Haya: a small Mongolian dinosaur with stones in its belly

Haya griva is a small ornithischian dinosaur from the Late Cretaceous of Mongolia. It is better sampled than many similarly sized dinosaurs: several specimens preserve skull and postcranial bones, and one skeleton is associated with a cluster of stones interpreted as gastroliths. This material reveals more anatomy and variation than a single jaw or isolated bone could. Even so, the exact branch of the ornithischian family tree on which Haya belongs remains unsettled.

Quick facts

A name from Mongolian tradition

Peter Makovicky and colleagues named Haya griva in 2011 from fossils found at Khugutzlavkhant in Dornogovi Province. The genus name refers to Hayagriva, a horse-headed deity in Mongolian Buddhism. The species name griva means “neck” in Sanskrit, echoing the name’s cultural reference. The name is not a description of the dinosaur’s head shape.

The holotype, IGM 100/2017, includes a partial skull and associated skeletal remains. Additional specimens from the same broad locality have expanded the anatomical record, including more complete skulls and postcranial skeletons. They are separate individuals, not missing pieces of the holotype. Their value lies partly in showing which features recur across specimens and which may vary with age or preservation.

The fossils come from the Javkhlant Formation. Its age has been discussed within the Late Cretaceous, with estimates commonly spanning parts of the Santonian and Campanian rather than one precisely dated horizon. Correlation of the Mongolian continental deposits continues to be refined, so a broad Late Cretaceous assignment is more secure than a single exact age for every specimen.

Several individuals reveal more of the anatomy

The known material includes multiple skulls and parts of the vertebral column, shoulder, pelvis and limbs. This makes Haya unusually informative for a small ornithischian, but the sample is not one complete skeleton assembled from a single animal. Researchers compare repeated bones to identify consistent anatomical traits while keeping specimens and their preservation histories separate.

The skull has a beak at the front of the jaws and cheek teeth suited to processing plant matter. The lower jaw and tooth rows help distinguish the genus from other small ornithischians. Some cranial elements are delicate or incompletely preserved, so the whole facial outline is still reconstructed. The fossils do not preserve skin colour, a crest or soft tissues that might have changed the external appearance.

In 2021, Daniel Barta and Mark Norell published a detailed osteological study that brought together the known cranial and postcranial anatomy. Their work sharpened comparisons among specimens and with other early ornithischians. A broad sample improves the anatomical description, but it does not automatically resolve phylogeny: evolutionary analyses still depend on which traits are scored and how incomplete relatives are represented.

Gastroliths and what they mean

Specimen IGM 100/2015 preserves a cluster of stones in the abdominal region, with a reported total volume of about 93 cubic centimetres. Such stones are called gastroliths when they are interpreted as swallowed objects retained in the digestive tract. Their position and association with the skeleton support that interpretation, though fossil burial can move loose stones and the context must be considered carefully.

Gastroliths may have helped a plant-eating animal process food mechanically, as stones do in the gizzards of some living birds. But a cluster of stones does not directly reveal the exact plants eaten, how often the dinosaur swallowed stones or how much digestion they aided. It is one piece of evidence about feeding, not a preserved meal or a full digestive system.

The same specimen is a useful reminder to distinguish an anatomical observation from its functional explanation. The stones and their position are part of the fossil record. Their role as gastroliths is a reasoned interpretation that fits their arrangement and comparison with other animals. A photograph or illustration can make the stones look definitive, but the behavioural details remain less certain than the bones themselves.

Growth and the possible hatchling

The sample includes individuals at different growth stages. Bone histology in an immature specimen indicates that the animal had not reached full skeletal maturity. A very small specimen has been interpreted as perinatal or hatchling-sized, but its exact age at death and developmental stage cannot be read as a calendar date from size alone. Bone growth and preservation vary among individuals.

No egg or nest has been securely tied to Haya. A very young skeleton can show that small individuals were present in the ecosystem, but it does not establish where they hatched, whether adults cared for them or whether the animals lived in family groups. Multiple specimens at one locality also do not by themselves prove a herd; fossils can accumulate through repeated deaths and sedimentary processes.

Growth comparisons help researchers decide whether differences among bones represent age, individual variation or distinct species. This is especially important for small dinosaurs, because a juvenile can lack features that become pronounced later. A larger sample reduces some uncertainty, while the lost or incomplete parts of each specimen leave others unresolved.

Position among early ornithischians

Haya has often been placed near early ornithopods or in groups sometimes called jeholosaurids. These labels have not remained stable across analyses. Some phylogenetic trees recover it among ornithopod-grade ornithischians; others place it in a broader set of early-diverging lineages. The fossils combine features that can be read differently depending on the sample of taxa and the character definitions.

It is safest to identify Haya as an ornithischian dinosaur while explaining that its precise relationships remain under study. The broad category is well supported by its anatomy; a confident claim that it was the direct ancestor of another named group would go beyond the evidence. Comparisons with the North American Thescelosaurus can help place it among small ornithischians, but the two genera are not interchangeable.

A small animal in a diverse ecosystem

The Javkhlant Formation preserves a diverse Late Cretaceous community in what is now Mongolia. The landscape and climate varied across the region; the fossils at a particular site record a local burial setting rather than every habitat the animal may have used. Haya was small compared with large-bodied dinosaurs, and estimates around 1.5 metres are based on the available skeletal proportions and comparisons, not a complete adult specimen measured from nose to tail.

Its beak and cheek teeth support a plant-based diet. The gastrolith cluster adds a possible digestive aid, but neither evidence source identifies a specific menu. Its gait is reconstructed from the limbs and comparisons with related animals; the sample does not record speed or social behaviour. Explore it alongside other ornithischians in the dinosaur catalogue, where the strongest story is the unusual breadth of its fossil sample and the questions that remain open.