Earlier in the last six blogs we looked at characters Nesbitt (2011) used to define Archosauriformes and nested clades up to Ornithodira with a special emphasis [his not mine] on the nesting of pterosaurs within all these clades. Nesbitt (2011) was satisfied that pterosaurs nested well here, but did not test competing candidates among the Fenestrasauria and Tritosauria, the only clades that actually provide a gradual accumulation of pterosaurian traits.
In Today’s Installment
Nesbitt (2009, 2011) nested Lagerpeton and Dromomeron at the base of the Dinorsauromorpha (Silesauridae + Dinosauria), robustly supported by five unambiguous synapomorphies, and these from a clade known only by the pelvis, anterior caudals and hind limb.
Closer to Pterosaurs?
Nesbitt (2011) found Lagerpeton closer to pterosaurs than to dinosaurs, chiefly due to ankle traits. Nesbitt (2011) lists these as 1) calcaneum and astragalus coosified; 2) ventral surface of calcaneum rounded like the astragalus; 3) no posterior groove of the astragalus; 4) calcaneum lacks any sort of tuber. All these traits are absent in basal dinosauriforms, including Marasuchus and Asilisaurus.
Unfortunately, Nesbitt (2011) considered the proximal tarsals (astragalus + calcaneum) fused to the tibia/fibula as homologous to the astragalus + calcaneum fused to each other distinct from the tibia/fibula. They are not. In fact, in Dimorphodon, they are separated from one another as two distinct condyles. A similar morphology is present in Sharovipteryx, which shares the three remaining traits (see above).
By the Way…
The large reptile tree found Lagerpeton to nest with Tropidosuchus, far from the Dinosauria/Dinosauromorpha. Nesbitt did not mention the ascending process of the astragalus, which is present on Lagerpeton and dinosaurs. However, in Lagerpeton this appears on the posterior surface, while in dinos the ascending process is an anterior structure. Also, Huehuecuetzpalli has a pterosaurian mesotarsal ankle composed of two larger proximal elements unfused to one another, which is distinct from traditional lizards.
Traits Shared with Dinosaurs…
Nesbitt (2011) reported the following traits lagerpetids share with other basal dinosauromorphs, and these are apparently absent in pterosaurs: 1) posterolateral portion of the femoral head ventrally descended; 2) a straight cnemial crest; 3) the longest metatarsal longer than 50% of tibial length; 4) metatarsal V ‘‘hooked’’ proximal end absent; 5) articular face for distal tarsal 4 subparallel to shaft axis; and 6) metatarsal V without phalanges and tapers to a point.
Well, 1) can’t comment on this because I’m not sure what this is; 2) shared with Tropidosuchus and Chanaresuchus; 3) ditto; 4) ditto; ditto; ditto.
I hate to keep pointing out the same Achilles heel in all these recent studies, but workers should not be afraid to add a few fenestrasaurs to their studies to get a fix on this situation instead of digging themselves deeper and deeper into the SOS.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.