SVP abstracts – First largepetid pectorals and forelimbs

McCabe and Nesbitt 2019 present
the first pectorals and forelimbs for Lagerpeton (Fig. 1), a taxon previously known from hind limbs and pelvic region, plus some dorsals, some caudal vertebrae only.

From the abstract:
“The posture of the earliest dinosaurs is thought to be bipedal whereas their
pseudosuchian relatives and stem archosaurs are thought to be typically quadrupedal.”
The large reptile tree (LRT, 1592 taxa) invalidates the traditional clade ‘pseudosuchia.’ All basal archosaurs are bipeds in the LRT. So are some tropidosuchids by convergence. Nesbitt has never tested a large enough taxon list to reveal this. He and others have been traditionally confused by this convergence.
“Therefore, the transition from quadrupedality to bipedality lies somewhere between the origin of Avemetatarsalia (bird-line archosaurs) and Dinosauria.”
The LRT invalidates the traditional clade Avemetatarsalia and shows exactly where quadrupeds became bipeds.
“However, studying this transition is hampered by the lack of forelimb fossils from many of the close relatives of dinosaurs and it is not clear if the morphology of the few dinosauromorphs that have forelimb material are unique or represent the plesiomorphic condition leading to dinosaurs.”
The LRT showed back in 2011 that Lagerpeton is a dinosaur mimic, related to the proterochampsid (Fig. 2) Tropidosuchus, not dinosaurs. So the premise of this abstract is completely wrong, based on invalid Nesbitt 2011 cladogram.
“New forelimb fossils of dinosaur relatives and careful assessments of their osteology is sorely needed to help address this knowledge gap.
We have forelimbs for dinosaur relatives. Pseudhesperosuchus is close. So is Turfanosuchus. We’re glad to see forelimbs for Lagerpeton. Just don’t imagine that they have anything to do with the origin of dinosaurs.
“Here we present the first pectoral (left scapulocoracoid) and forelimb (right humerus) bones of the important early dinosauromorph Lagerpeton chanarensis.”
Not a dinosauromorph.
“The bones were prepared from a concretion that only consisted of Lagerpeton bones and from the cynodont Massetognathus. We identify the bones as belonging to Lagerpeton because the distal end of the femur possesses an inflated crista tibiofibularis – a lagerpetid character state – and the newly recognized pectoral and forelimb bones are generally similar to those of the lagerpetid Dromomeron romeri and Ixalerpeton with tall and
constricted anteroposteriorly narrow scapular blade and a humerus with a highly asymmetrical proximal part of the humerus.”
Sounds good.
Figure 3. The closest kin of Tropidosuchus are the much larger Chanaresuchus (matching Nesbitt 2011) and the smaller Lagerpeton.

Figure 1. The closest kin of Tropidosuchus are the much larger Chanaresuchus (matching Nesbitt 2011) and the smaller Lagerpeton.

McCabe and Nesbitt continue:
“The scapulocoracoid of Lagerpeton has a tall, but anteroposteriorly narrow scapular blade more like Dromomeron romeri than Ixalerpeton.
And more like the omitted Tropidosuchus (based on the above description). McCabe, Nesbitt: why not test Lagerpeton against Tropidosuchus? Novas and Agnolin reported it as a proterochampsian, too. BTW, Ixalerpeton is a protorosaur in the LRT, based on the few bones known.
“The length of humerus and the proportions of the proximal and distal end in Lagerpeton are also more similar to that of Dromomeron romeri. Overall, the scapulocoracoids and humeri of lagerpetids are similar in proportion across taxa, but comparing the length of the forelimbs to the hindlimbs is hampered by the lack of articulated or unambiguously associated individuals of any member of the group. Currently, it is still not clear if the anatomy of the pectoral girdle and forelimb of lagerpetids, and thus posture, is unique for lagerpetids or represents the ancestral condition for dinosauromorphs.”
This pectoral girdle does not represent the ancestral condition for dinosauromorphs. Nor is it unique to lagerpetids (sans Tropidosuchus). McCabe, Nesbitt, look at Tropidosuchus before pushing your hypothesis over the edge.

References
McCabe MB and Nesbitt SJ 2019. The first pectoral and forelimb material assigned to the lagerpetid Lagerpeton chanarensis: comparing to other lagerpetids and other avemetatarsalians. Journal of Vertebrate Paleontology 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

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