25-5 Quantifying variation in Middle Cambrian trilobite species from Utah

Session: Refining the Cambrian—Biotas, Multiproxy Correlations, Workable Global Divisions, and Paleogeography: Discussions in Honor of Fred Sundberg

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Trilobites were a key component of the evolutionary radiations within Bilateria that occurred during the Cambrian period, and questions remain about the levels of morphological variation within trilobite species in the aftermath of the Cambrian radiation event. We used geometric morphometrics to study cranidial shape and quantify how it varied across time, space, and preservational style in well-preserved specimens of four abundant and morphologically well-characterized species of trilobites from the Middle Cambrian of Utah: Asaphiscus wheeleri Meek, 1873; Bathyuriscus fimbriatus Robison, 1964; Modocia brevispina Robison, 1964; and Modocia laevinucha Robison, 1964. Most of the specimens analyzed were collected by the internationally renowned Gunther family and are now housed at the Division of Invertebrate Paleontology, Biodiversity Institute, University of Kansas. Among key findings for various species, evidence for stasis over millions of years was uncovered, showing that even during this time of putatively high rates of evolution, within species evolution conforms to the punctuated equilibria model. The effect of preservational style on quantitative morphology was also examined, and specimens of the same species preserved under different preservational regimes were still found to group together. This highlights that geometric morphometric approaches can be successfully applied to Cambrian trilobites even when there are differences in preservation. Additionally, M. brevispina and M. laevinucha showed different patterns of variation in specific cranidial elements, although overall shape differences between the two species were not significant. In A. wheeleri, we identified two clusters that had not previously been identified by standard taxonomic approaches, indicating some evidence for intraspecific variation. We also observed a subtle shape difference in B. fimbriatus from different localities, comprising geographic patterns of intraspecific variation across close but distinct regions within modern-day Utah. Overall, this quantitative analysis of cranidial shape in four Middle Cambrian trilobite species provides new insight into their variation patterns, including evidence for stasis and geographic variation, and contributes to our understanding of trilobite evolution shortly after the classic Cambrian radiation interval.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10642

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