Meeting Abstract
Plesiosaurs are an extinct and iconic group of long necked marine reptiles from the Mesozoic. Neck elongation in plesiosaurs is almost unrivalled by both extinct and extant vertebrates, yet the mechanisms and selection pressures that underpin this elongation are poorly understood. Hox genes regulate axial regionalisation in vertebrates and the number of cervical regions is generally fixed within major groups of vertebrates. Recent work on cervical regionalisation in extant tetrapods has used 3D geometric morphometrics (GMM) as a proxy to delineate these regions governed by Hox gene suites, and has found that neck elongation in archosaurs occurs with the addition of regions beyond the basal number for amniotes; 3. We used 3D GMM and anatomical descriptions of plesiosaur cervical vertebrae in one species (Muraenosauus leedsii, R2863 BMNH) alongside comparisons to regionalisation within an extant phylogenetic bracket (archosaurs and squamates). Results from the GMM and comparative anatomical descriptions suggest that M. leedsii had 4 cervical regions. These regions consist of 1) the atlas-axis, 2) C3-C6, 3) C7-C37 and 4) C38-C42. These results suggest that M. leedsii had a derived regionalisation formula with the addition of a 4th cervical region, similar to modern archosaurs. Whilst our study so far entails only a single species of plesiosaur, our results suggest that plesiosaurs achieved such elongated necks predominantly by adding cervical vertebrae to region 3, rather than by changing Hox gene regulation to add more, novel, regions. A combination of further species included in this work flow, alongside a detailed examination of plesiosaur neck kinematics will allow for a more complete understanding of the role cervical regionalisation played in plesiosaur neck elongation.
