Tail Weaponry in Ankylosaurs and Glyptodonts: An Example of a Rare but Strongly Convergent Phenotype

The unusual clubbed tails of glyptodonts among mammals and ankylosaurines among dinosaurs most likely functioned as weapons of intraspecific combat or interspecific defense and are characterized by stiffening of the distal tail and, in some taxa, expansion of the distal tail tip. Although similarities in tail weaponry have been noted as a potential example of convergent evolution, this hypothesis has not been tested quantitatively, particularly with metrics that can distinguish convergence from long-term stasis, assess the relative strength of convergence, and identify potential constraints in the appearance of traits during the stepwise, independent evolution of these structures. Using recently developed metrics of convergence within a phylomorphospace framework, we document that convergence accounts for over 80% of the morphological evolution in traits associated with tail weaponry in ankylosaurs and glyptodonts. In addition, we find that ankylosaurs and glyptodonts shared an independently derived, yet constrained progression of traits correlated with the presence of a tail club, including stiffening of the distal tail as a precedent to expansion of the tail tip in both clades. Despite differences in the anatomical construction of the tail club linked to lineage-specific historical contingency, these lineages experienced pronounced, quantifiable convergent evolution, supporting hypotheses of functional constraints and shared selective pressures on the evolution of these distinctive weapons. Anat Rec, 303:988-998, 2020. © 2019 Wiley Periodicals, Inc.