Skull Growth and Prenatal Development in Baleen Whales: Teeth to Baleen Transition in Ontogeny and Phylogeny

In my dissertation, I aim to add a new ontogenetic perspective to the study of baleen whale’s (Mysticeti, Cetacea) skull evolution. In Chapter One, I analyze changes in dentition in closely-related toothed whales (Odontoceti, Cetacea), focusing on beaked whales, and trace the evolution of inactivating mutations in tooth-related genes in Mysticeti. While there is high variability in tooth counts in odontocetes, with many instances of convergent evolution among living and fossil taxa, all modern clades retain the ability to produce full adult dentition. Mysticetes instead present a variety of inactivating mutations in at least eight tooth- and enamel-related genes. Analyzing the complex inactivation patterns of these genes, I hypothesize that loss of function preceded the complete loss of adult teeth in baleen whales. In Chapter Two, I investigate the anatomical changes that occur in the skull of baleen whales in ontogeny, focusing on the teeth-to-baleen transition. The internal anatomy of 15 specimens of minke and humpback whales was analyzed using CT scanning. I provide qualitative descriptions of the specimens and study skull shape changes using 3D geometric morphometrics (GM) methods. Tooth germs resorb completely just before eruption of the of the baleen from the gums, and they are still present for a brief period along with baleen rudiments. GM analyses show that the rostrum progressively grows in length relative to the braincase, with the two parts of the skull showing different patterns of shape and size development. In Chapter Three, I combine developmental data with fossil evidence to directly analyze the connection between ontogeny and evolution of baleen whales using 3D GM methods. I also investigate possible differences in timing of ossification of skull bones between Cetacea and terrestrial artiodactyls. A general acceleration of skull development is likely responsible for modern mysticetes anatomy. The rostrum was the first part of the skull to increase its growth rate, allowing for the evolution of larger buccal cavity needed for bulk filter feeding. These changes in shape were not accompanied by shifts in the ossification sequence, as they appear mostly conserved among all taxa examined and not connected to their feeding adaptations.