The cranial musculature and the origin of the frill in the ceratopsian dinosaurs

Abstract

DOLLo (1884) in his fifth note on the Dinosaurians of Bernissart has discussed the muscles of mastication of certain dinosaurs in comparison with those of a rodent, a crocodile, and achameleon. This work has suggested to the writer a further study of the musculature of the skull, especially in the Ceratopsia, With a View of gaining an insight, if possible, into the origin of the peculiar defensive cranial armor of this remarkable group. These studies are based more particularly on the type skulls of Triceratops serratus and T. 12700748248 Marsh Which are preserved in the Peabody Museum at Yale, supplemented by an admirable palate, referred to the former species, in the American Museum of N atural History. The chief basis for comparative study has been the modern chameleon, Which, as the author Will show, exhibits some very remarkable points of convergence in structure toward the Ceratopsia. ' PART I. Jlluscula'ture. Jl/[uscles 0f mastication.

Functional Morphology and Evolution of the Ceratopsian Dinosaurs

“…The ongm and development of these structures constitute the principal (distinctive) evolutionary trends in the Ceratopsia, but other trends recorded in various taxa were 1) a general tendency for increased size (Colbert, 1948(Colbert, , 1949, 2) reduction of tail length, 3) significant shortening of the hind limbs (relative to body length), and 4) minor relative shortening of the forelimbs. Lull (1933) and Colbert ( 1948) correlated these latter changes with increased quadrupedalism related to (and perhaps required by) progressive increases in body size. It is quite obvious, however, as Colbert (1948) has noted, that the evolutionary modification of ceratopsian postcranial anatomy was very conservative in contrast to the remarkable alterations recorded in cranial morphology.…”

“…Each (Lull, 1908 and1933;Tait and Brown, 1928;Russell, 1935;Haas, 1955;Ostrom, 1964) noted this unusual manner of occlusion in advanced ceratopsians, but its importance in ceratopsian evolution has not been generally recognized. Fig.…”

“…The unique association of long squamosals and a short frill in Triceratops has generated some dispute over the phyletic placement of this genus. It apparently led Lull (1933) to suggest that Triceratops may have been derived from either Monoelonius or Arrhinoceratops (Lull and Gray, 1949, later Sternberg (1949) adopted the Arrhmoceratops-Triceratops alternative. Before these alternatives can be resolved, it is necessary to consider the probable function of the ceratopsian frill.…”

“…Ceratopsian frill function was first explored by Lull ( 1908) in Triceratops. This was followed in 1935 by L. S. Russell's analysis of Chasmosaurus and in 1955 by Haas' study of Protoceratops.…”

“…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.…”