Tuesday, October 13, 2020

Obscure controversies in Cenozoic marine vertebrate paleontology 2: the whale jaw that didn't disprove evolution

Note: This post discusses young earth creationism. I do not take young earth creationism seriously, and neither should you - though it is a serious threat to science communication and science education. Nothing is gained from me linking to the individual articles by creationist authors, so I am making no attempt to cite them or link to them. I do not want to give those trolls the attention (positive or negative) they so desperately crave. Since this is a science blog, I also will not tolerate any creationist nonsense here and will enthusiastically delete any creationist comments: engaging with creationists directly is a waste of time and elevates highly problematic pseudoscience; science and young earth creationism do not overlap except in a theology class or a court room.


Introduction

Seymour Island, Antarctica, is one of the few fossil localities in Antarctica that is not covered with ice year round. It's quite far north, and located off the east side of the Antarctic Peninsula, at about 64 degrees south - the same distance from the equator as Fairbanks, Alaska. Owing to the lack of ice cover, we know quite a bit about the geology - and it is richly fossiliferous. Seymour Island preserves a thick sequence of rocks dating from the late Cretaceous through the Oligocene - chronicling faunal change and recovery from the K/Pg extinction and the subsequent 30 million years of faunal change. Chief among these discoveries are scores of fossil penguins, the extinct toothed baleen whale Llanocetus denticrenatus (higher up towards the very top of the La Meseta Formation), and more recently - archaeocete whales.

The geographic location and geology of Seymour Island and the La Meseta Formation in particular. From Jadwiszczak and Mörs, 2017.

In 2011 the discovery of archaeocete whales, mostly basilosaurids, was announced. Among these was a surprisingly early occurrence of a basilosaurid whale, claimed as dating to 49 Ma - squarely in the Ypresian stage (56-47 Ma), otherwise known as the early Eocene. Now, there are some serious issues with this, and a brief recap of early whale evolution is needed for you to appreciate the controversy. The earliest known cetaceans, including Pakicetus and Ambulocetus, are from the Ypresian, with the oldest known cetaceans dating to about 53.5 Ma (Himalayacetus). Ypresian whales are chiefly all four-legged whales, including the long-legged and dog-like pakicetids, the crocodile-sized ambulocetids, the otter-like remingtonocetids, and the much larger seal-like protocetids. Pakicetids and ambulocetids were mostly restricted to freshwater thanks to fossil occurrence data and isotopic analyses, whereas remingtonocetids inhabited fresh and saltwater. Remingtonocetids were the first to leave Indo-Pakistan, being found as far west as Egypt - but protocetids were the first oceangoing cetaceans, and the first to reach the western North Atlantic and the Pacific. They still had functional hindlegs and could at least amble about on land clumsily like a seal. Most protocetids date to the middle Eocene. Basilosaurids had hindlegs, but they were tiny, no longer attached to the vertebral column, and likely vestigial as far as locomotion was concerned - and possibly retained for gripping mates during mating. These were therefore the first whales to be fully dedicated to a marine lifestyle. As a result, the same genera (and in some cases, species) have been reported from across ocean basins - the most extreme of which is a skull belonging to Zygorhiza from the Eocene of New Zealand, suggesting that Zygorhiza inhabited both the Atlantic and Pacific and both the southern and northern hemispheres (Kohler and Fordyce, 1997). We'll talk more about that specimen later on. In general, the transition from dog-like pakicetids to basilosaurids, until recently, was considered to take place over a 10-15 million year timescale. The date on the new fossil is only 4.5 million years younger than Himalayacetus. This would have some profound implications.

