Meeting Abstract
Variation in vertebral number and regionalization is an important component of bauplan diversity in vertebrates, but drivers and constraints on the evolution of this variation are poorly understood. Previous studies have suggested that, with the exception of some snakes, fishes, and salamanders, vertebrates do not exhibit pleomerism — the phenomenon whereby segmentation and body size are positively correlated. While it is often remarked that birds exhibit considerable variation in vertebral count, it has also been argued that birds, like mammals, lack pleomerism because of their highly regionalized and functionally constrained axial skeletons. Here, I present results from an ongoing survey of avian axial regionalization, currently comprising data from 957 skeletons for 516 species, to which I apply phylogenetically-informed Bayesian comparative methods to test for pleomerism. Sampling densely within and across clades reveals that most clades exhibit little or no variation in pre-synsacral count, suggesting some degree of constraint on pre-synsacral somitogenesis within clades, while pleomerism is strongly expressed in the synsacrum regardless of clade-specific differences in ecology and pelvic morphology. Intraspecific variation is most common in the synsacral and caudal vertebrae. Grebes may be unique among birds in exhibiting cervical pleomerism, suggesting that constraints on somitogenesis may be relaxed in particularly long-necked taxa, though this remains to be rigorously tested in non-avian clades. These results demonstrate that pleomerism is an important aspect of body size evolution in birds and highlight the importance of dense taxon sampling for identifying relevant clade-specific variation in more general macroevolutionary trends.
