Some Bits and Bobs of the Taxonomic History of Weltonia ancistrodon

 In which I unravel a bit of the taxonomic foundation for this particular species of cow sharks and get sidetracked by some of the bits and bobs I came across in the process.  I'm easily diverted, particularly since it's the people featured in any taxonomic story that I usually find most interesting.

Among the strangest of shark teeth are those of cow sharks (members of the Hexanchidae family).  Here are a couple fossil teeth of the cow shark Notorynchus primigenius that I found along the Calvert Cliffs.  These likely date from mid-Miocene Epoch, or perhaps roughly 16 to 14 million years ago.

These are all from the lower jaw of the fish with the largest cusp at the anterior end of each tooth.  The larger of the two is 20 mm in length.

The extant species in this family of sharks are distinguished from other sharks principally by the number of pairs of gill slits its members have (either six or seven - other sharks have five pairs) and an array of other attributes often characterized as primitive.  It is assumed that the extinct species with such teeth shared those various morphological attributes.  Perhaps most important for the fossil collector, the species in this family sport distinctive teeth, easily distinguished from all others.  This observation applies principally to teeth from the bottom jaw which, as seen above, are rectangular in shape with a row of several curved cusps headed, for the most part, by a prominent initial one (there may be cusplets in front of it).  Complicating things somewhat is the fact that teeth of any one individual cow shark can vary by its sex and age, and by the precise location in the shark's jaw from which the tooth came.  As a result, precisely ascribing an isolated cow shark tooth to a particular species can be difficult.  (See Bretton W. Kent, Fossil Sharks of the Chesapeake Bay Region, 1994, p. 17.)

As unusual as I find these teeth, those of one species of cow shark - Weltonia ancistrodon (Arambourg, 1952) - are, I think, the most striking of the lot.  The tooth from this species shown below (which may be partially repaired) dates, according to the dealer from whom I purchased it, to the Eocene Epoch (56.0 to 33.9 million years ago) and was found at Khouribga, Morocco.  It is 13 mm in length.

Obviously, the most startling feature of this tooth is the exaggerated first cusp that extends high over the other cusps on the tooth and curves back at its tip.  Remove that and the rest of the tooth bears the typical features of other cow shark teeth.

How did it come to have this scientific name?

Camille Arambourg (1885-1969) was the first to describe this species based on teeth he collected at Khouribga, Morocco.  Arambourg's journey into paleontology was fortuitous.  He trained in agricultural engineering, but, as a young man, while helping his father dig an irrigation system on the family farm in Algeria, he uncovered a trove Miocene fish fossils.  That sparked a life-long interest in the fossils of Africa and a distinguished academic career in geology and paleontology, which culminated in his appointment as professor of paleontology at the Museum of Natural History in Paris (Muséum National d'Histoire Naturelle).  (See entry for Camille Louis Joseph Arambourg at Encyclopedia.com, and Djillali Hadjouis, Camille Arambourg (1885-1969), Historiographie de Préhistoriens et de Protohistoriens Français du XX° Siècle, 2018.)

In Les Vertebres Fossiles des Gisements de Phophates (Maroc-Algerie-Tunisie) (Notes Mem. Serv. Mines Cart Geol. Maroc., 1952), Arambourg described teeth from various cow shark species.  Among the cow shark teeth, one kind was distinctive.  He asserted, "no other fossil form . . . combines similar characteristics.  In order to distinguish it, I therefore propose to name this new fossil:  N. ancistrodon."  (p. 44)  He placed it in the genus Notidanus.  The new species name, he noted, is a combination of Greek roots for "hook" and "tooth."

He also observed that this particular taxon was particularly abundant in strata from the Thanetian and Ypresian Ages.  These straddle the divide between  the Paleocene and Eocene Epochs and are dated 59.2 to 56.0 million years ago and 56.0 to 48.1 million years ago, respectively.  

