Flexicalymene (Trilobita) from the Cincinnatian Series (Upper Ordovician) of Ohio, Indiana, and Kentucky: A case study of microevolutionary pattern within a single species lineage in a sequence stratigraphic framework.

摘要

Deposits in the Cincinnatian Series are known world wide for their abundant and exceptionally preserved fossils. Flexicalymene (Trilobita) is well represented throughout the Series, in which it is known to show marked morphological variation. Recent advances in sequence stratigraphy have placed these fossils within a well-constrained temporal and spatial framework. The combination of these parameters has provided an excellent opportunity to study morphological variation of an individual species lineage over a variety of temporal and spatial scales, facilitating the exploration of links between morphology, environment, and time in order to test proposed theoretical models of evolutionary patterns within a paleo-shelf setting.; Sedimentological and taphonomic evidence suggest that event beds within butter shales of the Cincinnatian Series represent stacked components that correspond to different phases of the same depositional process, analogous with distal mud turbidites which form under similar flow dynamics associated with the deposition of fine-grained sediments during a mud-laden gravity flow below storm-wave base. These assemblages have high spatial and compositional fidelity, and represent an exceptional trilobite record at the highest taphonomic and temporal resolution available within the Cincinnatian Series.; Disparate patterns of morphological change in cranidial character complexes of Flexicalymene granulosa, in combination with transformations in the pattern of morphological change within the same character complex through time, presents a multifaceted picture of microevolution within a trilobite lineage. Several evolutionary patterns, such as gradualism, stasis, and punctuation, are represented. In contrast to other species of Flexicalymene, morphological changes in Flexicalymene minuens are associated with an allometric repatterning of its ancestors ontogeny in conjunction with a dramatic size decrease.; It is through the multivariate analysis of shape, integrated with the documentation of changes in discrete morphological characters that the subtle complexities of morphological variation and change through time can be documented. The overall result of approximately 6.5 million years of evolution is remarkable stability and a lack of innovation in form. Nevertheless, the evolution of Cincinnatian Flexicalymene attests to the diversity of evolutionary modes and patterns in one of the most tightly constrained study systems for morphological evolution available in the fossil record.