Recent fieldwork in the Purisima Formation, Part 2: a possible new species of Herpetocetus?

Back in late May, Dick Hilton (Sierra College) and I did a three day field trip collecting fossil vertebrates from a locality in the Purisima Formation I recently got a permit for. On the second day, excavated a large block of sediment with what I assumed at the time were sirenian bones. I had not seen the bones except in cross section, and because they were somewhat dense, I thought they might be from a sea cow. We quickly carved out a large block, and due to the cohesive nature of the sediment, we were able to wrap it in tinfoil and duct tape. It was one of the first finds of the day, and I thought there could definitely be something neat inside. Because we were only a half mile from the cars, I left my pack with Dick and hoofed it back to the car with the thirty pound block, and also to grab some gatorade I had left in my car. When I returned, Dick was taking a siesta, and after some lunch, we headed further down the beach. Only a few hundred feet down I spotted a distinctly potato-shaped thing sticking out next to a piece of bone: it rather looked to me like a tympanic bulla, and I climbed up a bit to check it out. It was in fact a tympanic, and when the rest of the piece came out, I was able to see that it was in fact a nearly complete squamosal, complete with the bulla and posterior process of the petrosal. Dick and I thought the trip had been a success just because of this specimen, especially because it was from a section of cliffs where neither of us expected to find anything.

Dick Hilton digging up a huge baleen whale tympanic.

I couldn't have been more wrong. Unfortunately, I didn't know that I was until after the SATLW (Aquatic Tetrapods) conference. I did have a day or two before the conference to prepare the squamosal, and it did indeed have a plug-shaped posterior process of the petrosal, indicating it belonged to the Herpetocetinae, which includes Herpetocetus, Nannocetus, (probably) Piscobalaena, and Cephalotropis (according to Steeman, 2007). After the conference, I opened up the duct taped jacket and began preparation. After a couple hours the exposed pieces were still not making sense, and then I found a couple of bones that looked like they were adjacent to one another. When I removed them, there was a tiny neck of bone connecting them – and after a little more preparation, I realized it was a Herpetocetus petrosal and posterior process. Damnit, another goddamn Herpetocetus.

The petrosal and posterior process of the new specimen, with the facial nerve canal labeled. Upper left is ventral, lower left is dorsal, and right is medial view.

The skull with (partially incorrectly) articulated petrosal of the new skull in dorsal (top) and ventral (bottom) views.

Once I had enough of the block prepared, I realized I had quite a bit of the ventral portion of a small braincase preserved. It includes both exoccipitals, one occipital condyle, the basioccipital, the right squamosal, and the complete petrosal. After preparation, the petrosal is most similar to petrosals of Herpetocetus. This may be a bit technical, but herpetocetine baleen whales have several peculiar features that define them as a group. The posterior process of the petrosal – which is typically an elongate strap of bone that connects with the skull posteriorly – is very short and plug-shaped in these animals. Additionally, the posterior process (which is rarely found attached in isolated fossil mysticete petrosals) is flat and contributes to the lateral side of the skull, instead of being 'hidden' in a trench between the squamosal and exoccipital bones. Secondly, some herpetocetines have a flattened anterior process that is blade shaped; this structure is typically conical and robust or knoblike in most other mysticetes. Clearly, this specimen exhibits both of these features. Additionally, Herpetocetus spp. exhibit a large triangular flange on the side of the bone, which overhangs the squamosal – also present in this specimen. Additionally, herpetocetines all have extremely small earbones relative to most mysticetes. Unfortunately, the neck of the posterior process appears to have been deformed slightly, and when the main portion is articulated correctly, the posterior process sits in its trough a little wonky, and when the posterior process is articulated correctly, the main portion doesn't articulate well.

The posterior process, squamosal, and tympanic of Herpetocetus bramblei.

The two alternate articulations of the petrosal showing correct articulation of the posterior process (left) and correct articulation of the body of the petrosal (right).

