The History of Trilobite Size Evolution

Macroevolutionary studies of body size evolution are becoming increasingly important in both fossil and extant groups, allowing for a much greater understanding of the history of life. Trilobites are an ideal group to examine large-scale patterns of size change; appearing in the Lower Cambrian, they radiated to their peak in diversity (~17,000 species), in the Upper Cambrian / Lower Ordovician, then began a steady decline punctuated by a second peak in diversity in the Early-Middle Devonian. The range of adult trilobite size has been well documented (varying from three to seven hundred millimetres), however only a few studies have attempted to quantify the distribution of trilobite body-size.

A database was constructed on the dimensions of specimens from museum and literature sources. Including more than 12,000 individuals; this represents the taxonomic, temporal and spatial range of the group. Comparisons with the Sepkoski database suggest that maximum and range size are positively correlated with generic diversity. Although the patterns within individual orders vary from the entire class, with the Redlichiida and Corynexochida displaying a negative correlation.

Finally, Kruskal-Wallis testing shows that trilobites undergo significant size shifts in the Cambrian, Ordovician and the Early Devonian with none in the later part of their history. The Early Devonian peak in size and diversity may in part be linked to increased predation pressure from vertebrate and ammonoid groups. Resampling of the dataset through rarefaction suggests that the patterns observed here is not a result of a sampling bias.