Tuesday, August 18, 2020

Obscure controversies in Cenozoic marine vertebrate paleontology 1: taxonomic feuding over the basilosaurid whales Zygorhiza, Pontogeneus, and Cynthiacetus

    Some of the most lively, and on occasion low-stakes and totally boring arguments in paleontology are over matters of taxonomy: what name do you use for a particular set of fossils? Taxonomic slapfights are common, because different researchers often represent different schools of thought and there are different philosophies behind what is needed to define a species or genus, what a type specimen should look like, what a type specimen means, how speciose genera should be, what families are (if anything), etc. Some of these questions may sound ridiculous given that genera and families are probably not biologically real – yet they are units that are counted through geological time by diversity measurements, so some meaning is ascribed to them for better or for worse. Some taxonomic disputes can rapidly get into the weeds, so to speak, and quickly lose the attention of even other seasoned researchers. But others highlight vastly different philosophical approaches – is “Torosaurus” the adult form (and therefore synonym of Triceratops? [narrator: it is] Is “Nanotyrannus” the juvenile form of Tyrannosaurus? [narrator: also yes]. These arguments are fascinating to me owing to nature of the dispute itself: do animals change shape as they grow? Many paleo aficionados are familiar with these examples, so I wanted to highlight three surprising examples most of you have not heard of within the field of paleocetology. Each of these three, again, highlights interesting contrasts in approaches to paleontology.

    One last point before this: many paleontologists and most science communicators are completely wrong about what the "validity" of a taxon is. Under the ICZN a name is valid if it is available and associated with an anatomical description or figure of a specimen in the literature. A valid name can have a horribly incomplete type specimen. What most have in mind when they think of validity is actually diagnoseability: is the fossil diagnostic?

The case of the crappy type specimen of Zygorhiza

    Type specimens are an established utilitarian concept in the life sciences. During the first century or so of biology, type specimens were generally not designated – meaning that species names were not tied to an individual specimen in a collection somewhere. Taxonomy in the late 18th and early 19th century was the wild west, and some conventions were honored on occasion, but naturalists frequently just re-named species all the time, so that for any taxon named before 1880 there’s likely going to be at least 10 or more synonymous names. Sometimes this was likely due to simple ignorance (someone had not read a paper published earlier) or due to the fact that frequently, few illustrations were ever published (wading through E.D. Cope’s paleocetological papers from the late 19th century is a bit of a nightmare), or sometimes research was conducted on the same taxon in parallel and published nearly at the same time, the consequences of which only to be noticed decades later. Type specimens were meant to solidify the concept of a taxon by tying it to a particular individual specimen with a catalog number – so that anyone wanting to know what Zygorhiza kochii looks like, they can go to that collection and look at it. But, some type specimens are better than others. The type specimen of Basilosaurus cetoides, for example, is diagnoseable but not fantastic: a single partial (but very elongate) lumbar vertebra. The type specimen of the Cynthiacetus peruvianus is perhaps the best of any basilosaurid: a nearly complete skeleton with a complete skull and mandibles. Frequently in paleontology, the first known fossil of a new species is usually pretty shitty. I don’t know what to call this phenomenon, but it’s very real.

Kellogg's skeleton of Basilosaurus cetoides, excavated in the 1890s, on display in the Sant Ocean Hall in the Smithsonian NMNH.

    So what is the type specimen of Zygorhiza? It has a pretty ridiculous history, to be honest – and I’ll spare the more boring elements and summarize it as briefly as possible. The first basilosaurid whales discovered in the southeastern USA were found on plantations in Alabama – discovered by slaves ploughing the fields, who took all the bones near the surface and made a big pile of them. There’s a whole bizarre story with Albert Koch, who purchased these non-associated basilosaurid specimens and strung them together to make the chimaeric and fantastical skeleton of Hydrarchos – which, like an early P.T. Barnum, paraded around the USA as a traveling exhibition. Koch originally named it Hydrarchos sillimani, after Dr. Benjamin Silliman, who was not involved, and quickly requested the species to be renamed. Koch did this, and re-named it Hydrarchos harlani (you can’t really do that in taxonomy, but as I said before, it was the ‘wild west’). This composite “skeleton” was eventually purchased by King Friedrich Wilhelm IV of Prussia, who put it on display in the Royal Anatomical Museum, but much of it eventually perished during World War II when the Museum fur Naturkunde in Berlin was bombed in 1945 (note: a separate bombing than what destroyed Stromer’s fantastic Egyptian fossil collection, including Spinosaurus and many archaeocetes, at Palaontologisches Museum Munchen in April 1944). More on Koch later…

The famous illustration of Albert Koch's archaeocete chimaera, Hydrarchos.

    Many contemporaries considered Hydrarchos to be a junior synonym of Basilosaurus cetoides, famously misinterpreted as a marine reptile by Harlan in 1835 (and ironically the namesake of the replacement species name Hydrarchos harlani, despite being a junior synonym). Harlan would later be corrected by Sir Richard Owen, who decided to re-name it Zeuglodon (it REALLY was the wild west!) and identified it as a whale after all. After being sold to the king of Prussia and quickly reidentified as Basilosaurus cetoides by various German paleontologists, Reichenbach (1847) noticed that one of the parts of the skeleton, a partial braincase, was much smaller and represented a different species. He  named this Basilosaurus kochii. A few years later, Muller (1851) named a different species, Zeuglodon brachyspondylus, based on a collection of non associated vertebrae with short bodies, differing from the lengthened vertebrae of Basilosaurus cetoides. Muller unfortunately also named the subspecies (we don’t really use these in vertebrate paleontology) Zeuglodon brachyspondylus minor for a small collection of cranial material including the holotype skull fragment of Basilosaurus kochii, named four years prior (again, you can’t do that). Frederick True (1908) thought that this “species” did not belong to Zeuglodon (ironically, not because the name was technically invalid owing to synonymy with Basilosaurus), and named it Zygorhiza. Despite being applied to a different species (Z. brachyspondylus minor), the earliest named species name available is kochii, and so the taxonomy stabilized around Zygorhiza kochii (with Z. brachyspondylus minor as a junior synonym) as the preferred binomial for the smaller basilosaurid whale from the Eocene Pachuta Marl of Alabama. For much more in depth reviews of this, I refer the reader to Kellogg (1936) and Uhen (2013A, 2013B).

Stitched panorama of the mounted skeleton of Zygorhiza kochii (reference specimen USNM 11962) in the old marine paleontology display hall at the Smithsonian NMNH in Washington D.C. before they decommissioned the gallery, photographed just before Hurricane Sandy in 2012.

    Since Kellogg (1936) published his monograph A Review of the Archaeoceti, the de facto reference specimen for Zygorhiza kochii has been USNM 11962, a beautiful partial skeleton including a very well-preserved skull with mandibles. Paleocetologists have more or less treated this specimen as the honorary holotype. However, as pointed out by Uhen (2013A), the holotype is that crappy braincase first discussed by Reichenbach (1847). That specimen (Mb Ma 43248) is pretty terrible – a small braincase missing the vertex and with broken earbones. Uhen (2013A) notes that because the earbones are damaged, the teeth missing, and the vertex broken away, the holotype is non-diagnostic. This might mean that Zygorhiza kochii is at the mercy of opportunistic taxonomists who might move to declare it a nomen dubium – despite nearly a century of work being done with a clear idea of what Zygorhiza kochii “means” in terms of its anatomy. This situation, while not necessarily an issue in paleocetology, has plagued the horribly overcrowded field of dinosaur paleontology recently – with competing researchers finding ways to declare old taxa as nomina dubia and substituting new specimens (often only marginally better) as type specimens. This often comes across as a transparently desperate ploy to get to name something – and has generated no shortage of controversy in recent years. In some cases, estranged paleontologists desperate to make their mark have even gone trawling through cabinets in established collections looking for any fossils that might just barely push the envelope of anatomical differences into the “new species” zone – or opportunistically given a species a new genus name (e.g. if it was not already the type species of a particular genus) and screwed colleagues out of the opportunity to do it themselves.

The holotype (left) and reference specimen (USNM 11962 - and proposed neotype, right) - of Zygorhiza kochii. Photo on the right taken during my first visit to the Smithsonian in fall 2012; image on left from Uhen, 2013A.

    Fear of a nomenclatural coup d’etat led Mark Uhen (2013B) to formally petition the International Commission on Zoological Nomenclature to designate a neotype for Zygorhiza kochii. What’s a neotype? A neotype is a “new type”, an action permitted by the ICZN if the holotype specimen is ever lost or destroyed. In this case, the holotype very much still exists – but it is so bad, Uhen (2013B) argued that a neotype specimen was needed in order to preserve the taxonomic stability of the species. He further argued that the distinction between the Zygorhiza holotype and other southeastern basilosaurids, like Dorudon serratus and Chrysocetus healyorum from the upper Eocene of South Carolina – is not obvious. Therefore, a neotype was needed – and he proposed to use USNM 11962, Kellogg’s beautiful skeleton at the Smithsonian, to be the neotype. While you can’t really designate a neotype if it’s not missing, the ICZN can make exceptions – hence the petition.

