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. / Doctor of Philosophy / Every animal's anatomy is determined by the developmental processes they undergo during their life history. Because of this, understanding these developmental processes is a key part of determining how animals have evolved into the many disparate forms we see today. Conversely, it is only through the fossil record that the actual history of anatomical evolution can be observed, so a holistic account of the pattern and process underlying evolution must integrate both developmental biology and paleontology. The pelvic complex—the hips and the vertebrae articulated with the hips—plays a fundamental role in how all nearly land vertebrates move (with the exception of legless groups). In some ways, the pelvic complex is very similar across all groups of mammals, reptiles, and their extinct close relatives: all retain the same three bones of the pelvis in roughly the same configuration with each other, and all possess a sacrum consisting of at least two vertebrae, the ancestral condition. Despite the retention of this basic plan, the pelvic complex of many groups is characterized by extreme deviations from the ancestral condition, with the three pelvic bones having highly disparate shapes and proportions. Therefore, the evolution of the pelvis and sacrum across mammals, reptiles, and their extinct relatives—especially among archosaurs (the reptilian group containing crocodylians and birds), which possess highly derived variations of the pelvic complex in response to the evolution of novel body plans—provides an excellent case study to explore the influence of developmental patterns on the evolution of animal form.
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 group-specific best practices for assessing growth in extinct reptiles. The second chapter uses the evolution of the sacrum among extinct mammal relatives as a case study to explore the evolution of novel anatomy along the normally conservative mammalian lineage, integrating comparative anatomy, evolutionary statistics, and developmental biology. The third chapter utilizes a new technique for imaging the soft tissues of developing embryos to explore the origins of the highly unusual pelvic complex in birds, finding that the distinctive bird anatomy arrives late in development, with ancestral—more typically "dinosaurian"—features being present at early-stage embryos of birds.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/106742 |
| Date | 03 June 2020 |
| Creators | Griffin, Christopher Thomas |
| Contributors | Geosciences, Nesbitt, Sterling J., Bhullar, Bhart-Anjan S., Xiao, Shuhai, Stocker, Michelle |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/vnd.openxmlformats-officedocument.wordprocessingml.document, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet, application/pdf, application/vnd.openxmlformats-officedocument.wordprocessingml.document |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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