New publication, or - The little whale that (almost) could: survival of Herpetocetus in the Pleistocene

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:

Research links whales - Otago Daily Times 4/13/2013

Dwarf whale survived well into Ice Age - Science Daily 4/4/2013

Otago Research reveals dwarf whale survived well into Ice Age - University of Otago press release 4/5/2013

Dwarf whales lived during Ice Age - Redorbit, 4/5/2013

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