Tuesday, June 9, 2020

The terrible fossil record of sea otters, part 2: un-fantastic otter fossils and where to find them

Make sure to take a look at part 1, where I summarized what we know about modern sea otter anatomy, behavior, geography, and molecular phylogeny.





And when you're done, you can find the gripping conclusion here in part 2.


Introduction

This post is organized into two sections: the first is a mostly comprehensive summary of the study of fossils of Enhydra-like otter fossils (Enhydrini) in mostly chronological order. The second part goes into my own field excursions in search of fossil sea otters. The last post in this series will discuss what we can tell from the fossil record about the evolution, diversity, paleoecology, and paleogeography of extinct sea otters and the recent evolutionary history of the modern sea otter.




The distinctive but unspectacular holotype molar of Enhydra reevei, from Mitchell (1966).


Enhydra reevei – the first fossil sea otter
                An isolated tooth from Plio-Pleistocene “rocks” in Suffolk, UK, was named Lutra reevei by Newton (1890) and later recombined as Enhydriodon reevei. Enhydra reevei is very fragmentary, but uniquely share with Enhydra lutris completely rounded cusps and a complete lack of any crests or ridges on its teeth. All other lutrines, including Enhydriodon and Enhydritherium, possess some sharp cusps and deep pits. A second tooth was discovered later and reported by Willemsen (1990).


The horrifying skull of the "bear otter" Enhydriodon dikikae, from Geraads et al. 2007.


Femora of giant extinct African otters (Enhydriodon?) compared with Enhydra
modified from Lewis (2008).

Enhydriodon – the old world giant otters
                Several species of Enhydriodon have been named – giant otters, some exceeding the size of Enhydra, from Europe, Africa, and India. Most are known only from jaw fragments or isolated postcrania. These were among the earliest Enhydra-like otters discovered in the fossil record, and all are late Miocene or Pliocene in age. The “bear otter” Enhydriodon dikikae was reported recently from the Pliocene (3.3-3.5 Ma) of the Awash Valley in Ethiopia, and titanic otter femora of uncertain affinities (perhaps belonging to this taxon) have also been reported rocks of the same age in the Omo valley of Ethiopia (Lewis, 2008). The femora preserve Enhydra-like aquatic adaptations, perhaps suggesting a less terrestrial existence than most nonmarine otters, likely encouraged by how much more wet and lush subsaharan Africa was during the Pliocene. This gigantic otter was probably 2 meters in length and had a massive, thickened skull approximately the size of a black bear.


The entirety of the southern California sea otter record (all Enhydra lutris
as reported by Mitchell, 1966.

Enhydra lutris from southern California
                A number of bones, mostly postcrania, were reported by Ed Mitchell in 1966 from Los Angeles county and the Channel Islands. Most of these were from various deposits of middle and Pleistocene age like the San Pedro Sand (500,000-200,000 years) or the Palos Verdes Sand (130,000-85,000 years). One tooth, however, was from the Timms Point Silt – which Mitchell thought at the time was Pliocene (we'll get back to the Timms Point Silt in Part 3). So, he discounted the tooth of Enhydra reevei as being a genuine sea otter because his southern California specimen was supposedly much older, thereby indicating a North Pacific origin for Enhydra. At the time this specimen was the oldest known Enhydra fossil worldwide. This was reinforced by the discovery of some very fragmentary specimens (a tooth and a fragmentary mandible, one from Kettleman Hills in the San Joaquin valley of California and the other from the San Mateo Formation of San Diego County, California) about ten years later, which Charles Repenning initially identified as Enhydriodon, and later reidentified as Enhydritherium.


The holotype mandibles of Enhydra macrodonta (A, C, D) and modern Enhydra lutris (B), from Kilmer (1972). You can really appreciate how much larger the molars of the extinct species are!

