Osteohistology of the large-sized Cretaceous crocodylomorph Stratiotosuchus maxhechti (Notosuchia, Baurusuchidae) indicates fast growth and niche partitioning with medium-sized theropods

Abstract

Among the wide diversity of notosuchian crocodyliforms, Baurusuchidae is remarkable by their morphology, indicating a terrestrial hypercarnivory habit during the Late Cretaceous of Gondwana. Commonly, amongst baurusuchids, the anatomy of the large-sized Stratiotosuchus maxhechti shows theropod mimic features in its skull and limb bones. Such similarities supported the hypothesis of competitive exclusion of medium to large-sized theropods by baurusuchids. However, a recent taphonomical approach claims that other lines of evidence should be considered (e.g., biomechanics); due to the biased fossil record found in the Bauru Group. This approach supported niche partitioning between theropods and baurusuchids instead of competitive exclusion. Here we performed a paleohistological analysis on a baurusuchid (S. maxhechti) to evaluate if the terrestrial ecology implies fast growth and if the microstructure is similar to what is found in theropods, with insights into a niche competition hypothesis. The samples were taken from one specimen with two femora and another one comprising the left ulna and tibia. All the four samples show the presence of fibrolamellar bone complex arranged in a cyclical growth. The growth zones are composed of woven matrix that gradually turns into a parallel-fibered bone, and is followed by lines of arrested growth (LAGs). There is also the presence of simple and anastomosed vascular canals. Compared to living crocodylians the growth shows a higher depositional rate and a more complex organization pattern. In this respect, the growth dynamics are like medium-to-large theropods. Stratiotosuchus lacks an External Fundamental System (EFS), which indicates that the material studied here represents individuals that died before reaching their maximum size, suggesting that they could take more than twelve years to to be fully grown. Comparing the body mass estimates by femoral circumference (∼178–180 kg) and the bone microstructure with theropods, they share a fast growth pattern by showing fibrolamellar bone complex. However, the body size diversity in both groups favors the niche partitioning hypothesis over the suggestion of competitive exclusion between baurusuchids and theropods.