    Two years later, a rebuttal was written by preeminent archaeocete paleontologist Philip Gingerich – who was Uhen’s Ph.D. adviser, to make things a bit strange. Gingerich is famous for discovering Pakicetus, Rodhocetus, Artiocetus, Maiacetus, and a Basilosaurus isis specimen with hindlegs, among other incredible archaeocete discoveries from Egypt and Pakistan – and has become ever more combative over the past 15 years. At my second SVP meeting I watched him deliver a particularly scathing talk critiquing recent papers on remingtonocetids by JGM Thewissen and colleagues – ironically, Gingerich’s other prominent Ph.D. student. I find it interesting that he’s been, at least publicly, lashing out at his former graduate students – it is, at minimum, bizarre behavior. There’s a lot more to the story, but I’ve been sworn to secrecy. Gingerich’s (2015A) rebuttal requests, quite frankly, that the ICZN not designate a neotype specimen for Zygorhiza and ignore Uhen’s (2013B) proposal. Gingerich reiterates the convoluted taxonomic history of Zygorhiza kochii, and uses a number of arguments.

1)      Dorudon serratus and Chrysocetus healyorum are not easily mistaken for Zygorhiza, since they are both slightly older (early late Eocene rather than latest Eocene), and the former is larger than Zygorhiza and the latter is smaller than Zygorhiza.

2)      The proposed neotype is from a different locality, 50 km away. [This is more of a technicality and means nothing scientifically since they’re from the same unit, but ICZN is pretty specific about specimens originating from the same locality in order to be part of the type series.]

3)      The type specimen and USNM 11962 are the same species based on size: there are probably three basilosaurids from the Jackson Group [the plot thickens immensely – see below!] and the smallest is Zygorhiza.

4)      Gingerich argues that since the type specimen and USNM 11962 both clearly represent the same species, and since the known sample of Zygorhiza kochii “should be thought of as a population of individual[s]…replacement of the existing holotype by a neotype will not solve any pressing problem.”

This last point is the most critical: at its core, Gingerich’s interpretation of the diagnostic value of the type specimen of Zygorhiza kochii is based entirely on its size. There’s some problems here: the first and most obvious is the fallout that would occur should somebody identify the holotype as a juvenile, in which case it could easily represent one of the other two basilosaurids. There is also the possibility that another smaller species could be discovered – but Gingerich (2015B) discounts this in a subsequent paper, saying he’s not holding his breath given that it hasn’t happened after nearly two centuries of collecting in the southeast. The last, and most serious problem, is that type specimens should be diagnostic, and ‘small size’ by itself is not a great start. However, to paraphrase my Ph.D. adviser R.E. Fordyce, “we shouldn’t judge the quality of historical type specimens and the judgement of researchers in the past based entirely on modern conventions” since conventions and attitudes change and are ultimately subjective. He advocates taxonomic conservatism: conserve existing names when possible, and quarantine them when conservation is not (e.g. designate nomina dubia).

Exceptions can be made for taxa that are already taxonomically stable: neotype or no, USNM 11962 serves as the ‘touchstone’ specimen for Zygorhiza – the de facto reference specimen. More extreme examples abound: Zarhachis flagellator was originally named based on a caudal vertebra by Cope – but Kellogg stabilized it in the 1920s based upon what he considered to be referable specimens. Is a caudal vertebra diagnostic today? Of course not! But Kellogg stabilized it and everyone has followed it through ‘taxonomic inertia’. So, I do sort of agree with Gingerich – a neotype is not needed – though I find virtually all of his reasoning problematic, and I think the last century of cetacean paleontology speaks for itself: Zygorhiza is stabilized. There is actually an argument here: names that have been in use for a long time and would cause quite a bit of confusion if fundamentally changed can be conserved  - though I forget if this is in the code or simply tradition.

Ultimately, my opinion on the matter is moot: in 2017, the ICZN declined to designate USNM 11962 as a neotype for Zygorhiza kochii.

The spectacular remains of Pontogeneus brachyspondylus, selected from the chimaeric assemblage of Hydrarchos. From Kellogg, 1936.

Pontogeneus or Cynthiacetus?

A third basilosaurid from the Eocene Yazoo Clay, Cynthiacetus maxwelli, was named in 2005 by Mark Uhen based on a well-preserved skull and mandibles with postcrania from the collections of the Mississipi Museum of Nature and Science. We actually had it on loan here for several years, and our museum’s benefactor Mace Brown spent quite a bit of time preparing off the hard limestone concretion that still encased it. It pretty clearly did not belong to Zygorhiza or Basilosaurus, and was pretty clearly separable: a bit smaller than Basilosaurus cetoides, and a LOT larger than Zygorhiza kochii. It differs chiefly from Basilosaurus in having normally proportioned lumbar vertebrae, like Zygorhiza – rather than the elongate, coke-can shaped vertebrae of Basilosaurus. The skull of Cynthiacetus maxwelli was not figured well by Uhen (2005), likely owing to its incomplete state of preservation. The teeth are nearly the same size as Basilosaurus, and I am not quite sure how the teeth differ between these species. The teeth are nearly double the size of their counterparts in Zygorhiza. One issue with basilosaurids is that the skulls all seem to be similar in most features – slight differences in proportions, a suture pushed over here or there, but basilosaurids *seem* to lack the more extreme anatomical diversity (disparity) of the skulls of early mysticetes and odontocetes.

The holotype (MNMNS 445) mandible and palate of Cynthiacetus maxwelli. This is a big, horribly heavy, slightly crushed, but reasonably well preserved specimen. The upper teeth are folded inwards. Photographed in 2019 at CCNHM just before it was returned to MMNS.

As it turns out, there is a different archaeocete taxon named from the same stratigraphic unit, also initially "discovered" amongst the chimaeric remains of "Hydrarchos" and named Zeuglodon brachyspondylus (later Pontogeneus brachyspondylus) by Muller (1851). Leidy (1852) shortly thereafter named Pontogeneus priscus, based off of an isolated cervical vertebra. Kellogg (1936) provided a figure and a description of the vertebra, classified it as Archaeoceti incertae sedis (uncertain position), remarked on its similarity to Basilosaurus, and even went so far as to refer some additional specimens to the taxon - but did not comment on its diagnoseability. Starting in the 1970s, paleocetologists moved away from naming all sorts of taxa based off of isolated vertebrae or even headless skeletons – with the exception of archaeocete paleontologists. Archaeocete paleontologists have been obsessed with finding archaeocete postcrania – and quite rightly so: for about 20 years it was a ‘cash cow’ so to speak in terms of scientific publishing: find any skeletons telling us about the land to sea transition in whales, and you’ve earned yourself a paper in Science or Nature. For fifteen years paleocetologists were searching for the first ancestral whale with ankles – and when that discovery was made, suspiciously in parallel, both Gingerich and Thewissen made the cover of Science and Nature (respectively) on the same day. Owing to this, there’s been a focus by archaeocete paleontologists on postcrania, and many descriptions of archaeocetes in recent years have skimped on the details of the skull (see my recent post on Ankylorhiza for more on this). Despite the more widespread use of postcranial features in diagnosing archaeocetes, defining and diagnosing a species based on an isolated vertebra is poor practice.

The beautifully preserved skeleton of Cynthiacetus peruvianus - a bit smaller than Cynthiacetus maxwelli, from the upper Eocene of Peru. From Martinez-Caceres et al., 2017.

    In a recent report on a partial skeleton of Zygorhiza kochii, Gingerich (2015B) reevaluated the taxonomy of Cynthiacetus and Pontogeneus. He pointed out that Muller (1851) indicated that “Z. brachyspondylus” differed from B. cetoides (which he referred to as Z. macrospondylus at the time) in vertebral shape and length. In 1852, Joseph Leidy described Pontogeneus priscus based on an isolated cervical vertebra from the Jackson Group in Louisiana – a little bit smaller than cervicals of Basilosaurus cetoides. Later, Leidy (1869) admitted that cervical vertebrae of “Z. brachyspondylus” were very similar and likely the same taxon. Kellogg synonymized these, and applied Leidy’s genus name to Muller’s species, recombining it as Pontogeneus brachyspondylus. Gingerich (2015B) made sure to mention that in a 1997 conference abstract, Uhen initially referred to MMNS 445 as Pontogeneus brachyspondylus – the specimen which he would later designate as the holotype of Cynthiacetus maxwelli in 2005. This meant that at some point, Uhen recognized that the Cynthiacetus maxwelli holotype had *something* to do with Pontogeneus.

