Arrhinoceratopsno Nose Horn

W. A. Parks (1868-1936) at the University of Toronto already had solid credentials in invertebrate paleontology, when, late in his career, he had the opportunity to study dinosaurs as well. Barnum Brown had already finished his collecting in Alberta. The Sternberg team had broken up, and C. H. Sternberg had returned to the United States. C. M. Sternberg in Ottawa had replaced Lawrence Lambe, who was now dead. Fortu-

Triceratops Skull
FIG. 4.12. Arrhinoceratops brachyops skull, Royal Ontario Museum. (From Dod-son and Currie 1990. Donna Sloan. Courtesy of the University of California Press.)

nately for the University of Toronto, Levi Sternberg came to work for them. With Parks participating, expeditions to Alberta were launched in 1920, and they came back successful. Despite the plethora of fossils described by Brown and Sternberg, there were still more to go around. For fifteen years (1920-1935), Parks made significant contributions to the study of duck-billed dinosaurs, armored dinosaurs, and meat-eating dinosaurs. Probably his greatest contribution was the description of the trombone-crested duck-bill, which he named Parasaurolophus zvalkeri in 1922.

In 1925, Parks described a new horned dinosaur from the Horseshoe Canyon Formation of the Red Deer River Valley near Bleriot Ferry, not far upstream from Drumheller. Little did he, or anyone else, dream that this would be the last new genus of the long-frilled Ceratopsia to be named from that day to this. (New short-frilled ceratopsians continued to be named as recently as 1995.) Parks named his skull Arrhinoceratops brachyops ("without a nose horn face, short face") (Fig. 4.12). This name, though technically well composed, certainly wins no popularity contests. I have yet to be approached by a bright-eyed, eager young lad bursting to share his trove of Arrhinoceratops brachyops lore with me. Nor have I seen Arrhinoceratops worn with pride on belt buckles, tie clasps, or other produce of the dinosaur knickknack industry. Arrhinoceratops remains a rare entity, only a single skull ever having been described, and only two significant papers appearing in print. Nonetheless, no one has ever questioned the validity of the taxon. Parks was terse in his description:

Supraorbital horn cores large, directed outwards and forward; nasal horn core absent; facial region short; crest relatively large, subquadrate, flat; squamosals long; parietals with oval fontanelles of moderate size; anterior process of jugal unusually long.

For Parks, the defining feature that plays so prominently in his name comes from the snout:

The nasal horn core is apparently absent, but the nasal bone is sharp above and somewhat rugose, suggesting that it may have carried a homy sheath. The nasal bone rises very abruptly, posterior to the suture with the rostral, suggesting the condition that maintains in Triceratops prorsus, but there is no trace of a horn core nor of an epinasal; neither does the surface of the bone indicate that a structure of this kind has been lost."

He also felt that the facial region in front of the orbits is very short. The skull is 1.5 m long and exceeds a meter in breadth across the frill (1.08 m)." The postorbital horn cores curve forward and diverge strongly laterally. The estimated height of the postorbital horn cores is about 41 cm. The frill is broad, with a straight rear border. The parietal fenestrae have an unusual oval form, with the long axis parallel to the central axis of the frill. Several features are important. The frill is very thin, several thickened portions attaining 30 mm in thickness, but most of it being 5-10 mm thick, according to Parks. The other feature is that both the dorsal and ventral surfaces are covered with vascular grooves in a reticular pattern, except for a smooth channel that extends from the parietal fenestra to the supratemporal fenestra.45 The squamosal is 72 cm long and nearly half that wide. Ornamentation on the free edge of the squamosal is modest. The left squamosal has a prominent fenestra in it. The epijugal is quite prominent and measures 10 cm in thickness.46

