The Contribution of Morphological Variation to Stratigraphic Longevity in Cambrian Trilobite Species

Numerous studies have shown that species duration is positively correlated with various aspects of species ecology. In particular, greater geographic range size may increase species longevity, either directly by decreasing the impact of local extinction or indirectly if factors that promote wide geographic ranges, such as dispersal ability or greater environmental tolerance, also increase survivorship. Morphological variation might also contribute to the range of environmental conditions a species can tolerate, increasing its likelihood of surviving environmental perturbations. Conversely, greater morphological variation may increase a species' ability to respond to local selection pressure, increasing the likelihood of divergence between populations. The degree of within-species variation would then be expected to correlate with rates of morphological evolution.

Despite the potential importance of morphological variation in structuring evolutionary patterns, spatially and temporally, little is known about how relative differences in variation across taxa contribute to differences in duration.

This gap in our understanding is ameliorated by comparing pairs of closely related trilobite species from the Steptoean (Furongian, Cambrian) of the Great Basin, USA. Relative durations are estimated using a composite section of continent-wide (Laurentian) occurrences of Steptoean taxa based on correlation using constrained optimization, but are not included in the correlation itself. Variation is assessed using geometric morphometric analysis of fossil material preserved in limestone, some silicified. Comparisons of cogeneric species with similar geographic range sizes but different durations (and vice versa) will allow an initial assessment of how morphological variation contributes to trilobite survivorship.