Vertebral size ratios and the ichthyosaurian vertebral column – a case study based on Late Jurassic fossils from North-East Greenland

Abstract

Vertebral centra are some of the most common fossils from ichthyosaurs and thus valuable for understanding these marine reptiles. This study sets out to provide further information on the dimensional ratios of centra and how these might be used to obtain more information about an assemblage of Late Jurassic disarticulated centra found at Kingofjeldet on Kuhn Ø in North-East Greenland in 2017. The centra are used to test whether vertebral ratios (H:W and H:L) can be used to assign disarticulated and possibly weathered centra to a region in the vertebral column. In order to evaluate this, the ratios of the centra from Greenland were compared with those of five articulated and well-known ophthalmosaurid specimens, as well as classical traits based on morphology. Assigning the correct position in the vertebral column from ratios is, however, not straightforward. Firstly, comparing different ichthyosaur taxa gives different possible positions for the disarticulated centra. Secondly, centra from different vertebral regions commonly display similar ratios. Thirdly, ratios are sensitive to alteration by taphonomic processes. The ratios of the centra hints towards an ichthyosaur with a more regionalised vertebral column being present in the Late Jurassic sea of North-East Greenland. Further studies are needed to improve our understanding of the significance of the degree of regionalisation of the vertebral column among ichthyosaurs.