MORPHOLOGICAL HISTORY OF DEVONIAN-TRIASSIC AMMONOIDS IN TIME AND MORPHOSPACE

Ammonoids are icons of boom-bust evolution in the fossil record, making them ideal subjects for tracking effects of extinction and recovery on morphologic variation. Principal components analysis (PCA) permits quantification and expression of morphologic variation in empircial, multidimensional morphospace. PCA of 21 shell parameters (including geometry, sculpture, aperture shape and suture complexity) in 597 genera of L. Devonian - to L. Triassic ammonoids show that eight morphotypes were established by the L. Devonian. A ninth occurred briefly, among the earliest members of the Ammonoidea, but all others persisted through the Permian. With a few exceptions, morphotypes were not exclusive to particular clades; i.e., morphotype persistence was not just a product of phylogenetic longevity. Although extinction events severely condensed the range of morphologic variation and eliminated most taxa and morphotypes, in each instance the effects were short-lived. PCA of 322 Triassic genera (13 characters each) shows that the Paleozoic morphotypes were reinvented within 10 myr of the P/Tr. The range of morphologic disparity (measured as variance in the first six PC axes) is stable from the L. Devonian - U. Triassic when sample size is corrected for. Several points stand out in the record of variance: 1) a sharp decline in the L. Devonian (due to loss of early, openly coiled "experiments"); 2) a sharp rise in disparity driven by the U. Devonian radiation of clymeniids; 3) significant low disparity through most of the Carboniferous but increasing in the Permian; 4) sharp decline in disparity during the U. Permian/L. Triassic, followed by a gradual increase through the Triassic.

The persistence and repeated re-evolution after mass extinctions of the same morphotypes across five orders spanning ~210 myr presents a strong argument for adaptive optima within morphospace. But external and phylogenetic influences cannot be dismissed. Developments such as the loss of the morphologically diverse, dorsally siphuncled Clymeniida and the rise of the suturally serrate Ceratitida during the Late Permian changed the course of evolutionary history and altered the frequency of morphotypes through time.