Untangling phylogenetic, geometric and ornamental imprints on Early Triassic ammonoid biogeography: a similarity-distance decay study

Ammonoids are diverse and widespread fossil, externally shelled cephalopods that flourished for more than 300 Myr before their total extinction 65 Ma ago. In spite of two centuries of intensive scientific studies, their mode(s) of life and long-distance dispersal abilities remain poorly known. Here, we address this by focusing on the latitudinal distribution of Early Triassic (approximately 250 Myr) ammonoids through similarity-distance decay analyses. We examine and compare rates of similarity-distance decay between various groups with respect to systematics, shell geometry and ornamentation to untangle phylogenetic, geometric and ornamental imprints on the observed biogeographical pattern. Our data do not support any phylogenetic and shell ornamentation influence, but rather demonstrate the significant effect of (sub-)adult shell geometry on the similaritydistance decay: most evolute morphs tend to have been more endemic than most involute forms. This contrasts with the classic hypothesis that long-distance ammonoid dispersal mainly occurred during the earliest planktonic stages, and thus that (sub-)adult morphological characteristics should not constrain large-scale biogeographical patterns of ammonoids. Although direct control by Sea Surface Temperature can be discarded, this result may indicate that at least some adult Triassic ammonoid morphs were skilled active swimmers capable of achieving long-distance migration, as observed for some present-day coleoid cephalopods.