Saturday, June 13, 2020

The Unecessary Death of Steneosaurus

Not a dinosaur, but a new paper on the classic crocodylomorph Steneosaurus exemplifies a troubling trend in recent vertebrate taxonomy.  Johnson et al. (2020) reexamine the original material of Steneosaurus, an aquatic croc from the Jurassic of France.  It hadn't been seriously looked at since the 1860s, so this is one of my favorite kinds of paleontology papers- restudying a fragmentary old specimen in a modern light.  What do they find?

We first get a detailed recount of its history, with two decades as Cuvier's "tête à museau plus allongé" (= head with a more elongated snout; I have to praise the authors for translating all the French to English, even in our spoiled era of Google Translate it saves time), before it was named Steneosaurus rostro-major by Geoffroy Saint-Hilaire in 1825.  Eudes Deslongchamps and son tackled it in the 1860s, where they viewed the specimen as too poorly preserved and so "stated that the taxon to represent the genus Steneosaurus should be either ‘Steneosaurus’ megistorhynchus Eudes-Deslongchamps, 1866, or ‘Steneosaurus’ edwardsi Eudes-Deslongchamps, 1868c."  Ha!  You don't get to just take somebody's genus and affix your new species as its type.  They were the last to examine the specimen in detail however, making that a pretty bad note to end on.

Johnson et al. then reexamine the type snout of Steneosaurus, correcting the species name by eliminating the hyphen, officially making it the lectotype, noting Steel had determined the posterior skull to be Metriorhynchus, and illustrating and redescribing the specimen.  Excellent work and very well done.  After eliminating Mycterosuchus nasutus, 'Steneosaurus' leedsi, 'S.' heberti and Lemmysuchus and other machimosaurins based on numerous dissimilar characters, the authors come to the contemporaneous 'Steneosaurus' edwardsi.

"As mentioned before, this was a second species that Eudes-Deslongchamps (1867–69) considered identical to S. rostromajor. These two taxa share a combination of features including:
1. A subcircular, moderately interdigitating premaxilla-maxilla suture.
2. Maxillae ornamented with irregular grooves.
3. A shallower mediolateral compression of the posterior maxillae, as opposed to ‘S.’ heberti (MNHN.F 1890-13).
4. Horizontally flat posterior premaxilla in lateral view.
5. Deep anterior and mid-maxillary reception pits that gradually become shallower towards the posterior maxilla.
6. Subcircular to circular alveoli that remain relatively the same size throughout the maxilla.
7. Teeth with well-pronounced enamel ridges at the base."

Well how cool is that?  They put in the hard work, found the matching more complete specimens, and now we have Steneosaurus edwardsi as a junior synonym of S. rostromajor, giving us a good look at what Steneosaurus really was after two hundred years.

Lectotype of Steneosaurus rostromajor (MNHN.RJN 134c-d) in dorsal (A, B) and ventral (C, D) views. (after Johnson et al., 2020).

But no.

Johnson et al. immediately say "it is important to note that many of these characters may, in fact, be related to sexual dimorphism, ontogeny and intraspecific variation."  True, but that could be said for basically every character supposed to diagnose Mesozoic croc genera, or theropod genera, pterosaur genera, etc..  Unless you have some specific example like 'enamel ridges have been shown to develop with age and both S. rostromajor and S. edwardsi are larger than S? leedsi or S? heberti with weak ridges', then it's just hand-waving.  And no, Johnson et al. never develop such an argument for one of those characters, let alone all seven.

Next, we get "In addition to the sexual dimorphism/ontogeny problem, one of the critical issues about MNHN.RJN 134c-d is that it is poorly preserved."  Sure, but you were still able to perform many comparisons.  Again, the authors never say any of their seven characters are taphonomic, so it's another objection without substance.

Yet the worst rationale for rejecting Steneosaurus is "in reality, the name Steneosaurus is extremely impractical. It was used for many metriorhynchid specimens (e.g. ‘Steneosaurus’ gracilis, ‘Steneosaurus’ palpebrosus and ‘Steneosaurus’ manselii) during much of the 19th century, largely in part due to Cuvier’s metriorhynchid skull region (MNHN. RJN 134a-b) being attributed to the teleosauroid rostral section (MNHN.RJN 134c-d). Indeed, the concise, classical definition of ‘Steneosaurus’ as we interpret it today was not given until the work of both Eudes-Deslongchampses (1868c, 1867–69)"

Substitute Megalosaurus in there to see how ridiculous it is.  That has had over 45 species assigned to it, and was named in the 1820s but didn't have a modern concept associated with it until the 1980s.  When Johnson et al. lament that for Steneosaurus "rather than comparing characters outright, comparison is by process of elimination (or the question of ‘what features does this specimen lack?’)", that perfectly describes the Megalosaurus paralectotype dentary.

"After the Eudes-Deslongchampses’ treatment, what was left was an undiagnostic, chimeric type specimen for S. rostromajor (MNHN.RJN 134) and the genus Steneosaurus was redefined using a new type species that was not accepted by some researchers. In addition, since the Eudes-Deslongchampses, there has been no attempt to rectify this taxonomic nightmare;"

You just showed it was diagnostic, Steel long ago got rid of the chimaeric portion, Eudes-Deslongchamps' stupid attempts to name new type species have no relevance, and you have done the work to finally rectify this taxonomic nightmare.

"Due to these three significant factors (uncertainty of variable characters, poor preservation and
unreasonable name), we have concluded that S. rostromajor, and therefore ‘Steneosaurus’ (MNHN.RJN 134c-d), cannot be confidently assigned to an existing teleosauroid species."

Nope, you just showed it can be assigned to the same species as S. edwardsi.

