Paleontologists have proposed a previously unrecognized connective-tissue structure in the cheek region of some dinosaurs, naming it exoparia. It may have linked the cheekbone region to the rear of the lower jaw and helped steady the jaw as it moved. The evidence comes from fossil bone surfaces and microscopic sections, but the tissue has not been identified as either muscle or ligament. Exoparia is a scientific hypothesis, not a newly found intact soft-tissue fossil.
What is exoparia?
Exoparia is the name Henry S. Sharpe and colleagues gave to a proposed connective-tissue structure bridging the zygoma—the cheekbone region—and the posterior mandible, or rear part of the lower jaw. Their 2025 paper in the Journal of Anatomy describes it as a hypothesis based on attachment evidence in fossil bones.
The researchers propose that the structure connected the skull and lower jaw near the craniomandibular joint. They do not claim to have found a preserved strip of tissue or to have established exactly what kind of tissue it was.
How did researchers infer a soft-tissue structure from fossil bones?
They examined attachment areas on the bones
The team studied the jugal, a bone in the cheek region, and the surangular, a bone toward the rear of the lower jaw, in ceratopsids, hadrosaurids, and tyrannosaurids. Bone surfaces can preserve features associated with where soft tissues attached, even when the tissues themselves are gone.
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Microscopic sections revealed collagen-rich fibres
Serial histological sections—thin slices examined under a microscope—showed an outer bone cortex rich in collagen fibres. The researchers compared this pattern with entheses, the areas where muscle, tendon, or ligament meets bone. In some specimens, the orientation of fibres in the fossil material also corresponded to visible striations on jugal flanges, or projections of the cheekbone.
Living birds provided a comparison, not a direct match
The team compared the fossil evidence with cheek and jaw attachments in chickens. Such comparisons help researchers interpret fossils, but a living bird is not a complete model for every dinosaur. The proposed exoparia is an inference from several kinds of evidence: bone morphology, microscopic attachment fibres, and comparison with a living animal.
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Was exoparia a muscle or a ligament?
That remains unknown. Sharpe and colleagues state that “a ligamentous or muscular identity for the exoparia cannot be determined with the available data.” The evidence supports a proposed connective-tissue attachment, but it does not resolve whether the structure was ligamentous, muscular, or otherwise organized.
These are different possibilities, not two confirmed descriptions of the same feature. A reconstruction that labels exoparia definitively as a muscle—or definitively as a ligament—would go beyond what the paper establishes.
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What might it have done?
Because the proposed structure lay near the joint between the skull and lower jaw, it may have helped stabilize the mandible against the cranium during jaw movement. The paper suggests that this kind of support would have been especially useful for dinosaurs that processed food by chewing. Its precise role is still unresolved; jaw stabilization is a proposed function, not a directly observed action.
Does this mean dinosaurs had cheeks?
Exoparia concerns connective tissue in the cheek region; it does not establish that dinosaurs had fleshy cheeks like humans or other mammals. The discovery also does not reveal the complete external appearance of a dinosaur’s face. It adds a possible internal soft-tissue structure to anatomical reconstructions, while leaving broader questions about facial shape and coverings unanswered.
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Why could exoparia change dinosaur reconstructions?
Many reconstructions of dinosaur soft anatomy use the extant phylogenetic bracket: researchers infer features by comparing fossils with living relatives, especially crocodilians and birds. Sharpe and colleagues argue that this approach can miss structures that evolved independently in dinosaurs or that later bird lineages lost or modified.
If further work supports the exoparia hypothesis, it would add a possible dinosaur-specific element to models of jaw mechanics and facial anatomy. It is a reminder that living animals are valuable comparisons, but they may not preserve every feature present in extinct groups. It does not, by itself, dictate how all dinosaur faces should be drawn.
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What the published evidence establishes—and what it does not
| Question | What the paper supports | What remains uncertain |
|---|---|---|
| Where was it? | A proposed connection between the zygoma and posterior mandible, inferred from attachment evidence. | The exact soft-tissue arrangement in life. |
| What kind of tissue was it? | A connective-tissue structure is hypothesized. | Whether it was muscular or ligamentous. |
| What did it do? | It may have helped stabilize the jaw during movement, particularly in chewing dinosaurs. | Its precise function and how that function varied among dinosaurs. |
| How broad was its distribution? | The study examined ceratopsids, hadrosaurids, and tyrannosaurids. | Whether the structure occurred in all dinosaurs. The University of Alberta describes it as present in “almost all dinosaurs,” but the peer-reviewed paper’s sampled groups and hypothesis are the narrower basis for the claim. |
When was the study published?
The paper first appeared online on 21 March 2025. Its final Journal of Anatomy issue publication is in volume 247, issues 3–4, pages 790–818. The authors include Henry S. Sharpe, Yan-yin Wang, Thomas W. Dudgeon, Mark J. Powers, S. Amber Whitebone, Colton C. Coppock, Aaron D. Dyer, and Corwin Sullivan.
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