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39915-AC8
Bridging the Biotic-Geologic Divide: Morphological Evolution in the Ordovician Trilobite Flexicalymene in its Temporal, Geographic, and Phylogenetic Context
The geological record of “deep time” serves as an informative natural laboratory for understanding patterns and processes of global change relevant the world of today. The exceptionally well-preserved Ordovician rocks of the Cincinnati region provide one such natural laboratory in which environmental changes apparent on timescales bridging the gap between the biological, measured in generations, and geological, measured in thousands and hundreds of thousands of years, are available for analysis. Recently a series of careful sedimentological, stratigraphic, and paleontological studies have provided the temporal and environmental context in which to study the detailed patterns of morphological change in well-preserved species lineages. The focus of this project has been to examine the temporal and environmental aspects of morphological variation in the trilobite genus Flexicalyamene, which is unusually abundant in rocks of the Cincinnatian Series.
This year has seen yielded three major successes. The first is the acceptance of publication of a paper in the extremely prestigious journal Evolution in which the pattern of morphological change in the eye position of Flexicalyame from the Kope Formation, stratigraphically low in the Cincinnatian Series is show to be paralleled by changes in the overall faunal composition based on a survey of all well preserved skeletonized taxa preserved in these rocks. As far as we are aware, this is the first time that as morphological trend within a species-lineage has been shown to track changes in ecology, both likely an indirect expression of changes in sea-level. We are confident that this paper is an important pioneer in further attempts to relate microevolutionary morphological change to changes in ancient ecological composition.
The second success is the acceptance of a paper on the evolutionary relationship between the subspecies Flexicalymene retrorsa retrosa and F. retrorsa minuens. This paper will be published in Evolution and Development, a major journal in the field. Flexicalymene retrorsa minuens from the uppermost 3 meters of the Waynesville Formation of the Cincinnatian Series (Upper Ordovician) lived approximately 445 million years ago and exhibited marked reduction in maximum size relative to its stratigraphically subjacent sister subspecies, Flexicalymene retrorsa retrorsa. Phylogenetic analysis is consistent with the notion that F. retrorsa retrorsa, was the ancestor of F. retrorsa minuens. Flexicalymene retrorsa minuens has been claimed to differ from retrorsa retrorsa “in size alone”, and thus presents a plausible example of global paedomorphic evolution in trilobites. Despite strong similarity in the overall form of the two subspecies, F. retrorsa minuens is neither a dwarf nor a simple progenetic descendent of F. retrorsa retrorsa. More complex patterns of global heterochronic paedomorphosis, such as a neotonic decrease in the rate of progress along a common ontogenetic trajectory with respect to size, coupled with growth cessation at small size, “sequential” progenesis, or non-uniform changes in the rate of progress along a shared ontogenetic trajectory with respect to size, can also be rejected. Rather, differences between these subspecies reflect are more consistent with localized changes in rates of character development than with a global heterochronic modification of the ancestral ontogeny. The evolution of retrorsa minuens from retrorsa retrorsa was largely dominated by modifications of the development of characters already evident in the ancestral ontogeny, not by the origin of novel structures. Factors promoting size reduction in retrorsa minuens appear to have been specific to this subspecies, because other co-occurrent taxa, including other trilobite species, do not show marked differences in mean size.
The third success is concerns results from the Master's thesis of Autumn Thompson, who has conducted a detailed analysis of geographical variation in calymenid trilobites collected along a paleoenvironmental transect from shallower water in the southern outcrop area of northern Kentucky to deeper water in southwestern Indiana. This study concentrated on beds within a single parasequence, probably coincident in depositional age to within a few thousand years. What has emerged is a clear pattern of depth-related morphological variation, in which individuals of Flexicalymene from individual localities cluster along a spatial trend that is clearly related to water depth. This trend relates to various features, most markedly the position of the eye, and mimics the temporal trend witnessed throughout the series as a whole. Therefore, morphological change within Flexicalymene can be linked directly to temporal change through the series. Indeed, the results of this study suggest that eye position can be used as a “barometer” of paleodepth along the ancient shelf. We are now exploring whether we can use this material for an analysis of seafloor temperatures across the ancient shelf.