Actually, I correct myself.  THIS is the worst rationale for rejecting Steneosaurus- "In addition, MNHN.RJN 134c-d was initially diagnosed based on significant orbital and temporal characteristics (from the metriorhynchid MNHN.RJN 134a-b), along with generic rostral ones.  Because the skull material is now known to be from a metriorhynchid, this ‘hybrid type specimen’ factor adds to the doubtful validity of Steneosaurus. According to Article 23.8 of the ICZN Code, ‘a species-group name established for an animal later found to be a hybrid (Art. 17) must not be used as the valid name for either of the parental species (even if it is older than all other available names for them)’ (this also signifies that the species name rostromajor is itself invalid). As such, MNHN.RJN 134c-d serves as an undiagnostic specimen; we, therefore, consider MNHN.RJN 134c-d to be a nomen dubium and, as such, Steneosaurus is treated as an undiagnostic genus."

If the term "parental species" didn't tip you off, Article 23.8 applies to hybrid individuals (those resulting from different species interbreeding), not type specimens chimaerically combined from multiple species.  The Article doesn't even say what Johnson et al. think- it says a name for a hybrid can't be used for either of the species that bred to make it, so that e.g. even if  a mule's scientific name was erected prior to that of horse's or ass's, it can't be the name for horse or ass.  And indeed even the cited Article 17 says that hybrids and chimaeras can be the basis of valid names- "The availability of a name is not affected even if 17.1. it is found that the original description or name-bearing type specimen(s) relates to more than one taxon, or to parts of animals belonging to more than one taxon; or 17.2. it is applied to a taxon known, or later found, to be of hybrid origin..."

If Johnson et al.'s interpretation were right, there goes Gojirasaurus, Protoavis, Chuandongocoelurus, Chilantaisaurus, Fukuiraptor, Coelurus, Alectrosaurus, Dakotaraptor, etc..

Before the big reveal, we have in the Conclusion what can only be described as a lie- "Through character comparison-and-elimination, the only taxon with which MHNH.RJN 134c-d could hypothetically be referred to is ‘S.’ edwardsi, but the two do not share any clear autapomorphic characters or a unique combination of characters."

What are your seven listed characters if not "a unique combination of characters"?  Does any other teleosaurid have them?  If not, they are unique.  In any case, we get the motivation for dumping Steneosaurus twice at the end of the paper-

"We believe that establishing teleosauroid taxonomy from the beginning with a series of ‘clean’ type species/specimens, with every nomenclatural act correctly formulated, is the best course of action, which we will highlight in a forthcoming paper (Johnson, 2019)."

"We believe that establishing teleosauroid taxonomy from the beginning with a series of ‘clean’ type species/specimens, with every nomenclatural act correctly formulated, is the best course of action. This will necessitate a revised teleosauroid taxonomy, in which species previously referred to the genus Steneosaurus are given new generic names. This work will be published by us in a separate contribution, based on the comprehensive teleosauroid phylogenetic analysis in Johnson’s PhD thesis (2019)."

Basically everything I hate about a current trend in vertebrate paleontology- just throw out old specimens and dishonor their authors who correctly reported what was new at the time to come up with your own names.  At least dumping Stegosaurus armatus or designating a neotype for Allosaurus fragilis could be claimed to save time and effort actually analysing the types, if you don't want to do the science to figure out if armatus is actually different from stenops or if fragilis can be distinguished from Saurophaganax.  But Johnson et al. already did all the hard work and found Steneosaurus edwardsi was S. rostromajor, they would just rather use Johnson's new genus name for the taxon.

And their reasons are just grasping at straws.  'Sure we identified these seven charactesrs uniquely shared by Steneosaurus rostromajor and S. edwardsi, but uhh.. could be sexually dimorphic?  Or anything could be individual variation.  Or ontogenetic?  Lots of things turn out to be ontogenetic.  Plus it's broken.  Sooo broken.  Sure we could evaluate characters, but who wants a taxon whose holotype isn't pristine?  Plus a lot of people had stupid ideas about Steneosaurus over the past two hundred years.  What do us scientists do when we have a complicated situation to resolve that was only partially understood historically?  Trash their names and give yourselves credit for new genera.'  Thus Steneosaurus gets the eternal identity of "all evidence points to it being Johnsonosaurus edwardsi, but ehhh... we just sort of ignore it now as Teleosauridae indet. and it's forgotten."

To conclude, Steneosaurus is really outside my wheelhouse.  But if Johnson et al.'s philosophy spreads, we're in danger of losing a lot of historical taxonomy and deserved credit to lazy or selfish authors.  Just look at Microraptor for example, whose holotype of M. zhaoianus lacks a decent skull.  Some decades down the line, what if cranial differences support various Jiufotang species and someone's like 'the postcranial proportions are unique between the M. zhaoianus type and M. hanqingi, but I want a complete type specimen, so Microraptor is an invalid undiagnostic nomen dubium, and instead I propose Mybetterraptorgenus hanqingi and M. gui.'  Just hope they don't pull a Wilson and Upchurch and claim 'Microraptor is invalid and co-ordinate suprageneric Linnean taxa must likewise be abandoned' and replace Microraptorinae with Mybetterraptorgenusinae.

References- Johnson, 2019. The taxonomy, systematics and ecomorphological diversity of Teleosauroidea (Crocodylomorpha, Thalattosuchia), and the evaluation of the genus 'Steneosaurus'. PhD Thesis, University of Edinburgh. 1062 pp.

Johnson, Young and Brusatte, 2020. Emptying the wastebasket: A historical and taxonomic revision of the Jurassic crocodylomorph Steneosaurus. Zoological Journal of the Linnean Society. 189(2), 428-448.