Parks presented very little analysis and unfortunately is remembered more for the errors he made. Lull expressed skepticism concerning some of the anatomical details, but more than fifty years passed before Arrhinoceratops was restudied, by a master's candidate at the University of Alberta, Helen Tyson. In Tyson's cogent analysis, Parks's failure to accept the existence of a true nasal horn core followed from his failure to see evidence for a separate center of bone growth, e.g., an epinasal. This despite the fact that a horn-like growth is as obvious as—dare we say it?—the noses on our faces. As Tyson put it, "To deny the presence of a horn core in Arrhinoceratops, which lacks such a suture but possesses a distinct horn-like organ, contributes neither to the problem of the homology of this structure nor to an accurate characterization of the genus." The emperor has no clothes! Furthermore, Tyson, benefiting from the progress made in understanding ceratopsian morphology over the decades since Parks's time, corrected some egregious errors. Parks failed to discern the dorsal limit of the rostral bone, which he thought contacted the nasal in front of his nonexistent horn.47 Similarly, he thought that the jugal reached far forward, almost contacting the premaxilla and cutting off the maxilla from the lacrimal, another unprecedented and erroneous interpretation. The sutures in the specimen are difficult to interpret, and he simply read too much into the cracks and fissures of a mature specimen.

Tyson performed a significant service by setting the record straight on these matters. Moreover, she proceeded to carry out some phylo-genetic analysis to place the position of Arrhinoceratops in a needed context. She unequivocally recognized Arrhinoceratops as a long-frilled ceratopsid. Moreover she saw evidence for a close relationship between Arrhinoceratops and Torosaurus. Both have thin parietal frills and squa-mosals with modest or absent epoccipitals. She cautioned against the assignment of isolated parietals or squamosals of thin construction, simple pattern, and prominent fenestrae to one genus or another without further confirming evidence. Accordingly, both genera remain rare."

Thus it is that the most recently described genus of chasmosaurine was described in 1925. The long-frilled horned dinosaurs remain of interest. New specimens continue to be described, and a new species, Chasmosaurus mariscalensis, was named by Lehman in 1989. Can it be that our knowledge of chasmosaurines is actually complete? That view is a little optimistic. For starters, a complete skeleton of even Triceratops would be nice. And I would be pleased to add skeletons of Torosaurus and Arrhinoceratops to the list. A population of Anchiceratops would also be nice. Chasmosaurines lived in what is now northern Mexico and on the North Slope of Alaska, but we do not yet know the generic identity of these animals. Finally, convincing intermediates among known chasmosaurines would clear up some mysteries, foremost among them the ancestry of Triceratops. Chasmosaurus is the earliest chasmosaurine, but not the most generalized by any means. Where is its ancestor and what did it look like? The job is certainly not finished.

Color Plates

Triceratops

PLATE I. Triceratops horridus encounters Tyrannosaurus rex. The meek shall inherit the earth. Late Cretaceous, Montana, Hell Creek Formation, circa 65 Ma. Chapter 3.

Chasmosaurus Dinosaur

PLATE II. A lone Chasmosaurus mariscalensis, wading in the swamps in search of fodder, tenses at the scent of danger. Late Cretaceous, Texas, Aguja Formation, circa 70 Ma. Chapter 4.

PLATE III. Styracosaurus albertensis, bellies filled, tranquilly survey the landscape, oblivious of nearby crocodile and Euoplocephalus in background. Late Cretaceous, Alberta, Judith River Formation, circa 74 Ma. Chapter 5.

What Did The Cretaceous Look Like

PLATE IV. Two bull Pachyrhinosaurus canadensis joust. Boys will be boys. Late Cretaceous, north slope of Alaska, Prince Creek Formation, circa 69 Ma. Chapter 6.

PLATE V. Two Lqjtoceratops gracilis refresh themselves, the young one not certain what to make of the turtle (Aspideretes) in the water. Late Cretaceous, Alberta, Scollard Formation, circa 65 Ma. Chapter 7.

PLATE VI. Two Psittacosaurus mongoliensis warily eye marauding troodontids. Early Cretaceous, Mongolia, Ondai Sair Formation, circa 100 Ma. Chapter 7.

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Responses

  • prima
    Where does Arrhinoceratops live?
    2 years ago
  • italia trevisan
    What does a triceratops nose horn look like?
    6 months ago

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