The cranial structure of the Triconodonts

Abstract

This paper is a study of the structure of the braincase in two closely related Mesozoic mammals:Triconodon mordaxandTrioracodon ferox. They belong to the order Triconodonta, subfamily Triconodontinae, and are from the English Upper Jurassic (Purbeck). One specimen of each species was available showing cranial structure, both from the collection in the British Museum. By chemical methods, both petrosals and the sphenoid of the specimen ofTriconodonand both petrosals of the specimen ofTrioracodonwere prepared. The material shows that the Triconodonta had a braincase of an essentially reptilian pattern. There was a persistent cavum epiptericum lying outside the ossified lateral wall (formed by the petrosal) of the braincase. The alisphenoid, forming the lateral boundary of the cavum epiptericum, formed no part of the braincase wall in this region. This was also true of the RhaeticMorganucodon, and may have been true of all pre-Cretaceous mammals. In basic construction the braincase of these mammals differs from that of an advanced therapsid only in the narrower cavum epiptericum in the former. This difference is due to the relatively larger size of the brain in the mammal. To convert a braincase constructed in this way into that of a modern mammal either the alisphenoid would have to be lost—leading to the condition found in the monotremes—or the lateral wall of the neurocranum would have to fail to ossify—thus incorporating the cavum epiptericum in the cranial cavity in the manner typical of marsupials and placentals. Although on these grounds alone the monotreme stock need not have separated from that which gave rise to the marsupials and placentals until early in the Cretaceous, other considerations suggest that the last common ancestor lived in Triassic times at the reptilian grade of organization. There seems, however, less reason than formerly to considerMorganucodonan ancestral monotreme. Finally, a reconsideration of all the evidence shows that there was no acceleration of evolutionary rates at the time the Mammalia came into existence.