530 FOSSIL VERTEBRATES/Mesozoic Mammals degree of medio-lateral separation from the mandible, whereas the middle ear bones are connected anteriorly to the mandible via the Meckel’s cartilage, even in adults The best available evidence suggests that loss of the Meckel’s cartilage may have occurred separately in modern monotremes and in the lineage of placentals plus marsupials, after the ancestor of the latter groups had split from eutriconodontans Elaborated Structure of Inner Ear Cochlea Mammals are most derived among living vertebrates in their complex inner ear cochlea and related hearing adaptation The petrosal bone has developed an enlarged bony housing for the cochlear part of the inner ear; this is crucial for sensitive hearing function, especially for high-frequency sound Development of the derived inner ear and its bony housing occurred in several evolutionary steps in the transition from cynodonts and early mammals, as evidenced by the precursor conditions of these structures in mammaliaforms Premammalian cynodonts have a cochlear cavity for the auditory function in the inner ear This cochlear cavity is small and globular; it does not extend anterior to the fenestra vestibuli in the reconstructed inner ear endocast The bony housing for the inner ear is formed by a mosaic of multiple bones Two of these bones, the prootic and the opisthotic bones, correspond to the homologous bones in extant nonmammalian vertebrates, which are developed from ossification of the embryonic auditory capsule in modern vertebrates None of the bones surrounding the inner ear are fused The bony housing of the inner ear in adults of the cynodont Thrinaxodon is formed by a mosaic of several bones, which can be either endochondral or intramembranous in embryonic origin among extant vertebrates By comparison, the inner ear of Early Jurassic mammaliaforms has an elongate bony cochlear canal that is much better developed compared to the cochlear cavity of premammaliaform cynodonts The elongate bony cochlear canal may indicate a greater sensitivity to high-frequency sound, which is very important in the hearing function of all extant mammals and was probably important for at least some of the earliest mammals The bony housing of the inner ear in stem taxa of mammals is formed exclusively by the petrosal, which is the single bone homologous to the fused prootic and opisthotic elements in premammaliaform cynodonts The petrosal is not only much larger than the prootic and opisthotic, but it also forms a distinctive structure of the promontorium The enlarged petrosal excludes the other cranial bones, such as the basisphenoid complex, the basioccipital, and the exoccipital, from the bony housing for the inner ear The mosaic of multiple bones for the inner ear housing of cynodonts is replaced by a single bone in the derived mammaliaforms, including modern mammals The differences between mammals and cynodonts in the inner ear and its bony housing are the result of their correlated structural transformation Enlargement of the promontorium is correlated with elongation of the cochlear canal The enlarged promontorium displaced the neighbouring sphenoid complex and basioccipital bone Inflation of the bulbous promontorium in the mammalian crown group is associated with coiling of the cochlear canal If the precursor condition in mammaliaforms is mapped on the cynodont-mammal evolutionary tree, there is a clear pattern of incremental evolution of mammalian characteristics in the inner ear cochlear canal and the inner ear bony housing The Larger Brain In early mammalian evolution, there is a clear tendency towards a larger volume of brain endocasts, and their morphological features are also better differentiated in the more derived taxa The relative brain sizes of the transitional mammaliaforms of the Early Jurassic are intermediate between the larger brain size of the derived Cretaceous mammals and the smaller brain size of the Triassic cynodonts Larger brain capacity of mammals compared to that of nonmammalian vertebrates indicates a larger volume of metabolically expensive neural tissues of the mammalian brain This can be further correlated to the development of a more elevated metabolism in early mammaliaforms than in premammaliaform cynodonts It could also be correlated with better sensory perception and elaborated neural control of the skeleto-muscular system for mastication and locomotion In the larger brain endocasts in the successively more derived mammaliaforms and mammals, the cerebral hemispheres are much better developed in the successively more derived groups The posterior part of the brain endocast tends to show a better differentiation of the mid-brain from the cerebellar structure in the mammalian crown groups: metatherians, eutherians, and monotremes Related to the tendency of increasing brain size, new structures in the braincase in the derived mammaliaforms are absent in more primitive cynodonts For example, the braincase of mammaliaforms has a bony floor to enclose the trigeminal ganglion of the cranial nerve V into the braincase (Figure 1: Sinoconodon) The enlarged brain is correlated with a posterior shift of the braincase relative to the jaw hinge and other skull structures In most premammaliaform cynodonts, the