Saturday 6 June 2015

Regaliceratops peterhewsi: A new species of Chasmosaur from the Late Cretaceous of Alberta.

Ceratopsids are among the most distinctive and easily recognized of Dinosaur groups, due to their large and heavily ornamented skulls, with large neck frills and horns. Despite their ubiquitousness in Dinosaur pop-culture they were a rather short-lived and geographically restricted group, found only in the Late Cretaceous of North America and parts of northeast Asia. The Ceratopsids are split taxonomically into two groups, the Centrosaurs, which went extinct in the Early Maastrichtian, about 5 million years before the end of the Cretaceous, and which tended to have enlarged nasal horns (horns on their noses) and ornate neck frills, and the Chasmosaurs, which survived till the End Cretaceous Extinction, and which tended to have enlarged orbital horns (horns above their eyes) and large but simple frills.

In a paper published in the journal Current Biology on 15 June 2015, Caleb Brown and Donald Henderson of the Royal Tyrrell Museum of Palaeontology describe a new species of Chasmosaur from the Late Cretaceous St. Mary River Formation in the Waldron Flats area of Alberta, close to the Oldman River and about 164 km south of Calgary.

The new specimen is named Regaliceratops peterhewsi, where ‘Regaliceratops’ means ‘Royal Ceratopsid’, in reference to both the ornamentation of the frill and the Royal Tyrell Museum, and ‘peterhewsi’ honours Peter Hews, who discovered the specimen from which the species is described, a single skull, complete but for the absence of the rostral bone and the lower jaw; the post cranial skeleton is unknown.

Photographs and Interpretive Line Drawings of the Holotype of Regaliceratops peterhewsi. (A–D) Nearly complete cranium in right lateral (A), left lateral (B), rostral (C), and dorsal (D) views. (A’–D’) Interpretive drawings of photographed views in (A)–(D). Areas in shadow, (C) and (D) left, are fully illustrated at right. Areas in white represent reconstruction (plaster/epoxy putty), and hatched areas indicate matrix. The following abbreviations are used: cvp, caudoventral process of premaxilla; ej, epijugal; en, external naris; isf, interseptal fenestra; itf, infratemporal fenestra; j, jugal; jn, jugal notch; m, maxilla; n, nasal; nhc, nasal horncore; ns, narial strut; ob, orbit; p, parietal; pf, parietal fenestra; phc, postorbital horncore; pm, premaxilla; por, postorbital ridge; pp, palpebral; ps, epiparietosquamosal; P#, epiparietal; s, squamosal; stf, supertemporal fenestra; S#, episquamosal; tp, triangular process. Scale bar represents 10 cm. Brown & Henderson (2015).

The skull morphology of Regaliceratops peterhewsi places it firmly within the Chasmosaurinae, however it shows a number of features common to Centrosaurs, most notably an ornate frill and a large nasal horn. These features have been seen in Chasmosaurs before, but largely in early members of the group, closer to the divergence between the two groups, and never to the extent seen in Regaliceratops peterhewsi.

A phylogenetic analysis of the Chasmosaurs carried out by Brown and Henderson suggests that they can be split into two distinct groups, an earlier Chasmosaurus-like group which were smaller and had features closer to those of Centrosaurs, and a later Triceratops-like group which were larger and had more highly developed Chasmosaur features (such as enlarged orbital horns and large simple frills). These two groups are share a common ancestor more recent than their common ancestor with the Centrosaurs, but before any of the known fossil species appeared, suggesting that a long ghost-lineage (a lineage that can be inferred from the gap in the evolutionary record, but for which no fossils are known) for the Triceratops-like group exists. Importantly the Chasmosaurus-like group appear to go extinct at about the same time as the Centrosaurs, suggesting that a common cause may have been responsible for the demise of the two groups, while the Triceratops-like group diversifies after the disappearance of the other two groups, which may indicate they were moving into ecological niches formerly occupied by Centrosaurs and Chasmosaurus-like Chasmosaurs.

Time-calibrated strict consensus tree of five most parsimonious trees for Chasmosaurinae utilizing the new epiossification homology scheme. Black bars indicate confident stratigraphic occurrence, whereas gray bars indicate less confidence. Bottom right: oblique view of Regaliceratops peterhewsi. Brown & Henderson (2015).

Regaliceratops peterhewsi is situated firmly within the Triceratops-like group, albeit quite early in the known group, and is the first known member of this group to show Centrosaur-like features. Among Mammals convergent evolution of horn ornamentation has been linked to convergent social behaviour. This is impossible to judge in Ceratopsids, an extinct group in which behaviour cannot be directly assessed, however it is possible that a Chasmosaur such as Regaliceratops peterhewsi moving into an ecological niche left vacant by the extinction of the Centrosaurs could also have developed similar social behaviour, resulting in convergent evolution of the skull ornamentation.

See also…

Ceratopsian Dinosaurs are thought to have originated in Asia in the Early Cretaceous, spreading to Europe and North America, and becoming the most important and diverse group of herbivorous Dinosaurs in North America by the end of the Period. Unfortunately...

Dinosaurs underwent dramatic increases in size over a number of years during their growth, and are presumed to have played a number of different ecological roles during this growth period. Numerous examples...

Ceratopsid Dinosaurs were a speciose group of large, herbivorous Dinosaurs from the Late Cretaceous of Asia and North America...

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