 

Creationists *hate* whales

The first implication was that this was *immediate* cannon fodder for Young Earth Creationists (YEC). If you're not familiar, especially for foreign readers, YEC is a big phenomenon here in the USA, largely driven by folks who subscribe to a literal interpretation of Genesis and much of the rest of the Bible. Most YEC folks are fundamentalist or evangelical Christians; I've never met a Catholic who was staunchly YEC (I was raised Catholic, for what it's worth). There is quite the spectrum of nonoverlapping beliefs amongst creationists in general - ranging from "a creator started everything off, and evolution took its course" (a fairly modern, liberal viewpoint accommodating most science) to theistic evolution (evolution is real, but was guided by a creator), to "evolution is real, but only for non-humans" (because we're created in God's image), to the extreme endpoint, which is YEC: the earth is under 10,000 years old and evolution is a lie. There are many disagreements amongst YEC folks, largely consisting of differences in which bits of science they're willing to admit are real and use to fit a narrative, and if the earth is precisely about 4,000 years old, 10,000 years old, or an undefined number of thousands of years. Creationism obviously stems from a belief system and "Creation science" is pseudoscientific rather than scientific. The failure of most creationists to agree on anything other than 'evolution is wrong' and parts of (or all) of the bible is right tells me everything you need to know: there is no consensus, and highlights the lack of scientific reproducibility. Nobody has reproducible results; reproducible results are what a consensus develops from. Creationists cannot agree on anything, including how much of evolution is real (all? some? none? if some, how much?) how old the earth is, or even what parts of the bible to interpret literally v. figuratively. My own perspective on YEC is that it's nonsense, and the bible was clearly never intended to be interpreted literally (there are talking snakes in there and stuff, and a LOT in Leviticus about not mixing different types of fabric together). Noah's story is lifted straight out of the Epic of Gilgamesh. These are parables intended to guide you on your spiritual and moral journey through your life - and have value in very much the same way Aesop's Fables do (in my opinion). 

 

Generalized depiction of archaeocete evolution using images of the actual fossil skeletons - young earth creationists (YECs) really, really hate these diagrams so I make sure to use them a lot. From Biologos.org.

Strict YEC adherents frequently push the narrative that scientists are lying to everyone about evolution, and it's never always the same reason: sometimes we're just stupid, and have yet to see the light, and other times there's a big conspiracy. This frequently dovetails with the "scientists are pushing the global warming conspiracy" and actually saying ludicrous shit like we're all in cahoots to make money off of criticizing oil companies, or that we're trying to lure young children into satanism, or something. We're really not that competent, and quite frankly, the inability of the white house over the past four years to keep any real secrets has deeply eroded my ability to take stock in any government-involved conspiracy theory. For example, if alien bodies and flying saucers were kept in a hangar in Roswell, Trump would have tweeted about it back in 2017. Paleontologist Jack Horner delivered a great public lecture at the 2010 SVP conference in Las Vegas where he said "you know how I know evolution is true? what my evidence is in favor of it? All of you-" he waved magnanimously at the crowd filled with paleontologists. "All of you are competing for grants and papers in top tier journals; disproving evolution would be the discovery of a lifetime. And that has not happened."

 What is the goal behind YEC and Intelligent Design? It's largely a far right wing, conservative Christian political movement aimed at legally weakening the separation of church and state in the United States. This is well-established by the "Wedge Document", drafted in secret in 1998 by the "Discovery Institute" (a pseudoscientific think tank) and publicly leaked in 1999. The document essentially laid out the Wedge Strategy: intelligent design could be used to sneak creationism into public schools, and if awarded a legal victory, could be used to push for further 'reforms' and widen the influence of evangelical christianity into US politics. Advances on many fronts, including abortion rights, gay rights, euthanasia, were sought by this plan. This more or less remains the dominant strategy of the Discovery Institute. Spreading Intelligent Design, Young Earth Creationism, and attempting to disprove evolution and/or natural selection are all part of the same game here, and it is not intended to convert scientists: it's intended to convince enough parents out there that their kids should not be learning about evolution in schools. YEC promoters are inherently distrustful, and are caught red-handed, again and again, deliberately misinterpreting or misusing science to prove to their flock - and undecided parents - that evolution is not real. Many of these folks are actually quite intelligent - you have to be smart to be a successful charlatan - and I've met my own fair share of sociopaths in science, and recognize the signs. They are often quite familiar with the science and know how to weasel things around in order to twist the narrative. YEC/ID proponents really hate whale evolution, because the evidence is so astoundingly wonderful. This evidence has largely accrued in the past thirty years - with a few kinks here and there - that YEC folks LOVE to whittle away at. You can tell how much they hate a particular topic based upon how much they write about it: there are a lot of articles in YEC "journals" about whale evolution.