The temporal region of the skull of Herpetocetus bramblei with the petrosal outlined in red.

However – it shows several features that differentiate it from all species of Herpetocetus as well as other herpetocetines like Nannocetus and Piscobalaena. Firstly, the anterior process is medially oriented – it is usually anteriorly facing instead. Second, the posterior process is very transversely narrow and elongate – it is typically more nearly circular in other species. Lastly, the most bizarre feature is that it has a very long anterior fissure of the facial nerve canal which is contorted into an S-shape – something I have not seen in any mysticete, fossil or modern.

Various mysticete petrosals in ventral view, showing two fossil rorquals (Plesiobalaenoptera and Balaenoptera sursiplana), a modern balaenid (Eubalaena japonica), the new specimen, and two other Herpetocetus specimens.

This is pretty exciting, and I am looking forward to preparing the other specimen, which includes part of a squamosal and a tympanic, and most likely a petrosal. It should not be too difficult to get these specimens written up and described.

Further Reading

Geisler, J. H. & Luo, Z.-X. 1996. The petrosal and inner ear of Herpetocetus sp. (Mammalia: Cetacea) and their implications for the phylogeny and hearing of archaic mysticetes. Journal of Vertebrate Paleontology, 70, 1045–1066.

Steeman, M.E. 2007. Cladistic analysis and a revised classification of fossil and recent mysticetes. Zoological Journal of the Linnean Society 150:875–894.

Steeman, M.E. 2010. The extinct baleen whale fauna from the Miocene-Pliocene of Belgium and the diagnostic cetacean ear bones. Journal of Systematic Palaeontology 8:63-80.

Whitmore, F.C., and L.G. Barnes. 2008. The Herpetocetinae, a new subfamily of extinct baleen whales (Mammalia, Cetacea, Cetotheriidae). In C.E. Ray, D.J. Bohaska, I.A. Koretsky, L.W. Ward, and L.G. Barnes (eds.). Geology and Paleontology of the Lee Creek Mine, North Carolina, IV. Virginia Museum of Natural History Special Publication 14:141–180.

More problems with Herpetocetus

Back in June at the Aquatic Tetrapods conference I coauthored a poster with Joe El Adli (San Diego Natural History Museum) and Jonathan Geisler (New York College of Osteopathic Medicine) on some of the taxonomic problems of Herpetocetus. Herpetocetus, as I've mentioned before, is an enigmatic small bodied mysticete whale which many bizarre and derived features, while retaining some primitive features as well. Fossils of Herpetocetus are fairly common in Northern California, particularly in the Purisima Formation - or maybe I just have a knack for finding them. Thus far, there is only one described species of Herpetocetus from California: Herpetocetus bramblei, named by Whitmore and Barnes (2008) from a very partial skull (basically just a squamosal with part of the exoccipital, parietal, and pterygoid) with a petrosal from the Purisima Formation. In summer 2007, I excavated a nearly complete skull of this same species from near the type locality, and last summer, I excavated a second specimen which lacked the braincase but included a complete rostrum. Since this topotypic material was collected, additional specimens from other localities in the Purisima Formation indicate that two additional undescribed species are present - one of the new species was discovered very recently, and I'll have more on that soon.

*Holotype, for the non-specialist, is the specimen which a new species is based off of. It should be representative of the new species in terms of its anatomy, and should be relatively complete enough to be comparable to other taxa. A type locality is where the holotype specimen originated.

There are several other described species of Herpetocetus from other corners of the globe - all from the Northern Hemisphere. The genus was first described from the Pliocene of Belgium (Herpetocetus scaldiensis) based on a partial dentary. A partial skull from the Pliocene Yorktown Formation was described as Herpetocetus transatlanticus, also by Whitmore and Barnes (2008). In the 1960's, an isolated tympanic bulla from Japan was named as the type specimen of Mitzuhoptera sendaicus, and a fossil mysticete skeleton with a skull, earbones, and dentary shared both the dentary morphology of Herpetocetus scaldiensis as well as the tympanic morphology of Mitzuhoptera sendaicus, and Oishi and Hasegawa (1995) transferred M. sendaicus to Herpetocetus, resulting in the new combination, Herpetocetus sendaicus. Each of these records is from either side of the Pacific (east and west) and the Atlantic (east and west).

