Fona: the small dinosaur that may have used underground shelter

Fona herzogae is a small plant-eating dinosaur from the Late Cretaceous of Utah, described in 2024 from a nearly complete skeleton and numerous additional bones. Its proposed underground lifestyle drew attention, but the evidence is subtler than a fossil preserved inside a clear tunnel. The anatomy and unusual burial of several individuals are consistent with use of underground shelter; the burrowing interpretation remains a hypothesis rather than a directly observed behaviour.

Quick facts

Fossils from the Mussentuchit Member

The fossils come from the Mussentuchit Member of the Cedar Mountain Formation in central Utah. The sediments formed in a warm, humid floodplain with rivers, clay-rich soils and periodic floods. The dinosaur lived during the Cenomanian, early in the Late Cretaceous. The holotype is NCSM 33548, a nearly complete skeleton preserved in partial articulation. Other sites have produced additional bones that expand what is known about the genus.

The name Fona honours Fo’na, a creator ancestor in Chamorro tradition. The species name herzogae recognises Lisa Herzog, who discovered one of the important fossil localities. The fossils were collected through fieldwork across several sites; they are not all parts of one individual. That distinction is especially important when comparing ages, body sizes and the association of juveniles at the Mini Troll locality.

What the Mini Troll site preserves

At Mini Troll, two immature animals and bones of a hatchling were found close together in a protected cavity-like deposit. Several specimens of Fona are preserved partly or fully articulated, which means bones remained joined in anatomical order. The concentration has been compared with the remains of animals using shelter below ground. It may also reflect the particular sedimentary conditions that protected delicate skeletons.

Researchers did not find a clear wall or infill that demonstrates an excavated burrow. In uniform clay, a cavity may be difficult to distinguish; it could have been dug by another animal or may not have been a burrow at all. The fossil arrangement therefore supports the possibility of underground refuge but does not show a tunnel with the certainty of a preserved trace structure. Taphonomy, the study of what happens to remains between death and burial, is central to evaluating the claim.

Could Fona dig?

Popular artwork often shows Fona rapidly scraping soil with its forelimbs, like a specialised mammalian digger. Its anatomy is not an exact match for animals that rely on powerful front-limb excavation. The elbow’s olecranon process and the position of the triceps muscle do not provide the long lever expected in highly specialised forelimb diggers.

The describing team discussed a different possibility: the animal may have loosened soil with its forelimbs and then used strong hind limbs to kick it away. Similar mechanics occur in some living birds and reptiles. This is a functional hypothesis based on bone shape, not an observed action frozen in the fossil. Biomechanical models and more comparative work can test how effective such digging would have been.

Even if Fona used burrows, their purpose is unknown. Shelter from heat or predators, seasonal refuge, nesting and shared use by young animals are possible explanations. A group of juveniles at Mini Troll is compatible with social use of a shelter, but it cannot establish their sex, family relationships or long-term social structure.

What the skeleton adds to the burrowing hypothesis

The case does not rest on one oversized claw. Several parts of the skeleton have been compared with digging animals, including the forelimb, shoulder and hind limb. Their proportions suggest a compact animal able to brace itself and move soil, but no single feature proves a specialised subterranean life. The interpretation depends on the combined anatomy, and the same traits could have served more than one function.

The fossils also differ in how they were buried. Partial articulation preserves bones close to their original connections and can indicate rapid burial or protection from disturbance. That pattern fits an animal dying in a sheltered cavity, but it can also arise from local sediment conditions after death. The proposed lifestyle is therefore supported by both functional anatomy and taphonomy, while neither line alone demonstrates that Fona routinely excavated a burrow.

Food and environment

Fona had a beak at the front of its jaws and small cheek teeth suited to plant matter. No stomach contents identify particular foods. Accidental ingestion of invertebrates cannot be ruled out, but there is no direct evidence that it was omnivorous. A herbivorous diet is the interpretation best supported by its jaws and teeth.

The Mussentuchit environment was a warm, humid floodplain with rivers, clay-rich ground and periodic flooding. Turtles, crocodylomorphs, small mammals and diverse dinosaurs lived in the region. Soft soil could have made digging easier, and rapid sediment infilling could have helped preserve articulated bones. However, the regional community is not proof that every named species shared the same exact habitat or time.

The body covering of Fona is unknown. Bristle-like structures have been proposed for some early ornithischians, but transferring them to this genus would be an artistic choice. Compare the animal with other taxa in the dinosaur catalogue, where the burrowing hypothesis can be considered against its anatomy, fossil arrangement and the limits of the evidence.