Happy Hunting of Horned Dinosaurs
Although they may have been a little slow on their feet, it seems safe to say that horned dinosaurs represent a flamboyant and fun group to discuss.
There are two kinds of dinosaur fossil which are particularly common in Alberta. The one group includes the duckbill dinosaurs and the other are the horned dinosaurs. These two groups are found together in many fossil dinosaur communities. The duckbills have actually been found in considerable variety in many parts of the world. Their remains appear suddenly, worldwide, in similar dinosaur communities. For this reason dinosaur specialists consider that they appear nearly simultaneously in the fossil record, with no hint anywhere of possible ancestors. This situation is contrary to the expectations of evolution theory.
The duckbill dinosaurs were large creatures which ran on two feet. Most were from 8 to 10 m long, but some were as long as 15 m. These were large heavy animals. Their diet was plants, and they had impressive arrays of elaborate teeth wedged tightly together to form a dental battery.
The fancy horned dinosaurs are the other major group of dinosaurs in upper Cretaceous rocks in western North America. Some experts interpret Cretaceous sediments around the world as the top layer of flood deposited material. The rocks below this level have no hint of any ancestor of these creatures. The two groups differ in several aspects, but it is the skull of the horned dinosaurs which is so conspicuous and easy to identify.
The horned dinosaurs were heavy four footed creatures of moderate size. The average body length was about 5m. This length included overgrown skulls. The one group of horned dinosaurs, the centrosaurs, had skulls that were about 1.5 m long. The other group, the chasmosaurs, had much longer skulls, up to 2-2.5 m long. Obviously these bony skulls were extremely heavy although the chasmosaur skulls usually featured open areas (windows) with no bone there. It is evident that these plant eaters were less than frisky on their feet. They did however exhibit some design features which enabled them to carry out all the tasks of life.
The back of the horned dinosaur skull is swept up into a frill which may be decorated with various spikes and bony knobs. On the face, such creatures may additionally sport various combinations of horns. These skulls sometimes extend as much as one third of the body length. It seems amazing that such top heavy creatures were able to manoeuvre at all. How could they turn their heads to find food, avoid obstacles and predators, or even find mates?
It so happens that these dinosaurs had a unique design feature which greatly enhanced the manoeuverability of their heads. They were provided with a ball and socket joint between the skull and the top of the backbone. Extending from the base of the skull was a unique, perfectly spherical ball which could be quite large, up to 95-113 mm diam. That is the size of a small grapefruit. (The technical term for this feature is occipital condyle.) Moreover the three top vertebrae of the backbone were fused together to make a strong socket for the bony projection from the skull. We see somewhat similar ball and socket connections between a vehicle and a trailer, but these connections are not as large. Thus the horned dinosaurs had a wonderful pivot for their heavy skulls. Another marvelous design feature was the teeth of these dinosaurs. Like the duckbills, the other prominent components of this community, the horned dinosaurs had densely packed arrays of teeth specially designed for efficient harvesting of tough plant material. Their most famous claim to fame however was the decorations on their faces and skulls.
The two groups of horned dinosaurs appeared simultaneously in the fossil record, neither ancestral to the other, and with no suggested ancestors in lower lying rocks. The centrosaurs had a short, solid or mainly so, frill swept up behind a short deep face. A horn over the nose, if present, was large. Horns over the eyes, if present, were small. The frill itself was often elaborately decorated with a scalloped edge of bone around the outside and with curved horns and spikes projecting in many directions. There were wild levels of variability between individuals within a population. No two individuals were ever alike. Despite the variation however, there are only a few distinctly different centrosaur designs. Most are considered to represent different genera with only one species each.
The other group of fancy horned dinosaurs is the chasmosaurs. These had a long shallow face with an extremely elongated frill swept up behind. Typically there were large windows (openings) in the bone of the frill. In life, these would have been covered with skin. The windows, of course, would greatly reduce the weight of the frill. This frill boasted very little decoration. Its size and shape were its chief claims to fame. Chasmosaurs had long horns over the eyes, but only a short horn over the nose. Like the centrosaurs, there was a lot of variability between individuals within a population and only in the genus Chasmosaurus have scientists described more than one species. The chasmosaurs, found from Alberta to Texas, occur over a much larger area than the centrosaurs. Also they occur not only in Campanian layers but also in the higher Maastrichtian rocks (the highest extent of Cretaceous sediments).
The famous Triceratops however fits neither the centrosaur nor chasmosaur group. It is actually something of an embarrassment to the experts. This species is found only in the very highest Maastrichtian rocks. If it were found lower than other horned dinosaurs, then the experts might have suggested that it was ancestral to the two different groups. As it is however, the scientists categorize Triceratops as a chasmosaur which originally had a long frill, but which “secondarily” underwent a reduction in the size of the frill.
A newly discovered horned dinosaur in Alberta also exhibits a blend of characteristics. Since it is found in rocks lower than the vast majority of horned dinosaurs, the experts suggest that it might resemble a species ancestral to the two groups. Thus if specimens with blended characteristics are found at a high level, they are said to be secondarily changed, and if they are found at a lower level, they are said to possibly represent an ancestor.
The fact is however that we do not see the pattern of variation that we should see if divergence (evolution of new groups) had taken place. With great diversity within populations, we should see a cluster of very similar species, and we do not see that. On the contrary we see species widely separated into separate genera.
The question on everybody’s mind is the purpose of the horns and frills. Many experts have suggested that much of the variation within a population reflected differences in gender and age, and that could well be. This does not explain why these animals would have such large frills and horns in the first place. Some have suggested sexual display or use for threatening posture. Others have suggested actual use for fighting. It seems hard to imagine how these extreme designs would be more useful and successful than something a little more moderate however. It seems apparent these features are designs, chosen by God and wonderfully crafted to enhance the richness and variety of their communities.
Thus Alberta celebrates her remarkable horned dinosaur fossil beds. The meat eating dinosaurs may be the most dramatic specimens, and the duckbills may show the greatest number of species, but the horned dinosaurs are not found in many other places and they certainly show the most astonishing designs.
[The material in this article, is presented in a briefer format in the new edition of our Tour Guide to the Royal Tyrrell Museum.]
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