Last week saw the publication of my most recent article, a
short paper in Naturwissenschaften regarding a new specimen of the archaic
baleen whale Herpetocetus from Northern California. The
new fossil is surprisingly young in geological terms for this genus – early to
middle Pleistocene. In this post I’ll discuss the new specimen and its
implications, as well as what exactly we know about the Pleistocene epoch in
terms of marine mammals. But before I talk much further about this study, I’ll
need to briefly introduce you to two different back stories.
Front page of the new publication in the journal Naturwissenschaften.
The first backstory is a discovery I made in
June 2007 – I spotted the cross-section of a baleen whale rostrum in a
sandstone cliff of the Purisima Formation near Santa Cruz. The tide was coming
in, and it would clearly take several afternoons to excavate – and in any
event, I had no idea which way the skull was pointing; it could have just been
the rostrum, which would be way less interesting than if it were everything
behind the rostrum – the braincase and earbones, which are certainly the most
informative part of a cetacean skeleton. The rostrum is informative as well,
but there would not have been much left given the way it was exposed.
Fortunately, I found a small boulder on the beach which appeared to include
part of the rostrum, matching the cross section in the cliff. I wrapped it up
and took it home – once prepared, it could tell me which direction the skull
was facing, and which end of the skull was in the cliff. Preparation of this
section indicated that the posterior part of the skull was in fact in the cliff
– so I returned two weeks later, and dug out the skull with my friend Chris
Pirrone. It was easy to excavate, but a pain to remove – we finished the jacket
at 6pm as the tide was coming in, and it took us four hours to move the ~150 lb
jacket a measly 300 feet. During this period of time, the jacket spent about
fifteen minutes in the surf while I rethought my life and doubted the outcome
of the excavation. Chris’ optimism saved the fossil – and the floormat from his
car as well, which we used as a makeshift stretcher. A few months later, back
in Montana (this was the last year of my bachelor’s degree), I had the fossil
mostly prepared. It included the entire posterior one-half of a skull, with
both petrosals and one tympanic bulla – and even all three middle ear ossicles
(malleus, incus, and stapes). At the time, no known skull of Herpetocetus was
yet known – but I knew it was something similar to Piscobalaena. The
fossil which would later become Herpetocetus bramblei, named by Whitmore
and Barnes (2008), was figured in Barnes’ doctoral thesis (1976) – and was
identical to my new specimen, in addition to being from the same locality. As
it turned out, I only needed to wait a year until the species would be
published, and it became immediately recognizable as Herpetocetus bramblei,
albeit much more complete than the holotype. This discovery (now curated and
cataloged as UCMP 219111 at UC Berkeley) catapulted me into the study of Herpetocetus
and other cetotheriids, and I started to find additional fossils of this whale
everywhere.
The second part of the backstory started when I
was in high school. There used to be a website titled “Collecting Fossils in California” at www.gtlsys.com, which I used to contact other
fossil collectors and find fossil sites to visit. Other much more experienced
collectors posted pretty incredible fossils they had found, from all over the US – but mostly from
the west coast. One collector in particular – Ron Bushell – lived at the time
in Eureka in Humboldt County in Northern California, and had for years been
finding incredible marine invertebrate and marine mammal fossils from
Plio-Pleistocene strata in Northern California. When I first visited the
website in 2004, Ron posted some sort of an odd bone he had partially prepared
out of a sandstone nodule from the Scotia Bluffs locality along the Eel River. I had no idea what
the find was at the time – and nobody else did either; some had suggested it
might be from a seal, or a sea cow, or even a cetacean. It surprised me that
there was no single, comprehensive collection of comparative images of bones
and teeth from all groups of marine mammals. At the time, to a high school
student who had assumed fossil marine mammals had attracted just as much study
as dinosaurs – it seemed like a reasonable assumption that such a volume
existed. Unfortunately, it does not. In 2007, I visited the Chapman’s rock shop
where the fossil currently resides – and identified it as a calcaneum of a
large walrus. Since my days in high school, Ron and I have been good friends,
and he gave me all sorts of expert advice on California fossil sites. Ron
made numerous other discoveries, including some fur seal mandibles I published
in JVP in 2011 (Boessenecker, 2011). Several of his other discoveries will
prompt further scientific study, including some Pleistocene sea otter fossils,
and a walrus forelimb from Crescent
City, California. Ron moved to Montana during my Sophomore year of college, and although he did visit campus
once, I unfortunately never had a chance to get out to his ranch near Helena – the road would be
snowed in by the end of September when I arrived in Bozeman, and would remain
snowed in nearly until I finished in the spring. One of his finds I saw for the
first time in January 2007 on a visit to Sierra College – a small baleen
whale skull in a hefty calcareous sandstone nodule.
