Trilobites and Cladistics

Statistical analysis of trilobite size over geological time: Sample size N > 2100 from Paleotological Database. The light boxes are quartiles, with tails showing overall range; sensu (Bell, et al., 2012). Note that y-axis is log scale. Bell concludes that trilobita adheres poorly to Cope’s rule. That is, while trilobites exhibit a fast size increase from the lower Cambrian to middle Cambrian, as well as into the Ordovician, size decreases thereafter. There is a gradual size decline through the Silurian. An initial size increase in the early Devonian is attributed to evolution of spiny exoskeletons in some species (Chatterton, 2006), noting also that the Frasnian/Famennian mass extinction removed all predatory species among orders Phacopida and Lichida, leaving generally more diminutive trilobites through the Carboniferous and into the Permian, when trilobites all went extinct.