It's not surprising that Arambourg came to use this species name given that prominent first cusp.  In fact, the word Ancistrodon had already been used taxonomically over the years as the genus name for an array of poisonous snakes:  copperheads, moccasins, and vipers.  Very appropriate for animals with fangs.  (According to the Integrated Taxonomic Information System, this genus name for snakes is no longer valid.  I would note that the scientific name that I believe is now applied to the shark species to which the tooth shown above belongs does not appear at all in the ITIS.)

In 1979, paleontologist David John Ward reconsidered the taxonomy of this species, giving it a new genus name:  Weltonia.  (Addition to the Fish Fauna of the English Palaeogene.  3. A Review of the Hexanchid Sharks with a Description of Four New Species, Tertiary Research, Volume 2, Number 3, 1979.)  Ward selected as the lectotype for this new species, the tooth that Arambourg had shown as figure 65 on plate 1 in his 1952 publication.  (A lectotype is the representative or type specimen for a species when the holotype was not selected by the original author.)  Here is the image of the lectotype tooth from the 1952 publication.  (I believe I have not violated any copyright laws by reproducing a small portion of the original publication and am doing so for educational, noncommercial purposes.)

Ward's professional life has contained the same sort of career change that marked Arambourg's.  He trained and practiced for several years as a veterinary surgeon, but his interest in fossils apparently took over, and he retired as a surgeon to pursue paleontology.  As far as I can tell, he doesn't have an academic degree in the field, though he is the author of many paleontology papers, including the one cited here, and is, at present, a scientific associate at The Natural History Museum, London.

In his description of W. ancistrodon, Ward took Arambourg to task.  First, he stated that Arambourg asserted incorrectly that the teeth with the prominent first cusp were from the shark's upper jaw.  They are, in fact, from the lower.  Further, the teeth Arambourg identified as coming from the species' lower jaw were actually from another cow shark genus altogether.

I am puzzled a bit by Ward's notation about where the teeth Arambourg described were held.  He wrote, "Depository:  (presumably) the Arambourg Collection, Paris Museum."  Presumably suggests that these teeth were, at least then, missing.  Indeed, I could not find them listed in the database of the fish holdings of the Muséum National d'Histoire Naturelle where Arambourg was paleontology chair and whose fish fossil collection he was instrumental in building.  So, it would seem that the identification of this new species rests presently on images reproduced in a 1952 publication, not the actual teeth (wherever they might be).

I am amused by Ward's brief explanation of the origin of the genus name he newly applied to W. ancistrodon.  Actually it's how he explained the origins of several new names he applied in this article that I found most amusing.  For W. ancistrodon, Ward wrote that the genus is "named after the American palaeochondrichthyologist Dr. Bruce Welton."  Full stop.  Nothing more about Dr. Welton.  In contrast, he observed of using Mr. D. Kemp's name for Notorynchus kempi that it was "in recognition of [his] field assistance."  A tad bit more expansive here than he was with the explanation of Weltonia, but not much.

Things become more interesting with Ward's explanation of two other new names.  For Hexanchus collinsonae, a new species in the established genus Hexanchus, Ward said that it was "named after the English palaeobotanist Dr. M. Collison in recognition of field assistance and stimulating conversation."  Hmmm.  For Hexanchus hookeri, another new species in this genus, Wards noted it is "named after the English palaeomammalogist Mr. J. Hooker in recognition of field assistance and lively discussions."

So, apparently, Ward didn't find Welton to be much of a conversationalist, certainly not stimulating or lively, a failing also true, it would seem, for Mr. Kemp.  This looks like a slight to me, whether intended or not.  Though, of course, any sting of the apparent slight to Welton is minimized because his name was given to a genus, a level higher than species in the taxonomic order of things.  Poor Mr. Kemp.

