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Platypterygius (Ichthyosauria) locomotory capabilities



From: Ben Creisler
bcreisler@gmail.com


A new online paper:

Maria Zammit, Benjamin P. Kear and Rachel M. Norris (2013)
Locomotory capabilities in the Early Cretaceous ichthyosaur
Platypterygius australis based on osteological comparisons with extant
marine mammals.
Geological Magazine (advance online publication)
DOI: http://dx.doi.org/10.1017/S0016756813000782
http://128.232.233.5/action/displayAbstract?fromPage=online&aid=9062970&fulltextType=RA&fileId=S0016756813000782


Reconstructing the swimming capabilities of extinct marine tetrapods
is critical for unravelling broader questions about their
palaeobiology, palaeoecology and palaeobiogeography. Ichthyosaurs have
long been the subject of such investigations because, alongside
cetaceans, they are one of the few tetrapod lineages to achieve a
highly specialized fish-like body plan. The dominant locomotory mode
for the majority of derived, post-Triassic ichthyosaurs is
hypothesized to have been caudal fin-driven propulsion. Limb-based
swimming has however been suggested for some highly autapomorphic
forms, such as the Cretaceous genus Platypterygius, which has a
remarkably robust humeral morphology and exceptionally broad
paddle-like limbs. To evaluate this atypical lifestyle model, we
conducted a comprehensive comparative osteological assessment of
Platypterygius in relation to extant marine mammals, whose analogous
skeletal frameworks provide a structurally compatible selection of
alternate propulsive strategies. Based on a proxy exemplar of the most
completely known species, P. australis from the Early Cretaceous of
Australia, the propodial shape, absence of functional elbow/knee
joints, tightly interlocking carpals, hyperphalangy and extreme
reduction of the pelvic girdle are most similar to cetaceans as
opposed to pinnipeds or dugongs. There is no obvious structural
consistency with aquatic mammals that use sustained forelimb-driven
swimming. The exceptionally broad fore-paddle (a product of
hyperdactyly) and extensive humeral muscle insertions might therefore
have had a cetacean-like role in enhancing manoeuvrability and
acceleration performance. We conclude that, despite its atypical
features, P. australis was most likely similar to other ichthyosaurs
in using lateral sweeps of the tailfin to generate primary propulsive
thrust.

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