THE RISE OF THE MAMMALIAN ORDERS During the Triassic, Jurassic and Cretaceous, the fossil record of the mammals is meagre ; later it is relatively abundant ; but even then there are blank intervals and imperfectly preserved records. As many existing orders of mammals began to diverge during the Age of Reptiles, when the record is most imperfect, it is difficult to reconstruct the earlier history of the mammalian orders from present data.
Probably by Upper Triassic times some of the smaller mammal like reptiles, a group already almost mammals, had crossed the line by acquiring a hairy covering and a new contact between the dentary bones of the lower jaw and the squamosal bone of the skull. With the coming of Lower Jurassic times the mammalian stock had split into three distinct groups: (I) the Allotheria or Multituberculata (q.v.), rodent-like mammals with gnawing front teeth and many-cusped grinding teeth; (2) the Triconodonta, small carnivorous forms with triconodont molar teeth, each crown consisting typically of three cusps in a fore-and-aft line and (3) the Pantotheria, or Trituberculata, very small insectivorous mam mals typically with sharp-cusped lower molars of the most primi tive tuberculo-sectorial type. The Allotheria were probably not closely related to any other order of mammals but were a peculiar, now wholly extinct group extending in time from the Upper Triassic to the summit of the basal Eocene. The Triconodonts also appear to be an isolated and extinct group, but certain of the Pantotherians, such as Amphitherium, appear sufficiently gen eralized to be the potential ancestors of all later mammals ex cept the recent monotremes which at present show distant rela tionships with the marsupials, but are still without known fossil ancestors, the features they share with the Allotheria being offset by many others that indicate wide differences. The marsupials may well be the descendants of some of the Pantotheria but actual connecting links are wanting. The same is true of the Placentals, which are first known in abundance in the basal Eocene of North America. Recently, however, the field parties
of the American Museum of Natural History in Mongolia have discovered several incomplete skulls of small mammals which appear to represent some of the Cretaceous forerunners of the centetoid insectivores and perhaps also of the most primitive carnivores or creodonts.
From comparison of the osteology and anatomy of the recent monotremes, marsupials and placentals, however, we may infer with high probability that the ancestral mammal was an egg laying vertebrate that retained a primitive reptilian type of shoulder-girdle with two complete coracoid plates on either side and with the beginnings of the neopallium in the brain. From such primitive types the marsupials (see MARSUPIALIA) evolved by developing, among other characters : a three-way, vaginal passage; (2) the true or allantoic placenta was early replaced by a false or yolk-sack placenta; (3) the corpus callosum or great cross-band between the opposite halves of the neopaliium was not developed; (4) all the milk teeth were suppressed except the hindermost premolars and (5) the dental formula of the adult dentition was reduced from a higher number to I CI P4 MI. On the other hand, the primitive placentals apparently never de veloped either the three-way vagina, or the yolk-sack placenta, but their true placenta early became highly developed, as did the corpus callosum ; the entire set of milk teeth was retained and the dental formula was early reduced to I. C+ P-9,- M4 The marsupial group may have been dominant in the Lower Cre taceous, when fossil records are practically blank. By the Lower Eocene the marsupials were already a defeated group which took to the trees and persisted in North America only in the form of the opossums, among the most primitive of all living animals; in South America, however, they gave rise to several extinct families (Borhyaenidae, Caenolestidae, Didelphidae) and in Aus tralia they became the dominant mammalian forms and gave rise to a great series of families, including the dasyures, bandi coots, phalangers, kangaroos and others.