DOES ECOLOGY IMITATE MORPHOLOGY AND TAXONOMY? CAMBRIAN-DEVONIAN TRILOBITES SUGGEST NO!

Trilobites rank among the most diverse and celebrated groups of animals in the fossil record, and their taxonomic and morphological diversity histories are well studied. However, our knowledge of large-scale trends in trilobite ecology primarily relies on the assumption that these proxies also reflect trends in ecology. For example, it is widely claimed that the Ordovician diversification of trilobites was intimately connected to a proliferation of novel trilobite life habits because of the increased morphological diversity during that period. However, such claims have not been tested directly using a robust analytical framework. Furthermore, it remains unclear to what extent these two iconic measures (taxonomic and morphological diversity) actually capture the ecological diversity of trilobites, among other animals.

This study uses an ecospace framework to represent life habits of ~500 Cambrian-Devonian trilobite genera. Ecological diversity is measured using life habit richness (number of unique life habits) and several metrics of disparity (mean Euclidean distance, maximum range, and convex hull area and volume.) Ecological diversity is compared to taxonomic richness and two measures of morphological disparity from other data sets. Morphological data comes from Foote (1993), who used elliptical Fourier analysis of trilobite cranidia to represent morphology. Paleobiology Database data confirm taxonomic coverage represents broader trilobite taxonomic trends.

The trend in life habit richness is distinct from trends in taxonomic richness and morphological disparity (and even ecological disparity), with peak richness in the Ordovician. Morphological and ecological disparity trends are also distinct, with morphological disparity increasing significantly during the Cambrian and maintaining relatively high levels throughout the Ordovician and Silurian before diminishing. In contrast, both measures of ecological disparity are greatest during the Cambrian and Ordovician, decreasing through the Silurian and Devonian, although the trend in mean Euclidean distance is noisier than that for total range. These analyses suggest that neither taxonomic nor morphological approaches adequately capture important changes in trilobite ecology.