The skull as I first saw it in the Sierra College prep lab in 2007, still with a fair
amount of calcareous matrix attached.
The fossil was donated by Ron to Sierra
College in Rocklin,
California, where Dick Hilton and George
Bromm began preparing the specimen – slowly. The sediment was so damn hard, but
fortunately soluble – and they were able to slowly acid prepare the specimen
with over the counter vinegar from the grocery store. Acid preparation took the
better part of a decade, and was finally completed in 2011. When I first saw
the specimen, I thought it looked remarkably primitive, and thought based on
the triangular supraoccipital shield – thought it was a middle Miocene
“cetothere” (aka “Kelloggithere”). You can imagine my shock when Dick Hilton
told me that the fossil was from the Pleistocene Falor Formation – it looked to
be about 10 million years too young! To be fair, I had not yet discovered the Herpetocetus
skull from Santa Cruz, and didn’t
really know how to recognize the taxon.
Comparison of UCMP 219111, the skull of Herpetocetus bramblei I collected in 2007 (and am currently/still studying), and the Pleistocene Herpetocetus sp. from Humboldt County.
After another year or two of study, it became clear in my
mind that
Herpetocetus was the last remaining true cetothere living in
the eastern North Pacific (and probably the Northern Hemisphere).
Herpetocetus
is present in very late Miocene and Pliocene strata from California
– including the San Mateo and San
Diego Formations of southern California.
Herpetocetus is also found in the early Pliocene Yorktown Formation of North
Carolina, as well as the early Pliocene of Belgium and Japan.
The youngest known
Herpetocetus – according to the literature – was from
the mid-late Pliocene San Diego Formation, approximately 2-3 million years in
age.
The skull of VMS-65 and a locality map (Fig. 1 from Boessenecker, 2013).
I returned to Sierra
College in 2008, and although the
fossil needed a fair amount of preparation – quite a bit of calcareous
sandstone still remained – I recognized the specimen as
Herpetocetus,
instead of a “kelloggithere” from older rocks. I asked Dick Hilton if I could
take the fossil on loan for a few weeks – I had a visit to the San Diego
Natural History Museum, the primary purpose of which was to make some
comparisons with
Herpetocetus material – skulls, mandibles, earbones –
at the SDNHM – ahead of my first talk at the annual SVP meeting, which was
coming up in October (read here for some information about that talk). Dick
agreed, and I brought the Falor Formation specimen along with the referred
Herpetocetus
bramblei skull to show Dr. Tom Deméré, the paleontology curator at the
SDNHM. Tom was fairly surprised to about the Pleistocene specimen – and agreed
with my identification (satisfying for a young researcher). It would be three
more years, however, until I saw the specimen fully prepared; each time I
visited Sierra College,
a little more matrix had been dissolved away. Finally, in summer 2011, the last
remaining matrix had been removed from the earbone, and study of the fossil was
able to commence. The fossil shares a number of features with
Herpetocetus
bramblei and others – including maxillae that “squeeze” out the premaxillae
and thus contact each other medially on the front of the braincase, just
posterior to the bony naris. That may sound a bit technical, but this is an
unusual feature in baleen whales and unique to whales like
Herpetocetus,
Piscobalaena, and
Nannocetus. Two other features, present in one
of the earbones – the petrosal (the one which houses the cochlea and
semicircular canals) – had two projections only found in
Herpetocetus.
