Meeting Abstract
The sacrum—the vertebrae that articulate with the ilium—is the nexus between the axial skeleton and the hindlimb. Reptiles added sacral vertebrae in several ways (e.g., sacralization of trunk vertebrae), increasing from the plesiomorphic 2 sacrals to >20 in some birds. However, apart from early-diverging “pelycosaur”-grade synapsid lineages, little is known of the mechanisms of synapsid sacral evolution. Dicynodont therapsids have a wide range of sacral counts (3–7+), with a trend of increasing absolute number of sacral vertebrae in younger clades. We explored the addition of vertebrae to the dicynodont sacrum and placed these patterns in the broader context of synapsid evolution. We established the identity of each sacral vertebra by the location of sacral rib—iliac articulation, either by direct observation in articulated specimens, or by the location of sacral scars on ilia. The three primordial sacral vertebrae are located dorsal to the acetabulum, with additional vertebrae added to the sacrum anteriorly and posteriorly. Sacral ribs decrease in size in posterior sacral vertebrae in all observed taxa. Using phylogenetic logistic regression, we found a statistically significant correlation between increase in sacral count and larger body size (p = 0.002), even given strong phylogenetic signal. Given that the number of presacral vertebrae is largely conserved across Dicynodontia, anterior sacral vertebrae are added by the addition of novel elements anterior to the primordial three. All crown mammals but xenarthrans are restricted to ≤3 sacral vertebrae, and sacrals are added exclusively from the caudal series. This suggests that dicynodonts were able to escape a constraint on patterns of regionalization in the column that was otherwise common in synapsids.