So, it came as no surprise that shortly after this announcement, YECs pounced on this press release - a preliminary announcement - and immediately claimed that this whale disproved evolution by forcing all of archaeocete evolution into a ~5 million year period. Evolutionary biologist JBS Haldane joked in the early 20th century that the discovery of a "rabbit in the Cambrian period" would disprove evolution. This is not quite that: this is a preliminary announcement of a specimen that's unusually old, but it does not actually pre-date the oldest known whales. It's still younger: it just would indicate that the whole sequence is a bit more telescoped. Or, another possibility - maybe something is wrong with the dates? Usually, one out-of-sequence fossil at a single locality suggests to reasonable paleontologists that there's more likely to be something wrong with the dating, rather than "holy shit this disproves everything we know!"

 

        Mandible and molar of an indeterminate basilosaurid whale, MLP 11-II-21-3, Eocene La Meseta Formation, from Buono et al. (2016).

Skull and mandible of Zygorhiza kochii, Eocene of southeastern USA, from Kellogg 1936 (Review of the Archaeoceti).

The Specimen in Question: Stratigraphy and Dating

Specimen MLP 11-II-21-3 was discovered in the TELM 4 horizon of the La Meseta Formation - also known as the base of the Cucullaea I allomember (allostratigraphy is similar to sequence stratigraphy, and has been used to subdivide the Pisco Basin/Formation in Peru as well; it's not used by everyone). This is a thick shellbed with many shells of the large ark clam Cucullaea raea as well as naticid (moon snail) and nassariid (dog whelk) gastropods. This shellbed has also produced the holotype of the astrapothere Antarctodon sobrali (Astrapotheres were strange elephant-like 'notoungulate' land mammals unique to South America, and otherwise known only from this locality in Antarctica) as well as many sharks, rays, bony fish, and some marsupials and other indeterminate notoungulates. Though no age-diagnostic plankton were found in association with the mandible, MLP 11-II-21-3 was definitely collected from this horizon, so evidence collected by other researchers from the same bed can be interpreted.


Stratigraphy and dating of the La Meseta Formation. On left: stratigraphic and geochronologic framework from Buono et al. (2016); on right: dinocyst biostratigraphy of the same strata from Douglas et al. (2014).

TELM 4, and other parts of the La Meseta Formation Strontium isotopes are frequently used to date Cenozoic marine strata: the ratio of 86Sr and 87Sr has varied through time. The gist of it is this: the radioisotope Rubidium 87, which naturally occurs in some rocks, decays to 87Sr by losing a beta particle. It's got a super long half life (49 billion years). As a result, 87Sr is added to seawater over the course of geologic time; 86Sr naturally occurs in seawater (for similar reasons as salt, calcium, and magnesium - weathering of terrestrial rocks) and therefore, through time, the ratio 87Sr/86Sr ratio increases with there being proportionally more 87Sr through time. This ratio jogged up and down several times during the Paleozoic and Mesozoic, but has more or less been an upward trend since the Cretaceous - meaning we can measure the ratio, and plug it into the curve and get a fairly accurate date. There are caveats, however: fossils sampled cannot be reworked. If reworked, you're actually dating a fossil that is older than the rock layer it was finally entombed within (we'll come back to this). Diagenesis (rocks getting messed up during burial, by compaction or groundwater chemistry) and metamorphism (heating and extreme burial causing mineral/chemical changes in the rock) can both screw with the ratio. If the curve oscillates, or goes flat (e.g. no change for several million years), the ratio is less informative and there are bigger error bars on the age determination. Lastly, because Strontium is similar enough chemically to Calcium, invertebrates frequently incorporate Strontium atoms into their shells, and therefore, ratios can be measured through destructive isotopic sampling of mollusk shells. 87Sr/86Sr ratios from shells collected from TELM 4 and the base of TELM 5 (just above the TELM 4 shellbed) are similar to TELM 3 (just underneath the TELM 4 shellbed), and suggest an age of 54-48.8 Ma for TELMs 2-5. This age corresponds to the early middle Eocene or Ypresian and Lutetian stages - and overlaps with the age of Pakicetus and Himalayacetus as well as some of the older protocetids, and if correct, is an objectively surprising date for a basilosaurid.

