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Functional Morphology and Evolution of the Ceratopsian Dinosaurs

John H. Ostrom
Evolution
Vol. 20, No. 3 (Sep., 1966), pp. 290-308
DOI: 10.2307/2406631
Stable URL: http://www.jstor.org/stable/2406631
Page Count: 19
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Functional Morphology and Evolution of the Ceratopsian Dinosaurs
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Abstract

Ceratopsian dinosaurs were characterized by a number of peculiar features that set them apart from all other tetrapods. The development of these features, reflected to varying degrees in the array of North American genera, constituted the major evolutionary trends within the group. Some of these have been noted before, others have not. The most conspicuous evolutionary developments were 1) extreme enlargement of the head, 2) two divergent trends of frill enlargement, 3) the development of large turtle-like beaks, 4) the perfection of unique shearing dentitions, 5) a general tendency for increased size, 6) increased quadrupedalism, and 7) development of either brow or nasal horns. The first four of these are related directly to problems of food-gathering or preparation, and are considered in detail here. The origin and enlargement of both long (Torosaurus type) and short (Triceratops type) parieto-squamosal frills is correlated with extreme enlargement of the jaw muscles (M. adductor mandibulae externus) that attached to the coronoid process. The resultant magnification of jaw power is believed to have been related to the unusual shearing dentition of advanced ceratopsians. Perfection of the shearing dentition involved 1) development of a long edentulous anterior jaw segment behind the beak, 2) a progressive increase in the number of tooth positions in the tooth row, 3) longitudinal compaction of tooth rows to unite individual teeth into long, continuous dental batteries, 4) increase in tooth replacement rate, 5) enlargement of tooth size, 6) increased transverse curvature of tooth root and crown, 7) lateral displacement of upper teeth relative to lower teeth, 8) increase in size of the central carina of tooth crowns, and 9) a progressive backward displacement of the entire tooth row. Except for Leptoceratops, all post-Protoceratops taxa reflect each of these modifications to varying degrees. Associated with these changes were structural alterations in the design of the mandibular lever. The short-frilled line is shown to have been characterized by progressive improvement in the jaw mechanics, resulting in progressively greater leverage for the jaw muscles in successive forms. This was effected by an increased height of the coronoid process, increased depression of the jaw articulation, and lowering of the angle of muscle application. The long-frilled line cannot now be analyzed completely, because of a lack of critical material of the latest members of the lineage, but limited analyses indicate that this line may not have been characterized by comparable mechanical improvements in the mandibular lever. This may be the reason for the development of much longer parieto-squamosal frills in the Torosaurus line, reflecting the greater size of the external adductor muscles. Exclusive shearing dentitions, efficient and powerful jaw mechanics, and extremely large jaw muscles equipped these reptiles to feed on a peculiar unknown diet. Whatever this diet was, it clearly was very resistant and probably very fibrous-perhaps like the fronds of cycads, or palms.

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