Fossil Hunt Diary

The format and illustrations of this post were inspired by “Metropolitan Diary,” a feature of the New York Times that runs each Sunday.  I love this weekly column which, over the years, has spoken to me in myriad ways, often prompting laughter, and, on occasion, tears.  It offers five brief, true stories written by Times readers about life in New York City.  These miniature essays, recounting some specific experience, capture the urban existence in tiny prose gems.  The five stories in this past Sunday’s (December 25, 2022) column, were selected by readers as the best of 2022.  They are quite wonderful.  Still, I was disappointed that one of my favorites from 2022, written by Sharyn Wolf and titled Rock, Rock, Rock (included in the December 18, 2022 column), failed to make the cut.  That one begins:

“Rock, rock, rock,” I heard a voice repeating.  “Rock, rock, rock”

I was walking up a trail into the ramble in Central Park when I came upon the voice’s owner:  a tall, slender man with a twist of silver hair over one eye.

I waited, not wanting to interrupt whatever it was that he was doing.

“Rock, rock, rock,” he said again in a monotone.  “Rock, rock, rock.”

The explanation that Wolf provides in the ensuing three short paragraphs is unexpected and delightfully funny.

These wee tales are hard to resist and their appeal has been enhanced over the past five years with deceptively simple drawings by artist Agnes Lee.  These appealing illustrations have a timeless quality to them.  Lee has written about her arrival in New York City and the process she follows preparing these drawings.  Though she is now leaving the city for the West Coast, her artwork will continue to grace “Metropolitan Diary.”

The story which follows is somewhat modeled on the Times column; it includes two of my illustrations influenced by Agnes Lee's.

Some further context is needed.  The setting is Purse State Park, a tree-lined Paleocene site on the Maryland shore of the Potomac River about which I’ve previously posted.  The man named Mel featured in this account was a committed, consummate fossil collector who was a mainstay in a fossil club to which I still belong.  A wonderful person, he has since passed away.

Here then is my fossil hunt diary entry:

Several days after joining a fossil club and attending my first meeting, I was hunting fossils at Purse State Park, wandering somewhat aimlessly and unproductively along the deserted shoreline.  I struggled to climb over, or around, the fallen trees lying in the water.

I had no idea what I was doing.

Suddenly, though, I spotted something in the water at my feet.  I reached down and picked it up:  a beautiful fossil shark tooth.  What kind was it?  As I stood there puzzling over my find, a voice startled me.

“Hey, what do you have there?”

I turned around and there was Mel, whom I'd met at the club meeting.

I extended my hand to show him the tooth.

“Otodus obliquus.  Nice one,” he said.

It was at that point that I realized what a truly amazing organization this club is:  I go out into the field, find something I'm not sure about, and a club member immediately appears to give me the definitive ID!

Peer Review and Iron Age Fossil Shark Teeth Collecting

Earlier this year, public health experts Nason Maani and Sandro Galea cautioned that a public policy for the pandemic promising to “follow the science” misconstrued science and the role it can and should play in the public arena.  (What Science Can and Cannot Do in a Time of Pandemic, Scientific American, February 3, 2021.)  The details of their cogent arguments aren’t relevant to this present post, but their comment about peer review is:

Peer review, designed to catch our mistakes at the best of times, can suffer from in-group bias and in any case has been put aside to an unprecedented degree in the explosion of preprint research during the pandemic.

And, so, when policymakers (at least, some) are claiming to be subordinating their actions to guidance from science, one of the safeguards that is intended to minimize egregious errors in scientific research is being relaxed.  Of course, it is.

I hadn’t thought much recently about scientific peer review until I came across some archaeological research on an Iron Age (8-9th century BCE) site in Jerusalem.  (Thomas Tütken, et al., Strontium and Oxygen Isotope Analyses Reveal Late Cretaceous Shark Teeth in Iron Age Strata in the Southern Levant, Frontiers in Ecology and Evolution, Volume 8, December, 2020.)  It’s quite a remarkable story about the potential good that peer review can do, and about the role serendipity can play in the scientific endeavor.

