Anatomical nomenclature of fossil vertebrates: standardized terms or ‘lingua franca’?

Abstract Whereas the anatomy of birds, domesticated mammals, and humans is described by standardized terminology, the anatomy of most fossil vertebrates is described by nonstandardized terminology. New fossil discoveries increasingly resolve the transitions between these living groups and their fossil outgroups, diminishing morphological differences between them, and vertebrate paleontologists can easily apply more than one system of anatomical terms to such groups. This plurality of systems has led to recent proposals to standardize anatomical terminology for tetrapods, either by applying avian and mammalian anatomical terminology to their respective stem groups (Sauropsida and Synapsida) or by creating an all-encompassing terminology for Tetrapoda from a combination of existing terminologies. The main rationale for implementing standardized anatomical terminology, which requires abandoning competing terminologies, is that it reflects homology and evolutionary descent, eliminates ambiguity, and enhances interdisciplinary communication. The proposed standardized anatomical terminology, however, entails many negative consequences, including reversing character trajectories, misrepresenting complex anatomical transformations and uncertain homologies, and requiring far-reaching terminological conversions. These negative consequences result from increasing the taxonomic scope of standardized anatomical terms that were developed for a specific group, but now: (1) apply to a broader hierarchy of character states; (2) involve additional phylogenetic interpretations or assumptions; and (3) are used for basal, often more generalized conditions. In contrast, traditional non-standardized anatomical terminology, although not strictly phylogenetic, is anatomical ‘lingua franca’ that has been in usage for nearly two centuries and is consistent, ubiquitous, and descriptive.