Enhydra macrodonta – the big-toothed sea otter
                Only a few years later (1972), an unusually informative sea otter was discovered at the Crannell Junction locality in Humboldt County, just about a mile south of Moonstone Beach. This locality was an old dumping site for Cal Trans, and there used to be a nice exposure of the “Moonstone Beach Formation” (still not formally recognized), which is middle Pleistocene in age, and likely to be about 500,000-700,000 years old. This fossil consisted of a pair of well-preserved mandibles stuck inside a soft clayey nodule; the teeth were proportionally wider than modern Enhydra lutris, and so it was named Enhydra macrodonta. The holotype, supposedly in the geological collection at the Geology Department at Humboldt State University, was not given a number, and upon my visit in 2008, was not labeled, and one of the mandibles was missing without explanation (nor did anyone in the department seem to know that such a critical fossil was in their care). As a matter of fact, owing to a label from a different fossil, I thought this was a second specimen, until reexamining photos during my Ph.D. I matched it with the less complete of the two holotype mandibles. According to Dr. Frank Kilmer, the collector demanded that the school return the fossil to her, which the school obliged - despite already being published (albeit uncatalogued) as a type specimen. Kilmer was under the impression that both mandibles were returned, yet somehow one of them ended up back at HSU. The whereabouts of the better mandible are unknown, as the collector’s identity is also unknown. It was, however, known only by a former invertebrate paleontologist turned barkeeper in Arcata, California, who would not return any of my phone calls. A partial cast of this missing mandible is preserved at UCMP, found among the fossils that were on loan to Repenning at the time of his murder in 2005 – but not specifically labeled as such.


The remaining left mandible of the holotype of Enhydra macrodonta, as I discovered it hiding, unlabeled, at Humboldt State University in 2008. Because 1) Kilmer (1972) only figured the left mandible in dorsal view, 2) the specimen had a label implying it was from a different locality, and 3) Kilmer wrote to me indicating that both mandibles were returned to the collector, it took me about five years to realize that this was in fact the left mandible of the holotype and not a second specimen.

Regardless of the strange history of the type specimen, it has made its way to the collections at UCMP (Berkeley) along with a bunch of other fossils I hand picked for salvage about ten years ago – many thanks to Dr. Pat Holroyd for managing the transfer, picking up the fossils, and getting them curated at UCMP. The remaining holotype mandible (and other referable specimens) will be under study in the future by Ash Poust (SDNHM) and myself. The uncertainty regarding the fate of these fossils gave me no shortage of anxiety over the course of my graduate and doctoral programs.

A femur of Enhydra sp., and possibly Enhydra macrodonta, from the middle Pleistocene Port Orford Formation of Oregon, reported by Leffler (1964).

                What do we know about Enhydra macrodonta? Not much, unfortunately. It’s got big teeth, it’s about the same size as Enhydra lutris, and is from the late middle Pleistocene. It may have had a higher bite force owing to the larger teeth, and perhaps a greater capacity for durophagy. But we know nothing about its skull, and scattered postcrania have not yet been analyzed. A couple of femora of Enhydra sp. were discovered in the 1960s and 1970s from rocks of the same age at Cape Blanco, Oregon, though only one has been described. These fossils have been widely misinterpreted as being Pliocene in age by careless researchers.


The California "Enhydriodon" mandible reported by Repenning (1976) with bona fide European Enhydriodon and Enhydra teeth for comparison. I've left the original caption in place. 

Enhydriodon from California?
A few scattered Pliocene otter specimens from California, originally identified as Enhydriodon (later identified as Enhydritherium). These were reported in the late 70s by Repenning, and reinforced Mitchell’s (1966) hypothesis that Enhydra-like otters have been living continuously along the Pacific coast over the past 5-6 million years, and had evolved in situ within the Pacific (see above). Repenning later revised this upon study of newly discovered sea otter fossils from Alaska (see below). 


A fossil Enhydra mandible fragment from the Gubik Formation of Alaska. Don't get too excited: the crappy mandible is the fossil! The complete one is a modern one with all the teeth pulled out. 3 is an impression of the molar roots of Enhydriodon lluecai, and 4 is the molar of Enhydra reevei, and 5 is a modern Enhydra molar. From Repenning (1983).

Scattered Enhydra fragments from Alaska
                The Gubik Formation on the North Slope of Alaska is Pliocene and Pleistocene in age, and consists of a series of transgressive deposits made during sea level highstands corresponding to interglacial periods. Each member of the Gubik represents a different highstand, and since the unit covers about 3 million years of geologic time, knowing only the formation name is supremely unhelpful for biochronologic purposes. A number of scattered specimens have been reported from the Gubik Formation (Repenning, 1983), including a fragmentary mandible with well-preserved lower molar alveoli. These alveoli are narrower than what you see in modern Enhydra lutris, and Repenning interpreted this latest Pliocene or earliest Pleistocene mandible a belonging to a primitive species of Enhydra that was close to Enhydriodon. The alveolar morphology also corresponds closely to that of Enhydriodon lluecai from the Pliocene of Spain. Repenning further speculated that the somewhat more gracile mandible of this Enhydra sp. from Alaska could correspond to the narrower molar of Enhydra reevei. Repenning, in the same paper, suggested his 1976 interpretation of in-situ evolution in the North Pacific from Enhydritherium could be in error, and that a dispersal from a European ancestor (e.g. Enhydra reevei) was plausible.