    Uhen (2005) declared “Z. brachyspondylus” and Pontogeneus priscus as nomina nuda, or naked names: taxa lacking a proper publication. As correctly pointed out by Gingerich (2015B), this doesn’t really apply to 19th century publications, and at the minimum, associating a name with an illustration is all that’s needed for a name to be valid. Uhen (2005) further argued that the holotype cervical vertebra of Pontogeneus priscus was so incomplete and similar to Basilosaurus, that it was not sufficiently diagnoseable – albeit incorrectly referring to it as a nomen nudum, rather a nomen dubium. Gingerich (2015B) further went on to indicate that, since there were only three apparent basilosaurids in the Jackson Group of Alabama, Mississippi, and Louisiana – a small species (Zygorhiza kochii), a large species (Basilosaurus cetoides), and a third medium sized species – the oldest available name for this taxon should be used. Therefore, Gingerich declared Cynthiacetus maxwelli to be a junior synonym of Pontogeneus brachyspondylus. Gingerich (2015B) further indicated that the P. brachyspondylus holotype had larger vertebrarterial foramina than Basilosaurus, and therefore is not easily confused with.

Comparison figure of the cervical vertebrae of Basilosaurus isis, Cynthiacetus maxwelli, and Pontogeneus brachyspondylus (this one happens to be Leidy's P. "priscus" holotype). From Gingerich (2015B). 

    There are of course, problem with this. The first and most important is that isolated vertebrae are not diagnostic. Within Neoceti, headless skeletons are not really diagnostic, and the few species erected on postcrania alone in the past few decades attract serious grumbling and eye rolling in private from other paleocetologists. Second, is that the ontogenetic status of the Pontogeneus priscus type specimen is uncertain, though it may have fused epiphyses. Third, it’s just a vertebral centrum and even for a vertebra is woefully incomplete; the argument that the vertebrarterial foramina are larger than in Basilosaurus is not defensible either. Fourth, unlike Zygorhiza and Zarhachis, Pontogeneus has not generally been in use, though Kellogg (1936) helpfully clarified the taxonomy and suggested it may be a third basilosaurid from the Jackson Group. Therefore, it does not deserve to be “grandfathered in” like Zarhachis flagellator. Lastly, and perhaps most critical, is that synonymy of Pontogeneus and Cynthiacetus relies on the interpretation that there are only three basilosaurids in the fauna. Gingerich admits this, and that the hypothesis can be tested by finding a second medium-sized basilosaurid – in that case, vertebrae of two similarly sized basilosaurids would not be distinguishable, and Cynthiacetus maxwelli would be clearly diagnoseable. By this line of reasoning, Gingerich (2015B) tacitly admitted that the holotype of Pontogeneus brachyspondylus is not diagnostic. It’s also a bit problematic to diagnose a taxon based on a fauna interpretation rather than morphology. In my opinion, there’s not much difference between naming a nomen dubium now, or resurrecting one that has not really been in use for nearly a century. Regardless, it’s not completely clear-cut, but I’ll be using Cynthiacetus maxwelli for the time being. How to fix it? Find more basilosaurids in the Eocene of the southeastern USA, expand the sample – maybe conduct a statistical analysis of cervical vertebra dimensions.

The holotype bulla (left) and a thoracic vertebra (right) of Basilotritus uheni from the Eocene of Ukraine. From Gol'din and Zvonok (2013).

Basilotritus or Platyosphys?

    Another study by Uhen on a large protocetid skeleton from the middle Eocene of North Carolina named Eocetus wardii, based on some really unusual vertebrae with a partial rostrum. At the time owing to its incompleteness, Uhen (1999) identified it as a protocetid closely related to Eocetus schweinfurthi from the middle Eocene of Egypt. Later, the discovery of a more complete skeleton from the late Eocene of Ukraine with basilosaurid teeth and identical vertebrae led to the naming of Basilotritus uheni by my friend Pavel Gol’din and colleagues, in honor of Mark Uhen’s earlier research on E. wardii; they referred this species to their new genus, recombining it as Basilotritus wardii. Their phylogenetic analysis pulled the taxon into the Basilosauridae. The vertebrae of Basilotritus are quite strange: the vertebrae are generally similar to other basilosaurids in shape but extremely thick with compact layering exposed in fractures – this is called pachyostosis. The bones are also osteosclerotic, meaning that there is minimal development of porous cancellous bone internally. The external layering is quite distinctive – and in the future, histological examination of theses bones is an absolute must (the external dense layering is reminiscent of an “EFS” – external fundamental system, the smoking gun histological determination of maximum size and cessation of growth). The thickening is also present in the neural arches and spines, and most unusually, the transverse processes of the lumbar vertebrae are nearly as long as the centra themselves – quite different from Basilosaurus, Dorudon, and Zygorhiza.

The spectacular original fossils of Platyosphys paulsonii, illustrated by Brandt (1873). The whereabouts of the specimens are unknown, and this image is all we have. 

    As it turns out, there were two eastern European taxa (named by famed cetologist J.F. Brandt in 1873) with similar vertebrae which Uhen (1999) made no mention of – Platyosphys paulsonii, based on three isolated vertebrae from the middle-late  Eocene Kharkov Formation of Ukraine, and Platysophys einori, from unnamed phosphate beds of the same age elsewhere in Ukraine. Given the location of the new partial skeleton of Basilotritus uheni and Pavel’s familiarity with obscure cetaceans of eastern Europe and the Caucasus, Gol’din and Zvonok (2013) fully discussed Platyosphys in the context of their new find. They indicated that the holotype vertebrae of Platyosphys paulsonii are lost – they apparently disappeared some time between the 1920s and World War II – and note that these vertebrae are generally similar, with some differences, and that they are also in general similar to Eocetus. Given that the specimen is lost, and comparisons are no longer possible, they declared Platosphys paulsonii a nomen dubium (Gol’din and Zvonok, 2013). Platyosphys einori, on the other hand, was not lost, and these authors figure the specimen and indicate that while it is generally similar to Eocetus and Basilotritus, it is not diagnostic owing to its incompleteness and lack of clearly observed internal structure, and also declare it a nomen dubium.

The spectacular holotype specimen of Platyosphys aithai from the late middle Eocene of Gueran, Morocco. From Gingerich and Zouhri (2015).

    Gingerich and Zouhri (2015) reported a new and unusual assemblage of archaeocete whales from the late middle Eocene (Bartonian stage) of Morocco (Aridal Formation, near Gueran), including three protocetids (one of which being Pappocetus), the new species of small basilosaurid Chrysocetus foudassii, new material of Eocetus schweinfurthi, and resurrected Platyosphys and named within it their new species Platyosphys aithai. Gingerich and Zouhri (2015) fundamentally disagreed with the nomen dubium decisions by Gol’din and Zvonok (2013), indicating that the survival of a type specimen to the present has no bearing on whether the name is valid or if the taxon is diagnoseable, which is technically correct under ICZN rules, which are (to be fair) extremely lax. Gingerich and Zouhri (2015) indicate that the illustrations from Brandt (1873) are sufficient to diagnose Platyosphys, the main characteristic being the elongated shelf-like transverse processes as well as the internal structure. This is fundamentally true – after all, the holotype specimen of Agorophius pygmaeus is missing (aside from a single tooth) and Fordyce (1980) redescribed it from the plates, and a new specimen was referred to it by Godfrey et al. (2016).

    Gingerich and Zouhri (2015) then reassigned Basilotritus uheni and B. wardii to Platyosphys, recombining them as Platyosphys uheni and Platyosphys wardii – though to be honest, I am surprised that they did not go further and declare Basilotritus uheni a junior synyonym of P. paulsonii (e.g. the sinking of Cynthiacetus, above). A few years ago, Gingerich presented an SVP talk about a new skeleton of Platyosphys aithai from Gueran, which included a well-preserved skull and articulated vertebral column and ribcage. I remember the ribs were extremely thick, like sea cow ribs – actually about what I would have predicted given the state of the vertebrae. The paper has not yet come out, and I am very much looking forward to it when it does. There’s been no followup by Pavel Gol’din and colleagues – and I know from correspondence that the annexation of Crimea by Russia in 2014 (following the Ukrainian revolution earlier that year) forced Pavel to leave his job at the Taurida National University in Sebastopol; he spent some time in Tel Aviv and Moldova before being hired again at the National Academy of Sciences in Kiev, where he’s continued much of his research on Paratethyan baleen whales. Needless to say, Pavel’s had a host of unfortunate career interruptions since describing Basilotritus.

