From Newsgroup: sci.bio.paleontology

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 rCyexopariarCO. 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.

Open Access:
https://onlinelibrary.wiley.com/doi/10.1111/joa.14242
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