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Sci Adv. 2017 Dec 13;3(12):e1701144. doi: 10.1126/sciadv.1701144. eCollection 2017 Dec.

A Triassic plesiosaurian skeleton and bone histology inform on evolution of a unique body plan.

Author information

1
Bereich Paläontologie, Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Universität Bonn, Nussallee 8, 53115 Bonn, Germany.
2
Osaka Museum of Natural History, Nagai Park 1-23, Higashi-Sumiyoshi-ku, Osaka 546-0034, Japan.
3
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan.
4
UMR 7179 CNRS/Muséum National d'Histoire Naturelle, Départment Adaptations du Vivant, 57 rue Cuvier CP-55, 75005 Paris, France.
5
Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan.
6
Dinosaur Institute, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA.

Abstract

Secondary marine adaptation is a major pattern in amniote evolution, accompanied by specific bone histological adaptations. In the aftermath of the end-Permian extinction, diverse marine reptiles evolved early in the Triassic. Plesiosauria is the most diverse and one of the longest-lived clades of marine reptiles, but its bone histology is least known among the major marine amniote clades. Plesiosaurians had a unique and puzzling body plan, sporting four evenly shaped pointed flippers and (in most clades) a small head on a long, stiffened neck. The flippers were used as hydrofoils in underwater flight. A wide temporal, morphological, and morphometric gap separates plesiosaurians from their closest relatives (basal pistosaurs, Bobosaurus). For nearly two centuries, plesiosaurians were thought to appear suddenly in the earliest Jurassic after the end-Triassic extinctions. We describe the first Triassic plesiosaurian, from the Rhaetian of Germany, and compare its long bone histology to that of later plesiosaurians sampled for this study. The new taxon is recovered as a basal member of the Pliosauridae, revealing that diversification of plesiosaurians was a Triassic event and that several lineages must have crossed into the Jurassic. Plesiosaurian histology is strikingly uniform and different from stem sauropterygians. Histology suggests the concurrent evolution of fast growth and an elevated metabolic rate as an adaptation to cruising and efficient foraging in the open sea. The new specimen corroborates the hypothesis that open ocean life of plesiosaurians facilitated their survival of the end-Triassic extinctions.

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