Skull morphology and histology indicate the presence of an unexpected buccal soft tissue structure in dinosaurs.

Abstract

Unlike mammals, reptiles typically lack large muscles and ligaments that connect the zygoma to the mandible. Dinosaur craniomandibular soft tissue reconstructions, often based on the rationale of extant phylogenetic bracketing, follow this general rule. However, descending flanges from the zygomata of hadrosaurs, heterodontosaurids, and psittacosaurids have been used to argue for a masseter-like muscle in these dinosaur taxa. We examined dinosauriform skulls for osteological indicators of connective tissue entheses on the zygoma and mandible, and subsequently sectioned 10 specimens for histological evidence. Osteological indicators were found on the zygoma in most sampled dinosauriforms, which range from rugosities to large descending processes, and morphologically resemble known muscular and ligamentous entheses. Similarly, rugose features oriented towards the zygoma were found on the mandible in sampled dinosauriforms, many having previously been interpreted as entheses for the adductor mandibulae muscle group. Serial histological sectioning of ceratopsid, hadrosaurid, and tyrannosaurid jugal and surangular rugosities reveals an external cortex rich in collagen fibres, strongly resembling entheseal fibres. Jugal entheseal fibres are usually oriented ventrally towards the surangular, and in hadrosaurids and tyrannosaurids these are parallel to macroscopic striations on the surfaces of the jugal flange. Histological sections of extant chicken buccal regions show similar entheseal fibres in the attachments of the jugomandibular ligament on the jugal and of the adductor musculature on the mandible. We hypothesise a strong connective tissue structure bridging the zygoma and mandible in dinosaurs, termed the 'exoparia'. This structure's size and proximity to the craniomandibular joint would be advantageous in stabilising the mandible relative to the cranium during jaw movement, particularly in dinosaurs thought to process their masticate. A ligamentous or muscular identity for the exoparia cannot be determined with the available data, but the size and shape of the zygomatic entheses in many dinosaurs are more consistent with a muscular attachment. Possible antecedents in non-dinosauriform archosaurs and derivations in modern birds may exist, but the homology of the exoparia is currently unknown. These results highlight the complex soft tissue evolution of dinosaurs and caution against simplified phylogenetic model-based approaches to tissue reconstruction that ignore contrasting osteological signals.