FUNCTIONAL MORPHOLOGY AND EVOLUTION OF THE CERATOPSIAN DINOSAURS.

Abstract

In 1902, H. F. Osborn suggested that the ceratopsian or horned dinosaurs would prove to be diphyletic with Torosaurus and Triceratops representing terminal members of two divergent evolutionary trends-one lineage typified by fenestrated parieto-squamosal frills and a second line featuring non-fenestrated frills. Subsequent discoveries have altered some of the details, but these two Lance-age genera are still considered distinct and as yet no other unquestionable contemporaneous genera have been discovered. Consequently, most phylogenies so far proposed (Lull, 1908; Lambe, 1915; Lull, 1933; Schlaikjer, 1935; Colbert, 1948; Sternberg, 1949), although differing in details, have adhered to a bilineal evolutionary history for the Ceratopsia. These lineages are now commonly referred to as the long-frilled and short-frilled lines. If the fossil record has been interpreted correctly and existing ceratopsian specimens do in fact represent two distinct evolutionary lineages, then there must have been a functional reason for these divergent trends. The object of this study was to explore the functional morphology of certain distinctive ceratopsian features in an attempt to assess the adaptive significance of recorded morphologic changes and perhaps thereby the why of ceratopsian evolution. In the following discussion, reference is made to specimens in the collections of several institutions, the names of which are abbreviated as follows: American Museum of Natural History (AMNH); National Museum of Canada, Ottawa (NMC); United States National Museum (USNM); Peabody Museum of Natural History, Yale University (YPM). CLASSIFICATION