Amniote paleontology and development: evolution of vertebral counts and other case studies

MUELLER, J.; SCHEYER, T.; BARRETT, P.M.; ERICSSON, P.; HEAD, J.; POL, D.; SANCHEZ-VILLAGRA, M.

Ghent, Bélgica

Congreso; European Evo-Devo Conference 2008; 2008

European sSciety for Evolutionary Developmental biology

Developmental investigations in fossil amniotes are usually hampered by their unique mode of reproduction, because the amniote egg makes the developing embryo spatially restricted and shows an only poor potential for fossilization. As a result, most ontogenetic studies of fossil amniotes have to focus on the postnatal stage, the only exceptions being occasionally fossilized eggs, or viviparous species (e.g. ichthyosaurs). Examples of investigations of fossil ontogenies in amniotes include paleohistology, biometric studies, and comparisons of ossification patterns in marine reptiles, which due to skeletal paedomorphosis and delayed onset of ossification are somewhat easier to study developmentally than other fossil amniotes.Alternatively, the amniote fossil record allows for developmental studies from a different perspective. The fact that so many different amniote phenotypes are preserved in the fossil record makes it possible to perform large-scale investigations of patterns within a phylogenetic framework. In combination with data obtained from ontogenetic studies of extant taxa, it is possible to gain insights into ancient developmental processes. For example, due to a combination of phylogeny, fossil anatomy, and extant ontogeny it could be shown that the lower temporal arcade of derived diapsid reptiles is only secondary.The examination of adult numbers of vertebrae in amniotes provides an indirect but powerful way to examine early evolutionary events, including somitogenesis. We conducted a comprehensive examination of presacral vertebral counts across Recent and extinct taxa, from literature and first hand observation of fossils. More than 500 fossil taxa were included, including dinosaurs, early synapsids and numerous stem members of recent clades or totally extinct groups. Characters were mapped to minimize the sum of squared changes with a rooted composite phylogeny for all the living and fossil taxa. Increase in body size has been attained in different lineages by either somatic growth, keeping the same number of segments, or by somitogenesis. Increases in the number of vertebrae occur in most groups regionally and not globally. The recorded patterns suggest an agenda of investigation of potential developmental constraints and canalization associated with different clades.