Osteohistological evidence for terrestrial adaptation and an extended growth period in mekosuchine crocodylians and its potential role in their extinction

The Australasian crocodylian fossil record is dominated by the extinct endemic clade Mekosuchinae, with known members spanning the Eocene-to-Holocene. This diverse clade exhibited high morphological disparity, from early members superficially similar to Crocodylus, to more terrestrial forms later in the clade's evolutionary history. However, it remains unclear why mekosuchines went extinct whilst Crocodylus continues to thrive in the region, and how the ancestors of this terrestrial clade reached Australasia without long-distance transoceanic dispersal. One possible explanation is that there was intraclade variation in the growth pattern and life habit of mekosuchine species, and that these, in turn, differ from those of Crocodylus. Here, we present the first osteohistological study of mekosuchines, evaluating the femora of the middle-late Eocene species Kambara taraina and late Oligocene-middle Miocene Baru (B. wickeni, B. darrowi). Bone compactness profiles are more characteristic of a terrestrial habit than those of extant crocodylians, with Baru darrowi demonstrating the most terrestrialized profile. Apposition rates are similar in absolute value to those of extant crocodylians, indicating a comparable metabolism; however, high appositional rates throughout mid-cortical cyclical growth marks suggest that mekosuchines were able (or needed) to maintain a moderate rate of growth for longer than extant crocodylians. Our results indicate that mekosuchines were more terrestrial than Crocodylus and that later-diverging mekosuchines showed increased terrestrialization; however, they do not indicate that early mekosuchines were well-adapted to transoceanic dispersal. Heightened terrestrialization and an extended moderate growth period might have made mekosuchines more vulnerable to late Cenozoic aridification in Australia than semi-aquatic Crocodylus.