I'm currently working on describing the sea otter skull from Walakpa Bay, Alaska, with Ash Poust, Morgan Churchill, and Chuck Powell.

Repenning also mentioned a partial skull of Enhydra from younger middle Pleistocene strata of the Gubik Formation, which he considered to represent the modern species. I first saw it in 2006 while browsing the California Academy of Sciences marine mammal fossil collection while they were still at the temporary downtown facility, and the late Curator Jean Demouthe dismissed the fossil as “just some late Pleistocene crap” and showed me some cetacean material. (The more I think about it the more I laugh; Jean was abrasive and formidable, and when I returned from NZ with my Ph.D. in hand she treated me very differently. She wasn’t for everybody, but liked her, and I do miss her.) Being 21 years old and naiive, I quickly agreed with her and moved on. It wasn’t for another year or so that I realized the importance of the fossil – since it’s apparently the only fossil cranium of Enhydra discovered anywhere. I have the specimen on loan now, and while I won’t divulge any of our team’s secrets, I will confirm that it is not Enhydra lutris, and we’re pretty excited about the implications of this fossil.





Skulls and mandibles of Enhydritherium terraenovae from the early Pliocene of Florida, from Berta and Morgan (1986) and FLMNH.

Enhyditherium – the red herring from Florida
                In 1985 a new genus of lutrine was named from the early Pliocene of Florida by Annalisa Berta and Gary Morgan – Enhydritherium terraenovae. This somewhat altered existing hypotheses of sea otter evolution and biogeography, as it was proposed to be the immediate sister taxon of Enhydra – therefore suggesting a North American origin for Enhydra, rather than a European origin from Enhydriodon. Additional specimens were reported from the west coast (in actuality originally referred to Enhydriodon by Repenning, and reidentified as Enhydritherium by Berta and Morgan). Enhydritherium was hypothesized to be an Enhydra-sized sea otter with a somewhat more river-otter like dentition.
                More completely preserved remains of Enhydritherium, including a pretty nice skeleton, were reported from Florida by Lambert (1997) – and still represents one of, if not the, most completely preserved fossil lutrines. He did not conduct a phylogenetic analysis, but noted that many of the localities where it had been found since 1986 had little marine influence, and he indicated that Enhydritherium was probably marine tolerant but not a marine specialist. In terms of feeding ecology, Enhydritherium most likely consumed a less mollusk-rich diet than Enhydra and was probably more reliant upon fish: Enhydritherium lacks the tiny forearms of Enhydra and is proportioned more like ‘normal’ lutrines. Regardless, in the absence of a cladistic analysis, evolution of Enhydra from an Enhydritherium-like ancestor in North America was the state of the science when I started researching sea otters over a decade ago.


An Enhydritherium mandible from the Pliocene of central Mexico, from Tseng et al. 2017.

                Subsequent discoveries and analyses have confirmed that Enhydritherium lived far within the continental interior, being discovered in terrestrial deposits in north central Mexico. Phylogenetic analysis by Wang et al. (2017) also seems to indicate that there is not a close relationship between Enhydra and Enhydritherium – the latter seems to some otters from the Miocene of Italy, and in one of their analyses, there seems instead to be support for a link between Enhydriodon and Enhydra. For these reasons, consideration of Enhydritherium
as a close relative or possible ancestor of Enhydra has been a 30 year long red herring.


The rugged, beautiful, and often foggy and eternally unforgiving coast of Humboldt County, California.

