Non-ceratopsid neoceratopsians are an interesting and varied group with a long history in dinosaur paleontology. For example, one of the most abundantly represented dinosaurs in the geologic record is Protoceratops andrewsi, a small-frilled, unornamented neoceratopsian that only reached about 2.5 meters in length. This neoceratopsian was postulated as the possible inspiration for the griffin, a mythical beast with lion and bird features whose legend arose out of central Asia, where skeletons of Protoceratops occur abundantly (Chapter 3). Protoceratops has been nicknamed the "sheep of the Gobi" because its specimens are common in Late Cretaceous deposits of Mongolia. Moreover, they were speculated to be defenseless fodder ("Mesozoic mutton") for their carnivorous contemporaries. Protoceratops is famous as one of the dinosaurs discovered in the original Mongolian expeditions led by Roy Chapman Andrews of the American Museum of Natural History (Chapter 3). For nearly 70 years, Protoceratops was credited as the egglayer of the numerous clutches found in the Gobi, until the discovery that the eggs were actually laid by its supposed egg predator, Oviraptor (Chapter 9). One specimen of Protoceratops contradicts its species' reputation as a passive, docile animal in that it may have fought a Velociraptor to the death (see Fig. 7.9). Enough skeletons of Protoceratops have been studied that some paleontologists think that they can discern sexual dimorphism within the species. The form with a more vertically raised frill has been arbitrarily identified as the "male" form. Other basal neoceratopsians share characteristics with Protoceratops, but it is recognized as a more derived form than some of the others.
As fascinating as Protoceratops and other smaller neoceratopsians are, ceratopsids stand out from the Late Cretaceous dinosaurs in many ways. They were among the largest herbivores of their time and the largest ornithischians of the entire Mesozoic, some having reached 8-meter lengths and weights of 7 to 8 metric tonnes. More importantly, with regard to their biology and aesthetics, ceratopsids also carried the most beautiful "advertisements" ever seen in dinosaurs. Their visually appealing traits included single or multiple horns that were hooked or straight, positioned either on or above the nares and orbits (both seen in Triceratops), or on the periphery of the frill. Broad or long frills, as well as parietal fenestrae, seen as large holes in the frill in some species, were other attributes that best express ceratopsid diversity. Other details of the skeletal anatomy that distinguished them from their neoceratopsian predecessors were:
1 a femur longer than the tibia; and
2 10 sacral vertebrae that fused to form a cohesive unit in the hip, which helped to support some considerable musculature.
The many varied members of this group can be further categorized on the basis of squamosal lengths into the clades Chasmosaurinae (= long squamosals) and Centrosaurinae (= short squamosals). Chasmosaurines include Anchiceratops, Arrhinosaurus, Pentaceratops, the previously mentioned Triceratops, Torosaurus (Fig. 13.8), and Chasmosaurus (Fig. 13.9). Two primitive genera of the chasmosaurines are Pentaceratops and Chasmosaurus. Among the traits that distinguished them from more advanced genera in their clade are very large fenestrae in their parietals. Avaceratops, Brachyceratops, Centrosaurus, Monoclonius, Pachy-rhinosaurus, and Styracosaurus are all centrosaurines, the most basal of which are Avaceratops and Brachyceratops. These two are not crowned by the horns on the frills seen in more advanced forms such as Styracosaurus (Fig. 13.10). All genera of chasmosaurines and centrosaurines found so far occur only in Upper Cretaceous strata of western North America; the centrosaurines have an even narrower geologic range and geographic distribution. As discussed later, centrosaurines are
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