A fast start: Evidence of rapid growth in Trialestes romeri, an early Crocodylomorpha from the Upper Triassic continental beds of Argentina based on osteohistological analyses.

Abstract

The palaeobiology aspects of early Crocodylomorpha during their origin in the Triassic are poorly known, despite occupying an important palaeoecological role in continental environments. In this study, we report the microstructural features of appendicular bones of two specimens of Trialestes romeri, a non-Crocodyliformes Crocodylomorpha from the Upper Triassic from the NW of Argentina. Our goals are to infer aspects of life history (i.e. age estimation and maturity events), inter-elemental variation and the growth dynamics within a phylogenetic framework. The samples include the humerus and the ulna of the holotype (PVL 2561) and the humerus and the femur of a referred specimen (PVL 3889). All elements are mostly composed by the fibrolamellar complex with a variable distribution of parallel-fibred bone in their cortex. Furthermore, they possess a uniform and homogeneous vascularisation, formed mainly by a laminar pattern. The humerus of the PVL 2561 records two lines of arrested growth (LAGs) and no cyclical growth marks were registered in the ulna. Meanwhile, the humerus and the femur of PVL 3889 exhibit a single LAG. Hence, a slight disparate inter-elemental variation is reported in both individuals. The absence of an external fundamental system (EFS) and a homogenous distribution of the bone matrix and vascularisation suggest that both specimens did not attain sexual and somatic maturity. Overall, the histological features of T. romeri indicate a rapid growth rate just like other early crocodylomorphs (Terrestrisuchus, Saltoposuchus), whereas others possess a rather moderate (Crocodylomorpha indet. BP/1/8484 specimen) or slower (Hesperosuchus) growth rate. These findings reveal that rapid growth rates were widely present among early crocodylomorphs and their early occurrence in the evolutionary history of Crocodylomorpha might suggest that it is the plesiomorphic condition.