Concluding remarks

    These three taxonomic controversies for Basilosauridae highlight several different questions. What standards should holotypes have? Are 150 year old drawings of isolated vertebrae really sufficient to diagnose a taxon? At what lengths should we go to preserve obscure old names? Which are worth preserving? I am still of the opinion that isolated vertebrae are not diagnostic, and you can make the same argument that I did above for Pontogeneus – since Platyosphys has not been in continuous use or widely recognized since the late 19th century, it doesn’t really pass the same test as Zarhachis, for example – and in the time being, the new taxon Basilotritus was erected on clearly better material. Raising the spectre of lost specimens, preserved only as drawings in a tome from the 1870s, seems like a major “gotcha!” to me. I have no skin in the game, but have some sympathy for Uhen and Gol'din: they were motivated to move the field forward and nominate diagnoseable specimens as holotypes, not by taxonomic piracy. One issue is that isolated vertebrae cannot be diagnostic at the species level. If a specimen is not diagnostic at the species level, it cannot really be diagnosed as a species. The vertebrae of Platyosphys are perhaps diagnostic at the genus level. In my opinion, much of these recent taxonomic opinions issued by Gingerich seem to be examples of taking the ICZN at face value and defining taxa at the very limits of what is permitted – what we *can* do versus what we *should* do. We *can* name taxa based on non-diagnostic remains, but we *should* use diagnostic remains instead. These arguments are far removed from ‘best practices’ and perhaps unfair to paleontologists wanting to move the field of cetacean paleontology forward. At the same time - if an old holotype is diagnoseable, we shouldn't circumvent it - for that is the path to nomenclatural anarchy (not that I'm saying that's taken place).

Further Reading

Gingerich, 2015A: https://www.biotaxa.org/bzn/article/view/13742

Gingerich, 2015B: https://deepblue.lib.umich.edu/handle/2027.42/113064

Gingerich and Zouhri, 2015: https://www.sciencedirect.com/science/article/pii/S1464343X1530039X

Gol'din and Zvonok, 2013: https://www.cambridge.org/core/journals/journal-of-paleontology/article/basilotritus-uheni-a-new-cetacean-cetacea-basilosauridae-from-the-late-middle-eocene-of-eastern-europe/291BF670BF4B3D57664D25C9CBDC8E79

Godfrey et al. 2016: https://bioone.org/journals/journal-of-paleontology/volume-90/issue-1/jpa.2016.4/A-new-specimen-of-Agorophius-pygmaeus-Agorophiidae-Odontoceti-Cetacea-from/10.1017/jpa.2016.4.short

Kellogg, 1936: http://publicationsonline.carnegiescience.edu/publications_online/archaeoceti.pdf.

Martinez-Caceres et al., 2017: https://bioone.org/journals/Geodiversitas/volume-39/issue-1/g2017n1a1/The-anatomy-and-phylogenetic-affinities-of-Cynthiacetus-peruvianus-a-large/10.5252/g2017n1a1.short

Uhen, 1999: https://www.cambridge.org/core/journals/journal-of-paleontology/article/new-species-of-protocetid-archaeocete-whale-eocetus-wardii-mammalia-cetacea-from-the-middle-eocene-of-north-carolina/7F100C92C1A92CC11FCE5B04E4B46949

Uhen, 2005: A new genus and species of archaeocete whale from Mississippi. Southeastern Geology, 43:3:157-172.

Uhen, 2013A: A review of North American Basilosauridae. Bulletin of the Alabama Museum of Natural History, 31:2:1-45.

Uhen, 2013B: https://bioone.org/journals/The-Bulletin-of-Zoological-Nomenclature/volume-70/issue-2/bzn.v70i2.a14/Case-3611Basilosaurus-kochii-Reichenbach-1847-currently-Zygorhiza-kochii-Mammalia-Cetacea/10.21805/bzn.v70i2.a14.short

Note: I've left out many of the 19th century references here, since very few of you will read them, and I need to wrap this up and go to work. The most thorough account of the 19th century nomenclatural history of archaeocetes can be found in Kellogg (1936), so if you're really so desperate that you want *more*, I refer you to Kellogg.

Sunday, August 9, 2020

Ankylorhiza tiedemani, a giant dolphin from the Oligocene of South Carolina, part 2: the ecology, evolution, and swimming adaptations of Ankylorhiza

Don't forget to check out Part 1 here.

Disclaimer: this post includes a lot of irreducible terminology, particularly when it comes to the phylogenetic section – there simply are no useful or meaningful simple synonyms of words like “paraphyletic”. I’ll try to sketch these out, but especially for the phylogenetic terms, I suggest consulting the UCMP online phylogenetics glossary https://ucmp.berkeley.edu/glossary/gloss1phylo.html

Life restoration of Ankylorhiza tiedemani, artwork by yours truly! 

The skull and teeth Ankylorhiza – a killer dolphin?

At first glance, the skull of Ankylorhiza tiedemani bears some gross similarities with Squalodon: it has a somewhat elongate rostrum, procumbent incisors, some triangular posterior cheek teeth (probably molars and/or premolars), large temporal fossae, and long zygomatic processes. But on closer inspection, the similarities seem to end. Ankylorhiza has a more derived dentition than Squalodon: most of the teeth are single rooted and none appear to be completely double rooted: the posteriormost cheek teeth are bilobate or have incompletely split root lobes (the posteriormost teeth are typically the last to become single rooted, both in cetaceans and in pinnipeds), whereas multiple teeth are double rooted in Squalodon. Ankylorhiza tiedemani also generally lacks large accessory cusps, and a couple minute bumps are present on the cutting edges of the last two teeth – and that’s it; in Squalodon, about half the dentition has accessory cusps like a basilosaurid whale. Squalodon, like most crown Odontoceti (modern odontocetes), has a narrow rostrum base and a wide vertex (top of the braincase); in Ankylorhiza, the base of the rostrum is wide, and the vertex is quite narrow – both are features shared with other early odontocetes like Agorophius and the xenorophids, as well as archaeocete whales. Ankylorhiza tiedemani also has quite a bit of parietal exposed at the vertex of the skull: in crown odontocetes, the parietal is completely hidden by the bones of the facial region migrating backwards with the blowhole towards the top of the skull. And that brings us perhaps to one of the most critical differences: the bony naris in Ankylorhiza is very far forward, in front of the orbits and out on the base of the rostrum – approximately the same location as in some xenorophids, and yet posterior to the position in admittedly more plesiomorphic odontocetes like Simocetus and Ashleycetus where it is far out on the rostrum.*

The early evolution of telescoping in odontocetes - maxilla in gray, showing the overriding of the frontal, from Geisler et al. 2014. Ankylorhiza is at a near-identical stage of telescoping as Patriocetus, the third skull from the right.

 *Interestingly, the position of the blowhole does not correspond 1:1 with the phylogenetic placement of xenorophids, Ankylorhiza, and simocetid-grade dolphins: in xenorophids, the blowhole is slightly posterior to the base of the rostrum, despite being the earliest lineage of these three on the cladogram; in the next diverging lineages, the simocetid-grade dolphins, it is far out on the rostrum, and in Ankylorhiza and other “agorophiid” grade dolphins, it is just in front of the orbits. This shouldn’t be terribly surprising as these lineages all more or less appear in the fossil record at the same time and all are contemporaneous, so none are directly ancestral to one another. Instead, these Oligocene odontocetes seem to represent the spokes of a wheel frozen in time just after an explosive radiation of early odontocetes some 35 million years ago – most of which would go extinct, with a few surviving ‘experiments’ and a lot of dead ends. This is also the best explanation for the seemingly haphazard distribution of dental features (e.g. as discussed for Ankylorhiza v. Squalodon above).

So, what was Ankylorhiza eating? Its teeth are relatively large, and bear sharp cutting edges unlike modern dolphin teeth. Further differing, but shared with many extinct marine tetrapod lineages – like the giant pliosaurids, for example – are longitudinal ridges on the enamel (I’ve referred to this in the paper as ‘fluted’ enamel). A recent paper by McCurry et al. (2019) found that these ridges do not seem to correspond to internal tooth structure, enamel thickness, etc. and therefore are likely to be related to the efficiency of puncturing rather than distribution of force/pressure during biting (as has been demonstrated with rugose enamel, for example). The McCurry paper is thought provoking and surprising, indicating that by whatever means, the external shape is important for aquatic feeding regardless of how the tooth is “built” and has re-evolved many times among aquatic tetrapods. It does, however, need to be evaluated in the context of enamel micro/ultrastructure, which to my knowledge has not yet been attempted but would shed considerable light on this topic.

The teeth of CCNHM 103, the best preserved specimen of Ankylorhiza tiedemani. A-B are the upper teeth, C-D are the lower teeth; while the lower dentition is not complete, most positions are preserved. The lower teeth highlight the thick cementum the best: contrast rc1? with rpc4: in the former, the cementum has spalled off from the dentine, and in rpc4, the thickness of the cementum is visible relative to the much thinner dentine 'core' on its own. The cementum is the light tan/buff colored tissue.

