-analysis/88512187D088C94D924BFF0ACEE78B43
Craw and Waters 2018. Long distance kelp-rafting of rocks around
southern New Zealand, New Zealand Journal of Geology and Geophysics, 61:428-443. https://rsnz.onlinelibrary.wiley.com/doi/10.1080/00288306.2018.1492424
Domning 1978. Sirenian evolution in the North Pacific Ocean. University of California Publications in Geological Sciences 188:1-176.
Domning 1989. Kelp evolution: a comment. Paleobiology, 15:53-56. https://www.cambridge.org/core/journals/paleobiology/article/abs/kelp-evolution-a-comment/51069F5673A8EB585E1C19661E231D1C
Domning et al. 2007. Steller's sea cow in the Aleutian Islands. Marine Mammal Science 23:976-983. https://onlinelibrary.wiley.com/doi/10.1111/j.1748-7692.2007.00153.x
Estes and Steinberg 1988A. Predation, herbivory, and kelp evolution. Paleobiology 14:19-36. https://www.cambridge.org/core/journals/paleobiology/article/abs/predation-herbivory-and-kelp-evolution/8BD83F331294C4B30E470A301660624E
Estes and Steinberg 1989. Response to Domning. Paleobiology 15:57-60. https://www.cambridge.org/core/journals/paleobiology/article/abs/response-to-domning/55141B6BE582E02693DB4010D6C6D359
Estes et al 2005. Evolution of large body size in abalones (Haliotis): patterns and implications. Paleobiology 31:591-606. https://www.cambridge.org/core/journals/paleobiology/article/abs/evolution-of-large-body-size-in-abalones-haliotis-patterns-and-implications/530B5BCEF0273104082B3B779E4E834F
Frey et al. 2012. Seaweed-assisted, benthic gravel transport by tidal currents. Sedimentary Geology 265-266:121-125. https://www.sciencedirect.com/science/article/abs/pii/S0037073812001005
Geiger and Groves 1999. Review of fossil abalone (Gastropoda: Vetigastropoda: Haliotidae) with comparison to recent species. Journal of Paleontology 73:872-885. https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/review-of-fossil-abalone-gastropoda-vetigastropoda-haliotidae-with-comparison-to-recent-species/D2BCDF9E75306D14955DE0C202F9F25D
Kawai et al. 2013. Ancestral reproductive structure in basal kelp Aureophycus aleuticus. Scientific Reports 3:2491. https://www.nature.com/articles/srep02491
Kenyon 1969. The sea otter in the eastern Pacific region. North American Fauna 68: 1-352. https://digital.library.unt.edu/ark:/67531/metadc700973/
Kiel et al. 2024. Early Oligocene kelp holdfasts and stepwise evolution of the kelp ecosystem in the North Pacific. Proceedings of the National Academy of Science 121:e2317054121. https://www.pnas.org/doi/10.1073/pnas.2317054121
Lee 2003. Molecular phylogenies and divergence times of sea urchin species of Strongylocentrotidae, Echinoidea. Molecular Biology and Evolution 20:1211-1221. https://academic.oup.com/mbe/article/20/8/1211/1081403
Rathbun 1926. The fossil stalk-eyed Crustacea of the Pacific Slope of North America. United States National Museum Bulletin 138:1-149. https://www.biodiversitylibrary.org/page/8429305
Santos et al. 2016. New data on the ontogeny and senescence of Desmostylus (Desmostylia, Mammalia). Journal of Vertebrate Paleontology 36:e1078344. https://www.tandfonline.com/doi/full/10.1080/02724634.2016.1078344
Sarko et al. 2010. Estimating body size of fossil sirenians. Marine Mammal Science 26:937-959. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1748-7692.2010.00384.x
Starko et al. 2019. A comprehensive kelp phylogeny sheds light on the evolution of an ecosystem. Molecular Phylogenetics and Evolution 136:138-150. https://www.sciencedirect.com/science/article/abs/pii/S1055790319300892
Vermeij 2012. The evolution of gigantism on temperate seashores. Biological Journal of the Linnean Society 106:776-793. https://academic.oup.com/biolinnean/article-abstract/106/4/776/2452394
Vermeij et al. 2018. The coastal North Pacific: origins and history of a dominant marine biota. Journal of Biogeography 46: 1-18. https://onlinelibrary.wiley.com/doi/10.1111/jbi.13471
Waters and Craw 2017. Large kelp-rafted rocks as potential dropstones in the Southern Ocean. Marine Geology 391:13-19. https://www.sciencedirect.com/science/article/abs/pii/S0025322717302025
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