Palaeobiogeograhic relationships of Early Carboniferous ammonoid assemblages in North America
Final Report Abstract
Carboniferous ammonoid taxonomy, stratigraphy, and biogeography with focus on the Late Early Carboniferous (Visean) assemblage were studied in sections of the Ouachita Foreland Basin (American Midcontinent) and the Antler Foreland Basin (Western United States). A monograph of the early Late Visean ammonoid assemblages of the Antler Foreland Basin (West-Central Utah, East-Central Nevada), which is currently being completed, will contain 16 species of the genera Bollandoceras, Calygirtyoceras, Girtyoceras, Dimorphoceras, Glyphiolobus, Kazakhoceras, Goniatites, Entogonites, and Praedaraelites. These taxa were discovered from sections, in which eight successive ammonoid horizons could be separated, all of which contain well-preserved faunas mainly characterised by species of Goniatites and girtyoceratids. The successive Visean ammonoid faunas from Arkansas/Oklahoma on the eastern side and Utah/Nevada on the western side of the North American Craton are, despite some differences, closely related. Differences are mainly present in the diversity, with the western occurrences being much more diverse in genera and species. Further differences can be recognised in the temporal distribution of the ammonoid occurrences. Late Asbian ammonoids are diverse in the Antler Foreland Basin, but rare, or absent in Arkansas. The ammonoid faunas of the Antler Foreland Basin occupy, in a global context, a unique position. The use of genus occurrences in the most important palaeogeographic units in a cluster analysis of the late Asbian occurrences shows that the Antler Foreland Basin is very distinct from the occurrences of the North Variscan Realm (Rhenish Mountains, North England, and South-West Portugal, which are closely related to each others) and also to the North African occurrences. For an interpretation of this pattern, further data have to be included, for instance the presence or absence of species in the various regions, or the morphology of species of distinct genera. As data are analysed, it is clear that differences between the regions with rich late Early Carboniferous occur become more pronounced. The ongoing study shows that Brigantian and Serpukhovian faunas from the Antler Foreland Basin are even more different than those in Europe and North Africa than in the Asbian. In this project, genus-level palaeobiogeography will be analysed using distinct morphological characters occurring in the genera common most regions. For this purpose, several analytical methods have to be developed for ammonoid morphometrics, (1) the quantitative analysis of the ontogenetic development of conch geometry, (2) Fast Fourier Transform (FFT) techniques for analysing conch shapes, and (3) landmark and semilandmark analysis of morphological characters such as the suture line.
