Evolution of growth strategy in alligators and caimans informed by osteohistology of the late Eocene early-diverging alligatoroid crocodylian Diplocynodon hantoniensis.

Abstract

Among living crocodylians, alligatoroids exhibit a wide range of body sizes and a biogeographic distribution that spans tropical-to-subtropical climates. The fossil record of alligatoroids, however, reveals even greater diversity, including multiple examples of gigantism and a broader distribution that extends into polar latitudes. Osteohistological studies on extant alligatoroids show that living alligators and caimans both exhibit seasonal growth, with roughly comparable growth rates. However, alligators and caimans diverged from one another over 60 million years ago; the dearth of studies on extinct alligatoroids makes it unclear if the shared condition in extant taxa reflects convergent responses to rapid climatic changes in the recent past or represents the ancestral condition in alligatoroids. Additionally, sample sizes are often limited to one or two individuals, especially in extinct crocodylians, obscuring any intraspecific variation present. To address this uncertainty, we conducted the largest monospecific osteohistological study of an extinct crocodylian to date, based on a sample of nine femora, providing unique insight into the intraspecific variation in growth of the early-diverging alligatoroid Diplocynodon hantoniensis from the late Eocene of the UK. The bone microanatomy of D. hantoniensis shows moderate compactness, with a well-defined medullary cavity, and osteohistological features that are generally consistent with those of extant alligatoroids. Samples vary greatly along a continuum in the degree of remodelling and vascularity, highlighting both the importance of evaluating intraspecific variation and limitations of basing histological assessments on singleton samples. Ontogenetic assessment indicates that our sample captures a range of skeletally immature to mature individuals, approximately corresponding to femoral size, but with notable exceptions possibly driven by sexual dimorphism. Body size estimates for D. hantoniensis (1.2-3.4 m) fall within the typical range of living American alligators (Alligator mississippiensis). Reconstruction from cyclical growth marks indicates a similar overall growth rate between D. hantoniensis and A. mississippiensis. As in extant alligatoroids more generally, this is determinate, seasonally-controlled growth. Femoral circumference scales positively with femoral length in D. hantoniensis, demonstrating similar allometry to A. mississippiensis. This differs from some other extant crocodylians (e.g. Crocodylus niloticus and Crocodylus johnstoni) and suggests conservation of allometric relationships in alligatoroids. This in-depth look into an early diverging alligatoroid indicates that seasonality and growth rates present in extant members were established near the base of the clade. Furthermore, it highlights the importance of including larger samples of singular species in order to capture potential variation when making clade-wide interpretations.