Morphological evolution in Cetacea: Skull asymmetry and allometry of body size and prey.

摘要

Cetaceans include baleen (Mysticeti) and toothed whales (Odontoceti). Both groups have oddly shaped skulls compared to other mammals, but odontocetes are further distinguished in having directional skull asymmetry; the right facial region is expanded relative to the left. Although an odontocete synapomorphy, asymmetry differs among odontocetes. Previous untested hypotheses attribute differences to convergence, body size, and apex height. I examine skull asymmetry using Procrustes transformation on two dimensional landmark coordinates collected from most extant species, and on three dimensional landmarks taken from a subset of species. I demonstrate that differences cannot be attributed to body size or apex height; rather, patterns of asymmetry show a strong phylogenetic signal. I characterize asymmetry among odontocete lineages, and argue for a single origin of asymmetry with subsequent alteration in different lineages. Asymmetry of soft tissue associated with production of echolocation clicks are likely associated with skull asymmetry, but comparative dissection and computed tomography data are currently too limited to elucidate this association.;I also collected body size data (length and mass) of extant cetaceans from a large number of sources. Adult (determined from length at sexual maturity) body sizes of extant species are summarized, and multiple models for predicting size of fossil cetaceans from skull measurements (condylobasal and dentary lengths; occipital condyle and foramen magnum areas; paired occipital condyle, rostral, and zygomatic widths) are developed. Models were confirmed with fossils of known (skeletal) length, and body size estimates are reported for numerous fossil species. In addition, data on prey sizes taken by generalist feeding odontocetes were used to explore correlations between odontocete and prey sizes. As with most predator-prey relationship, odontocete size is positively correlated with prey size; larger odontocetes feed on a wider range of prey sizes; and few odontocetes feed on prey as large or larger than themselves (limited to killer whales and other globicephalines). The ability to estimate body size in fossil cetaceans, and size of their prey, provides a new tool for examination of cetacean evolution. Work in progress will apply these tools to a broad examination of marine mammal feeding guild evolution through the Cenozoic.