200-147 million years ago




The skull modifications of early mammals included the solidification of the skull, the conversion of the small ancestral bones of the reptilian jaw joint into middle ear bones, the loss of tooth replacement, and the development of chewing teeth.

In the early mammals, the braincase increased in size and here was additional solidification of the separate ancestral elements of the skull. The components of the modern temporal bone underwent greater fusion. The bony parts of the otic capsule, referred to as the periotic and opisthotic in cynodonts, joined to form the petrosal. The base of the petrosal extended anteriorly and contained 2 depressions for the ganglion of cranial nerve V and the floccular lobe of the cerebellum. The ancestral components of the modern sphenoid bone also underwent greater fusion. In mammals, the pterygoid was reduced, the orbitosphenoid center of ossification filled in area of medial orbit which was previously open, forming the optic foramen, the basisphenoid fused to the pterygoid and parasphenoid, the ectopterygoid expanded superiorly to make a lateral wall which contacted dermal bones, and the parasphenoid was greatly reduced in size. Mammals also develop a styloid process of the temporal bone and a condylar foramen for cranial nerve XII. Primitive mammals possessed a cavum epiptericum which surrounded the trigeminal nerve ganglion. In modern mammals it is still identificable, although it has fused to the surrounding bone.

In Mesozoic mammals, the only jaw joint was that between the dentary and the squamosal bones. The smaller quadrate and articular were no longer involved in jaw articulation, but instead functioned in the middle ear as the malleus and incus. The angular continued to support the tympanum, which must have been large, given the size of the angular. The jaw joint in most early mammals jaw joint was located above the level of the tooth row (but not in Sinoconodon). After the earliest mammals, the prearticular, angular, and surangular were no longer located in a groove in the lower jaw.

Primitive mammals such as Morganucodon possessed a tooth count of 5,1,4,4/4,1,4,4. These teeth showed mammalian modification in that they were replaced only once (with no replacement of molars) and that occlusion of upper and lower teeth allowed more efficient chewing. . In mammals, canines, premolars, and incisors are replaced once while molars are not replaced at all. Since cheek teeth are permanent, it is an advantage to have matching surfaces on upper and lower teeth (occlusion); the occlusion of mammalian molars and premolars are an important characteristic.

Hadrocodium was a small mammal known from the Early Jurassic about 195 million years ago Its braincase is enlarged in the alispheniod and parietal regions (wider in each case). Its addition of the malleus and incus as ear bones is inferred from the absence of the postdentary trough which would otherwise contain them in the lower jaw. This trough is present in other early mammals such as Morganucodon, kuehneotherids, and Haldanodon. The swelling which contains the inner ear in the petrosal bone is larger than in other mammaliforms. It estimated body weight would have been about 2 grams, about the same size as the smallest living bats (2 grams) and insectivores (2.5 grams). The advanced features of Hadrocodium put it closer to the main mammalian lineage than Morganucodon, Sinoconodon, Adelobasileus, and Haldanodon. It is the sister group to mammals with living representatives and triconodontids (Luo, 2001).