One, the anterior process – is tapered and bladelike; in most other mysticetes,
this process is blunt. The other feature – the lateral tuberosity – is
triangular and projects truly laterally (i.e. towards the side of the skull),
whereas in most mysticetes it faces ventrally when the bone is in the skull
still – and it underlaps the squamosal bone of the skull, a condition unique to
Herpetocetus (and possibly in
Caperea as well; e.g. Fordyce and
Marx 2012). Because the fossil is relatively incomplete, I was only comfortable
identifying the fossil to
Herpetocetus sp.
The earbones of Pleistocene Herpetocetus sp. specimen VMW-65 above and Herpetocetus bramblei below (Figure 2 of Boessenecker, 2013).
Before I continue, a few words about the Falor
Formation. It’s exposed in a narrow NW-SE directed belt inland of Arcata in Humboldt County, California. It’s predominantly
shallow marine, and was deposited in a long embayment presumably separated by
headlands from the larger Eel River Basin to the south. The Eel River Basin began to subside around
the Mio-Pliocene boundary, depositing the absurdly thick Wildcat Group,
starting with the Pullen Formation (latest Miocene-early Pliocene, Purisima Fm.
equivalent) and ending with the middle Pleistocene Carlotta Formation, which
appears to be conformably overlain by late Pleistocene coastal sediments. Now,
within the Falor basin, there are no marine rocks exposed older than latest
Pliocene; the Falor Formation unconformably overlies Mesozoic basement rocks.
An ash bed lies near the base of the Falor Formation; this ash has been
chemically fingerprinted and matches the Huckleberry Ridge Tuff, a well known
2.1 Ma eruption of the Yellowstone supervolcano. While at MSU, we went on several field trips where we
went and looked at the tuff itself and associated basalt flows; the eruption
was so large that ash traveled far enough west to reach the Pacific coast
(correlative ash has also been found in deep sea cores from the middle of the
North Pacific, suggesting it traveled around the earth by prevailing winds
before being deposited on the seafloor). This 2.1 Ma date provides the lower
age boundary for the new Herpetocetus fossil. Because little
syndepositional folding/deformation of the Falor Formation is known, deposition
is interpreted to have been completed by 700,000 years, when deformation
associated with the Mad River fault zone became active. This provides an age
range of 2.1 Ma to 700 Ka for the Falor Formation Herpetocetus.
Examples of some of the weirdness in Pliocene marine mammal faunas: Denebola brachycephala, technically Miocene but present in the Pliocene of California as well - a warm-water beluga relative, now extinct; Callorhinus gilmorei, a very modern-looking pinniped but quite a bit smaller than its modern descendant; Odobenocetops, the walrus faced whale - 'nuff said; Acrophoca, the long snouted seal from Peru/Chile; and Auroracetus, a relative of the modern La Plata river dolphin, but from the Pliocene Yorktown Formation of North Carolina. I could include many more examples, but they wouldn't fit.
Time to switch gears, and give some context to Herpetocetus
in the Pliocene. The Pliocene was a time of change – although I suppose you
could probably say that about any epoch, really. The Pliocene was a
transitional period between the warm climate of the Miocene and the icy climate
of the Pleistocene. The Tethys and Paratethys seaways finally closed up, the
Panama land bridge formed, causing changes in global circulation and upwelling
– followed by increased northern hemisphere glaciation and cooling climates.
Pliocene marine mammal assemblages were a bit of a weird mix – definitely a
transitional assemblage between that of the Miocene and the Pleistocene, but
not in the same sense as climate and geography. Pliocene marine mammal
assemblages include a menagerie of species of modern and archaic aspect.
