Mammals (Pre-Quaternary), Extinctions of representatives of a few other lineages of nonmammalian synapsids, which were the basal stocks of another but lesser evolutionary radiation Later, in the Early Triassic, large and small herbivorous forms, primarily dicynodonts, became prominent elements of terrestrial faunas In parallel, the surviving carnivorous therapsids radiated, producing a variety of new lineages of large and small carnivores as well as some herbivorous species In comparison to Permian terrestrial faunas, synapsids did not regain their dominant position in Triassic terrestrial vertebrate faunas Analysis of the evolutionary radiation of the carnivorous therapsids during the Triassic reveals trends in modification of skull structure and morphology of the postcranial skeleton that increasingly came to resemble those characterizing modern mammals (Kemp, 2005) For example, the lower jaw was modified through expansion of the dentary bone and the reduction or losses of other bones characteristic of early synapsid or reptilian jaw structure These were preliminary steps toward acquisition of an articulation of the dentary bone of the jaw with the squamosal bone of the skull, a key skeletal character for distinguishing modern mammals from reptiles Limb posture began to be modified from a primitive, sprawling stance to a more typically mammalian stance with the elbow and knee tucked close to the body and the limbs brought into an upright posture A few million years before the end of the Triassic, some synapsids evolved a functional articulation between the dentary and squamosal Many textbooks and popular articles present the acquisition of this structure as the hallmark of the Mammalia and identify the end of the Triassic (approximately 251 Ma) as the time of origin of the group In current phylogenetic classifications, however, the clade Mammalia is restricted to a crown group including the common ancestor of monotremes (the echidna and platypus), marsupials, and placentals and all its descendants Phylogenetically, some of the very advanced nonmammalian synapsids with a dentary–squamosal jaw articulation and other mammal-like specializations lie outside the crown group; they are sister lineages In many recent research papers, these very advanced nonmammalian synapsids are referred to as mammaliaforms During the Late Triassic, the composition of terrestrial faunas underwent a major change that played a significant role in reshaping the courses of evolution of synapsids and other vertebrates All the lineages of Triassic synapsids, except for a few lineages of mammaliaforms and one group of herbivorous nonmammalian synapsids, the tritylodonts, became extinct by the end of the Triassic All the surviving lineages were represented by species of relatively small body size During this interval two groups of reptiles, the rhynchosaurs and later several lineages of dinosaurs became the dominant terrestrial vertebrates Identification of the causal factors behind the Late Triassic decline of therapsids and the contemporaneous increasing diversity of dinosaurs as well as the mass extinctions in both the marine and terrestrial realms marking the end of the Triassic continues to be a matter of debate It has been argued that the early dinosaurs were in some way competitively superior in their interactions with the therapsids and this was the cause of extinction of these lineages More likely, the decline in diversity and demise of some therapsid lineages in the Late Triassic is attributable to changes in the environment Taking advantage of the losses of these lineages, dinosaurs radiated opportunistically Extensive eruptions of flood basalts in rifts developed by the initial separation of North America from Europe and Africa (the Central Atlantic Magmatic Province) and associated modifications of the atmosphere appear to have been distal causal factors of the Triassic–Jurassic boundary mass extinctions Mammalian Evolution During the Age of Dinosaurs During the Jurassic and Cretaceous, popularly dubbed the Age of Dinosaurs, mammaliaforms and early mammals were not dominant members of terrestrial faunas Functional interpretations of their dentitions suggest that most were probably carnivorous, feasting on small prey including small terrestrial vertebrates and invertebrates For the most part, they were very small animals, in the size range of modern mice and rats A few evolved larger body sizes, rivaling modern opossums or raccoons, but these species were exceptions to the rule Recent discoveries reveal that in spite of their relatively small size during the Jurassic, mammaliaforms evolved diverse ecomorphological specializations (Luo, 2007) For example, two lineages of closely related mammaliaforms evolved aquatic specializations, one with a beaver-like flattening of their tails and another with an otter-like transverse compression of their tails Members of another mammaliaform lineage developed folds of skin linking their neck, limbs, and tail to produce membranes resembling those of modern gliding squirrels For the most part, these mammaliaform lineages became extinct by the middle of the Cretaceous One group that appears in the fossil record in the Jurassic, the multituberculates, evolved a remarkably rodent-like style of dental specialization and is interpreted as having been omnivorous or herbivorous in dietary preferences This interpretation is strengthened by the persistence of the group into the Paleocene of the Northern Hemisphere Then, in the late Paleocene as rodents began their evolutionary radiation, the taxonomic diversity of multituberculates dwindled By the end of the Oligocene, when almost all families of rodents had differentiated, multituberculates had become extinct Tracing the course of mammalian evolution during this phase of synapsid evolution is complicated by distinct biogeographic differentiation Unlike the Permian and Triassic, when most of the terrestrial regions of the world were parts of one supercontinent, Pangaea, through the Jurassic and Cretaceous terrestrial areas and their faunas were fragmented In part, this was the product of plate tectonic processes that broke up the Pangaean supercontinent and shifted the positions of the resulting continental blocks Additionally, the Age of Dinosaurs was a time characterized by extensive marine transgressions when shallow seas covered many areas and added to fragmentation of terrestrial areas By the beginning of the Cretaceous, when the currently sparse fossil record of these faunas gives us our first real picture of biogeographic differentiation, there is evidence of distinct northern (North American–Eurasian) and southern (Australian–Antarctic–South