Other dating methods used include biostratigraphy using dinoflagellate cysts - which are a type of plankton. Dinoflagellate biostratigraphy for the La Meseta Formation is calibrated using paleomagnetic dating for the Southern Ocean - the study of changes in magnetic polarity of the earth's magnetic poles, reversing and switching back to normal throughout earth's history. Dinoflagellate biostratigraphy actually suggests a younger age. The dinocyst Enneadocysta partidgei, in particular, suggests an age of 45 Ma, and in total, the cyst fauna suggests an age no older than 49 Ma - the date assigned in the original press release, and coveted by creationists. Now, it is best recognized as an extreme possible end point in a murky cloud of dates. Buono et al. briefly cite an earlier paper by Douglas et al. (2014), who conclude that based on dinocysts and paleomagnetism, that TELM 4 is about 41-46 Ma in age. Buono et al. follow these dates instead, which is much more consistent than the cetacean fossil record. At least one article written by YEC hacks wildly misinterprets the way in which Buono et al. cited this paper, claiming that the evolutionists* are trying to sweep it under the rug. The YEC proponent who wrote that failed to understand that most of my colleagues who wrote that paper do not speak English as a first language (Dr. Monico Buono is Argentinean), and occasionally there will be some subtle language differences. The second is that, had this YEC actually read the Douglas et al. paper, there is a pretty clear outline of the dinocyst biostratigraphic framework in the supplementary materials that they did not read. OOPS. Buono et al. (2016), being scientists, did actually read the paper they were citing. The YEC critic did not. And, no, as I said above, I refuse to link to the YEC article itself. But I bet you can find it in five minutes on google if you're so motivated - it was published as a response to Buono et al. (2016).

*if you ever hear someone use the word "evolutionist", run!

 

Strontium curve and dates from the La Meseta Formation. The Strontium curve from the middle to late Eocene is pretty wavy, meaning that dates could slide forward or backwards in time based on small errors in the ratio, or snap to a different part of the curve altogether! There is quite a spread in ratios for TELMs 2-4 and many points are off the curve altogether, which immediately suggests limited confidence. Young earth creationists know this caveat about Strontium isotope dating just as well as geologists do, and are lying to your face about it. The method HAS drawbacks, but YECs don't tell you that geoscientists acknowledge these caveats, because they'd rather push the narrative that scientists are lying to you (oh, the irony). From Ivany et al. (2008).

What could account for the unusually old Strontium dates for TELM 4 and 5? Buono et al. point out that many of the fossil invertebrates from TELM 4 and 5 are reworked, and therefore if those shells were the ones sampled, the age is going to be misleadingly old. It's difficult to rework plankton, on the other hand, since the tiny size and delicate skeletons quickly abrade and become unrecognizable. Given the possibility or likelihood of reworking, it's better to use the plankton. That brings us to another really critical point: the biostratigraphic framework, though already existing to some degree, was published three years after the initial press release on the discovery. So, in a sense, the last author on the paper, Thomas Mörs, did not have all the biostratigraphic data at the time when he made that press release. Lastly, Buono et al. also point out that the middle Eocene is a period where the Strontium curve has some oscillations - and therefore, Strontium ratios might be inaccurate or have quite a spread on them (e.g. if there is a 'valley' on the graph, the ratio could either be on one of two slopes of the graph - the older part or the younger part, which have the same ratio) This is very much the case - see the Sr curve from Ivany et al. (2008) and the range of dates for the La Meseta Formation from the different members.

 

Identification of the fossil: is it a basilosaurid?

This is an important question to ask, given that it's just a mandible. The mandibles of protocetids and basilosaurids are not that different - though the large size of this specimen led the authors to identify it as a basilosaurid rather than a protocetid. There is considerable overlap between the smallest basilosaurids and the largest protocetids - the basilosaurid Zygorhiza has a skull the same size as the protocetid Georgiacetus, for example - and this mandible is about 60 cm long as preserved. That's actually within the size range of protocetids, to be honest. Just like both groups, it has a long mandibular symphysis, double rooted teeth, embrasure pits, and a large mandibular foramen. One of the only diagnostic traits is that it's got a big cheek tooth with numerous accessory cusps - which is a basilosaurid feature. BUT - we don't have many protocetids that are transitional in dental anatomy between a protocetid and a basilosaurid, suggesting there are still some fossils out there to find. Something that is transitional between a protocetid and a basilosaurid could very well have a tooth like this, but pre-date basilosaurids by several million years. I'm not saying that this is the case - I would have made the same identification as Buono et al. - I'm just saying that these alternative hypotheses are possible given the fragmentary nature of the specimen.

