Two mechanisms can be detected in evolution leading to differentiating ontogenic patterns and adult morphology: Heterochrony, changes in the timing of developmental characters; and heterotopy, displacements of homologous structures in development. To explore aspects of their patterns and their interrelationship, both phenomena were exemplarily studied in land vertebrates, particularly in its most debated taxon, Testudines.
In one part of the thesis the organogenesis of different tetrapod taxa was compared. Therefore a standard system to study vertebrate embryos was developed based on clear anatomical definitions of developmental characters. The study includes recommendations on how to use such characters within a phylogenetic framework, how to handle them in embryological collections and how to deal with them in molecular oriented laboratories.
In total 104 organogenic characters were compared among embryos of 23 tetrapod species, including 15 turtle species. Using the Parsimov algorithm we were able to detect heterochronic shifts that autapomorphically support particular nodes in alternative topologies. Our mbryological data and the analyses performed best support a sistergroup position of Testudines to all remaining living sauropsids, and a basal position of marine turtles within Cryptodira. The heterochronic shifts detected could – with caution – be correlated to differentiating feeding and ocomotion behaviours in early life history of mammals and reptiles or to the count of vertebrae in the respective axa. As this study exclusively deals with a neontological kind of data we could not differentiate whether turtles evolved within “Anapsida” or on the stemline of Sauria ithin Diapsida.
For two amniote species – the turtle Emydura subglobosa and the mammal Tachyglossus aculeatus – organogenetic patterns were exemplarily described in detail and their timing was orrelated to ossification, myogenesis or early crawling behaviour. Alternative topologies were tested among Mammalia to detect the robusticity of heterochronic data – resulting in a critical interpretation of this non- independent kind of data for phylogenetic approaches.
Another part of the thesis is concerned with the heterotopic aspect of evolution. In the studies mentioned above I recognised several heterochronic shifts to include head related characters. Uncertainties about the phylogenetic position of turtles within Tetrapoda are mainly based on the arrangement of temporal bones; and studies on organogenesis did not result in a definitive solution. Hence, the anatomy of the cranium associated musculature in turtles was comparatively studied.
A clear definition of muscular structures in general and a review on all studies dealing with cranial musculature in turtles are presented. The homology of 88 muscular units are discussed. I focussed on the evolution of jaw musculature within Testudines and performed phylogenetic analyses having several alternative topologies as phylogenetic frameworks. The hardly comparable jaw muscle anatomy within Sauropsida does not suggest a solution for the position of turtles; hence, a non-homologous arrangement of jaw musculature in the major tetrapod taxa and a position of turtles outside of Sauria are proposed. The jaw muscle characters best support the currently most accepted arrangement of turtle subgroups, presuming 1. outh American and Australasian chelids (Pleurodira) to be monophyletic groups and 2. soft-shelled turtles Trionychidae) being the sister taxon to all remaining living cryptodires. The value of soft tissue characters for phylogenetic reconstruction is discussed and a system for muscle development and evolution is proposed that most adequately reflects the plastic ehaviour of this structure in time and space.