SAILBACKS THROUGH DEEP TIME: A COMPARATIVE OSTEOHISTOLOGICAL ANALYSIS OF HYPERELONGATE NEURAL SPINES IN ARIZONASAURUS BABBITTI AND OTHER AMNIOTES

In amniotes, hyperelongate neural spines (HENSs), which often form a “sailback” along the axial column, have arisen independently at least 12 times, widely scattered phylogenetically and temporally. Suggestions for functions of HENSs include use as thermoregulators, display organs, and/or biomechanical support. Osteohistology may be informative for comparing hypotheses of HENS function, but while histological analyses of HENSs have been published for stem-mammals and dinosaurs, there are no data on ctenosauriscids, a group of Early-Middle Triassic pseudosuchians which evolved HENSs, or on extant taxa with HENSs, like squamates and ungulates.

Here, the osteohistology of neural spines in the ctenosauriscid Arizonasaurus babbitti, and extant taxa Bison bison and Basiliscus basiliscus are compared with taxa with previously described HENSs, including Dimetrodon gigahomogenes and Spinosaurus aegyptiacus. Analysis of internal features is used to correlate with possible functions, with extant taxa that use HENSs for display (e.g., T. cristatus) or biomechanical support (e.g., B. bison) serving as a framework for the relationship between structure and function. Under this framework, a higher ratio of trabecular:cortical bone signals more tissue development, concentrations of Sharpey’s fibres show points of tendon attachment, and position of lines of arrested growth (LAGs) show change in rate of growth through ontogeny.

A. babbitti shows a higher ratio of trabecular:cortical bone than any of the living taxa, and is not found to have a significant change in amount of LAGs over the neural spine shaft, indicating relatively constant growth through ontogeny. Sharpey’s fibres were not found in distal spine sections, or concentrated in bundles as in B. basiliscus. Cortical vascular canals were observed in A. babbitti, with pathways to the external bone surface indicating blood flow through the neural spines. These results suggest that A. babbitti likely had more tissue on its HENSs than modern squamates. However, our results also show extinct taxa have extremes in these metrics of internal bone structure or combinations of features that fall outside of the envelope of extant ones, suggesting both that convergent evolution of HENSs in these species may not represent functional convergence, and that HENSs in extinct taxa may serve functions beyond those seen in living taxa.