MORPHOMETRIC EVOLUTION OF PALEOZOIC BRACHIOPODS - THE EFFECTS OF ENVIRONMENT AND ECOLOGICAL INTERACTIONS ON SHELL MORPHOLOGY

Abstract

Atrypida (Brachiopoda, Rhynchonellata) are ancient marine invertebrate fossils that are well preserved, abundant and diverse in the mid-Paleozoic (ecological evolutionary unit (EEU) P3). Atrypides thus serve as useful tools for morphological shape study. While qualitative studies have been performed for solving problems in brachiopod taxonomy, phylogeny, evolution and ecology, quantitative studies have been lacking.

After qualitative taxonomic identifications, morphological shape of the Silurian-Devonian Eastern North American atrypids was analyzed using geometric morphometrics, placing the results in evolutionary and ecological perspectives. Landmark analysis was performed on dorsal valves, ventral valves, anterior and posterior regions. These data were used for comparison with morphological distances, calculated as pairwise Procrustes distances. Evolutionary divergence times were determined from atrypide phylogeny and from stratigraphic zonations in the Michigan Traverse Group. Maximum likelihood tests were performed to determine evolutionary rates and modes of morphological divergence within and between genera. Episkeletobionts on hosts were examined to determine how morphological variation affected these assemblages.

Morphometric results suggest that morphological distances within each genus was as large as those observed between genera. Thus, referring individuals to a particular genus, based on shell shape alone is challenging. Diversifying selection has been acting on morphological divergence of these generic pairs and morphometric distances are consistent with the prior phylogenetic arrangement. Short term changes observed in individual lineages, that gets averaged out in time when compared to other genera, together with considerable morphological overlap observed between genera in P3 EEU, suggests a pattern consistent with stasis, as expected in EEUs.

The Traverse Group atrypid species lineage, which represents a 5 m.y. time span, exhibited lightly constrained morphological evolution, reflecting a stasis-like pattern. Greater encrustation preferences on one of two morphotypes on the dorsal valve and posterior region, suggest greater surface area facilitated by relatively deeper profile hosts in their reclining life orientation. Overall, the integration of various paleontological datasets reveal that the seemingly homogenous group of atrypide brachiopods exhibit subtle but significant evolution in their shell shape that is correlated with several kinds of ecological differences.