Ankylorhiza has two more interesting dental quirks. The first, and most readily interpreted, is that it has thickened cementum on the tooth roots. Cementum is one of the three major dental tissues, along with enamel and dentine – and is the only tissue that can grow externally on the tooth after tooth eruption. Enamel is only developed in the “crypt” and after eruption, dentine is deposited but from the outside in. Cementum, on the other hand, binds the root of the tooth to the bone of the jaw and can be deposited continuously throughout a mammal’s lifespan. In some cetaceans, cementum can be unusually thick – in some sperm whales, half of the radius of a tooth might be cementum. Larger odontocetes tend to have thicker cementum also. There is some experimental evidence that cementum can help teeth in modern mammals sustain higher point loads without fracturing, and thickened cementum in extinct sperm whales has recently been interpreted as improving bite force capabilities and tooth survivability against fractures, which makes sense given that many extinct big-toothed sperm whales were likely macrophagous killer sperm whales (e.g. Zygophyseter, Acrophyseter, Albicetus, Brygmophyseter, Livyatan, etc.). In Ankylorhiza, we similarly interpreted thickened enamel as a sperm-whale like adaptation for increased bite force. Another consideration, however, is body size: it’s also possible that owing to how physiologically expensive it is to produce larger teeth, that tooth size may lag behind skull size, and as the alveoli become larger in later postnatal growth, cementum is used to fill in the gap. This is a concern Brian and I both independently had – a line of investigation for someone else in the future!

Two of the procumbent lower incisors of CCNHM 220, our specimen of Ankylorhiza tiedemani from the lower Oligocene Ashley Formation. In CCNHM 103, the crown is worn down to the gumline - which we interpret as the result of dental ramming.

The last dental feature are the procumbent incisors: this is certainly not unique to Ankylorhiza, as procumbent tusk-like incisors are also present in Squalodon and Phoberodon, and waipatiids, most clearly in Otekaikea huata – though similar teeth are also preserved in the two species of Waipatia. Functional hypotheses for these apical tusks in Squalodon, Phoberodon, and waipatiids have not been proposed in any published article. We noted that the anterior incisors of Ankylorhiza are quite heavily worn in CCNHM 103, and the lower first incisor is worn down to the gumline, with a possible wear facet corresponding to the upper first incisor, which is not completely procumbent – but curves anteroventrally. It’s also possible that this is coincidental and that these teeth did not contact at all, given the shape of the upper jaw. Regardless, extreme wear – the most extreme in the entire dentition – suggests extreme use. We propose that the tusks in Ankylorhiza could have received this extreme wear from being used by ramming prey – this could be an effective way to kill large prey, and is a method that modern delphinids use to dispatch prey (or, in the case of bottlenose dolphins, to commit “porpicide”, seemingly for fun).

A dorsolateral view of the skull of CCNHM 103, showing the enormous temporal fossa for the temporalis muscles. This must have afforded Ankylorhiza a punishing bite force.

In addition to these dental features, the temporal fossae are enormous – each left and right is individually much larger than the endocranial volume for the brain, so perhaps ½ or more of the internal volume of the braincase end of the head is completely occupied by temporalis and masseter muscles to close the jaws. These muscles are quite a bit reduced relative to the condition in basilosaurid whales, of course – but are significantly larger in comparison to xenorophid dolphins and crown odontocetes, indicating that their large size is associated with adaptation to macrophagy in Ankylorhiza. Even further evidence comes from the range of motion at the cranio-vertebral joint: if you take the atlas vertebra (missing in CCNHM 103, but I borrowed one from a different specimen) and moved it to the approximate limits of cranial flexion (dorsal movement of the skull) and extension (ventral movement), the range of movement is nearly identical to that of Orcinus and Pseudorca – which seems correlated with grip and tear feeding, where these large delphinids rip their prey (that is too large to swallow whole) into manageable sized portions by shaking the prey back and forth. Neither modern delphinid has sharp cutting edges or serrations, so this may have been somewhat more efficient in Ankylorhiza.

Altogether, available evidence indicates that Ankylorhiza tiedemani was a macrophagous “killer dolphin” – the first odontocete to evolve into this niche, and the first truly large odontocete to evolve. Ankylorhiza is known from the Ashley Formation, and therefore evolved within only about 4-5 million years of the oldest known odontocetes (Simocetus rayi, ~33 Ma). However, as detailed elsewhere on this blog, odontocetes probably evolved as early as the late Eocene, and there’s probably an additional 5 million years of missing fossil record unaccounted for by fossils permitting the divergence of agorophiid-grade dolphins from basilosaurids. This niche did not disappear of course – it was refilled by Squalodon in the early Miocene – as well as by the macrophagous Scaldicetus-grade sperm whales (Acrophyseter, Brygmophyseter, Livyatan, Zygophyseter) in the middle and late Miocene. These whales at some point died out in the latest Miocene or Pliocene, and this niche was not refilled until some time in the Pleistocene by the modern killer whale.

The skull, vertebrae, and forelimb of Ankylorhiza tiedemani. From Boessenecker et al. 2020.

Ankylorhiza and the evolution of swimming in Neoceti

The feeding ecology is actually not the main thrust of the paper – the main point of the paper is the implication for locomotion and the evolution of swimming. I won’t talk too much about archaeocetes here – there is not too much of a consensus on the swimming style of legged archaeocetes – but to review the situation in basilosaurids, the earliest whales committed to a pelagic lifestyle – there are rectangular vertebrae at the tip of the tail indicating the presence of a caudal fluke, and the tiny vestigial hindlimbs could not have served any role in locomotion. The flippers still have a moveable elbow joint, and the finger bones are elongated and spindly. The scapula is dramatically broader than earlier archaeocetes, generally resembling modern cetaceans. The humerus is very long (longer than the radius/ulna) and little modified from terrestrial mammals – with a long deltopectoral crest. The vertebrae have very smooth changes in proportions, meaning that they are not ‘regionalized’ like in many modern odontocetes. Basilosaurids also have a high vertebral count, with somewhat serpentine bodies – and long thoraces, with large numbers of thoracic vertebrae and ribs. In modern cetaceans, there is a tendency for a reduction in thoracic number and an increase in the number of caudal vertebrae.

Functional anatomy of the postcranial skeleton of Ankylorhiza tiedemani. The top shows the proportions of vertebral centra (bodies) of this skeleton - length, width, and height - and a lot can be inferred from these diagrams, pioneered by coauthor Emily Buchholtz, which I have lovingly nicknamed "Buchholtz diagrams". This plot more or less shows that Ankylorhiza was a better swimmer than a basilosaurid, mysticete, or modern sperm whale, with some vertebral adaptations in parallel with belugas and beaked whales. The second and third are PCA plots, analyzed by Morgan Churchill, which are sort of a "kitchen sink" approach to analyzing measurements of bones. The vertebral plot shows that Ankylorhiza plots with modern odontocetes, close to sperm whales, but slightly closer to archaeocetes. The forelimb plot shows Ankylorhiza clustering with archaeocetes and beaked whales rather than members of the Delphinida. From Boessenecker et al. 2020.

We approached the analysis of postcranial evolution in two ways: we conducted principal components analyses (PCA) of forelimb and vertebral measurements, and also conducted an ancestral character state reconstruction. The PCA is a type of statistical analysis and basically seeks to identify what measurements explain most of the variation in shape within a particular dataset. In this case, we got very different results for the flipper and vertebral column. The vertebral anatomy was similar to modern odontocetes – slightly closer to archaeocete whales, but clustering with odontocetes. The flipper anatomy on the other hand, clustered with basilosaurid whales (and to a lesser extent, the Amazon river dolphin and beaked whales) to the exclusion of most modern odontocetes. Ancestral character state reconstruction takes a single character from a cladistic analysis – for example, if the transverse processes of the lumbar (lower back) vertebrae point sideways or downwards a little bit – and reconstructs where changes probably occurred from one condition to another on a cladistic tree. It’s generally a good idea to exclude these characters from the analysis producing the tree, so that the tree itself is not influenced by the character in question (basically avoiding circular reasoning). We found that over a half dozen anatomical features present in modern cetaceans must have evolved twice – once in baleen whales, and once in echolocating whales. Some of these features that Ankylorhiza, stem odontocetes, and stem mysticetes share with basilosaurids include ventrally deflected transverse processes of the lumbar vertebrae, a long humerus with a long deltopectoral crest (owing to similarly long humerus being present in toothed mysticetes [Mystacodon, Fucaia] and eomysticetids [Eomysticetus, Yamatocetus, Waharoa, Toharahia]), a wide caudal peduncle. The wide caudal peduncle is recorded by equidimensional – rather than narrow – mid-caudal vertebrae. This is an adaptation for efficient up/down movement of the tail – all of the propulsive force is in the flukes, at the end of the tail (a long lever arm) and minimal effort is wasted on moving the tail stock: it slices vertically through the water. Such a peduncle evolved independently in many fish (tuna/mackerel) and many sharks, but the peduncle is instead horizontal given the side to side movement.