Tütken and his colleagues had prepared a research article on a multitude of fish teeth (over 10,000) they had found in the fill dumped into a pool to create the foundation for a private house built in Jerusalem some 2,900 years ago.  Their focus was on what these teeth could reveal about the trade in fish at the time.  As they write, “Identification of fish trade in antiquity is traditionally based on the presence of ‘exotic’ fish remains in archaeological sites, which are distantly located from the original habitat of the fish.”

Among the thousands of fish teeth the authors analyzed were 29 shark teeth.  Not surprisingly, in drafting their article, the authors operated on the logical assumption that any remains, such as the shark teeth, discovered in archaeological layers had a provenance contemporaneous with other remains and objects found at the site.

Then came the peer review process.  One reviewer spotted something curious about the shark teeth depicted in the original piece.  As Tütken said,

We had at first assumed that the shark teeth were remains of the food dumped nearly 3000 years ago, but when we submitted a paper for publication, one of the reviewers pointed out that one of the teeth could only have come from a Late Cretaceous shark that had been extinct for at least 66 million years.  (As quoted in The City of David and Sharks’ Teeth Mystery, Goldschmidt Conference, Phys.Org, July 4, 2021.)

How fortunate that one of the reviewers of the original paper had some exposure to fossil shark teeth and was brought up short by the distinctive shape of a shark tooth among those depicted in the original article draft.  I assume the tooth that sparked this realization was from the genus Squalicorax (I don’t think that’s made clear in anything I’ve read, but, as I note below, it’s a safe assumption).  Ah, the value of a breadth of knowledge among peer reviewers, knowledge that ranges beyond the narrow focus of the paper under review, thus avoiding what Maani and Galea describe as “in-group bias.”

A bit of paleontological understanding is useful to appreciate the impact that this Iron Age research story has had on me and why I think the Squalicorax teeth saved the day.  This genus of sharks, which existed in the Late Cretaceous (from roughly 100 to 66 million years ago), sported teeth so distinctive that they are very easily recognized by even the least experienced collector of fossil shark teeth.  These shark are commonly known as Crow sharks.  Pictured below is a specimen of S. kaupi I found many years ago in Big Brook, New Jersey.  It is 15 mm on its longest axis.

Tütken and his colleagues, using data on the strontium isotope ratios and oxygen isotope ratios found in the teeth of extant fish from different bodies of water in the eastern part of the Mediterranean, determined that all of the shark teeth in their study were not contemporaneous with the Iron Age structure.  Indeed, the isotope ratios in those teeth matched those determined by others for Late Cretaceous fossil shark teeth found in Israel and Jordan.  (Fish teeth retain the isotope ratios present in the waters when the fish were alive.  Thus, a time signature is carried by the teeth and, if read, can separate contemporary from ancient teeth.)

The researchers largely ruled out the possibility that these Cretaceous teeth weathered out of local formations.  Instead, it appears likely they were discovered and removed from a site some 80 kilometers away.  Which raises the key question:  Why were these fossil shark teeth here at this Jerusalem site?  A question to which the authors do not have an answer.  There is no evidence these were being used as ornaments or as tools.  As a result, Tütken has said,

Our “working hypothesis is that the teeth were brought together by collectors, but we don’t have anything to confirm that. . . .  We know that there is a market for shark’s teeth even today, so it may be that there was an Iron Age trend for collecting such items. . . .  However, it’s too easy to put 2 and 2 together to make 5.  We’ll probably never really be sure.  (As quoted in Phys.Org.)

Fossil shark tooth collectors at work three millennia ago?  Amen to that, I’d like to think so.

I am puzzled by one aspect of the discussion in the paper as published.  Tütken et al. write that, at archaeological sites in the area, shark vertebrae are the primary shark remains that are found, not teeth.  That just doesn’t square with my experience working geologic formations that contain shark fossils because teeth vastly outnumber vertebrae.  Even if the research being cited applies to sites to which contemporary sharks were brought, teeth in the specimens would still grossly outnumber vertebrae.  Were sharks decapitated and only their bodies transported to other sites?  Quite curious.

