MONOCLONIUS\ Styracosaurus'. Triceratopsl These names evoke such images of large, powerful, dare I even say sophisticated plant-eating dinosaurs. Their huge heads, bristling with sharp spikes and luxuriant bony frills, their solid, four-legged bodies, all suggest rhinoceros-like body plans run amok in the Cretaceous. Clearly these were animals to be reckoned with, not meek victims ready to bare their necks to the glistening teeth of a predator. Horned dinosaurs of the family Ceratopsidae are uniquely North American, as far as we know. They ranged from Mexico to Alaska but have not yet been found outside the bounds of our North American continent. Ceratopsids conveniently, and for the most part unambiguously, divide themselves into two groups or subfamilies, which we may designate as the Centrosaurinae, or short-frilled ceratopsids, and the Chasmosaurinae, or long-frilled ceratop-sids. As Triceratops is a chasmosaurine, we might say that the chasmo-saurines are slightly better known. The centrosaurines enjoyed their greatest diversity slightly earlier than the chasmosaurines and are primarily found in Alberta and Montana, whereas the chasmosaurines showed their greatest diversity slightly later in time and extend from Mexico to Alaska. If we treat the chasmosaurines before we consider the centrosaurines, there is no implication that the latter are more derived—the course of the evolutionary history of the ceratopsids presents a fork in the road, not a ladder of progress.
The first known and greatest of the "horn faces" was of course Triceratops itself. This mighty plant-eater is one of the all-time favorites among connoisseurs of Mesozoic saurians.1 It was also the beneficiary of some powerful public relations. "Three-horned face" is known as one of the last of all dinosaurs, and it was a fitting contemporary of that redoubtable predator and stalker of children's dreams, Tyrannosaurus rex (Plate I). Triceratops was also exceptionally abundant, or at least its remains were preserved with a high frequency. Barnum Brown estimated that he had seen "no less than five hundred fragmentary skulls and innumerable bones referable to this genus" while working in Montana between 1902 and 1909.2 His statement may have been slightly hyperbolic, but it remains true that, even with only about one-tenth that number of specimens in museums, this is still one of the most abundant dinosaurs that we know. Triceratops is of surpassing importance because it was the first horned dinosaur to be described on the basis of a complete skull. Prior to the description of Triceratops in 1889, it was not clear, despite nearly two decades of finding ceratopsid fossils, that there were great reptilian herbivores with horns on their faces. The concept of horned dinosaurs nucleated around Triceratops. Yet scales did not drop suddenly from the eyes of pioneering paleontologists. Triceratops, like many dinosaurs before and since, was born in controversy and error.
The Chasmosaurinae are generally recognized by the prominence of the paired horns over the eyes combined with a modest horn over the nose, and by having a long frill with a long squamosal bone. The position of Triceratops within the Chasmosaurinae has been controversial until recently, as we will see, because its frill is much shorter relative to basal skull length than that of any other chasmosaurine. The solid frill of Triceratops is also a very unusual feature.
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