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.

Sixth Triennial conference on the Secondary Adaptation of Tetrapods to Life in the Water

Two weeks ago today, I delivered a presentation on the first day of the 2011 Aquatic Tetrapods conference in beautiful San Diego, California. I had eight hours on my drive down to San Diego the day before to worry about how screwed I was: the last time I gave that presentation, it was only 20 slides longer, and took 50 minutes to deliver. I practiced it once Monday morning in front of Joe El Adli (our gracious host), and Yale students Rachel Racicot and Daniel Field - I barely fit it into 20 minutes. It all worked out fine, and in the actual presentation, I finished the last conclusion slide right as it counted down to zero. This was a fresh break from SVP tradition, which dicates that cetacean research is presented on the afternoon of the last day, giving you all week to worry about the presentation. This time, I was able to relax during the entire meeting.

My title slide for my taphonomy presentation.

It would be very difficult to summarize all the research presented, but I might be able to summarize a few of the highlights that stick out in my mind. Julia Fahlke (University of Michigan) gave a fascinating talk Monday Morning about basilosaurid and protocetid cetaceans with asymmetrical crania, and implications for the evolution of hearing underwater. I'll admit, when I read the abstract I was skeptical, but her presentation was pretty compelling - I'll wait to say more until it gets published, though. Brian Beatty presented some details of his research on meningeal ossification in cetacea, which appears to not be homologous to that in many other mammals. Larry Barnes showed us a new Paleoparadoxia skeleton the LACM has been working on from the Monterey Formation; it's virtually complete, with a very large, gnarly looking skull. Olivier Lambert gave a talk coauthored by Giovanni Bianucci on a new large assemblage of bizarre ziphiid fossils dredged from the seafloor off the coast of Spain; boy, there are some real freaks. Our Australian colleague Erich Fitzgerald presented a new juvenile aetiocetid skull from the Oligocene of Washington State which he's been slowly preparing with acid; needless to say, it's a beautiful specimen. Manuel Martinez, a Peruvian who is Christian de Muizon's Ph.D. student, presented on an incredible new toothed mysticete from the Oligocene of Peru - I won't give any details, but lets just say this will be a very, very important specimen.

Frank Fish gives a mini presentation on locomotor adaptations of various marine mammals, using an assortment of articulated limbs (a walrus forelimb is seen in the foreground).

On wednesday, we went over to the San Diego Natural History Museum for an osteology workshop on aquatic tetrapods. Due to the research focus of Tom Demere (Paleo curator) and Annalisa Berta, the majority of material out on display was from modern and fossil marine mammals. There was a great assortment of wonderful stuff out, and it was amazing to be there with so many other marine mammal (and otherwise) researchers there.

Several cetacean researchers are in this photo: Toshiyuki Kimura (foreground), Mette Steeman (behind "Tosh"), and Giovanni Bianucci (background, left) and Joe El Adli (background, right).

At the workshop, I caught Brian Beatty red-handed demonstrating his very technical method to determine the relative height of the bony tentorium (here on a skull of the Amazon river dolphin, Inia geoffrensis).

For a while, Daryl Domning gave a short presentation on the locomotion and forelimb of sirenians (using a Manatee forelimb skeleton).

Here, some of the brightest minds in cetacean paleontology scrutinize one of the weirdest fossil mysticetes: a new species of Herpetocetus under study by Joe El Adli. From left to right: Mette Steeman, Joe El Adli (standing), Felix Marx (leaning over skull), Meredith Rivin (background), Giovanni Bianucci (in glasses), and Olivier Lambert (in red).

Manuel Martinez, a Peruvian researcher studying with Christian de Muizon in Paris, did not waste a chance to photobomb. Here he is photographed with a cast of the skull of the bizarre extinct edentulous* walrus Valenictus chulavistensis.

Daryl Domning again gives a short presentation, this time on sirenian crania. Here he is showing a cast of the skull of the world's largest sirenian, the extinct Pliocene species Hydrodamalis cuestae (the ancestor of the "modern" Steller's Sea Cow).

My (soon to be) coauthor Morgan Churchill photographs an articulated hindlimb of the modern walrus.

Rachel Racicot (and Daniel Field) hung out with me for a bit in the type room at the San Diego Natural History museum while I photographed some fossil pinniped material. Skulls of gigantic Hydrodamalis cuestae sit on the table behind Rachel.

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.

Herpetocetus skull #3, part 2 - Excavation Day 1

Unfortunately, when I discovered this fossil in its mutilated state, it was two days before I planned on driving back to MSU at the end of the summer, and my last scheduled day of fieldwork for summer 2009, so it was absolutely out of the question to excavate the fossil then. So, I had to return sometime to excavate it. I didn't have the option of doing so over Thanksgiving, Winter, or Spring Break, so I waited a whole year. As it turned out, not much erosion had occurred locally over since initial discovery; just to make sure, I'd visit the fossil every couple of weeks.

I finally had the time to excavate it in mid August; I had some ambitious plans for my last week of summer in California: Saturday and Sunday up at Lake Tahoe, monday and tuesday excavating the fossil, wednesday at the Santa Cruz Museum of Natural History, Thursday at California Academy of Sciences, Friday at University of California Museum of Paleontology, packing up my car on Saturday, and Sunday thru Tuesday on the road.I found out a couple days before hand I would be able to enable to enlist the help of my longtime friend and avocational paleontologist, Chris Pirrone, a local attorney in the San Jose area. Chris helped me excavate the first Herpetocetus skull (which, if you attended my SVP talk in 2008, was the focal point of that research), and has helped me with a few other excavations in the Santa Cruz area.On Monday I was all by myself removing overburden. Surprisingly, after only about four hours of chopping at the cliff face with a railroad pick, all the overburden was gone. So, I started to carefully remove rock an inch at a time by tapping my clam knife with a rock hammer. Eventually, I found the premaxillae, and was able to determine the angle the rostrum went into the cliff (and, more importantly, the fact that the rostrum did in fact exist). If the rostrum paralleled the cliff face, then it would make for an easier excavation, as I would not need to dig as large a hole. Unfortunately, the rostrum dove in there pretty steeply. Here's what it looked like after the first day of the excavation: admittedly there's not much there - and that's exactly the way I prefer it for an excavation. The more fossil bone you expose during the excavation, the higher the chances of damaging that bone. So it's important to figure out during an excavation what the fossil is, which way it's oriented, and roughly where each feature should be; surprises during excavations are usually made by damaging bone.

I've outlined in red the anterior portion of the rostrum (premaxillae are the skinny medial elements, while the maxillae are the wide lateral elements). I exposed some bits of the premaxillae here and there, but just enough to know 1) the angle of the skull going into the cliff and 2) the tip of the rostrum. Oddly enough, the midline of the rostrum didn't match up with the midline of the skull exposed in the cliff. As it turns out, there was a fracture running obliquely through the rostrum - you can see it, near where the red lines end. It's actually offset by about five or six inches.

To be continued...