PALEOGEOGRAPHIC SIGNIFICANCE OF MUSCLE SCARS IN GLOBAL POPULATIONS OF LATE CRETACEOUS OSTRACODES

Ostracodes are among the most useful groups of fossils for applications in paleogeography and plate tectonics, mainly because of their provincial distributions. Analyses by many workers of the distributions of some groups of ostracodes, typically cytherocopine taxa at the generic level, demonstrate that shallow marine ostracode faunas may differ markedly in coeval deposits, even within short geographic distances. These analyses indicate that many shallow marine taxa are unable to cross deep water barriers. An intimate relationship exists, therefore, between plate tectonics and biological evolution. This relationship can reveal clues not only about evolving plate tectonic configurations, but also about the phylogenetic relationships among taxa and the resulting taxonomy.

This study focuses on Late Cretaceous shallow marine ostracode genera. Whereas many of these genera are restricted paleogeographically, several, for example Brachycythere, Veenia, Curfsina, Mosaeleberis and Aysegulina (known formerly as Limburgina) among others, are reported to occur on widely separated continents in spite of widely recognized faunal barriers. Brachycythere and Curfsina have been described from such widely separate localities as North America, Europe, India and Australia; yet despite their external similarity, the muscle scar patterns from each geographic region show systematic differences. Although soft parts are rarely preserved, the muscle scar pattern is determined by the configuration of the soft part morphology and is thus of great phylogenetic and taxonomic value. These geographic and phylogenetic patterns indicate at least two possibilities: either there was convergent evolution of the exterior morphology or the exterior morphology is a plesiomorphic state, whereas the muscle scar patterns are apomorphic. In either case, recognition of these clades as discrete taxa contributes to their usefulness in paleogeographic and plate tectonic studies.