Filling a key gap in growth patterns of Pseudosuchia through the osteohistology of <i>Dynamosuchus collisensis</i> (Ornithosuchidae: Archosauria)

Abstract Ornithosuchidae are among the least studied pseudosuchians from a histological perspective. We present the first long-bone osteohistological data for the ornithosuchid Dynamosuchus collisensis, based on mid-diaphyseal thin sections of the humerus and femur, supplemented by a rib. The long bones show cortices dominated by highly vascularized woven–parallel complex and parallel-fibred bone, with three well-defined annuli and a fourth incipient one near the periosteal margin. Medullary expansion occurs through endosteal resorption, and secondary osteons in formation are present. The rib displays lower vascularization, suggesting reduced growth rates in axial elements. Open neurocentral sutures, the absence of an EFS and the lack of peripheral slowdown in cortical deposition indicate that the holotype was skeletally immature, and probably sexually immature, at the time of death. This suggests that the individual had not yet reached its maximum body size and would probably have attained a larger size, consistent with a role as a top-tier predator in Carnian ecosystems. These features support sustained rapid growth, contrasting with the predominantly slow growth of late-diverging crocodylomorphs and more closely resembling early loricatans and poposauroids. Because Ornithosuchidae occupy an early-diverging position within Pseudosuchia, these data support the hypothesis that fast growth may represent a plesiomorphic condition within the clade.