The influence of developmental patterns on vertebrate evolution, with the evolution of the sacrum and pelvis as a case study

Patterns of organismal development—including embryonic morphogenesis, postnatal ontogenetic trends, and growth—are fundamental to understanding the evolution of phenotypes. Consideration of these processes is necessary to construct a holistic and rigorous account of morphological evolution. Therefore, the integration of both the fossil record (actualistic observations of changes in morphology through Earth history) and observations of extant clades (comparative patterns of pre- and postnatal development) is required to study the evolution of form across deep time. The pelvic complex plays a central role in tetrapod locomotion with the exception of taxa lacking hindlimbs (e.g., whales, snakes, caecilians). This complex, composed of the pelvic girdle (ilia, ischia, and pubes) and the sacrum (vertebrae articulated with the ilia via sacral ribs) is in some aspects highly conserved through amniote evolution: all terrestrial amniotes with functional hindlimbs retain the same bones of the pelvic girdle in roughly the same configuration with each other and a sacrum consisting of at least two vertebrae, the ancestral amniote condition. Despite the retention of this basic plan, the pelvic complex of many groups is characterized by extreme deviations from the ancestral condition. Therefore, the evolution of the pelvis and sacrum across amniotes—especially among archosaurs, which possess highly derived variations of the pelvic complex in response to the evolution of novel baupläne—provides an excellent case study to explore the influence of developmental patterns on the evolution of morphology.

The first chapter of my dissertation reviews the methods that have been used to study reptilian growth and maturity in the fossil record. I clarify often ambiguous terminology and highlight clade-specific best practices for assessing growth in extinct reptiles. The second chapter uses the dicynodont sacrum as a case study to explore the evolution of novel morphologies along the normally constrained synapsid lineage that are convergent on several archosaurian innovations, integrating comparative anatomy, comparative phylogenetic methods, and developmental biology. The third chapter utilizes a new technique (modified CLARITY protocol) for imaging the soft tissues of developing embryos to explore the morphogenesis of the avian pelvic complex, finding that the derived avian condition arrives late in development, with plesiomorphic—more typically "dinosaurian"—character states being present at the outset of musculoskeletal development.