Evolutionary and developmental patterns of Cretaceous uncoiled ammonites

点击翻译按钮获取中文摘要

The heteromorph ammonoids (i.e., ammonoids that deviate in their coiling from the planispiral, regular coiling) belong to the most enigmatic fossil groups, but their origin, palaeobiology, and functional morphology is still a matter of debate. Little is known about possible reasons why ammonoid iteratively deviated from normal coiling and instead realise a putatively disadvantageous conch construction with respect to buoyancy control and locomotion. Furthermore little is known why they became successful clades in terms of taxonomic diversity and morphological disparity especially during the Cretaceous. Within the proposed project, these uncoiled ammonoids will be analysed with respect to their morphospace occupation by the use of relevant characters such as conch geometry, shell ornamentation, and suture line. Selected taxa (depending on availability of material) from the Cretaceous will be studied including possible sexual dimorphism, intraspecific variability, and variation in ontogenetic traits. Their wide geographical distribution and high abundances, fast evolutionary rates, and the preservation of the entire ontogeny due to accretionary shell growth made especially Mesozoic ammonoids a perfect fossil group to study evolutionary scenarios with respect to palaeoecological perturbations. The main goal of this project is (1) to test various hypotheses of the origin of Cretaceous heteromorph ammonites, (2) to evaluate the species and genus concepts as well as validity of higher taxonomic entities and their phylogenetic relationships, (3) to investigate palaeoecological parameters and their possible influence on the conch geometry, and (4) to compare diversity and disparity patterns with respect to ontogenetic traits. Here we make a first attempt to analyse the intraspecific variability of Cretaceous heteromorph ammonites and discuss them with respect to their deviation in coiling and conch form from monomorph ammonoids. With this study we will improve the understanding of the phylogenetic relationships, palaeobiology, palaeoecology, and stratigraphic range of the Cretaceous uncoiled ammonites. The Cretaceous period is characterised by transgressions/regressions, warming/cooling events and oceanic anoxic events which made it ideal to study the appearance and distribution of specific heteromorph shell morphologies and hopefully detected characteristic patterns. In comparison with normally coiled ammonites from contemporaneous deposits the influence of distinct palaeoenvironmental changes (water depth, temperature, oxygen) and their effect on the heteromorph shell geometry can be tested.