Patterns of Morphological Evolution in the Bird Skull

Summary Birds show a great variation in skull and beak shape. This morphological diversity known for different species, also exist among chicken breeds. A goal of evolutionary morphology is to understand how this adult disparity is generated in development. Another is to understand how different selective environments affect the tempo and mode of morphological change. It is unclear how much of the diversity in the bony adult skull across bird species is already laid out in the cartilaginous embryonic skull, the chondrocranium, and how variable chondrocrania are in their development and morphology. This is because traditionally, the morphology of the chondrocranium and embryonic skull development in vertebrates have generally been considered conservative. The chondrocranium is largely resorbed during development and only a few of its elements are retained in the bony skull.This thesis investigates patterns and processes leading to skull shape diversity in birds.The first of two main objectives of this dissertation is to summarize and analyze available knowledge from extensive descriptive studies of chondrocranial development in several species, including the domestic chicken, and to identify possible variations in the embryonic skull and examine how these relate to adult skull morphology. The second aim is to examine the influence of humans on avian skull and beak morphology and the rate of phenotypic change in these traits. In the first chapter, the chondrification sequence of the embryonic skull (i.e., the chondrocranium) was reconstructed for ten bird species and, in addition, the morphology of the chondrocranium of additional 11 bird species was considered. The reconstructed developmental sequence of the chondrocranium was analyzed in a phylogenetic framework for possible group-specific developmental patterns and chondrocranial morphology. We found similar developmental patterns among bird chondrocrania, with peri- and prechordal regions being most conserved. In terms of anatomy, they were largely uniform, with some variation, e.g., in fenestrations. The morphological diversity of the adult skull was reflected in the orbito-nasal region of the Kiwi chondrocranium and appears to be present in the chondrocrania of other bird species. In addition, there was evidence of group-specific developmental patterns or morphology, albeit few. The second chapter deals with the embryonic development of the skull in domestic chicken (Gallus gallus f. domestica ), and its variation and correlation with skull variation in the adult, focusing on the hypoglossal foramina in the Red Junglefowl and various chicken breeds. It was found that, regardless of the preparation method used, there was general agreement regarding the chondrification sequence of most elements. In contrast, there were inconsistencies regarding the origin of the fenestra basicranialis posterior, the presence of the intertrabecula and cranial ribs, and whether certain cartilaginous elements develop as outgrowth of other cartilaginous structures or have an independent chondrification center. Furthermore, we found that the number of hypoglossal foramina varies in the described chondrocrania and that there are also differences in the bony skull of the Red Junglefowl (Gallus gallus ) and between and within different chicken breeds, as well as bilateral asymmetry within individuals. The third chapter contributes to the growing field of urban evolutionary biology with data on morphological changes of beak dimensions and the rate of phenotypic change in urban bird populations in Switzerland. Although we found that beak dimensions in general have changed little over the past century, there is evidence for an increase in beak length in Geneva Blackbirds and a trend toward a decrease in beak width in Bern Chaffinches. Rates of change in our sample compared to other estimates from anthropogenic and natural contexts appear similar in magnitude. This may indicate that selection pressures in urban habitats are similar to those in the wild.This dissertation has shown that developmental patterns are largely conserved in embryonic skull development, but that chondrocrania nevertheless exhibit some of the morphological diversity of the adult skull. Furthermore, we showed that beak dimensions of urban birds have changed little with two exceptions, and the urban rates of phenotypic change were within the range of evolutionary rate estimates from natural habitats. Keywords: Aves, chicken, heterochrony, ontogeny, skull, chondrocranium, disparity, variation, hypoglossal foramina, homology, urban evolution, morphological change, rate of evolution, haldanes, darwin