Can vertebral remains differentiate more than one species of Australian Cretaceous ichthyosaur?

Abstract

All currently described Cretaceous ichthyosaur remains in Australia have been referred to the single taxon Platypterygius australis M’Coy 1867, primarily on the basis of skull morphology. This research set out to determine if specimens could be attributed to P. australis on the basis of vertebral data alone. Vertebrae were measured and subjected to morphometric analysis for specimens previously attributed to P. australis, along with several unattributed specimens that either lack skulls or where existing skulls have not been described. Centrum length (CL) plots, together with a principle components analysis and multivariate analysis of variance, suggest that different specimens can be distinguished from each other on the basis of vertebrae. Although our analyses using only basic centrum dimensions are heavily impacted by size, and thus growth stage of individuals, two specimens previously attributed to P. australis on the basis of skull morphology alone plotted separately from each other in all analyses, with one plotting consistently near the outgroup taxon, Ophthalmosaurus natans (Marsh, 1879). Hence, the data are consistent either with very large intraspecific variation within vertebral patterns, or support the possibility that more than one Australian species occurs in the dataset. CL and multivariate data showed that juvenile ichthyosaurs plot differently relative to all subadults–adults, not only in their proportionately smaller vertebrae, but also in having consistently different shapes with little difference between the length of dorsal and caudal vertebrae. One subadult individual (RGC1) also showed similarity in length between dorsal and caudal vertebrae and has abnormal rib facets. We posit that it could either be an aberrant P. australis, or an otherwise unknown taxon. Although additional material is required, questions raised by this study suggest the need to test the postcranial remains of additional specimens, especially those with well-studied skulls, so as to identify the level of intraspecific variation or any sexual dimorphism, and to determine if any as yet undetected ichthyosaur taxa occurred in the Australian Early Cretaceous.

Vikram Vakil [[email protected]], Gregory E. Webb [[email protected]] and Alex G. Cook [[email protected]] School of Earth & Environmental Sciences, The University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia.

Key words:

• marine reptiles
• ichthyosauria vertebrae
• morphometrics
• taphonomy

Acknowledgments

Special thanks to Kristen Spring, Heather Janetzki and Andrew Rozefelds (QM), and Michelle Johnston (KK) for assistance and access to material. Espen Knutsen (QM) is thanked for his invaluable comments on an earlier version of this manuscript. Benjamin P. Kear (Uppsala University) also provided invaluable comments on all versions of the manuscript. Lene Liebe Delsett (University of Oslo) and a second anonymous reviewer contributed constructive reviews. Funding for this Master degree research was provided by the parents of Vikram Vakil and The University of Queensland.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplementary research materials for this article can be accessed at https://doi.org/10.1080/03115518.2020.1853809.