Convergent evolution of mysticetes and odontocetes, as reconstructed by our Ancestral Character State Reconstruction (ACSR), with diagrams showing the different flipper skeletons at top. From Boessenecker et al. 2020.

What could have driven such extensive convergence in the postcranial skeleton of odontocetes and mysticetes? This situation does seem a bit different than in pinnipeds, which have remarkably different postcranial features and locomotion. We hypothesize that two major evolutionary “ratchets” appeared in cetaceans: the first was a trade of hindlimbs for tail flukes, highlighted by the evolutionary transformation of protocetids and basilosaurids, and the second was the ‘locking’ of the elbow in the cetacean flipper. With these two adaptations, best interpreted as key innovations in cetacean evolution, there was no going back to any form of quadrupedal paddling, and cetaceans were stuck with fluke-powered swimming. The locking of the elbow meant that, for the most part, the foreflippers would be reduced to hydrofoils for steering rather than being used to generate thrust (like a sea lion or penguin). We think the latter of these features caused a cascade of convergences later on within Neoceti.

Prior to this analysis, these features were assumed – and quite reasonably so – to have evolved in the common ancestor of mysticetes and odontocetes. There’s an interesting ‘perfect storm’ of coincidences that took place to make this sort of knowledge gap and discovery possible:

1)      1) Because the postcranial skeleton of baleen whales and odontocetes share more in common with eachother than either does with archaeocetes, researchers who work on early Neoceti have tended to be “headhunters”.

2)      2) Archaeocete researchers have historically sort of ignored cranial anatomy and spent much more effort on postcranial anatomy. Archaeocete researchers typically (with some minor exceptions) have not worked on Neoceti, and vice versa* – reinforcing point 1 above.

3)      3) Few fossils of early Neoceti, up until now, included either well-preserved forelimbs or vertebral columns, reinforcing point 1 above. Because the scattered remains of early Neoceti didn’t really give us a “transitional blueprint” to postcranial change, neocete specialists didn’t particularly recognize or appreciate differences between stem odontocete and modern odontocete skeletons. Nor did they look for them – reinforcing point 1 above.

no caption needed

*There are a few notable exceptions: Mark Uhen has worked on both groups, and the French/Belgian working group has studied both archaeocetes, odontocetes, and early mysticetes (Lambert, Muizon, Martinez-Caceres, et al.).

So, based on missing data, and descriptive/research practices informed by that missing data, there was a bit of circular reasoning taking place. This situation allowed for the unappreciated significance of Ankylorhiza being preserved with such a complete postcranial skeleton.

Our preferred tree topology - with various relevant clades outlined in color rectangles. From Boessenecker et al. 2020. Ankylorhiza is placed within the red box, a possible monophyletic Agorophiidae.

Who is Ankylorhiza related to?

Ankylorhiza tiedemani was originally placed in the genus Squalodon – but no longer is. Which, begs the question – what is Squalodon, anyway? The short answer is, nobody knows – no modern cladistic analysis has included multiple species of Squalodon, so we’re not quite sure if the genus is monophyletic. Nobody is certain if the family Squalodontidae is monophyletic, and if it is, what other genera belong within it. Many putative squalodontids, including Patriocetus, Phoberodon, Prosqualodon, Neosqualodon, and others – have all shown to be different lineages of stem odontocetes in cladistic analyses. At present, the best we can do, is define Squalodon as large-bodied odontocetes with mostly symmetrical skulls and large triangular molars with accessory 2-4 cusps and rugose enamel and a number of distinctive earbone features – and for the time being, restrict the family to the genus. Squalodon seems to be one of the later diverging stem odontocetes, just outside the crown group, but also appears occasionally within a more broadly defined Platanistoidea – as redefined by Christian de Muizon in the 1980s, including Platanista, squalodelphinids, extinct platanistids, and squalodontids. In recent years, this seems less tenable as the only analyses to recover this relationship do so only at the expense of removing undescribed species from Charleston Museum (originally coded in Geisler and Sanders, 2003) and shorter character lists. When more characters and taxa are included – squalodontids fall out into the stem group as a paraphyletic grade of dolphins between Ankylorhiza and the modern sperm whale, often intermingled with a similarly paraphyletic Waipatiidae (also recovered on some occasions as a clade within Platanistoidea, but frequently also smeared into a paraphyletic grade on the stem).

Comparison of Ankylorhiza with Squalodon, Phoberodon, and the "earthquakes squalodontid" - an Oligocene taxon from NZ that is similar to both Phoberodon and "Prosqualodon" hamiltoni from the early Miocene. From an old SVP talk of mine.

Ankylorhiza may be superficially similar to Squalodon, but shares much more in common with Agorophius pygmaeus and Patriocetus kazakhstanicus, including a wide base of the rostrum and a narrow intertemporal constriction. In our analyses, Ankylorhiza formed a clade with Agorophius (though, surprisingly, not the very similar Patriocetus) – and therefore, in the future, could be the basis for a redefined Agorophiidae. For the record, many whaleontologists wince when they hear the word Agorophiidae – historically it’s been an even bigger wastebasket than Squalodontidae: virtually all stem odontocetes aside from Squalodontid- and waipatiid-grade odontocetes (including the xenorophids!) were placed into a hilariously paraphyletic Agorophiidae.

Patriocetus kazakhstanicus, a very similar but smaller odontocete from the Oligocene of Kazakhstan (from Dubrovo and Sanders, 2000), which differs principally in having more archaic teeth with thinner cementum - large teeth of this morphology are known from Oligocene localities here in South Carolina, and at least one Patriocetus-like dolphin appears to have been the same size as Ankylorhiza but is known only from fragments.

Another surprising result of our analysis was the clustering of three purported squalodontids – Squalodon, Prosqualodon, and Phoberodon – into a clade! However, this was only in one of the two analyses with different weighting schemes. In this analysis, waipatiids “broke apart” and became paraphyletic. Interestingly, in the other analysis, the opposite happened: squalodontids fell apart and the waipatiids became monophyletic. This is likely because the two groups are remarkably similar in morphology and evolutionary grade and 1) may actually be the same clade after all, owing to frequent phylogenetic clustering and 2) in the past have mostly been separated based upon size and some features that may be related to size.

Future Directions

There’s two different suites of future studies I envision. The first are immediate questions to be answered about Ankylorhiza itself. There’s a second species of Ankylorhiza awaiting description! That should be fun to tackle. There are juvenile specimens that can tell us about the growth of Ankylorhiza. There are other specimens of both species that tell us a little bit more about the feeding behavior and ecology of Ankylorhiza – including specimens with serrated teeth, and mandibles. Paleohistology of Ankylorhiza would be a useful endeavor. FEM and reconstruction of the volume of the temporalis could tell us about the bite force capabilities of Ankylorhiza, as can microwear analyses. I am dying to know what the microwear of the incisor tusks looks like!

Other major directions are concerned with broader impacts of our work: is there evidence elsewhere that has been overlooked before? What about the postcranial skeleton of Mirocetus – the second most completely known stem odontocete (yet, arguably, one of the poorest understood). Other studies contrast with ours: Amandine Gillet’s recent paper looked at this differently and concluded that vertebral morphology is quite divergent within Neoceti. Why is this? Maybe each study emphasized different aspects of vertebral anatomy? A clear future direction for research is a renewed effort to find and describe Oligocene odontocete skeletons with postcrania: clearly, not identical to modern cetaceans. What the hell did the flipper of xenorophids look like? We still don’t have one of those. Does anyone else belong in the Agorophiidae? Are the Waipatiidae and Squalodontidae monophyletic or even separate clades? A major overhaul of squalodontid morphology and phylogeny is needed, though recent efforts to redescribe Phoberodon and Prosqualodon are nice initial contributions.

What drove Ankylorhiza to extinction? Within about 5 million years or so, the first macrophagous physeteroids appeared, re-filling the vacant niche. We don’t really have a good handle on what caused apparent turnover between the late Oligocene (e.g. Chandler Bridge Formation assemblage – dominated by “Agorophiidae”, “Waipatiidae”, and Xenorophidae) and the early Miocene (dominated by squalodontids, early platanistoids, and other longirostrine odontocetes – e.g. Belluno Sandstone of Italy).

Lastly, and this is a broad one - this should be a major call for paleocetologists to consider their biases in the study of fossil cetaceans. What parts of the skeletons are you focusing on, perhaps at the expense on other parts? I can tell you, it can be very aggravating as a neocete specialist trying to find anything out about the earbones or basicranium of basilosaurid or protocetid whales, let alone decipherable figures (the landmark monograph on Cynthiacetus peruvianus is a notable exception). Likewise, a neocete worker has a similarly difficult time finding much of anything out about the postcranial anatomy of even well-preserved odontocete and mysticete specimens (this is decidedly worse in the crown of each clade). Each group of researchers have their own set of historical biases, in the absence of which, these findings would have been announced decades ago. Lucky for me, I guess!

