Bipedal Poposaurus

[Jocelyn Falconnet <j.falconnet@gmail.com>]

Gauthier, J.A., Nesbitt, S. J., Schachner, E. R., Bever, G. S. & Joyce, 
W. G. 2011. The bipedal stem crocodilian Poposaurus gracilis: inferring 
function in fossils and innovation in archosaur locomotion. Bulletin of 
the Peabody Museum of Natural History, 52(1), p.107-126.

We introduce a spectacular new specimen of a Late Triassic stem 
crocodilian identified as Poposaurus gracilis. It is part of a poorly 
known group, Poposauroidea, that, because of its striking similarities 
with contemporaneous stem avians (“dinosaurs”), has long puzzled 
archosaur paleontologists. Observed vertebrate locomotor behaviors, 
together with exceptional preservation of distinctive anatomical clues 
in this fossil, enable us to examine locomotor evolution in light of new 
advances in phylogenetic relationships among Triassic archosaurs. 
Because this stem crocodilian is unambiguously an archosaur, a diapsid, 
a tetrapod and a choanate sarcopterygian, we can safely infer major 
components of its locomotor behavior. These inferences, together with 
form-function constraints, suggest that P. gracilis was a fleet-footed, 
obligately erect-postured, striding biped. That behavior seems to have 
been superimposed on the ancestral archosaur's innovative locomotor 
repertoire, which includes the capacity to “high walk.” These novelties 
persist in a recognizable form in archosaurs for at least 245 million 
years and are widely distributed across Earth's surface in diverse 
ecological settings. They thus qualify as evolutionary innovations 
regardless of significant differences in diversification rates among 
extant diapsid reptiles.


http://www.bioone.org/doi/abs/10.3374/014.052.0102