Aquilops: an early horned dinosaur from North America

Aquilops americanus was a small horned dinosaur relative from the Early Cretaceous of Montana. It is known from one partial skull with lower-jaw fragments, yet that specimen records one of the earliest well-supported neoceratopsians in North America.

The animal did not resemble a miniature Triceratops. It lacked the enormous brow horns and broad solid frill of later ceratopsids. Its most distinctive feature was a sharply hooked beak core on the front of the upper jaw, combined with a low skull and a modest cheek region.

Aquilops is securely known from a juvenile skull. Its body, adult size and external covering must be reconstructed from relatives.

Quick facts

Discovery and name

The skull was discovered in 1997 during fieldwork in Carbon County, Montana, in rocks of the Cloverly Formation. It entered the collections of the Sam Noble Oklahoma Museum of Natural History as OMNH 34557. Andrew Farke and colleagues described it as a new genus and species in 2014.

Aquilops combines Latin words for eagle and face, referring to the hooked beak. The species name americanus marks its North American origin. The name is anatomical, not evidence that the dinosaur hunted like an eagle.

The Cretaceous Period included several exchanges of dinosaur lineages between Asia and North America. The age and relationships of Aquilops make it useful evidence for one such episode.

The holotype skull

The specimen preserves much of the front and roof of the skull, including the beak-bearing rostral bone, parts of the upper jaws and cheek region, and fragments of the lower jaws. The reconstructed skull measures about 84 millimetres long. Several sutures and surface textures indicate that the individual had not finished growing.

The rostral bone forms a pronounced downward-curving hook. Behind it, the upper jaw carried small teeth suited to cropping and processing plants. A projection on the cheek bone was present but far less developed than the broad ornaments of later ceratopsians.

No associated vertebrae, pelvis, limbs or tail were found. Every full-body reconstruction therefore combines this real skull with proportions inferred from other small neoceratopsians.

Classification among horned dinosaurs

Ceratopsians were ornithischian dinosaurs with a unique rostral bone at the tip of the upper jaw. Early members were generally small bipeds, while later lineages produced heavier quadrupeds with enlarged skulls. Aquilops belongs within Neoceratopsia, closer to forms such as Leptoceratops and ceratopsids than to the earliest Asian ceratopsians.

The first analysis placed it near the base of Neoceratopsia and found its closest comparisons in Asia. Later datasets can shift individual branches as new characters and taxa are added. The broad conclusion remains that it represents an early North American member of a group whose early record is richest in Asia.

It was not an ancestor that simply grew into Triceratops. More than thirty million years and many branching lineages separate them. Both inherited the ceratopsian beak, but their skulls and bodies served different scales and feeding systems.

How large was Aquilops?

The known skull has been used to reconstruct an animal around 60 centimetres long and perhaps 1.5 kilograms in mass. Those figures describe a plausible immature individual, not a measured adult. A commonly repeated adult length of roughly one metre is an extrapolation from relatives.

Juveniles can have proportionally larger heads and shorter snouts than adults. Scaling the body directly from the skull may therefore distort total length. Without a growth series or postcranial skeleton, a narrow adult range would imply more knowledge than the fossil supplies.

The dinosaur size comparison explains why length and mass ranges should remain separate and why an estimate is not an exact measurement.

Beak, teeth and feeding

The hooked rostral bone supported a keratinous beak in life. Its core establishes the shape only approximately because the horn covering did not fossilise. The beak could crop vegetation, while the cheek teeth sliced or crushed food before swallowing.

Small body size would have kept the head close to low plants. Ferns, cycads, conifers and early flowering plants were available in Early Cretaceous ecosystems, but no gut contents or coprolites have been tied to the holotype. Illustrations showing one preferred plant are ecological possibilities rather than direct observations.

The jaw does not show the extensive dental batteries of later ceratopsids. Feeding mechanics were simpler, and the animal probably selected manageable plant parts instead of shearing large volumes of tough vegetation continuously.

Walking on two or four legs

Most close small-bodied relatives are reconstructed as primarily bipedal, with relatively long hind limbs and a balancing tail. Aquilops is usually shown that way. The inference is reasonable but indirect because none of its limb bones are known.

It may have lowered the forelimbs while feeding or moving slowly, but habitual quadrupedal support cannot be tested from the skull. Trackways from the Cloverly Formation are not automatically attributable to this genus. A footprint requires suitable anatomy, size, age and association before it can be assigned even tentatively.

The Cloverly environment

The Cloverly Formation records rivers, floodplains and seasonally variable terrestrial habitats. Its fauna included the large predator Acrocanthosaurus, the dromaeosaurid Deinonychus, armoured dinosaurs such as Sauropelta and herbivorous ornithopods.

A tiny ceratopsian shared the landscape with animals capable of killing it, but there is no preserved encounter involving the holotype. Concealment, alertness and rapid movement are plausible defences. Social groups, calls and specific display colours remain unknown.

Biogeography between Asia and North America

Early ceratopsians are especially diverse in Asia, whereas the North American record before advanced ceratopsians is sparse. The relationships of Aquilops support dispersal from Asia into North America by the Early Cretaceous. Later ceratopsian movements may have occurred in separate waves.

Land connections at high northern latitudes changed as sea level, climate and tectonics changed. A single genus cannot identify the exact route or date of crossing. It does show that small neoceratopsians had reached the continent well before the spectacular horned dinosaurs of the Late Cretaceous.

Growth and the juvenile skull

Several features led the describing researchers to regard the holotype as immature. The eye socket is proportionally large, the skull bones retain juvenile textures and some contacts had not fully fused. These signs are more informative together than any one character on its own.

Growth could have changed the depth of the snout, the relative size of the eye and the prominence of the cheek. Later ceratopsians often transformed their skull ornament substantially as they matured. There is no adult Aquilops skull to show whether its hooked beak became deeper or whether the cheek projection enlarged.

This matters for taxonomy as well as appearance. Young individuals of related species can look different enough to be mistaken for separate animals. The diagnostic combination in Aquilops was chosen from characters considered unlikely to be caused only by youth, but a larger sample would test that judgement.

Comparison with other small ceratopsians

The comparison shows why familiar small horned dinosaurs cannot simply supply every missing bone. Protoceratops is younger and more specialised, while Psittacosaurus lies on another early ceratopsian branch. They constrain a plausible range of body plans rather than provide a perfect anatomical template.

No evidence for horns, herds or combat

The skull lacks the enlarged brow and nasal horns associated with ceratopsid combat and display. Its cheek projection may have changed the outline of the head, but there is no evidence that it functioned as a weapon. The beak was part of the feeding apparatus, not a horn.

Only one individual is known, so group living cannot be inferred from an Aquilops bonebed. Related dinosaurs sometimes occur in groups, but shared ancestry does not require identical behaviour. A herd scene can illustrate one possibility only if its caption makes the uncertainty clear.

What can be reconstructed safely?

The shape of the preserved skull, the hooked beak core and the arrangement of visible bones are direct evidence. Tooth form strongly supports plant eating. Immature bone surfaces support a juvenile interpretation, although the exact age in years is unknown.

Body length, mass, limb proportions and bipedal gait come from comparison. Skin texture, colour, quills and behaviour are still less certain. Some early ceratopsians preserve filament-like structures, but none are known from Aquilops itself.

This hierarchy does not make the reconstruction useless. It shows which parts can change when a skeleton is found. The partial skull already establishes a distinctive animal and fills an important geographical gap in the dinosaur catalogue.