Regardless, this won’t be the last you hear of Ankylorhiza – so stay tuned!

Further Reading/References

Boessenecker et al. 2020: https://www.cell.com/current-biology/fulltext/S0960-9822(20)30828-9

Dubrovo and Sanders, 2000: https://www.tandfonline.com/doi/abs/10.1671/0272-4634(2000)020%5B0577%3AANSOPM%5D2.0.CO%3B2

Geisler et al., 2014. https://www.nature.com/articles/nature13086

Gillet et al. 2019: https://royalsocietypublishing.org/doi/full/10.1098/rspb.2019.1771

McCurry et al. 2019: https://academic.oup.com/biolinnean/article/127/2/245/5427318

Saturday, August 1, 2020

Ankylorhiza tiedemani, a giant dolphin from the Oligocene of South Carolina, part 1: bringing clarity to 170 years of confusion

Part 1 of a 2 part series on our new study describing the most complete skeleton of an early echolocating whale, Ankylorhiza tiedemani. This first post covers the background and introduction to the taxonomy of this ancient dolphin - the next one will dive deeper into feeding ecology, locomotion, and the evolution of early dolphins. Check out Part 2 here.

My first visit to College of Charleston in 2012 - checking out some Oligocene toothed and toothless mysticetes for my Ph.D. research on eomysticetid whales. I had no idea that, just a few years later, I would end up working on this tremendous collection at CCNHM!

A couple of weeks ago we published a new paper in the journal Current Biology on a very large dolphin from the Oligocene of South Carolina – Ankylorhiza tiedemani. I’ve been working on this study with my coauthors Morgan Churchill, Emily Buchholtz, Brian Beatty, and Jonathan Geisler on and off (but mostly on) since summer 2017. It’s based on a fossil collected in the 1990s by CofC alum Mark Havenstein, but serious research on this taxon – affectionately known as “Genus Y” for decades – actually began in the early 1970s with the discovery of specimen ChM PV 2764, a well-preserved skeleton from the Chandler Bridge Formation of South Carolina, discovered and excavated by the late Al Sanders. Al was the Natural History curator at Charleston Museum until 2012; I first met Al a few months later in October 2012 on my very first visit to Charleston.

Al Sanders (right) and Ewan Fordyce (left) at CCNHM in 2012 discussing CCNHM 108 on the table, which five years later would become the holotype specimen of Coronodon havensteini.

Al had no advanced degrees, but was an unparalleled expert in paleontology and natural history of the southeastern USA and his knowledge, discoveries, and body of published research commanded respect in spite of that. Al was a titan in marine mammal paleontology. He was also well-situated to inherit a flood of fossils, as Charleston had significant population growth after World War II, and with the baby boom in full swing, suburbs and subdivisions were being built on the outskirts of Charleston at a rapid rate. Now, here’s the interesting bit: there were suburbs built much earlier, from 1900 to 1940 or so, that are within about five miles of downtown: West Ashley (where I used to live), North Charleston, and Mount Pleasant – but all of these are inboard of I-526, which is a ‘ring’ highway that makes a halo about 5 miles radius from downtown Charleston. As it happens, within this region the Oligocene rocks are quite buried: 5-30 meters (or more) deep, with a healthy cover of Pleistocene overburden (chiefly the Wando Formation). In the areas where phosphate mining was most extreme – West Ashley outside 526 and the area of the North Charleston airport (Charleston International/Joint Base Charleston) – the rocks are quite shallowly buried here, only 2-3 meters down – 4 meters was about the maximum depth that the strip mines during the heyday of phosphate mining would go, past that it was too much work (recall they were stripping off overburden by hand, horse, ox, and plow). By the late 1960s, subdivisions were being constructed north of the airport in North Charleston, near another zone called the Fort Bull Bulge, where the Oligocene deposits are only 2-5 meters below the surface. In one of these subdivisions in Summerville, South Carolina, Al Sanders and company found a spectacular skeleton including a nearly complete skull and vertebral column of an obscenely large dolphin.

The spectacular cast skeleton of ChM PV 2764 - "Genus Y" - on display in the old Natural History hall at Charleston Museum, which has since been revamped thanks to the efforts of our colleague Matt Gibson, NH curator at Charleston Museum. This specimen is now best identified as Ankylorhiza sp. or Ankylorhiza sp. 2. 

This dolphin was dubbed the informal name “Genus Y” in a landmark paper by Frank Whitmore and Al Sanders in 1977 – which was able to review all published Oligocene cetaceans (worldwide) in about 16 pages. They also made reference to a “Genus X” and in notes and labels spread throughout Charleston Museum collections, there is also a Genus Z, Genus A, B, C, and so on. This was necessary as there was so little published at the time on these sorts of cetaceans, Al’s early research was truly groundbreaking. At the same time, Al faced an “embarrassment of riches” issue: there was almost too much to study. In about 15 years, Al and others collected about 10-20 lane cabinets worth of fossils, and many unopened jackets and unprepared fossils await attention or are currently being worked on by our good friend Matt Gibson, the current Natural History curator of Charleston Museum. Sadly, Al only published a few of his cetacean discoveries – these did, however, crucially included the publication of Eomysticetus whitmorei, which was the cornerstone paper that made my own Ph.D. research possible. Others included the naming of the ur-dolphin Ashleycetus planicapitis (one of the most plesiomorphic dolphins ever discovered), a redescription of Xenorophus sloanii (in the same paper, no doubt a lifelong goal of Al's that would unlock the taxonomy of the more diverse collection of South Carolina xenorophids), and the naming of Micromysticetus rothauseni (proposed to be a "cetotheriopsid", later strongly recovered within Eomysticetidae by my Ph.D. research). When visiting Charleston Museum in 2012, I got to see specimens of the “Charleston toothed mysticetes”, which Al and Larry Barnes were supposed to have published. An independently discovered skull was later published as Coronodon havensteini in 2017 by Jonathan Geisler, Brian Beatty, Mace Brown, and myself, and Al was fortunately alive to see a name applied to one of these toothed mysticetes – though Ankylorhiza is the “one that got away”, so to speak. On the other hand, it’s critical to remember that Al published on a wide variety of fossils and geology and had a long, remarkable, and successful career – this included naming the Chandler Bridge Formation, revising the entire record of Plio-Pleistocene mammals from the state, and the entire record of dinosaurs and other Mesozoic reptiles from South Carolina. While this study did not describe Al’s Genus Y skeleton (ChM PV 2764), it would not have been possible were it not for that discovery, so thanks to Al.

The presumed holotype tooth of Saurocetus gibbesi, from Allen (1924), along with a lower postcanine of CCNHM 103, Ankylorhiza tiedemani.

The story *really* begins in 1848 with the discovery of an isolated tooth from the Ashley Phosphate Beds, which THE Louis Agassiz named Saurocetus gibbesii – a big triangular molar with some serrations, longitudinally fluted enamel, and possibly double-roots. The specimen was never figured, and the specimen number was not cited by Agassiz, and then lost for about 50 years (because of course that happened to an unfigured specimen of unknown catalog number…). Some three decades later, a partial rostrum (snout) of a skull of a very large dolphin – though not containing any teeth resembling Saurocetus – or, any well-preserved teeth for that matter – was dredged from the Wando River on a phosphate barge, and presented to the American Museum of Natural History by a Mr. I.B. Tiedeman, and published in 1887 by J.A. Allen as Squalodon tiedemani. Of course it was placed in Squalodon; it had a similar rostrum with some European specimens of Squalodon. Generally, any large odontocetes with heterodont teeth were placed into Squalodon, and Remington Kellogg attempted to sort through this quagmire in 1924 when he named Squalodon calvertensis – an actual species of Squalodon. On Squalodon tiedemani, Kellogg wrote that it was much larger than any known “squalodont”, and was perhaps the first to cast doubt on squalodontid affinities, noting “A careful comparison of the types of Squalodon atlanticus and Squalodon tiedemani has failed to convince the writer that these two cetaceans are closely related to one another.” Kellogg did mention that the incompleteness of the specimen meant that “considerable uncertainty exists as to whether or not Squalodon tiedemani should be placed nearest the squalodonts or the zeuglodonts [=basilosaurids]”.

The holotype rostrum of "Squalodon"(now Ankylorhiza) tiedemani - from Allen (1887).