How diagnostic are bullae and dentaries? I've already addressed problems with the jaw morphology of herpetocetines (here and here), and mysticetes in general. If you recall, there are two problems concerning the dentary of Herpetocetus spp. in particular: 1) The dentary of the possible sister taxon Nannocetus is not yet known, and dentaries substantially older than Herpetocetus (and possibly belonging to Nannocetus) are nearly identical to Herpetocetus (see below image), indicating that this general morphology is possibly characteristic of a larger group of whales. 2) Some species of Herpetocetus have dentaries that are very difficult to tell apart and lack autapomorphic characters (unique derived features), and thus are not suitable as holotypes. This logically results in the implication that Herpetocetus scaldiensis, which is based on a jaw, is the type species of Herpetocetus, and thus the species and genus may be taxonomically invalid or nomina dubia (means dubious name in latin).

The first figure of our poster, showing comparative drawings of various fossil herpetocetines.

Earbones have long been used for taxonomic purposes, and in many cases have been designated as holotypes. Sir Richard Owen designated many isolated bullae from the Plio-Pleistocene Red Crag of eastern England as holotypes (all of which have been sunk; e.g. Balaena definata). It is unclear how diagnostic earbones are for baleen whales: petrosals (otherwise known as periotics - the inner ear bone) have all sorts holes and knobs and crests and are rather easy to tell apart from genus to genus. A recent paper published by Eric Ekdale, Annalisa Berta, and Tom Demere (2011) indicate that earbones of extant mysticetes are diagnostic to the species and are easily told apart. Additionally, Steeman (2010) reexamined a large suite of earbones previously described by taxonomic mad man P.J. Van Beneden, who is largely responsible for constipating the entire field of mysticete systematics for over 100 years. Steeman (2010) found that many of these earbones - specifically petrosals - may be diagnostic tools, and generally reached a similar conclusion like Ekdale et al. (2011). But what about bullae?

The second figure from our poster, showing variation in tympanic bulla morphology from various herpetocetines. Note the overall similarity between Herpetocetus spp.

Bullae of three species of Herpetocetus have been described: H. scaldiensis, H. transatlanticus, and H. sendaicus. In our poster, we figured all known bullae (described or undescribed), including both the holotype of Mizuhoptera sendaicus and the referred specimen of Herpetocetus sendaicus, and a new bulla of Herpetocetus bramblei. Additionally figured are bullae of Nannocetus and Piscobalaena, also herpetocetines. We concluded, as we hope that you will when looking at this figure, that the bullae of different Herpetocetus species do not vary significantly from species to species. They are, on the other hand, diagnostic at the family level: they are clearly distinct from all other bullae of (described) cetotheriids. However, a bulla that is only distinct at the genus level is inadequate to be used as a holotype. This suggests that Mizuhoptera sendaicus, unsurprisingly, is probably a nomen dubium. It also indicates something interesting is going on with the skulls of mysticetes, or at least cetotheriids: tympanics are slightly less informative than the petrosals. It might be possible someday to quantify how phylogenetically useful different anatomical regions are, aside from just counting up the number of characters used per anatomic region in a cladistic analysis. Who knows, maybe someone has already thought of that and developed a method.

Further Reading:

New published article (Part 1): herpetocetine jaws, and an example of finding a "simple" research project

New published article (Part 2): taxonomic problems with Herpetocetus and "cetotheres"

References:

El Adli, J., Boessenecker, R.W., and J. H. Geisler. 2011. Taxonomic problems of and relationships among species of the fossil baleen whale genus Herpetocetus. Sixth Triennial Conference on Secondary Adaptation of Tetrapods to Life in Water Program with Abstracts: 23.

