DISSERTATION RESEARCH: The Evolution of Crocodylian Cranial Development

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Crocodylians (crocodiles and alligators), often labeled 'living fossils, are commonly thought to have an ancient and conservative skeletal anatomy and lifestyle. However, modern crocodylians are more diverse than often appreciated and their skulls, in particular, can range from short and wide to incredibly long and slender. Prior research has shown that crocodylian skull shape is correlated with dietary and habitat specializations, but it remains unclear how these shapes come to be. The formation of crocodylian skull diversity is tied to development: the changes that occur in an individual as it grows from a fertilized egg to an adult. The process of development is controlled by genes, and changes to how genes are expressed can modify developmental pathways and adult anatomy. By combining data from fossils, crocodylian embryos, and modern developmental biology techniques, this project will reveal which changes in developmental pathways were key to the evolution of the crocodylian skull, and the evolutionary diversity of crocodylians. Insights from this research may also inform general connections between genomic, developmental and organismal complexity in vertebrates. To capitalize on the public interest in crocodiles and their relatives (dinosaurs and birds), this project will develop K-12 activities for students and teachers in conjunction with the Harvard Museum of Natural History (HMNH). These activities explore key concepts in paleontology and developmental biology and will be tied to the Next Generation Science Standards. The HMNH provides a unique venue to reach a diverse urban audience from across the Boston Metropolitan area and visitors from around the world. Unique morphological features distinguish major groups of animals like crocodylians, mammals, and birds. These features are important for the particular lifestyles and habits of these groups and have been critical for their survival over millions of years. The formation of this diversity is caused by the evolution of novel genes, novel functions for genes, or modification to the developmental pattern of gene expression. Integrating fossils, embryos, and developmental genetic techniques, this study will reveal developmental mechanisms involved in the evolution of crocodylian cranial diversity. This project will study the expression patterns of key genes involved in the development of the skull in extant crocodylians using in situ hybridization. Developmental stages from eight target species, which broadly sample the phylogeny and diversity of snout shapes in modern crocodylians, will be analyzed. The project aims to experimentally replicate ancestral and derived gene expression patterns and to use geometric morphometrics to quantify the relationship between expression patterns and skull shapes. These functional studies in alligator embryos will test the likelihood that particular changes in development were the cause of evolutionary changes in anatomy. This project integrates paleontology, anatomy, and developmental biology to link changes in anatomy to genetic inheritance through development. This integrative methodology will lead to a greater understanding of critical mechanisms underlying the evolution of anatomical diversity and novel features of organisms.