MULTITUBERCULATA, an order of extinct mammals. Their relationships to other groups are best considered by giv ing a brief outline of the Mesozoic mammals.
Although the Cenozoic Era is popularly known as the age of mammals, about two-thirds of mammalian evolution took place during the Mesozoic. The remains of Mesozoic mammals are among the rarest, most fragmentary and most valuable of all fossils. These earliest mammals were very small, averaging about the size of a rat. It is clear that they were locally abundant and included insectivorous, carnivorous and herbivorous types.
The oldest known mammals are from the late Triassic (or possibly earliest Jurassic) of England, Germany and South Africa. These belong to two quite distinct groups both referred to the Allotheria (see below). Both are very aberrant, however, and the true ancestral Triassic types of mammals are still unknown. The Jurassic mammals are much better known, and are di vided into four orders, all extinct. Characteristic of the whole Mesozoic and also of the oldest Cenozoic (Paleocene) were the Allotheria or Multituberculata, ranging from the upper Triassic into the true Eocene and known to have occurred in Europe, Africa, Asia and North America. They had a single pair of en larged incisors below and two or three pairs, of which only one was enlarged, above. The cusps of the molars were arranged in two or three straight longitudinal rows. In the Jurassic each molar had only five or six cusps, but in some of the later types as many as 3o cusps occurred on a single tooth—whence the name. The upper premolars were generally composed of conical cusps, less regularly arranged than those of the molars, and, except in a few aberrant forms, the lower premolars were developed into large, compressed, sharp-edged shearing teeth. They appear to have been vegetarians. Some show a superficial resemblance to the living rat-kangaroos of Australia, but recent work indicates that the multituberculates constituted an entirely separate sub class, not ancestral to any later mammals and related to the latter only through common ancestry in the Triassic or even earlier.
In the second Jurassic order, the Triconodonta, the molar teeth have three cusps each, arranged in a straight longitudinal line. Although small, these mammals were predaceous carnivores. They were confined to the Jurassic and probably represent an other aberrant offshoot of the earliest mammalian stock.
A third Jurassic order, the Symmetrodonta, includes mammals long classed with the Triconodonta, although recent work indi cates they are a distinct order related rather to the Pantotheria than to the Triconodonta. Confined to the upper Jurassic, so far
as known, they are characterized by triangular teeth, typically with three cusps.
The last order of Jurassic mammals, the Pantotheria, was the most important phylogenetically. The dentition, while more primitive and different in details, has a broad resemblance to that of the most generalized insectivores and marsupials. The typical dental formula was It; Mi, although various genera show marked modifications from this, chiefly by reduction. The upper molars were triangular with a large internal cusp and usually one main external cusp and several lesser ones. The lower molars typically had four main cusps arranged in an asymmetrical tri angle, followed by a heel, or talonid, with one cusp. The panto theres apparently represent the stock which gave rise early in the Cretaceous to the marsupials and placentals. The earliest and most primitive pantothere was Amphitherium from the English middle Jurassic (Stonesfield).
Lower Cretaceous mammals are known only from a few poorly preserved teeth from the English Wealden, of slight importance, but the upper Cretaceous mammals are relatively well known. The multituberculates continue as more advanced forms referred to the family Ptilodontidae, with more numerous molar cusps. The other Jurassic forms, however, have disappeared and are replaced by the earliest members of the Insectivora and Mar supialia (qq.v.).
The Cretaceous marsupials, known at present only from North (and possibly South) America, are all closely related to the living opossums, but are much more varied in size and other characters. They probably represent a widespread complex of primitive mar supials from which the later and more specialized South American and Australian forms were derived.
Of the upper Cretaceous Insectivora, only a single genus, Gypsonictops, is known with certainty from North America, but in Asia the expeditions of the American Museum of Natural History have brought to light eight skulls. These are very close to the ancestral stock from which the majority of placental mammals were derived. Zalambdalestes, a peculiar long-snouted form, and Deltatheridium, a very primitive and generalized type, are the most important.
The following is an outline classification of the major groups of the Mammalia: