53 - Structure of Modularity and Integration Among Elements in the Catarrhine Vertebral Column

Poster Board Number: 53
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Abstract Body : The vertebral column contributes to organismally important functions such as locomotion and protection of the central nervous system. As a result, it is both generally conserved in its overall structure and highly variable in its morphology. Morphological variation, especially in the neural arch, is linked to posture, locomotion, and dietary ecology across taxa and is vital for identifying such behaviors in the fossil record. Understanding how this variation evolves is thus especially important. Recent research has shown that some taxa exploit existing patterns of phenotypic integration in order to achieve characteristic vertebral morphologies. Phenotypic integration refers to covariation among traits and is thought to constrain evolution along specific pathways. However, phenotypic integration itself evolves, and selection can lead to the formation of modules—traits that are strongly integrated with each other to the exclusion of others. Patterns of integration and modularity thus significantly influence the morphologies we observe in living and fossil taxa. Here, we characterize patterns of modularity within and between subaxial cervical and thoracolumbar vertebral elements in four catarrhine taxa (Macaca=97; Hylobates=76; Pan=92; Homo=151) with highly varied locomotor behaviors to determine whether these patterns are shared across catarrhines or whether there is significant divergence among taxa, which may reflect strong selective pressures. We estimated the covariance ratio (CR) and effect size of covariation between linear measurements of the neural arch and its corresponding body and between the neural arches of adjacent vertebrae. Our results indicate a general lack of modularity both within individual elements (CR=0.494-1.9955) and between adjacent vertebral elements (CR=1.1825-2.2815), with no estimates significantly different from a hypothesis of generalized integration. Covariation between adjacent neural arches tends to be higher than within vertebrae, a pattern confirmed by the effect sizes, but which is rarely significant. In general, thoracic elements tend to display the highest overall covariation between vertebrae and lowest overall covariation within vertebrae, though the specific region of highest or lowest covariation varies among taxa. Homo may display modularity between corresponding vertebral bodies and neural arches in the lumbar region (CR=0.494-0.9873), unlike other taxa. Overall, our results find both shared and divergent patterns of integration and modularity in the catarrhine vertebral column. The thoracic region may be particularly constrained, likely as a result of the rib cage. Humans are unusual in vertebral modularity patterns, with changes in the lumbar region potentially reflecting novel selection pressures. These results also suggest that vertebrae need to be considered in sequence rather than as individual elements, as intervertebral covariation interactions may be more important than intravertebral ones.