The Antarctic plesiosaurian record is critical for understanding the evolution of elasmosaurids in the southern hemisphere. Elasmosaurids exhibit some of the most remarkable modifications of the vertebrate axial skeleton given their extreme elongation of the cervical region. Despite a considerable amount of information available on vertebral counts within Plesiosauria throughout the decades, we have a considerably more limited understanding of the diversity of cervical vertebral shapes in elasmosaurids and how these have changed throughout ontogeny and phylogeny. Here, we compile the largest known morphometric dataset on elasmosaurid cervical vertebrae, including data on juveniles and adults, to answer some of those long-standing questions. This dataset also includes newly recovered materials from Antarctica, which we describe herein. Using multivariate statistical approaches, we find that the two major elasmosaurid cervical morphotypes, the elasmosaurine anteroposteriorly elongate (can-shaped) and the aristonectine anteroposteriorly short and dorsoventrally tall (disk-like), evolved towards opposite regions of the morphospace from the plesiomorphic ‘Cimoliasaurus’-grade condition. We also find a marked ontogenetic shift from the disk-like to can-shaped morphologies, which is especially pronounced in elasmosaurines, but more limited in aristonectines. Furthermore, we find that juvenile aristonectines occupy a specific region of the vertebral morphospace, distinct from any other group or ontogenetic stage, thus suggesting that reversal to the ‘short-necked’ condition in elasmosaurids is mostly characterized by ontogenetic predisplacement in aristonectines. Finally, we find that it is possible to discriminate between vertebral shapes of distinct taxonomic groups regardless of ontogenetic stage, which suggests a greater diversity of Antarctic elasmosaurids than previously recognized.

Phylogenetic analysis. The codification of the specimen in the matrix of O’Gorman (2019a) with the codification of Jucha by Fischer et al. (2021) is available in the files “OG(2019a) & Fischeretal(2021).nex” and “OG(2019a) & Fischeretal(2021).tnt”. Statistical analyses. The whole dataset employed in our statistical analyses of cervical and dorsal vertebrae are compiled in the file “Dataset.xlsx”. All the compiled cervical vertebrae specimens are listed in Supplementary Table 1. The whole dataset of PAST files to cervical vertebrae are available in the files: “Supp cervical adult morphotypes.dat”; “Supp cervical adult genus level.dat” and “Supp cervical juvenile genus level.dat”. All the analyzed dorsal vertebrae specimens are listed in Supplementary Table 2. The whole dataset of PAST files to cervical vertebrae are available in the files: “Supp dorsal adult genus level.dat”. We plotted the VLI values in histograms and performed the normality test Shapiro-Wilk (W) to determine whether we could perform non-parametric analyses.