Ekdale, E.G., A. Berta, and T.A. Demere. 2011. The comparative osteology of the petrotympanic complex (ear region) of extant baleen whales (Cetacea: Mysticeti). PLOS One 6:1-42.

Oishi, M., and Y. Hasegawa 1995. Diversity of Pliocene mysticetes from eastern Japan. The Island Arc 3:436–552.

Steeman, M.E. 2010. The extinct baleen whale fauna from the Miocene-Pliocene of Belgium and the diagnostic cetacean ear bones. Journal of Systematic Palaeontology 8:1:63-80.

Whitmore, F.C., and L.G. Barnes. 2008. The Herpetocetinae, a new subfamily of extinct baleen whales (Mammalia, Cetacea, Cetotheriidae). In C.E. Ray, D.J. Bohaska, I.A. Koretsky, L.W. Ward, and L.G. Barnes (eds.). Geology and Paleontology of the Lee Creek Mine, North Carolina, IV. Virginia Museum of Natural History Special Publication 14:141–180.

New published article (Part 2): taxonomic problems with Herpetocetus and "cetotheres"

The "cetotheres" have long been a troubled group of fossil baleen whales. Typically, they have throughout there long and confusing taxonomic history been treated as a wastebasket group to include all extinct mysticetes that lack the synapomorphic (i.e. distinguishing) features of the extant groups of baleen whales (gray whales, Eschrichtiidae; rorquals, Balaenopteridae; and right whales, Balaenidae). For a very long time, this group included strange early-diverging mysticetes such as Parietobalaena, Pelocetus, Diorocetus, Aglaocetus (all Miocene mysticetes from the Chesapeake Group of Maryland and Virginia), Cophocetus from Oregon, Cetotherium from the Miocene of the Ukraine, and my favorite mysticete, the problematic Herpetocetus (among others). Many authors during the latter half of the twentieth century doubted that this was a natural grouping, and I suspect that the proliferation of this notion in the literature has more to do with taxonomic laziness on the behalf of mysticete systematists than anything else.

While the taxonomic problems associated with "cetotheres" are a topic for a different post altogether, a brief summary is warranted for the backdrop of the implications in my recently published article. With the advent of cladistics, some studies found that 'cetotheres' are a paraphyletic group of stem-mysticetes (i.e. that they are an unnatural group characterized by primitive rather than derived features). For several years it seemed that the term 'cetothere' should be shit-canned for all eternity, until Bouetel and de Muizon (2006) published a large study on a small Herpetocetus-like 'cetothere' from the Pliocene and latest Miocene of Peru, called Piscobalaena (after the Pisco Formation). They found that some 'cetotheres' form a natural monophyletic group (i.e. a group that is defined on derived features that includes all the descendants of a common ancestor). Because this clade included Cetotherium rathkei, they called this clade the Cetotheriidae sensu stricto, and other 'cetotheres' the cetotheres sensu lato. This same relationship has been supported by several other phylogenetic analyses.

The lectotype jaw of Herpetocetus scaldiensis. From Bouetel and de Muizon, 2006.

Within the true cetotheres, Herpetocetus is the most derived member, and also the youngest surviving member. As previously mentioned, it was based on a lower jaw from the Pliocene of Belgium. When it was described in 1872, a type specimen was never selected, and the lower jaw was selected as a 'lectotype' over thirty years later. The jaw of this animal is pretty distinctive,
Subsequently, many authors have used the distinctive jaw morphology to refer isolated jaws to the genus Herpetocetus. A nearly complete mysticete skeleton from Japan, including a skull, was identified as Herpetocetus due to its jaw morphology. Subsequently, fossils of Herpetocetus have also been reported from the Pliocene and latest Miocene (6-2 Ma) of California, and the early Pliocene (3-5 Ma) of the east coast (North Carolina), basically indicating a 6-2 million year record only in the Northern Hemisphere.

