Avaceratops: a young skeleton, an uncertain adult

Avaceratops lammersi is a ceratopsid from the Judith River Formation of Montana, known chiefly from the partial skeleton of a young, rapidly growing individual. The type preserves substantial parts of the body but lacks the nose and brow-horn cores. Many familiar adult restorations use another skull, MOR 692, whose assignment to Avaceratops is disputed.

The genus is therefore more informative than a simple picture of a small horned dinosaur suggests. Its bones document one juvenile individual, while the adult size, mature horns and exact position within Centrosaurinae remain open to interpretation.

Quick facts

Discovery, people and the name

Eddie Cole discovered the bones in 1981 at Careless Creek Ranch in Wheatland County, Montana. Peter Dodson examined the material, and later fieldwork recovered additional elements of the skeleton. Anthony Fiorillo also participated in the excavation. Dodson formally named the genus and species in 1986 and designated the specimen ANSP 15800 as the holotype.

The name Avaceratops means “Ava's horned face”. It honours Ava Cole, the wife of the collector, and combines her name with the Greek roots used in ceratopsian names. The species name lammersi recognises the Lammers family, whose land contained the quarry.

The bones were not preserved as a fully articulated skeleton. They lay scattered in sandstone from an ancient channel, together with charred wood and bones of other vertebrates. The type elements are close in size and preservation and are reasonably interpreted as one individual. This association supports a coherent anatomical description, even though it cannot supply the missing portions.

Classification and the effects of growth

Avaceratops is an ornithischian ceratopsian in Ceratopsidae. It is commonly placed within Centrosaurinae, the branch that also includes Centrosaurus, Styracosaurus and Pachyrhinosaurus. Some analyses place it among Nasutoceratopsini, near Nasutoceratops and other early-diverging centrosaurines.

That placement is not final. The holotype was young, and the frill, horns, edge bones and skull sutures of ceratopsids changed substantially during growth. A juvenile may appear more primitive than the adult branch to which it belongs. Earlier studies treated Avaceratops as one of the most basal ceratopsids and even considered it near the common ancestry of centrosaurines and chasmosaurines. Broader analyses and later fossils shifted that view toward an early centrosaurine, perhaps a nasutoceratopsin.

The genus is not demonstrated to be the direct ancestor of Triceratops or of all ceratopsids. Similarities can reflect an early branch or juvenile anatomy rather than direct ancestry. Its exact diagnosis is difficult because the holotype lacks a complete set of adult skull characters.

One accepted species and unsettled referrals

Most current treatments recognise one species, Avaceratops lammersi. The proposed spelling A. lammersorum did not replace the original name. A past combination, Monoclonius lammersi, also did not gain broad acceptance as the best classification.

The more important uncertainty concerns the boundary of the genus. ANSP 15800 is immature and does not show every mature feature needed to diagnose the animal beyond doubt. Secure adult material from the same horizon could confirm that Avaceratops is a distinct genus or reveal which adult form grew from this juvenile.

Two isolated squamosal bones, USNM 4802 and USNM 2415, have been referred to the genus. A larger partial skull, MOR 692, was assigned in a 1999 study and later became the basis for many adult restorations. Subsequent authors noted that it differs in some details, comes from a younger horizon and shares no decisive diagnostic features with the type. It is more cautious to call it an indeterminate close centrosaurine than a proven adult Avaceratops.

What the holotype preserves

ANSP 15800 includes parts of the parietal and left squamosal bones of the frill, premaxillae, maxillae, jugal, pieces of the lower jaw, quadrates, palate and an incomplete braincase. It also preserves many vertebrae and ribs, shoulder bones, both humeri, a left radius and ulna, both femora, tibiae and fibulae, plus parts of the hands and feet. For a ceratopsid known from one young animal, this is an unusually useful combination of skull and postcranial evidence.

The nasal region and brow-horn cores are absent. Their form and length cannot be read directly from the type. Reconstructions with prominent horns usually borrow them from MOR 692 or from other centrosaurines. A caption should make that comparison explicit, rather than imply that the holotype preserves adult horns.

Microscopic study of ANSP 15800 found actively forming fibrolamellar bone, consistent with rapid growth. The small skeleton was not an almost mature dwarf. An exact age in years cannot be calculated confidently because the ribs were heavily remodelled and growth-arrest lines were unevenly preserved.

Size, frill and limbs

The holotype is estimated at about 2.3–2.8 metres long. This is the size of the known juvenile, not a reliable adult maximum. A frequently cited adult length near 4.2 metres depends largely on assigning MOR 692 to the genus. Since that referral is uncertain, adult size remains unresolved. The young individual probably weighed several hundred kilograms; estimates around a tonne apply only to a hypothetical adult reconstruction.

The preserved frill was short and broad. Its parietal bone lacks a strong central notch, and the rear margin has low waves or scallops. Large marginal ossifications were not fused to the frill. These traits were once treated as primitive adult characters, but age may explain at least some of them. Damage also leaves open the possibility of very small parietal openings, so a completely solid adult frill is not proved.

The left squamosal is broad, with a smoothly curved rear edge and several external bumps. The bumps may have strengthened the bone or contributed to its external outline, but soft tissues did not survive. No single function can be assigned from the bone alone.

The rest of the skeleton supports a sturdy quadruped. The forelimbs carried the front of the head and body, while the hind limbs supplied much of the propulsive force. The elbow process and deltopectoral crest are less developed than in some adult relatives, a pattern that could reflect immaturity rather than a unique adult gait. The pointed terminal toe bones are ordinary support structures, not evidence of predatory claws.

Montana habitat, food and behaviour

The Judith River Formation formed on a low coastal plain west of the inland sea that divided North America. Rivers, sand bars and floodplain deposits accumulated near the Careless Creek site. Charred wood records plant material in the channel deposit, but the quarry is a burial setting, not proof that a group of Avaceratops lived together there.

As a ceratopsid, it was herbivorous. A beak cropped vegetation and cheek teeth processed it. No stomach contents or genus-assigned coprolites reveal a specific menu. Feeding low on plants is plausible from the body plan, but exact plant preferences are not demonstrated.

The horns and frill of mature ceratopsids could have contributed to display, recognition, competition or defence. The juvenile type does not preserve mature horn cores, so it cannot show how adult Avaceratops used them. The quarry contains bones from multiple animals and is not a single-species herd deposit. Parental care, permanent herds, courtship and exact defensive behaviour remain hypotheses.

Common errors in reconstructions

The holotype is not a fully grown animal just over two metres long. Bone histology indicates that it was still growing quickly. Nor are its adult brow horns directly known: the type lacks the relevant cores, and the skull commonly used to supply them is not securely referred to Avaceratops.

A completely solid frill and a precise adult length are also uncertain. Finally, the genus is not a proven direct ancestor of Triceratops. The body of the juvenile is well represented, but the adult face, colour, soft tissues and exact branch position need more evidence. The dinosaur catalogue lists it as a distinct taxon while preserving those limits.

Avaceratops is best understood through the combination of a substantial young skeleton and a difficult adult reconstruction. Its type documents growth and body anatomy particularly well; the mature skull and horns that often define its public image remain the less secure part of the story.