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
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
Lambert, 1997. https://www.jstor.org/stable/4523861
Kilmer, 1972. https://www.biodiversitylibrary.org/part/42552#/summary
Poust and Boessenecker, 2018. https://www.tandfonline.com/doi/abs/10.1080/02724634.2017.1317637?journalCode=ujvp20
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.
Tseng et al. 2017. https://royalsocietypublishing.org/doi/10.1098/rsbl.2017.0259
Wang et al. 2017. https://www.tandfonline.com/doi/abs/10.1080/14772019.2016.1267666?journalCode=tjsp20
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.
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.
ReplyDeleteA fascinating article explaining this slightly niche subject area to a layman with a keen interest in mammalian paleontology. Thank you for your enlightening article
ReplyDelete