Anatomy informs geology: Hydrodynamic dispersal of alligator bones, with implications for taphonomic interpretations of fossil deposits of crocodylians, dinosaurs, and other morphologically novel taxa

Abstract

Distinctive anatomical features of bones can influence not only how these structures perform in living animals but also the tendency of elements to be transported by flowing water after death. Such transport can be critical in the concentration of fossils from animals that live near freshwater habitats, providing important context for interpreting the composition of paleocommunities. Measurements of the tendency of flowing water to disperse skeletal elements have been collected for diverse taxa, including mammals, turtles, and birds. However, these extant models may not be entirely appropriate for many morphologically distinct extinct lineages, such as non‐avian dinosaurs. To expand the range of models available for evaluating the influence of hydrodynamic transport on the assembly of fossil deposits, we used a flow tank to measure the water speeds that disperse bones from a subadult American alligator (Alligator mississippiensis), with the skull and mandible tested in multiple starting orientations. Alligator bones are sorted into three main dispersal groups: early (vertebrae, most girdle elements), intermediate (ribs, most limb bones), and late (pubis, femur), with the skull and mandible varying between intermediate and late depending on orientation. Late dispersing elements tended to be heavy or very flat. These results can refine interpretations of the taphonomic context for deposits of fossil crocodylians and morphologically similar taxa (e.g., choristoderes, phytosaurs) and provide an additional comparative model for deposits of non‐avian dinosaurs. Moreover, variation in hydrodynamic sorting across lineages highlights how distinctive anatomical features can influence the concentration of fossils, shaping understanding of assemblage composition and paleofaunal evolution.

Variation in the sacrum of phytosaurs: New evidence from a partial skeleton of Machaeroprosopus mccauleyi

“…PEFO 31219 includes a complete skull, partial mandibles (Ballew, 1989), 22 presacral vertebrae, 3 sacral vertebrae, and 6 caudal vertebrae, pelvic elements, a complete left femur, osteoderms, and ribs. Assuming a pattern of hydrodynamic dispersal for bone elements similar to what has been demonstrated in modern crocodylians (Blob et al., 2022), it appears that little hydrodynamic dispersion of elements took place after death. The absence of most of the limb elements and the tail suggests the passage of some time for decay and the removal of these elements (de Araújo Júnior & da Silva Marinho, 2013; Syme & Salisbury, 2014; Toots, 1965), but not enough time for the skeleton to be completely disarticulated (Behrensmeyer, 1991).…”

Introduction to Dinosaurs: New Ideas from Old Bones papers honoring Peter Dodson

“…In this volume, Blob et al (2023), for example, explored the intersection between anatomy and depositional processes as it impacts the vertebrate fossil record. Using skeletal elements of the extant crocodilian, the American alligator ( Alligator mississippiensis ) they employed laboratory experiments to measure water speeds required to transport skeletal elements (Blob et al, 2023).…”

“…In this volume, Blob et al (2023), for example, explored the intersection between anatomy and depositional processes as it impacts the vertebrate fossil record. Using skeletal elements of the extant crocodilian, the American alligator ( Alligator mississippiensis ) they employed laboratory experiments to measure water speeds required to transport skeletal elements (Blob et al, 2023). From these experiments, they showed how features of anatomy have a role in the formation of the fossil record and the bias it can introduce to understanding faunal composition, ancient biodiversity, and evolution (Blob et al, 2023).…”