Stellasaurus ancellae

Stellasaurus ancellae was a centrosaurine ceratopsid dinosaur from the Late Cretaceous of North America, known from remains recovered from the Two Medicine Formation in Montana, United States. Formally described in 2020 by Wilson, Ryan, and Evans, this taxon represents a particularly interesting form within the evolution of Campanian centrosaurines, as it combines anatomical traits seen in forms such as Styracosaurus, Einiosaurus, and Achelousaurus. Its generic name means “star lizard,” referring both to the striking arrangement of the cranial ornamentation and to a cultural homage to David Bowie’s “Starman,” while the specific epithet ancellae honours fossil preparator Carrie Ancell.

Stellasaurus ancellae belongs to Centrosaurinae, a group of ceratopsids characterized by short, deep skulls, prominent nasal horns, and parietal frills ornamented with highly variable bony projections. The holotype, catalogued as MOR 492, comes from Campanian levels of the Two Medicine Formation and preserves diagnostic cranial elements, especially those related to the ornamentation of the nasal, supraorbital region, and parietal. The original description interprets it as a transitional centrosaurine within the so-called Styracosaurus lineage, positioned both morphologically and stratigraphically between Styracosaurus albertensis and Einiosaurus procurvicornis.

From a morphometric perspective, Stellasaurus ancellae would have been a medium- to large-sized ceratopsid, comparable in general proportions to other derived centrosaurines from the Campanian. Its body length can be estimated at approximately 4.5–5.5 metres, with a hip height of around 1.5–1.8 metres and a mass probably within the range of 1.5–2.5 tonnes, although these figures should be considered approximate, as no complete associated postcranial skeleton is known. Its body would have been quadrupedal, robust, and low-slung, with a broad trunk, powerful limbs, a relatively short tail, and a proportionally large head, as is typical of centrosaurine ceratopsids.

The most distinctive feature of Stellasaurus lies in its cranial ornamentation. It possessed an elongated, erect, and slightly recurved nasal horn, closer in general form to that of Styracosaurus than to the strongly forward-curving nasal horn of Einiosaurus. The supraorbital horns were greatly reduced, while the parietal frill displayed a distinctive combination of ornamental processes: elongated and straight parietal projections in the posterior region, together with shorter projections in lateral positions. This mixture of characters gives it a highly recognizable cranial silhouette, with a visually spiny and “star-like” profile consistent with the meaning of the genus name.

Like other centrosaurines, Stellasaurus ancellae was a quadrupedal herbivore adapted to feeding on low- to mid-height vegetation in continental environments. Its rostral beak, dental batteries, and powerful jaws would have allowed it to crop and process tough plant material, while the cranial ornamentation may have served functions related to visual communication, species recognition, sexual selection, or intraspecific competition. The complexity of its horns and parietal frill reflects the great diversity of visual signals developed by North American ceratopsids during the Late Cretaceous.

Stellasaurus lived in the ecosystems of the Two Medicine Formation, a geological unit deposited during the Campanian in northwestern Montana and nearby areas, within the ancient island-continent of Laramidia. These environments included floodplains, seasonally semi-arid areas, nesting grounds, and systems influenced by variable climatic conditions, where numerous dinosaurs coexisted, including hadrosaurids, tyrannosaurids, small theropods, ankylosaurs, other ceratopsids, and a diverse vertebrate fauna. The Two Medicine Formation is especially important for understanding the regional evolution of centrosaurines, as it has yielded taxa such as Einiosaurus, Achelousaurus, and forms related to Styracosaurus.

This replica represents Stellasaurus ancellae as a centrosaurine with a robust body, quadrupedal posture, short and powerful skull, prominent nasal horn, and a parietal frill ornamented with distinctive bony projections. Its reconstruction integrates the available information on its cranial morphology, evolutionary position, geological context, and approximate morphometry, highlighting one of the most distinctive ceratopsids of the Two Medicine Formation and a key species for illustrating the evolutionary diversity of horned dinosaurs from the Late Cretaceous of North America.

Approximate measurements of Stellasaurus:

Scale 1:35 — Complete model
Length: 145 mm
Height: 67 mm
Width: 37 mm
Snout-to-tail length: 145 mm

Scale 1:20 — Complete model
Length: 250 mm
Height: 116 mm
Width: 63 mm
Snout-to-tail length: 250 mm