Wednesday, 22 October 2014

Gotta catch ‘em all – Pterosaur taxonomy

Despite the name being brand spanking new, i.e. Aerodactylus Vidovic and Martill 2014, the specimens have been known to science for over a century and a half!
Type specimen of Aerodactylus scolopaciceps
Aerodactylus is a new genus name for the species Pterodactylus scolopaciceps Meyer 1860. If you don’t know your Velociraptor from your Deinonychus and you’re worried this has gone over your head, don’t worry… P. scolopaciceps was synonymised (subsumed under the name) with Pterodactylus kochi in the 1880’s by Zittel – who famously (in pterosaur research) has a pterosaur wing with soft tissues named after him. Zittel (1883) even suggested that P. kochi might be synonymous with P. antiquus. The name P. scolopaciceps was briefly used by Broili (1938), who considered the species to be valid due to the preservation of many of the features listed by Meyer (1860) in a second specimen. However, the name fell out of use again, and by the time Wellnhofer (1970) performed his review of German pterodactyloids the name had become a mere footnote.

Much later, in 2013 Chris Bennett published a paper that followed Zittel’s suggestion and synonymised P. kochi with P. antiquus, including the two specimens referred to P. scolopaciceps. This synonymisation was based on the growing support from Mateer (1976), Jouve (2004) Bennett (1996) himself, and new evidence from principal component analyses (PCA) and Nopcsa curves. It seemed a closed case. However, PCA is not statistics or repeatable with an altered dataset, unless a population shows a normal distribution. Likewise, Nopcsa curves are open to interpretation. Although PCA can be used to support, or refute hypotheses (whilst being treated with appropriate caution) a cluster does not necessarily provide meaningful data to a taxonomist. So, a -small, unassuming- question mark hung over the synonymy. 
Referred specimens of Aerodactylus scolopaciceps

Whilst studying many Pterodactylus specimens for a much larger project, I noticed that there were a few slight differences between some specimens of P. kochi (note: this was before Bennett’s paper). So when Bennett (2013) synonymised the species with P. antiquus I set to work, constructing hypotheses and testing them. Ideally I would have used a discriminant function analysis (DFA), which is a bit like a PCA but far more appropriate for answering taxonomic problems. Unfortunately data limitations meant that DFA was not an option. Instead, I used an exhaustive bivariate technique, which used statistics to tell me if removing or adding specimens significantly improved or diminished the relationship support for a group. The results were clear, the hypothesis of P. kochi and P. antiquus being different from specimens now called Aerodactylus scolopaciceps was supported, while confusion over P. kochi’s taxonomic validity remains.
Cladogram of Pterodactyloidea from Vidovic & Martill 2014

A cladistic analysis (which deserves its own article) demonstrated that Aerodactylus scolopaciceps is more closely related to Ardeadactylus longicollum, Aurorazhdarcho micronyx, Gladocephaloideus and Cycnorhamphus than Pterodactylus. The group containing Aerodactylus was named Aurorazhdarchidae, which increases our understanding of the family level diversity in the Bavarian plattenkalks. Aurorazhdarchidae is made up of pterosaurs traditionally considered ctenochasmatoids or archaeopterodactyloids with long necks, depressed rostra (concave beaks) and long pteroid bones.

The species name for Aerodactylus scolopaciceps comes from its superficial resemblance to a Woodcock or Snipe. In the paper we suggested that it could have occupied a similar ecological niche, probing sediments, or browsing on ground level for invertebrates. Admittedly in our paper we provide little evidence for these interpretations, but we would welcome a palaeoecological study on these specimens. Considering it is one of the best represented pterosaur taxa (with soft tissue) in the Solnhofen Plattenkalk the possibilities for future research are vast.

Finally, why name the genus after Aerodactyl?
Well, as with most things in science, it was a conversation in a pub that was responsible.
Bennett SC (1996) Year-classes of pterosaurs from the Solnhofen limestone of Germany: Taxonomic and systematic implications. Journal of Vertebrate Paleontology 16: 432-444.
Bennett SC (2013) New information on body size and cranial display structures of Pterodactylus antiquus, with a revision of the genus. Palaeontologische Zeitschrift 87: 269-289.
Broili F (1938) Beobachtungen an Pterodactylus. Sitzungsberichte der Bayerischen Akademie der Wissenschaften, Mathematisch-naturwissenschaftliche Abteilung 1938: 139-154.
Jouve S (2004) Description of the skull of a Ctenochasma (Pterosauria) from the Latest Jurassic of eastern France, with a taxonomic revision of European Tithonian Pterodactyloidea. Journal of Vertebrate Paleontology 24: 542-554.
Mateer NJ (1976) A statistical study of the genus Pterodactylus. Bulletin of the Geological Institutions of the University of Uppsala 6: 97-105.
Meyer H (1860) Zur Fauna der Vorwelt: Reptilien aus dem lithographischen Schiefer des Jura in Deutschland und Frankreich. Frankfurt. 1-84.
Wellnhofer P (1970) Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Suddeutschlands. Bayerische Akademie der Wissenschaften, Mathematisch-Wissenschaftlichen Klasse, Abhandlungen 141: 1-133.
Zittel KA (1883) Über Flugsaurier aus dem lithographischen Schiefer Bayerns. Palaeontographica 29: 47-80.

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