 

The Verdict

So, does this specimen destroy evolution and eliminate the pesky land-to-sea transition of whales that YECs hate so badly? Not even close. To broadly paraphrase Bertrand Russell - 'extraordinary claims require extraordinary evidence'. The extraordinary claim is that MLP 11-II-21-3 disproves whale evolution. Here are my conclusions/responses to this absurdity:

    1) MLP 11-II-21-3 is from a stratigraphic unit with two slightly offset suites of dates, one of which seems to be affected by reworking. 

    2) On a smell test level, which is more likely: that a single specimen destroys everything we know about the chronology of whale evolution, but *only* if you ignore half of the available dating? Or, when you look closely at the data and realize one dating method has some problems, and that the other dating method somehow actually does line up OK with the rest of the basilosaurid fossil record?

    3) Even if both sets of dates were not subjected to reworking, they're not that far off: we're talking about a difference of 4-5 million years. It would certainly telescope whale evolution further, and whale evolution is already quite rapid.

    4) This is so laughably far from finding a "rabbit in the Cambrian" that this point is not really worth considering further - other than, if genuine, all it really tells us is that we need to go out and find some more whales and refine dates for existing discoveries. We may find older examples of protocetids that could accommodate an older maximum age for Basilosauridae.

    5) While not 100% precise, the ability of these two methods to provide ballpark ages that are actually reasonably close tells me that the science is *working* rather than failing. You need to step back a moment and give it a big picture look: if these YEC jerks were right, why would the two methods independently give ages only a few million years off? Why wouldn't one say "Triassic" and the other "Pleistocene"? In my opinion, it's because the methods work, but sometimes there are wrinkles. Which doesn't mean that the "evilutionists" are lying: we're being totally honest and transparent, and these YEC folks are going apeshit trying to make mountains out of methodological molehills. 

    6) I know that these YEC folks know exactly what they're doing - and they're doing it because they're shameless charlatans on a crusade against secularism and science in the name of transforming America into a theocracy. They've said it themselves, no need to take my word for it (e.g. the wedge document/strategy). Their intention is to sow confusion about science and give just enough ammunition to scientifically uninformed Christian folks who may be honestly looking for the truth - and being directed to crazy right wing pseudoscientific garbage. So remember, YECs writing this tripe know what they're writing is carefully cultivated garbage. 

    7) That brings us to the moral of the story. Scientists: be careful, and don't step too far away from the data. We're all desperately searching for amazing fossils that make it into high profile journals - don't strain credulity and ruin your credibility. Competitive science needs to calm down every now and then, and to paraphrase Ferris Bueller - science "moves pretty fast. If you don't stop and look around once in a while, you could miss it."


References/Further Reading

Buono et al., 2016:  https://bioone.org/journals/ameghiniana/volume-53/issue-3/AMGH.02.02.2016.2922/Eocene-Basilosaurid-Whales-from-the-La-Meseta-Formation-Marambio-Seymour/10.5710/AMGH.02.02.2016.2922.short

Buono et al., 2019: http://www.aps-polar.org/paper/2019/30/03/A190617000001

Douglas et al., 2014: https://www.pnas.org/content/111/18/6582

Fordyce and Marx, 2018: https://www.sciencedirect.com/science/article/pii/S096098221830455X

Ivany et al. 2008: https://pubs.geoscienceworld.org/gsa/gsabulletin/article/120/5-6/659/2280/Eocene-climate-record-of-a-high-southern-latitude

Jadwiszczak and Mörs, 2017. https://polarresearch.net/index.php/polar/article/view/2635

Lastly, the wedge document - National Center for Science Education: https://ncse.ngo/wedge-document

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.

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*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