Integrated microanatomy and microstructure of the maxillary tooth plate reveal a reinforced feeding system and tougher diet in Late Triassic Hyperodapedontinae (Rhynchosauria, Archosauromorpha)

Hyperodapedontine rhynchosaurs possessed a unique oral apparatus, long interpreted as an adaptation for processing abrasive and resistant plant material. However, the microanatomical and histological evidence supporting this interpretation remains poorly documented. Here, we investigate the maxillary tooth plates and dentition of Brazilian Late Triassic hyperodapedontines across ontogenetic stages using histological thin sections, scanning electron microscopy, and high-resolution computed microtomography, and compare them with Middle Triassic rhynchosaurs to assess functional implications. Hyperodapedontine exhibits clear dental adaptations for processing harder foods, including accelerated secondary dentin deposition and three special dental features (serrations, beaded edges, and flutes), which together enhance tooth functionality under intense wear. Patterns of secondary dentin deposition indicate a dietary shift from juveniles to adults, with adults exhibiting more structurally reinforced teeth. Osteohistology of the maxilla reveals an inner and mid cortex composed of woven-fibered bone and woven-parallel complexes undergoing extensive remodeling. The outer cortex on the occlusal surface is mainly composed of an avascular lamellar bone, except for the longitudinal groove floor and walls, which are formed by a nearly avascular parallel-fibered bone. The presence of an avascular to nearly avascular bone layer in the occlusal surface is consistent with a protective role against occlusal loading stress. Variation in the thickness of the parallel-fibered bone lining the longitudinal groove reveals that the medial region was structurally reinforced, indicating that the food breakdown would primarily occur in this portion of the maxillary groove-dentary blade occlusion system. Together, these characteristics reinforce previous hypotheses that Late Triassic Hyperodapedontinae were better adapted for processing tougher food resources than Middle Triassic rhynchosaurs.