Archaic marine mammals include late surviving “Scaldicetus”-grade sperm
whales (more on this below), dusignathine walruses (such as Dusignathus
seftoni), albireonid dolphins (Albireo savagei), and others. Herpetocetus
technically falls in this category (but read further). The Pliocene is also
the first appearance of many modern genera – in the Pacific, Eschrichtius
(the gray whale), Balaenoptera (rorqual whales), Tursiops (bottlenose
dolphins), Callorhinus (northern fur seal), and Hydrodamalis (Steller’s
sea cow) all appear during the Pliocene. In the Atlantic,
Balaena (bowhead whale), Megaptera (humpback whale), Lagenorhynchus
(white-sided and related dolphins), Pseudorca (false killer whale) and
other modern critters show up.
Three broad groups further adding to the Pliocene “mix” existed (some
members of the aforementioned archaic and modern groups outlined above may also
fit into these latter three categories). The first are members of extant groups
with wildly different biogeographic ranges than today. This includes beluga
relatives adapted to temperate and subtropical waters of the Pacific and Atlantic (Denebola from Baja California and Bohaskaia
from North Carolina, for example), South American “river dolphins” in North America (the pontoporiids Auroracetus
bakerae, cf. Pontoporia), Asian ‘river dolphins’ in North America
(the lipotid Parapontoporia), and tusked odobenine walruses in temperate
and subtropical waters (Valenictus in southern California and Baja, and Ontocetus
in the Carolinas, Florida, and Morocco). These discrepancies in distribution
indicate that some unknown process drove rather extreme changes in marine
mammal biogeography. The second of these groups are close relatives of modern
marine mammals – either within the same genus or in closely allied genera –
that are dwarfs relative to modern species; examples include the fur seal Callorhinus
gilmorei, the dwarf right whale Balaenula, and various small-bodied
species of Balaenoptera (e.g. “Burtinopsis” from Japan). The last group are
marine mammals with seemingly bizarre or novel adaptations, which seem to be
more prevalent in Pliocene oceans than today. These include the sharp-snouted
seal Acrophoca, double-tusked walruses like Dusignathus, the
aquatic sloth Thalassocnus, the walrus-faced whale Odobenocetops,
the strange “skimmer porpoise” from the San Diego Formation, and the
ziphiid-convergent dolphin Australodelphis from Antarctica. Herpetocetus is
pretty strange, and given the unique configuration of its jaw joint, which
prohibited it from opening its jaws much past ~30 degrees – so Herpetocetus
sort of belongs to this ‘bizarre’ category as well. To conclude – although
strange, Herpetocetus more or less fits with what we know about this
motley crew of strange marine mammals of the Pliocene ocean.
Stratigraphic range of various cetotheriids, with Herpetocetus range extension bolded (Fig. 3 of Boessenecker, 2013).
In contrast, the Pleistocene fossil record of marine mammals
– as crappy as it is – is nearly entirely composed of modern genera and species
(or extinct species within modern genera). Some exceptions include the strange
globicephaline pilot whale Platalearostrum (possibly Pliocene in age)
and the sea lion Proterozetes ulysses (probably a species referable to Eumetopias).
Previously published baleen whales from the Pleistocene include humpbacks and
other rorquals from Japan (Megaptera, Balaenoptera), gray whales
from California (Eschrichtius), and bowhead whales from the arctic and
north atlantic (Balaena). Another important distinction is that
the Pleistocene is the first period in the history of cetaceans when most
baleen whales trended towards the gigantic body sizes we see today. Indeed, if
you only go as far back as the Pliocene, most mysticetes are comparatively
small; in the San Diego Formation assemblage, there is a big ~10-15 meter
balaenopterid, and a similarly sized eschrichtiid – and all other mysticetes
are smaller in size, right down to diminutive Herpetocetus at roughly 4
meters. So there are giants in the Pliocene (and late Miocene), but they are
rare, and on average body sizes are much smaller. With these two pieces of
information in hand – the dominance of extant species in the Pleistocene and
the trend towards modern gigantism – a picture is painted of the modern baleen
whale fauna appearing in the Pleistocene after the members of the strange Pliocene
fauna went extinct.
