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Abstract
Cope's rule defines lineages that trend towards an increase in body size through geological time. The trilobite family Asaphidae is one of the most diverse of the class Trilobita and ranges from the Upper Cambrian through to the Upper Ordovician. The group is one trilobite clades that displays a large size range and contains several of the largest trilobite species. Reduced major axis correlations between the lengths of cephala and pygidia and the total sagittal length of complete individuals have high support and were used to standardise all incomplete specimens to total axial length. Phylogenetic studies into Cope's rule tend to use supertrees, composite trees or a single tree selected through a fit criterion. Here, for the first time, all trees recovered from a maximum parsimony analysis were analysed equally. Maximum likelihood was used to fit four evolutionary models: random walk, directional, Ornstein–Uhlenbeck (evolution towards an adaptive optimum) and stasis. These were compared equally using Akaike weights. Fitting of evolutionary models by maximum likelihood supports stasis as consistently the most likely model across all trees with low support for directionality.
Acknowledgements
Claire Mellish and Andrew Ross (NHM), Sarah Stewart (NSM), Tim Ewin (BCM), Jeff Liston (GLAHM), Matt Riley (BGS), Jan Bergström (NRM), Petr Budil (CGS) and Vojtěch Turek (NM) are thanked for giving access to specimens in their care. Richard Fortey, David Jablonski, Graeme Lloyd, Manabu Sakamoto and Shawn Whiteman are thanked for discussion and advice. Alistair McGowan, Alan Owen, Nigel Hughes and Claude Monnet are thanked for their comments that greatly improved the manuscript. This work was funded by Natural Environment Research Council grant NER/S/A/2005/13593.
