New postcranial material of the early caseid Casea broilii Williston, 1910 (Synapsida: Caseidae) with a review of the evolution of the sacrum in Paleozoic non-mammalian synapsids.

Aaron R H Leblanc,Robert R Reisz,Brian Lee Beatty

doi:10.1371/journal.pone.0115734

Abstract

Here we use the description of a new specimen of the small caseid synapsid Casea broilii that preserves the sacral, pelvic and hind limb regions in great detail and in three dimensions, as a unique opportunity to reevaluate the early stages in the evolution of the sacrum in the lineage that led to mammals. We place this new material in the context of sacral evolution in early caseid synapsids and conclude that the transition from two to three sacral vertebrae occurred in small-bodied species, suggesting that it was not an adaptation to heavy weight bearing. Furthermore, we compare descriptions of sacral anatomy among known early synapsids, including caseids, ophiacodontids, edaphosaurids, varanopids, and sphenacodontians and review sacral evolution in early synapsids. Based on the descriptions of new species of caseids, edaphosaurids, and varanopids over the past several decades, it is clear that a sacrum consisting of three vertebrae evolved independently at least four times in synapsids during the Late Carboniferous and Early Permian. Furthermore, similarities in the morphologies of the sacral vertebrae and ribs of these early synapsids lead us to conclude that an anterior caudal vertebra had been incorporated into the sacral series convergently in these groups. Given the repeated acquisition of a three-vertebra sacrum in early synapsids and no apparent link to body size, we argue that this sacral anatomy was related to more efficient terrestrial locomotion than to increased weight bearing.

Highlights

The evolution of the mammalian vertebral column, especially its degree of regionalization, has attracted attention in molecular, developmental, and evolutionary biology [1,2,3]
Muller et al [2] suggested that this was indicative of developmental constraint in vertebral formulae in Paleozoic synapsids, a feature that is consistently found in modern mammals
Unlike previous descriptions of the sacral series of the type genus for Caseidae [6, 10], this new specimen of Casea broilii provides a three-dimensional view of the sacrum and shows that it is a well-integrated unit, where the distal ends of all three sacral ribs contact each other and form a single continuous plane of contact with the ilium

Summary

Introduction

The evolution of the mammalian vertebral column, especially its degree of regionalization, has attracted attention in molecular, developmental, and evolutionary biology [1,2,3]. Whereas much of the focus has been on the development and evolution of the presacral portion of the vertebral column in synapsids [1, 3], very few studies have discussed the evolution of the synapsid sacral region This is in part due to a lack of documentation of the variation in sacral counts for Paleozoic synapsids, making it difficult to determine the ancestral condition for major synapsid clades [2]. Such a study would yield important information on the variation in the numbers of sacral vertebrae, but would provide information regarding terrestrial adaptations in early synapsids, given the functional significance of this part of the axial skeleton

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