Press release image for the study showing the stratigraphic range of familiar marine and terrestrial mammals from the Pacific coast of the United States; I put this together as a sort of infographic, using part of a life restoration I borrowed from another project.
Enter Herpetocetus – a stranger in a strange land, if
you will. A peculiar little whale that couldn’t open its mouth very far open,
dwarfed by more recently evolved giants, in a rapidly cooling ocean. Why did
Herpetocetus persist so much longer than other members of the Pliocene fauna?
For one, the Pleistocene record is so terrible, we don’t know if other Pliocene
taxa held on for longer – and this new fossil suggests, maybe they did.
However, what we do know, if we look outside cetaceans, is that the modern
pinniped assemblage had already appeared in this region. Based on the California
and Oregon fossil record, we
already have fossils of modern California
and Northern Sea Lions (Zalophus and Eumetopias, respectively),
harbor seals (Phoca), with elephant seals (Mirounga) to follow in
the late Pleistocene. The sea otter Enhydra also first appears during
the middle Pleistocene, presumably an immigrant from Asia.
We don’t know much about Pleistocene odontocetes in this region – but nevertheless,
it appears as though Herpetocetus persisted alongside many marine
mammals familiar to us today. The occurrence of Herpetocetus in the
Pleistocene suggests two separate possibilities: either Herpetocetus was
(even more) unusual and persisted alongside modern species after an extinction
of marine mammals at the end of the Pleistocene – in which Herpetocetus can
be viewed as a “lost” member of the modern fauna (like the Steller’s sea cow)
that almost survived – or, this discovery indicates that the Plio-Pleistocene
pattern of faunal turnover among marine mammals was more complex than
previously noted. To be fair, nobody has really attempted to address this issue
before (one exception would be Valenzuela-Toro et al., 2013).
Another interesting aspect of this study is what it means
for the hypothesis that the pygmy right whale, Caperea marginata, is an
extant cetothere. In the Fordyce and Marx (2013) phylogenetic analysis, Caperea
appears as a sister taxon to Herpetocetus and Nannocetus, and is
deeply nested within the “true” cetotheres. The presence of one of these
putative sister taxa within the Pleistocene lends some indirect support, by
establishing the plausibility of one of these cetotheres surviving to the
modern era. If Herpetocetus (a proven cetothere) could survive to within
the last one million years, then Caperea – tentatively identified as a
cetothere – plausibly could have survived as a member of that group. In the
paper I went so far as to highlight Herpetocetus as a northern
hemisphere analog of Caperea – in a way, both are tiny, have weird
anatomy, and are far removed from other contemporary cetaceans in the northern
and southern hemispheres, with no close relatives, presumably doing something
weird and different in terms of behavior and ecology. I'd love to talk about the feeding strategy and mechanism for the extinction of Herpetocetus, but that research isn't quite finished yet.
So what’s next? More Herpetocetus research is on its
way (including new species!), as is a much, much larger study of Plio-Pleistocene faunal change in
marine mammals. So, stay tuned.
This study in the news:
References:
Boessenecker, R.W. 2013. Pleistocene survival of an archaic dwarf baleen whale (Mysticeti: Cetotheriidae). Naturwissenschaften 100:4:365-371.
Fordyce, R.E., Marx, F.G. 2012. The pygmy right whale Caperea marginata-the last of the cetotheres. Proceedings of the Royal Society B 280:20122645.
Valenzuela-Toro AM, Gutstein CS, Varas-Malca RM, Suarez ME, Pyenson ND (2013) Pinniped turnover in the south Pacific Ocean: new evidence from the Plio-Pleistocene of the Atacama Desert, Chile. J Vertebr Paleont 33:216–223.
Whitmore FC, Barnes LG (2008) The Herpetocetinae, a new subfamily of extinct baleen whales (Mammalia, Cetacea, Cetotheriidae). Va Mus Nat Hist Spec Pub 14:141–180
