MORRISON MAMMALS: TRICONODONTA 29 



Order TRICONODONTA Osborn 



Definition. — Incisors small and three or four in number. Canines present. Mo- 

 lars always with three main cusps arranged in a longitudinal series, the central cusp 

 of the lower molars somewhat higher than the other two and the anterior and posterior 

 cusps subequal. Internal cingulum present. Lower molars shearing obliquely up the 

 internal slope of the upper molars. Mandible without angular process, with distinct 

 masseteric and pterygoid crests and high strong coronoid. Zygomata stout, not ex- 

 panded laterally, arising from posterior ends of alveolar processes of maxillae. 



Distribution. — Middle and upper Jurassic, England. Upper Jurassic, North 

 America. 



The triconodonts are confined to the Jurassic, but here play an important part. 

 They are the carnivorous mammals of the period and are relatively abundant in each 

 of the three best known Jurassic mammalian faunas. One of the two Mesozoic mam- 

 mals first brought to scientific attention was a triconodont. At first confused with 

 Amphitherium, a pantothere, it was placed in a distinct genus, Phascolotherium by 

 Owen in 1838. The essential unity of the group was clearly recognized by Owen, who 

 in 1 87 1 placed Phascolotherium, Triacanthodon, and Triconodon together. In 1887 

 Marsh proposed and accurately defined the family Triconodontidae, to include Tri- 

 conodon, Priacodon, and possibly some other genera not specified. Marsh, however, did 

 not clearly differentiate between triconodonts and symmetrodonts, a confusion which 

 has been shared by most subsequent writers. Thus he at first referred a typical tricono- 

 dont, Priacodon ferox, to the symmetrodont genus Tinodon. This mistake he soon cor- 

 rected, but in 1887 he still referred the triconodont Phascolotherium to the family 

 Tinodontidae, although with doubt. 



There is some question whether Marsh intended to include the Triconodontidae 

 in his order Pantotheria. He nowhere made a clear statement on this point, but there is 

 some reason to believe that he did not. Thus (1887, p. 345) he says in defining the 

 Pantotheria "Premolars and molars imperfectly differentiated" while of the Tricono- 

 dontidae he says (p. 341 ) "In this group the premolars are unlike the molars." He also 

 states that the pantotherian angle is without inflection, whereas it was then universally 

 believed that the triconodont angle was inflected. It is clear that the order Pantotheria 

 was not defined with the triconodonts in mind, and it is probable that he meant to leave 

 the latter incertae sedis, for they are obviously excluded from the Allotheria also. 



Osborn (i888a) at first referred the triconodonts to his "Prodidelphia" along 

 with the Amphitheriidae, Peralestidae, and Kurtodontidae. He enlarged the family 

 Triconodontidae, including three subfamilies. The subfamily Amphilestinae was 

 Marsh's Triconodontidae with the addition of Amfhilestes, which Marsh had not 

 mentioned, and of Amphitylus, a form which, as Osborn later recognized, belongs in a 

 different order. Osborn's Phascolotheriinae of 1888 was identical with Marsh's Tino- 

 dontidae, including Phascolotherium and Tinodon, and his Spalacotheriinae was 

 Marsh's Spalacotheridae, including Sfalacotherium, and Menacodon. The term Tri- 



