RATES AND PATTERNS OF DIVERSIFICATION IN THE HISTORY OF ANIMAL SKELETONS

A major feature of the fossil record is the rapid proliferation of anatomical innovations early in the evolutionary radiation of major taxa. Consisting of 182 possible pairwise character combinations of skeletal design elements, the multivariate ‘skeleton space’ not only describes the range of constructional features present in animal skeletons, such as shells, tusks, and bones, but may also provide insight into the rates and patterns of morphologic evolution. Preliminary estimates show that Recent animals exploit between 170–176 skeletal designs, out of a total of 181 options achieved by all animals living and extinct. Remarkably, nearly all of these designs (n=174; 96.1%) were established by the end of the Cambrian Period; Cambrian disparity therefore reached or surpassed modern levels despite having evolved relatively early in the Phanerozoic Eon. In contrast, post-Cambrian expansion was minor: five hundred million years of subsequent taxonomic and ecologic diversification yielded at most seven additional options, primarily involving the evolution of three-dimensional machine parts and the rise of vertebrates. These include the first appearances of flexible solids in the form of intervertebral discs in Middle Ordovician jawed fishes; one element and remodeled solids in the form of belemnite guards and articulating limb bones of primitive amphibians (e.g., the tarsals of Ichthyostega), respectively, in the Devonian Period; and two element solids in the form of paired horn-like outgrowths adorning the crania of several species of Permian mammal-like reptiles. At least five skeletal designs are apparently no longer exploited, with perhaps two (40%) of these disappearing during intervals of mass extinction. The filling of occupied skeleton space by the end of the Paleozoic Era and lack of any obvious post-Paleozoic expansion challenges the conventional “cone of increasing diversity” through geologic time and suggests unusual morphologic flexibility followed by constraint in the evolution of animal skeletons.