Sadly, the age of the “S.tiedemani holotype was completely unknown, and for a long time, widely assumed to be Miocene. By the 1970s, fossils of superbly large squalodontids from the Chesapeake Group (Pungo River Formation and Calvert Formation) were being identified at the Smithsonian as S. tiedemani, first and foremost a phosphatized skull with clam borings from the mid Miocene Pungo at the Lee Creek Mine (Boreske et al., 1972), identified by Frank Whitmore. Sometime later, a giant Squalodon skeleton from the mid Miocene Calvert Formation of Virginia was identified in Alton Dooley’s Ph.D. thesis as S. tiedemani – but he later changed his mind, concluding later that “S.” tiedemani has something to do with the “Genus Y” skeleton of Al Sanders. Dooley named this new skeleton from Virginia as Squalodon whitmorei in 2003. A following study by Frank Whitmore accordingly reversed course (Whitmore and Kaltenbach, 2008) and assigned the reworked Boreske et al. skull to S. whitmorei. However, they also referred a couple of large odontocete specimens including a titanic dentary from the rivers of Charleston to S. whitmorei. Not to skip ahead, but in our supplementary info, we concluded it was probably actually Ankylorhiza, reidentifying the Charleston mandible as Ankylorhiza sp.

The absolutely monstrous holotype skull of Squalodon whitmorei, which is a bit more derived and a bit larger (S. whitmorei likely had a skull at least 15-20 cm longer than CCNHM 103, and is roughly the same size as the larger, undescribed species of Ankylorhiza represented by ChM PV 2764).

My first look at CCNHM 103, in October 2012 after the Society of Vertebrate Paleontology meeting in Raleigh, NC. 

In the late 1990s Mark Havenstein and colleagues discovered an enormous dolphin skeleton during construction of the Crowfield Plantation subdivision, sandwiched between Summerville, Ladson, and Goose Creek in South Carolina – about a 15 minute drive from my house. The skeleton needed to be removed quickly, so as not to interfere with construction activities; considerable piecing was needed in order to put it back together. The benefactor of our Mace Brown, acquired the specimen sometime later and spent years piecing the skull and skeleton back together – and in 2010, the skeleton was donated and became one of the first specimens to be catalogued into our collection as CCNHM 103* (our catalog begins at CCNHM 100). When I first visited CCNHM in October 2012, I met Mace and saw this specimen on display – on the same hanging platform it occupies right now. I thought “sweet jesus that’s a big dolphin” but was too preoccupied with examining eomysticetid remains and the fossil I would eventually help publish as Coronodon havensteini five years later (unbeknownst to me at the time!) to consider that specimen any further. That same week, I didn’t even examine Genus Y at Charleston Museum; I wouldn’t see it in person until 2015. Nor would I examine CCNHM 103 in detail until 2016.

*CCNHM is the original acronym, after “College of Charleston Natural History Museum”, and appears in the Geisler et al. (2014) publication on Cotylocara (CCNHM 101). Shortly thereafter, the museum was renamed (in early 2015 I believe) the Mace Brown Museum of Natural History, after Mace’s voluminous contributions to the collection and well-being of the museum.

Coauthor Jonathan Geisler examining the teeth of CCNHM 103 back in 2017. The braincase and rostrum of the larger, undescribed species, CCNHM 1075, sits off to the right hand side. The orange specimen is our referred skull from the Ashley Formation, CCNHM 220.

We went back and forth on the naming for a while, we were uncertain if there was one or two species of “Genus Y”, and CCNHM 103 had been nicknamed “Genus Y not” or Genus Y°” by Brian. I tend to be a lumper rather than a splitter (I did most of my paleontological education at Montana State University after all), so it took a bit of convincing and lots and lots hand wringing between the initial members of the team. Alternative hypotheses were: 1) Genus Y and Genus Y not were different species; 2) different sexes of the same species; and 3) different growth stages. Some specimens do have a significantly wider vertex than others – it’s quite narrow in CCNHM 103, somewhat narrow in ChM PV 2764, and surprisingly wide in the largest specimen, CCNHM 1075. Since the occipital shield on the back of the skull anchors in the neck muscles that stabilize the head, it’s perhaps not surprising that this might scale with body size (bigger head, after all). However, we finally settled on a few consistent differences: in CCNHM 103 and the holotype of “S.” tiedemani, the rostrum is turned up a bit and dorsoventrally thickened, with the first incisor positioned dorsal to the second; secondly, the cheek teeth of CCNHM 103 differ from Genus Y proper (ChM PV 2764 and CCNHM 1075) in lacking large accessory cusps – they are there, but instead take the form of tiny, 1-2 millimeter wide “beads” of enamel on the cutting edges. So we settled eventually on the two species idea, and sure enough, our phylogenetic analysis grouped the specimens precisely how we hypothesized – which was a satisfying vindication. At this stage, however, we were full steam ahead on naming a new genus and species.

CCNHM 220, our less spectacular specimen of Ankylorhiza tiedemani from the Ashley Formation - still a very important specimen as it documents this same species as being present in the early Oligocene.

At some point I couldn’t shake the idea that we very likely were not dealing with a completely new species, even though Al Sanders had concluded otherwise. I had extensive discussions on lumping v. sinking with my Ph.D. adviser Ewan Fordyce – and Ewan is very much in favor of stabilizing old names with newer, better preserved and more anatomically informative fossils that improve the diagnosis. After all, he had done this for many taxa, and declaring others nomina dubia or cetaceans of uncertain affinities (incertae sedis). This was necessary given his arrival on scene in the late 1970s, where the New Zealand cetacean fossil record had been almost criminally abused by various workers, and much of his dissertation through U. Canterbury consisted of mopping up this enormous mess. Ewan on occasion cited the extreme example of Zarhachis flagellator – originally based on a caudal vertebra (doubtfully diagnostic), later stabilized with the referral of a skeleton by Remington Kellogg. The name is now indelibly associated with the nicely preserved skeleton that Kellogg published, and so if someone really wanted to declare it a nomen dubium and commit some opportunistic taxonomic piracy and name a new genus and species off the skeleton (as is frequently done in the frankly overcrowded realm of archosaur paleontology) – there’s actually enough of a case to ‘preserve’ Zarhachis under ICZN rules. After all, this sort of thing played out recently with Zygorhiza kochii – the holotype of which is a piece of junk braincase without any redeeming diagnostic features other than size, and meanwhile everyone mentally pictures Kellogg’s beautiful skeleton (USNM 11962) when they think of Zygorhiza. The taxonomic viewpoints of Gingerich v. Uhen on Eocene basilosaurids would make for a great post… later. While I would not go so far as to did what Kellogg did with Zarhachis (and I imagine even the most conservative paleocetologists today would not either), I was determined to find if we truly had something new or something old. In the end, it was a bit of both.

A slide from an updated SVP presentation showing the features linking CCNHM 103 with the holotype of Ankylorhiza tiedemani, AMNH FM475. Also, these are both shown to scale: CCNHM 103 is a bit smaller and slightly less robust - this is a BIG dolphin!

After trawling through all the papers on early large odontocetes I could, I re-read Allen (1887) and remarked upon the similarity between CCNHM 103 and the “S.” tiedemani holotype: the “double decker” incisors, the expanded premaxillae, the upturned rostrum, and very large size. Jonathan Geisler visited the AMNH and identified another similarity: clusters of foramina within the embrasure pits that the lower teeth fit into when the mouth was closed. He also determined that there is some adhering matrix – indicating that the holotype of “S.” tiedemani originated from the Ashley Formation, which is early Oligocene – 28-30 myo, a little older than our skeleton CCNHM 103 from the 23-24 myo Chandler Bridge Formation. So, that settled it for me: we agreed that CCNHM 103 was referable to “S.” tiedemani, necessitating a new genus name, because there’s no way this thing was related to Squalodon – tacitly acknowledged 96 years ago by Kellogg. So we assigned it to the new genus Ankylorhiza, and referred CCNHM 103 to that species. The other species of Ankylorhiza, represented by Al Sanders’ Genus Y skeleton ChM PV 2764 and our specimen CCNHM 1075, is not yet named.

Further Reading

Allen, 1887. http://digitallibrary.amnh.org/handle/2246/1611

Allen, 1924. https://academic.oup.com/jmammal/article-abstract/5/2/120/837661?redirectedFrom=fulltext

Boessenecker et al., 2020. https://www.cell.com/current-biology/fulltext/S0960-9822(20)30828-9?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982220308289%3Fshowall%3Dtrue

Boreske et al., 1972. https://www.jstor.org/stable/1302923

Dooley, A. C. 2004. A new species of Squalodon (Mammalia, Cetacea) from the middle Miocene of Virginia. Virginia Museum of Natural History Special Publication 8:1-43.

Whitmore, F.C., and J.A. Kaltenbach. 2008. Neogene Cetacea of the Lee Creek Phosphate Mine, North Carolina. Virginia Museum of Natural History Special Publication 14: 181–269.

Whitmore and Sanders, 1977. https://academic.oup.com/sysbio/article-abstract/25/4/304/1653287?redirectedFrom=fulltext