My desperate and minimally fruitful search for fossil sea otters in the Pacific Northwest
                It took me about 3-4 years of collecting Pliocene marine mammal fossils before I became wise to the fact that I had never found, nor heard of, a fossil sea otter from the Purisima Formation in northern California – despite preserving all manner of other modern marine mammal genera (Callorhinus, Tursiops, Phocoena, Balaenoptera, Eubalaena, among others). When I asked other paleontologists, they either shrugged (“I hadn’t really thought about it” or “We don’t find them in this other Pliocene unit either…”) or referred me back to Repenning’s paper and the Berta and Morgan (1986) papers. But, none of those are Enhydra proper. Where were the actual Enhydra fossils? They show up in Holocene middens and scattered late and middle Pleistocene deposits, but not a shred of evidence from the embarrassingly well-sampled San Diego Formation of southern California. I spent the following ten years searching the Purisima Formation near Santa Cruz and Half Moon Bay for unusual fossils and not once did I turn up any marine carnivore fossils that were not from fur seals or walruses.
                What do you do when you want to find something rare? Go and look where other people have found them! I spent a number of field trips, chiefly on a huge detour through Oregon on my many road trips to/from California and Montana at the beginning and end of summer break from MSU, visiting a handful of otter-bearing localities in Humboldt County, California, and Curry county, Oregon, chiefly of middle Pleistocene age. These visits were mostly from Summer 2007 to 2011, and have not been back since. On one trip, I spent three days in Humboldt County by myself staying at a KOA Kampground Kabin (yes, they’re really dedicated to naming everything with a K) and a Motel 6 in Arcata, scouting out all these different localities in a less frenzied fashion then before, where I thought “I still have a six hour drive to get home to San Francisco tonight…”. Quick story: I am a super light sleeper, and while the KOA cabin was great, my neighbors at the Motel 6 were a loud trailer trash family whose kids would start giggling every 30-40 minutes and the dad would yell “goddamnit I told you to shut the fuck up” a couple of times. It was super effective! And by effective, I mean it wasn’t, because I looked at my watch and saw it was 6:20 in the morning and I hadn’t slept a wink. It was because of this insomnia that I eventually cut this trip short.


Ash Poust and Lee Hall looking for Pleistocene mammal fossils at Moonstone Beach in Humboldt County. That was a very long, damp day.

                My first time looking for otters was a stop on a big road trip in 2008 to the Moonstone Beach locality. It is hard to find, as the trail is not visible from the beach; you need to spot a 3 foot section of overgrown trail from the beach, make it up 15 feet of slipper algae covered rocks in between waves, and then wind your way up a 100’ tall hill to a 20 foot tall, 150 foot long cliff that is invisible from the beach as well. On my first visit, in 2007 I believe, I could not find the trail and I did not make it to the locality. The abundance of poison oak killed my interest in exploring or hacking my way through the undergrowth. However, I planned things out better for my summer 2008 visit, got in touch with a fossil collector who had donated many significant finds in the area (Ron Bushell), who explained how to find it. I’ve visited it three or four times since, and mostly just found fish bones, a crumby deer astragalus, and a fragment of a probable cormorant humerus – but no marine mammal fossils.
                The next day on the same trip in 2008, I tried a locality about 100 yards away and higher up section that was a little road cut. Invertebrate paleontologists (Zullo, Durham, Wolfe) had collected a partial harbor seal mandible and a giant ground sloth claw core from the locality back in the 70s, so I thought it might produce some sea otters. I wasn’t wrong: after about a half hour of looking, I found a beautifully preserved upper molar of Enhydra macrodonta! The only known upper molar for the species, as it happens. So I bagged up about 200 lbs of gravelly matrix and brought them down to my car in the hopes of finding some more teeth or bones. By this point I had run out of water and desperately needed lunch: it was a rare week where the sun was shining every day by 10am – Humboldt County is usually bathed in perpetual fog. Make no mistake though – I get pretty overheated doing fieldwork if its 65F and sunny out. I am really used to, and built for, field paleontology on the foggy coast. By the time I drove my little Honda down to the beach to wet-screen all the matrix, I had a pounding headache. So I ate my lunch, popped some advil, and napped in the shade on the beach behind a large boulder. After a couple hours I felt better and started screening. I didn’t find a fucking thing. Just rocks and roots.


Crannell Junction in the 1970s, when the exposure still existed, compared to the past few years, where there is a fully forested hillside instead. 


Camel rock on two different days, from slightly different vantage points. The tide variation is so extreme that you would never know there's a sand bar you can walk on most of the way - but it's only exposed a few times a year.


The tiny little fissure-fill like remnant of Moonstone Beach Formation exposed out at Camel Rock.


The cave through Camel Rock.

