Ontogeny and evolution of the elasmosaurid neck highlight greater diversity of Antarctic plesiosaurians

Abstract

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 (disc‐like), evolved towards opposite regions of the morphospace from the plesiomorphic ‘Cimoliasaurus’‐grade condition. We also find a marked ontogenetic shift from the disc‐like to can‐shaped morphology, 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, and that the diversity of Antarctic elasmosaurids was greater than previously recognized.

Body reconstruction and size estimation of plesiosaurs

“…O’Keefe and Hiller [203] discovered complex ontogenetic allometry in elasmosaurids. Brum et al [204] enhanced this conclusion by revealing a morphological shift in cervicals from disc-like to can-shaped in all elasmosaur groups. For polycotylids, O’Keefe et al [205] and Byrd [206] reported allometric growth in propodials and girdle elements respectively.…”

“…In cervical width-BSL regression, three out of eight samples included in the dataset are juveniles. The growth pattern of pliosaur vertebrae has not been studied, but it is possible that cervical ratios of juveniles are different from those of adults, as in elasmosaurids [203, 204]. In cervical width-BSL regression, the juveniles gather at lower ends of the axes due to their small sizes, and this may lead to biased estimates.…”

A plesiosaurian autopodial element (Plesiosauria indet.) with remarkable articular subchondral surfaces from the Maastrichtian type area (south-east Netherlands)

“…Representatives of the family Polycotylidae Williston, 1908 in general had short, stout necks and an elongated rostrum (Druckenmiller & Russell, 2008; Frey et al, 2017; Fischer et al, 2018). Similar to polycotylids, elasmosaurids possessed numerous pointy teeth, but had extremely long necks and a short rostrum (Everhart, 2006; Kubo et al, 2012; Brum et al, 2022). Both groups had four morphologically similar flippers that enabled them to move efficiently through the water using all limbs simultaneously, a way of propulsion also used by marine turtles and penguins often referred to as subaqueous flight or ‘flying under water’ (Robinson, 1975; Massare, 1994; Caldwell, 1997; Carpenter & Sanders, 2010; Muscutt et al, 2017).…”

Ankylosaurian body armor function and evolution with insights from osteohistology and morphometrics of new specimens from the Late Cretaceous of Antarctica

“…Despite such limitations, several vertebrate remains have been discovered in Antarctica over the past few decades. These discoveries include plesiosaurs (e.g., O'Gorman et al 2019; Brum et al 2022), pterosaurs (Kellner et al 2019), and dinosaurs (Lamanna et al 2019); for an updated survey of fossil vertebrates in the Upper Cretaceous of Antarctica, see Reguero et al (2022). Importantly, this includes one ankylosaurian species, Antarctopelta oliveroi Salgado and Gasparini, 2006— although there is some dispute on the validity of this taxon (Arbour and Currie 2016; Rozadilla et al 2016).…”

“…Since 2007, the project PALEOANTAR (organized by the Museu Nacional/Universidade Federal do Rio de Janeiro, Brazil) has been working in the Antarctic Peninsula to shed light on the diversification and evolutionary history of Antarctic ecosystems in the deep past (Lima et al 2021; Kellner 2022; Santos et al 2022), including studies on Mesozoic vertebrates (e.g., Kellner et al 2011, 2019; Brum et al 2022) and invertebrates (e.g., Pinheiro et al 2020; Videira-Santos 2020; Piovesan et al 2021). Here, we report on new ankylosaurian materials recovered from Antarctica during the PALEOANTAR expedition in 2015.…”