Cymbospondylus (Ichthyopterygia) from the Early Triassic of Svalbard and the early evolution of large body size in ichthyosaurs

Abstract

Ichthyosaurs were a highly successful group of marine reptiles in the Mesozoic. The ichthyosaur radiation is part of the recovery from the Permian-Triassic mass extinction. In the Early Triassic, this group underwent extensive global radiation, filling ecological niches for the first time that were later occupied by various other lineages of marine amniotes. However, the evolution of body size in ichthyosaurs is not fully understood, as most large-bodied taxa originate from the Middle Triassic and later, and are mostly known from only a few specimens. In this study, we describe three articulated posterior dorsal vertebrae (IGPB R660) of the ichthyosaur Cymbospondylus sp. from the latest Olenekian Keyserlingites subrobustus zone of the Vikinghøgda Formation of the Agardhdalen area, eastern Spitsbergen, Svalbard. We numerically estimated the total body length of IGPB R660 from dorsal vertebral centrum length using a comparative dataset of other species of the genus and two different allometric analyses. This approach yields total length estimates of 7.5 m and 9.5 m for the individual, respectively, the highest for any unambiguous Early Triassic ichthyosaur find. Earlier, higher estimates of 11 m were based on taxonomically and stratigraphically inconclusive material but do not appear unreasonable based on evidence provided in this paper. Our study underscores both the rapid ecosystem recovery after a major mass extinction and extremely rapid increases in body size in ichthyosaurs after their adaptation to a secondarily aquatic lifestyle.