SVP 2018: More complete post-crania for Dromomeron

Smith, Irmis, Nesbitt and Turner 2018 report
on new Dromomeron material from the Petrified Forest, Late Triassic, Chinle Formation. Previously Dromomeron was known from a femur similar to that of Lagerpeton, a traditional dinosaur ancestor that nests with the chanaresuchid, Tropidosuchus in the large reptile tree (LRT, 1308 taxa; Fig. 1; Novas and Agnolin 2016).

Smith, et al., write:
“Discovery of the non-dinosauriform dinosauromorph Dromomeron romeri from the HQ established that early dinosauromorphs were contemporaries with dinosaurs for a substantial part of the Late Triassic, refuting hypotheses of rapid transition to dinosaur dominated faunas. This discovery was based on hindlimb elements, which diagnosed
Dromomeron as a member of Lagerpetidae, the sister group to all other dinosauromorphs.
Despite a flurry of new discoveries, the anatomy of lagerpetids remains poorly known. We
describe new specimens of Dromomeron romeri, including an articulated post-cranium,
further revealing lagerpetid anatomy.”

Importantly the foot of Dromomeron is different than that of Lagerpeton.
“An articulated metatarsus is short relative to the tibia, in contrast to the elongate metatarsus of Lagerpeton chanarensis and most dinosauromorphs. MTIII is the longest, with sub equal MTIV/MTII, and MTI 50% the length of MTII. This configuration differs starkly from the highly modified metatarsus of Lagerpeton.” 

Interesting that these authors seem to have ignored
the abstract from two years ago by Novas and Agnolin 2016, who also found Lagerpeton nested as a derived proterochampsid, confirming of an earlier blogpost here in 2011. No other proterochampsid has a longer metatarsal 3 than 4.

Archosauriform taxa with metatarsal 3 the longest:

  1. Basal Dinosauria
  2. Gracilisuchus (and no other crocs, which might mean Romer made a mistake)
  3. Most erythrosuchids through poposaurs (many derived exceptions)
  4. Champsosaurus

So maybe Dromomeron is a dinosaur. Not a lagerpetid.
That seems to be the most likely solution at this stage. No one wants to ‘Pull a Larry Martin’ and make a decision based on one or a few traits. Let’s see what a suite of traits gets us.

Tropidosuchus in its two variants. In the holotype (above) the humerus is more robust and pedal digit 4 is gracile, as in Chanaresuchus (Fig. 3). In the referred specimen of Tropidosuchus (below) the humerus is smaller and pedal digit 4 is longer than 3, as in Lagerpeton. The rise to a bipedal configuration appears to coincide with the change in pedal proportions.

Figure 5. Tropidosuchus in its two variants. In the holotype (above) the humerus is more robust and pedal digit 4 is gracile, as in Chanaresuchus (Fig. 3). In the referred specimen of Tropidosuchus (below) the humerus is smaller and pedal digit 4 is longer than 3, as in Lagerpeton. The rise to a bipedal configuration appears to coincide with the change in pedal proportions.

References
Smith N, Irmis R, Nesbitt SJ and Turner AH 2018. New material of Dromomeron romeri (Archosauria, Dinosauromorpha) from the Upper Triassic Chinle Formatin of New Mexico provides insight into the evolutionary morphology of early dinosauromorphs. SVP abstracts.
Novas FE and Agnolin FL 2016 Lagerpeton chanarensis Romer (Archosauriformes): A derived proterochampsian from the middle Triassic of NW Argentina. Simposio. From Eventos del Mesozoico temprano en la evolución de los dinosaurios. Programa VCLAPV. Conferencia plenaria: Hidrodinámica y modo de vida de los primeros vertebrados. Héctor Botella (Universitat de València, España) 2016

https://pterosaurheresies.wordpress.com/2016/11/13/you-heard-it-here-first-lagerpeton-is-not-a-dinosauromorph/

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