So you can see, when I first thought long and hard about these early Late Miocene (10-12 Ma) Herpetocetus lookalikes, why I was somewhat confused. Herpetocetus also has distinctive earbones and skulls (based on specimens associated with jaws), and there aren't any earbones or skulls with the typical "Herpetocetus morphology" that occur any older than 6 Ma (there is one undescribed skull from the 6.8 Ma Santa Cruz Mudstone I've identified as Herpetocetus aff. bramblei). What is known from the 10-12 Ma Santa Margarita Sandstone, in addition to the jaws in question, is Nannocetus eremus.

The holotype braincase of Nannocetus and a quick and dirty reconstruction based on the rostrum of Herpetocetus sendaicus.

Nannocetus is a really tiny (greatest width across the skull is about 10 inches) weird mysticete, originally described in 1929 by Remington Kellogg. A second specimen from the Santa Margarita Sandstone was described by Whitmore and Barnes (2008), and is the only other known 'true cetothere' from the Santa Margarita. However, Nannocetus is not yet known by a jaw; could Nannocetus be the rightful owner of the two dentaries I described?

If so, then the supposedly distinctive anatomy of the lower jaw of Herpetocetus is not distinctive, and raises important questions about referring isolated dentaries based on their morphology. Additionally, this problem raises an even more important issue: what, then, of fossil baleen whales described solely based on isolated lower jaws? Most of them are probably invalid, because lower jaws *might* only be diagnostic at the supraspecific level (i.e. at the level of a genus or subfamily - whatever the hell those are). "But Bobby, the type species of Herpetocetus is based only on a lower jaw!" Aw, crap. That's right. We've now come full circle: Herpetocetus may or may not be a valid name in the first place, if jaw morphology is insufficient for taxonomic purposes.

Before us mysticete taxonomists go off ready to sink Herpetocetus as a nomen dubium, there are a few important things I pointed out in the article which should be remembered: 1) The fossil dentaries DO show a couple of features distinct from Herpetocetus, including a mandibular foramen with a a lanceolate opening, unlike Herpetocetus. 2) Although highly likely, it is possible that these dentaries are not Nannocetus. However the age discrepancy does mean something in and of itself. 3) Distinctive skull fragments showing some synapomorphies of Herpetocetus were also in the "type series" described in 1872, so it is unfair to say that it was based only on a lower jaw.

With those exceptions in mind, I hope my new article has established some caveats for mysticete workers. Additionally, this work has identified the possibility that mysticete jaws are perhaps diagnostic to the generic level; this still means they are unsuitable as holotypes, but that they are by no means useless - the jaws of mysticetes tell us quite a bit about the animal's feeding and its relationships (although they are not as fine-tuned as, say, parts of the skull). Hopefully future fossil mysticete holotypes will be designated only on material that is really diagnostic, and hopefully will include comparable elements like earbones, braincases, and (also hopefully) the posterior end of the lower jaw.

New published article (Part 1): herpetocetine jaws, and an example of finding a "simple" research project

The new issue of PaleoBios, the paleontology journal published by UCMP at UC Berkeley, includes my new article on herpetocetine jaws from the Santa Margarita Sandstone in central California. During the fall of 2008, I was in my first semester of graduate school, and was taking a difficult, time-intensive course on advanced stratigraphy. At the time, I already had two articles I had been working on: one on my undergraduate research concerning a new fossil vertebrate assemblage from the Purisima Formation, and another on Plio-Pleistocene fur seals from the Wildcat Group in northern California (both are currently in press). However, neither was in any shape to be published anytime soon - one was only half written, and the one that was nearly finished needed a lot of work (i.e. quality control). I needed a new manuscript to work on, to give me something to do that semester aside from stratigraphy, which had started to eat away at my brain. In other words, I needed to start something fresh from scratch, with a clear beginning, a clear end, and a clear message.

