By Mike Thorn
 

This article briefly reviews the literature published to date on the poorly known ceratopsid genus Arrhinoceratops. It is based on web pages which can be found, along with information, reviews and references for many other ceratopsid genera, at the Ceratopsia Home Pages http://www3.cybercities.com/c/ceratopsia/index.htm

Introduction

Arrhinoceratops is a relatively obscure ceratopsian, the only undisputed specimen being a reasonably well preserved skull found five kilometers upstream from Bleriot Ferry on the Red Deer River in Alberta, Canada and described by Parks in 1925. The sediments where it was found are part of the Horseshoe Canyon Formation, placing it in the earlier half of the Maastrichtian age (approximately 68 to 71 million years old).

The name Arrhinoceratop means \"without a nose horn\", and arises from Parks\' original description which mistakenly asserted that the animal lacked a nasal horn core. However, a more recent reassessment has concluded that there is a forward pointing nasal horn core (Tyson, 1981).

To date, only one unequivocal species of Arrhinoceratops, namely Arrhinoceratops brachyops, has been described, and that is based on one specimen (the skull ROM 796, formerly ROM 5135, at the Royal Ontario Museum, Canada). A second, poorly preserved skull (ROM 1439) was assigned to this genus (Lull, 1933) but it was never described and is now lost (Tyson, 1981). A second species, Arrhinoceratops utahensis, from the southern USA was described but has since been referred to Torosaurus utahensis (Gilmore, 1946, Lawson, 1976).

Description fig1. Drawing of the anterior part of the skull of Arrhinoceratops brachyops

KEY:
EJ - Epijugal
J - Jugal
L - Lacrimal
Mx - Maxilla
N - Nasal
PF - Prefrontal
PO - Postorbital
PMx - Premaxilla
QJ - Quadratojugal
Q - Quadrate
R - Rostral
SQ - Squamosal

The skull on which the genus is based measures 1.5 m and is reasonably complete, although there is no associated lower jaw. A single tooth was preserved and although the tip has been lost, it appears to have been blunt rather than pointed as are the teeth in Triceratops. The skull seems to be from an adult animal as most of the sutures between the bones have co-ossified. The face is short relative to other chasmosaurinae and the frill is relatively short and broad, and is rounded posteriorly in a fashion similar to Triceratops and Torosaurus.

The supra orbital horn cores are of modest size, arising close together and then diverging from each other, making an angle of about 120 degrees.

The squamosal and parietal bones are thin and lack epoccipitals and the parietal bones have small round or oval fenestrae, all features which make it similar to the genus Torosaurus (Lehman, 1996, Tyson, 1981). There is a graded series of frill shapes between Arrhinoceratops and Torosaurus, ranging from the short, relatively broad frill of Arrhinoceratops to the long, narrow frills of the larger specimens of Torosaurus. There is complete stratigraphic separation of the two genera, Torosaurus being from the younger Lance, Hell Creek and North Horn Formations and it has been suggested that Torosaurus evolved from an Arrhinoceratops like ancestor (Tyson, 1981).

Problems with the original The original description of Arrhinoceratops made by Parks was brief and contained a number of errors (Tyson, 1981). In particular, he claimed the following features as diagnostic of the genus :

1.There was no nasal horn core
2.The anterior process of the jugal was unusually long
3.The rostral bone projects backwards to touch the nasal bone
4.The rostral bone projects backwards onto the premaxilla

The lack of a nasal horn core has been disputed by Tyson. Parks claimed that a true horn core should have a separate center of bone growth and cited the lack of any sutures to delimit the horn core as evidence that the horn core was absent. He did note that there was a forward projecting bone on the nasal and that it\'s rugose texture suggested that it was covered with a keratinous sheath in life (Parks, 1925). Tyson observed that the skull was of an adult and that the sutures had been obscured by aging. There is a distinct forward pointing horn like structure similar to that found in all other currently known chasmosaurines and the absence of visible sutures does not mean that it is not a proper horn core (Tyson, 1981).

Parks\' exaggerated the size of the anterior process of the jugal in his original description. It is not particularly long relative to other chasmosaurinae and is in fact relatively shorter than that found in the genus Chasmosaurus. However, the process is much shorter on the left hand side of the skull than on the right. Tyson suggested that this is possibly abnormal and a result of damage to the sutures around the jugal when the animal was young. Such damage causes overgrowth of the surrounding bones which eventually obscures the original damage. It is also possible for skulls to be asymmetrical without any pathological cause (Tyson, The rostral bone does not project back to the nasal as described by Parks. Although the expected suture between the rostral and premaxilla has been obliterated, there is a clear change in the angle of the snout where the end of the rostral bone would be expected to be (Tyson, 1981).

Tyson also disputed that the rostral bone projected backwards onto the premaxilla. This would have been a feature unique amongst the ceratopsia, but in fact seems to have been down to a straight forward misinterpretation of a deep crack, shallow sulcus and grooves in the premaxilla by Parks as sutures.

Tyson did however note that the part of the premaxilla which Parks interpreted as an extension of the rostral bone was covered in deep grooves which suggest it may have supported a keratinous sheath in life. This could have acted as an accessory cutting organ (Tyson, 1981).

Systematic palaeontology (after Tyson,1981)

ARCHOSAURIA, Cope, 1869
ORNITHISCHIA, Seeley, 1888
CERATOPSIA, Marsh, 1890
NEOCERATOPSIA, Sereno, 1986
CERATOPSIDAE, Marsh, 1888
CHASMOSAURINAE, Lambe, 1915
Genus Arrhinoceratops Parks, 1925

Type species - Arrhinoceratops brachyops Parks, 1925
Type specimen - ROM 796, skull without lower jaws
Type locality - 3 miles above Bleriot Ferry, Red Deer River, Alberta, Canada
Referred specimens - none
Known distribution - Horseshoe Canyon
Formation, Upper Cretaceous
(Maastrichtian), Alberta

Diagnostic Features

1.brow horn cores large and directed forwards and outwards

2.nasal horn core short but massive, the posterior edge merging smoothly with the dorsal profile of the face, the apex of the horn directed anteriodorsally

3.face relatively short

4.rostral bone small, the premaxillary septum has a shallow depression and two posterior flanges that invade the external narial openings

5.epijugal is large and directed ventrolaterally

6.lateral temporal fenestrae are sub triangular in shape

7.antorbital fenestrae reduced


8.squamosal is long and tapering, reaching to the posterior border of the frill

9.frill is short but broad, with oval parietal fenestrae of moderate size

References

Gilmore, C.W., 1946, Reptilian fauna of the North Horn Formation of central Utah, U.S. Geological Survey, Professional Paper, 210-C:29-52.

Lawson, D.A., 1976 Tyranosaurus and Torosaurus, Maastrichtian Dinosaurs from Trans-Pecos Texas, Journal of Paleontology, 50:158-164.

Lull, R.S., 1933, A revision of the Ceratopsia or horned dinosaurs, Peabody Museum of Natural History Memoirs 3(3):1-175

Parks, W.A., 1925, Arrhinoceratops brachyops, a new genus and species of ceratopsia from the Edmonton Formation of Alberta, University of Toronto Studies, Geological series, 19 p 5-15

Tyson, H., 1981, The structure and the relationships of the horned dinosaur, Arrhinoceratops Parks (Ornithischia:Ceratopsidae), Canadian Journal of Earth Sciences, 18:1241-1247

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