Anyway, score a big one for peer review.  Tütken and his colleagues are quite gracious in their comments in the published paper about the impact of the reviewer’s observation about a Cretaceous fossil tooth which they label a “game changer.”

It led us to perform additional analysis and, ultimately, to rework the narrative of the manuscript, changing the story from shark exportation from the Nile delta to fossil shark finds in cultural layers.  This was a textbook example how a good review should work and we are very appreciative.

Sexual Dimorphism ~ Another Challenging Variable

This post was prompted by the recent publication of a fascinating article by paleontologist Jordan C. Mallon about our ability or, more specifically, our inability to distinguish between males and females in the available fossils for several prominent dinosaur taxa.  (Recognizing Sexual Dimorphism in the Fossil Record:  Lessons from Nonavian Dinosaurs, Paleobiology, 2017.)

Sexual dimorphism is present when characteristics of the males and the females of a given species differ.  These differences may be reflected in male and female morphology including such features as body shape (e.g., display features such as crests), body size, or coloring, and may also be found in variations in behavior.  Sexual dimorphism is an important attribute that is central to evolutionary biology, particularly as it relates to sexual selection.

With extant animals, some aspects of sexual dimorphism may be obvious as with the African lion species (Panthera leo) whose males sport a distinctive mane and may be somewhat larger than the females.  Pictured below are mounted specimens on display in the Hall of Mammals at the Smithsonian’s National Museum of Natural History.

A word of caution.  Having living animals available for study offers no certainty that we will, in fact, accurately identify sexually dimorphic traits.  The lions in the Hall of Mammal, as posed, promote the widely accepted canard that the female lions do all of the hunting for the pride and the males just lounge around.  It was only in this decade that careful tracking of male and female lions in Kruger National Park (South Africa) using GPS and other technology showed that females hunt in open areas while males do their hunting in denser vegetation shielded from observation.  (Scott R. Loarie, et al., Lion Hunting Behaviour and Vegetation Structure in an African Savanna, Animal Behaviour, Volume 85, 2013.)

Certain patterns of sexual dimorphism have been detected in extant animals.  For example, with respect to size and morphology, biologists Ehab Abouheif and Daphne J. Fairbairn observe, “In most species of animals, females attain larger body sizes than do males (e.g., most spiders, insects, fish, amphibians, reptiles), whereas in most birds and mammals, males are the larger sex.”  (A Comparative Analysis of Allometry for Sexual Size Dimorphism:  Assessing Rensch’s Rule, The American Naturalist, Volume 149, Number. 3, March, 1997, p. 540, references omitted.)

But all bets are off with extinct animals known from the fossil record.  Mallon is not alone in positing that the barriers to identifying the sex of extinct species are particularly high.  Armed only with fossils, we find ourselves without much of what might help to distinguish males from females including gender-specific behavior as well as some morphological attributes such as sex organs and coloring.  Without the information that observation of living animals provides, paleontologists face a daunting challenge.  As evolutionary ecologist Robert J. Knell and his colleagues have written,

Identifying sexual dimorphism in animals known only as fossils is often difficult; specimens of a particular species are sometimes rare, unique, or unavailable, and reliably identifying sex in fossils is often difficult or impossible.”  (Robert J. Knell, et al., Sexual Selection in Prehistoric Animals:  Detection and Implications, Trends in Ecology and Evolution, 2012.)

On the one hand, if the sex-related differences preserved in the fossil record are slight, then, as Knell et al. note, one must work with large samples (IF such sample sizes are actually available).  Apparently, it hasn’t been unprecedented for claims to be made about the sexes of dinosaur fossils based on bones from just two specimens (which is clearly an inadequate sample size).  At times, it may be differences in the sizes of features that distinguish males from females, not the presence or absence of such features in either of the genders.

