| dc.description.abstracteng | The following three chapters can be regarded as thematic complexes of studies on phenotypic variations and variability in order to elucidate their origins on a systematic hierarchy-level in echinoids.
Chapter 2 seeks systematically valuable characters in the microstructure of recent atelostomate echinoids (Spatangoida, Holasteroida). Within the last 145 Myr years, the atelostomate group of echinoids has become an important component of the marine benthos. Systematic studies within this echinoid group are predominantly based on test morphology (Kroh & Smith, 2010). An important feature of echinoids is their spines. Attempts have been made to use the variation in their spines for systematic purposes but without any meaningful results, leading to them being regarded as a poor variable and thus, insignificant morphological character. In this study, 973 spines of 74 atelostomate taxa have been investigated in detail with respect to their microstructures. The results revealed several homologies shared among the particular orders of holasteroids and spatangoids. However, a single character (pattern of perforation of the inner cylinder) was found to be reliable to distinguish between spines of spatangoids and holasteroids. Interestingly, a single outlier in this respect was found within the holasteroids [Corystus relictus (de Meijere, 1903)], which was similar to the pattern found in spines of spatangoids. An anomaly in a single spine of this taxa, combining both patterns of perforation, suggests that the deviant holasteroid taxa bears the genetic potential to develop both character states.
Chapter 3 considers a group of fossil echinoids (regular echinoids; Phymosomatidae), which has traditionally been neglected in palaeolontigical studies of evolution. This may be due to their less rigid skeleton architecture and, accordingly, their relatively poor fossil record (Kier, 1977). Moreover, regular echinoids reveal a low phenotypical variation in comparison with irregular echinoids, which hampers traditional approaches in phylogenetic studies. Thus, only little is known about their evolutionary relationship, and virtually no knowledge exists about evolutionary processes leading to their diversity. In this study, ontogenetic trajectories among three species of the genus Gauthieria are recognised: G. radiata (Sorignet, 1850), G. spatulifera (Forbes in Dixon, 1850), and G. princeps (von Hagenow, 1840). These trajectories enable comparisons among the three species and thus reveal heterochronic processes within their evolution. As mentioned above, detailed phylogenetic results which are crucial for drawing conclusions about heterochronic development are completely missing from this group. Nevertheless, this work shows that comparisons of ontogenetic variations are useful tools to uncover evolutionary links between otherwise only low variable taxa.
Chapter 4 studies variations within a species of the Late Cretaceous irregular echinoid Micraster. In this work, populations from different habitats from the early Coniacian are investigated, two populations from the Münsterland Cretaceous Basin (Germany) and one population from the North Cantabrian Basin (Spain). Variations on different levels are investigated, from between populations and habitats to variations within-individuals. The aim of this work was to discuss and trace back the mechanisms of the respective variations, in terms of being largely genetically influenced, or either as being a sign of phenotypic plasticity, and to test for the presence of stochastic variations as a result of developmental instability.
Micraster is regarded as a well-known example of a progressive modifying lineage. The influence of the environment on variations in this phenotype were neglected by previous studies. However, this study demonstrates the possibility to distinguish between largely genetic influenced variations and variations due to phenotypic plasticity, and to further explore developmental instabilities in this fossil taxa. | de |