The right dentary of UCMP 85431. Scale bar =10cm. From Boessenecker (2011).

A month earlier, I had given a presentation (coauthored with Jonathan Geisler) on a new skull of Herpetocetus bramblei I had collected from the Purisima Formation in 2007. While ruminating on possible projects, I suddenly remembered a partial lower jaw from the Santa Margarita Sandstone that looks a lot like the lower jaw of Herpetocetus*. The problem is, the Santa Margarita Sandstone is early Late Miocene in age (10-12 Ma), while the oldest known bona fide specimens of Herpetocetus are latest Miocene to earliest Pliocene in age (~5 Ma). In fact, the oldest known described specimen is the fragmentary type specimen of H. bramblei, which is right about 5.33 Ma. So, this specimen (UCMP 85429) is MUCH older than any known specimen of a "true" Herpetocetus.

*All species of Herpetocetus have relatively similar lower jaws, and currently the lower jaw has only been described for the type species, Herpetocetus scaldiensis (the type specimen of which is a lower jaw - more on this later).

The left dentary of UCMP 85429; scale bar = 10cm. From Boessenecker (2011).

The lower jaw of baleen whales has long been assumed to be a fairly diagnostic element, at least in certain groups. Herpetocetus scaldiensis certainly has a very distinctive mandible. Other fossil mysticetes certainly have distinctive mandibles as well. Many fossil mysticetes (H. scaldiensis, Balaenoptera davidsonii, Archaeschrictius ruggieroi, etc.) have been described just off of their lower jaws. Are dentaries really that diagnostic? Perhaps. Demere (1986) used mandibular features to reevaluate "Eschrichtius davidsonii" from the San Diego Formation, which was a chunk of a 30% complete mandible, missing the anterior and posterior ends. Being able to refer a new dentary to this taxon, he demonstrated that the "davidsonii" morphotype was actually a rorqual, and assignable to the genus Balaenoptera, which led to its recombination as Balaenoptera davidsonii. Clearly, mandibular morphology is important, and can be used to assess the taxonomy of certain groups, and 'fix' the taxonomy of certain problem taxa. Are these specimens the oldest known records of Herpetocetus?

Reconstructed lower jaw of Herpetocetinae genus and species indet., based on UCMP 85429 and 85431. From Boessenecker (2011).

Reflecting upon the known fossil record of "true cetotheres" - relatives of Cetotherium and Herpetocetus (an entire other topic worthy of its own post) - I was able to formulate some interesting questions I could ask (and attempt to answer), which would make a nice core of an article. With these objectives in mind, I started taking copious notes on the anatomy of the fossil specimens, and eventually typed these notes up into an anatomical description for the article. By the end of the semester, I had a manuscript that was about three-quarters finished. During spring 2009, I wrapped up the discussion, and constructed some figures. I had a few people look at it and make some comments. Morgan Churchill looked at it late in spring, and had some of the most constructive and useful edits. That summer, I TA'd the Geology Field Course for MSU, and during some of the weeks, I took my manuscript out into the field with me, along with a red pen. In between helping students, I sat out in the hot sun, and critically examined every sentence in the manuscript. I even drafted one of the figures (on vellum, with nice pens) around the campfire one night after a few beers. By the end of the second to last project at field camp, I had all my edits finished, and I sent it off to PaleoBios for review.



To be continued...

Boessenecker, R.W. 2011. Herpetocetine (Cetacea:Mysticeti) dentaries from the Upper Miocene Santa Margarita Sandstone of Central California. PaleoBios 30:1:1-12.

Deméré, T.A. 1986. The fossil whale, Balaenoptera davidsonii (Cope 1872), with a review of other Neogene species of Balaenoptera (Cetacea: Mysticeti). Marine Mammal Science 2:277–298.