                I also tried visiting two other nearby localities where otter material has turned up. I visited the Crannell Junction site, and it is difficult to imagine that there ever was bare rock exposed here: the hillside is fully vegetated and studded with 30-40 foot tall trees! I pulled up in my car, looked around for about 10 minutes, and concluded the spot hadn’t had any rock exposed since at least the 1980s based on the size of the trees – and the fact that Ron Bushell had never collected there since he began in the 1990s. The other locality is called Camel Rock (also called Little River Rock, and there is a second spot called Camel rock further north), and is a large sea stack with two humps (hence the name); between the humps is about a 5 meter wide wedge of Moonstone Beach Formation shelly sandstone that was deposited between the two humps when they were just submarine rock outcroppings. Getting there is extremely difficult, and my story is a bit like something out of the 1980s adventure movie The Goonies. It sits nearly 400 meters from the high tide line on shore, and is accessible only during extremely low spring tides, or by boat: but the waves are generally too large for a craft under ten feet long, and the rocks are too sharp for an inflatable. There is a sand bar you can walk on, but it is difficult to follow, and it is rarely ever exposed – you kind of have to just wade through the frigid water and read the waves. The water isn’t clear, and there are tons of enticing boulders that *look* like you can climb on and hop from boulder to boulder. However, there are deep channels scoured out around these; I doubted the waves, and walked towards one of these boulders, and fell into a channel up to my waist (my cell phone and camera were inside a waterproof box). I decided to follow where the waves seemed shallow, and indeed – the water was only knee deep. I had about an hour until low tide, giving me a two hour window. I finally made it across the submerged sand bar, and now had about 50 yards of climbing over slippery algae and barnacle covered boulders (a perpetual hazard in my west coast fieldwork: slip on the algae, cut yourself up on the razor-sharp barnacles). Once I reached the base of the sea stack, to my dismay I realized that there were no exposures accessible from the ground – the deposit was shaped like a V, at least ten feet above my head. And the rocks are all super slippery, and impossible to climb. I was extremely disappointed. I began to pack up and turn around – the beach seemed so far away, and I grew anxious about the tide – is it rising? Am I going to get stuck out here? And then I saw light – literally – there was a cave, underneath the saddle in between the two humps! I crawled underneath, and I was on the other side of the island! I climbed up some grass, spooked a bunch of sea gulls, and found a very small exposure at the top of the saddle; some fossilized fish bones, and a marine mammal rib fragment, were sitting out, mixed with modern fish bones left by the gulls. I also found a gull nest – and let me tell you, gull chicks look like little black and white spotted dalmatian puppies. They were SO CUTE. I’m not sure what the permitting process is like for these sea stacks, so I didn’t collect anything, and decided against coming back; the outcrop is so small, the odds of finding something there are so small relative to Moonstone Beach, and the locality is so dangerous and difficult to access, that it’s just not worth it.


Ash Poust (left) and Lee hall (right) searching for middle Pleistocene marine mammals in the Port Orford Formation in coastal Oregon in 2009.


                I spent another few trips over the next three years, and didn’t turn up much more than scraps – aside from a beautiful mandible of the sea lion Proterozetes ulysses found by Ash Poust in Oregon (and published by us a few years ago – Poust and Boessenecker, 2017), and a couple of associated sea lion vertebrae encrusted with barnacles (same locality), which I published in Palaios in 2013. I spent a lot of time searching high and low for these damn otters, in some of the only places they had been reported from! Fossil otter remains are quite rare: famous collector Doug Emlong found only a single specimen from the Oregon locality. Most specimens collected by paleontologists are one-off examples where they got lucky; the only person to my knowledge, to find multiple sea otter fossils in their own lifespan, was Mr. Ron Bushell, who collected about a half dozen specimens from Moonstone Beach in the 90s. Ron would visit Moonstone Beach after it rained and he lived nearby in Eureka and could visit many times a year. No vertebrate paleontologists lived in Humboldt County, and aside from students and invertebrate paleontologists, there wasn’t really anyone out looking for them. That being said: I am absolutely certain that scientifically significant sea otter fossils from Moonstone Beach exist in private collections, awaiting scientific study if the collectors are willing to donate them. Also, thanks to the multiple grant funding agencies who have rejected my grant applications to look for more sea otters at these localities!

Further Reading
Berta and Morgan, 1985. https://www.jstor.org/stable/1304931
Repenning, 1976. C. A. Repenning. 1976. Enhydra and Enhydriodon from the Pacific Coast of North America. Journal of Research of the United States Geological Survey 4(3):305-315.
Willemsen, 1990. A new specimen of the otter Enhydra reevei (Newton, 1890) from the crag of Bramerton, Norfolk. Bulletin of the Geological Society of Norfolk, 39:87-90.

2 comments:

  1. I awoke near the shore of Humboldt Bay in Eureka to see a sea otter backfloating by. Never saw another there, but some river otters.

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  2. A fascinating article explaining this slightly niche subject area to a layman with a keen interest in mammalian paleontology. Thank you for your enlightening article

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