Cope′s Rule and Phylogenetic Trends in the Nautiloidea

One of the most commonly reported patterns in the history of life is the tendency of body size to increase over geologic time. Known as Cope’s Rule, this pattern is traditionally depicted as anagenetic change within a single lineage towards large body size – the classic ladder-like progression from Hyracotherium to Equus among horses in the Cenozoic Era, for example. More recent arguments propose that many examples of Cope’s Rule may be better viewed as evolution from ancestral small body size, the lower limit of which is bounded by constraints in morphospace. These so-called ‘passive’ trends have found support across a wide variety of clades ranging from planktonic foraminifera to rodents. The present work explores long-term phylogenetic trends in the evolution of body size in coiled nautiloids of the order Nautilida, a long-lived stock of shelled mollusks which originated in the Devonian Period and represent almost all Mesozoic and Cenozoic nautiloids. Analysis of the fossil record indicates that the Nautilida evolved relatively large body sizes (measured here as shell diameter and simple geometric and allometric estimates of shell volume) early in their history and away from boundary conditions at the lower size limit. An expansion of total size range or variance occurred during the great Carboniferous radiation of superfamilies Tainocerataceae, Trigonocerataceae, Aipocerataceae, and Clydonautilaceae. Nautiloids were nearly extinguished by the terminal Triassic mass extinction; the lack of a vigorous post-extinction radiation of survivors coincided with a substantial contraction in variance during the Jurassic Period. Both minimum and maximum body size increased during the subsequent Cretaceous and Tertiary/Quaternary periods, which might suggest the presence of an ‘active’ trend – indeed, Cope’s Rule in the strict sense. Although more intensive sampling is required to confirm these patterns, it appears that no single overarching trend dominated the history of the Nautilida.