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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.
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