On the other hand, “when dimorphism is strong, there is a risk that different sexes will be described as different species.”  (Knell, et al.)  That is, relying on morphology to identify species, as is a common and necessary practice in paleontology, poses the risk that significant differences in fossils, possibly related to sexual dimorphism, might be treated as indications of the presence of different species, not the two sexes of the same species.

From his analysis, Mallon concludes that, with the dinosaur fossils at hand, paleontologists, for the most part, cannot tell whether an individual dinosaur was male or female.  In this vein, I would add that one must ignore the nicknames given some of the iconic dinosaur fossil skeletons such as the Field Museum’s Tyrannosaurus rex which is known as “Sue,” the Triceratops horridus cast on display in the Last American Dinosaurs exhibit at the Smithsonian’s National Museum of Natural History which is fondly called “Hatcher” and its companion T. rex cast in the same exhibit carrying the nickname “Stan.”  (The National Museum has Hatcher's fossil bones, while its Stan is just one of many casts that can be found worldwide.  Stan's original fossil bones are in the Black Hills Institute.)  These nicknames honor the collectors who found the original fossils – Sue Hendrickson, John Bell Hatcher and Stan Sacrison.  In all fairness, the Field Museum’s T. rex was nicknamed Sue by Peter Larson because he believes this specimen was, in fact, female.  (See, Jacqueline Ronson, Sue the Tyrannosaurus Has a Sexual Identity Crisis, Inverse Science, December 1, 2016.)

Yes, there are some dinosaur fossils that can be identified as female because they carry eggs or have a medullary bone which, in contemporary birds, supplies the calcium used to fashion egg shells.  Mallon concludes that, for the taxa he studied (nine prominent species including Tyrannosaurus rex, Allosaurus fragilis, a couple of Stegosaurus species, and Protoceratops andrewsi), the available evidence offers “no support for sexual dimorphism.”  Mallon takes pains to make it clear he is not saying that such dinosaurs were not sexually dimorphic, simply that, in his considered view, the fossil record is inadequate to make such a claim.

This is certainly a thought-provoking, revisionist conclusion and it’s not just relevant for dinosaurs.  Mallon lays out the conditions under which sexual dimorphism might be successfully detected in different fossil taxa.

For me personally, his article brought me up short.  I have to admit that for the years that I’ve collected vertebrate fossils (principally shark teeth), I have never pondered the gender of the animals and how that might be reflected in the fossils I have at hand.  So much for my intellectual curiosity and my efforts at taxonomy.

Considering just fossil shark teeth, several of the most important variables that might account for differences in the teeth under study are:

• natural variation among individuals within the same species (hey, this variability is the material that natural selection work with),
• chronological age of the specimens (not only might teeth from younger sharks be smaller, they might also differ in terms of shape),
• position of the teeth in the jaws (most shark families are heterodonts, that is, teeth vary by location in the jaws; depending upon the shark, teeth from the upper jaws can differ from those in the lower jaw, and/or they can vary by location within a specific jaw),
• differential influence of the environment on the health and development of individuals from the same species,
• taphonomic processes (i.e., what happens to an organism in death) which may introduce variability in the fossils that emerge millions of years later, and
• sexual dimorphism.

This list is based primarily on paleontologist Bretton W. Kent’s Fossil Sharks of the Chesapeake Bay Region (1994, p. 2).  Though he does include sexual dimorphism as a factor behind some variation in fossil shark teeth, as far as I can tell, it’s only in a very few species that sexual dimorphism has been identified.  He cites it in just two fossil shark species found in the Chesapeake Bay region – Hexanchus (cow sharks), and Rhizopriondon (sharpnose sharks).

Ultimately, it’s somewhat ironic that, despite his warnings and the unlikelihood of detecting sexual dimorphism in most of my collected fossils, Mallon’s piece has prompted me to be sensitive to that possibility.