Introduction
The domestic cat, Felis domestica, belongs to the Mammalia, a synapsid lineage that has its roots in the Triassic Period. Mammals can be readily diagnosed based on living representatives.
Among their more distinguishing features are the presence of an insulating covering of hair, a diaphragm that separates the pleural and abdominopelvic cavities, and mammary glands (to which they owe their name). Such soft tissue features, however, fossilize only under exceptional circumstances, and recognition of the most ancient mammals has been based on other features. Paleontologists have traditionally relied mainly on three skeletal features in determining whether a fossil species had achieved the mammalian condition: (1) the presence of three middle ear ossicles (nonmammalian synapsids have one or, rarely, two); (2) a jaw joint between the squamosal bone of the skull and the dentary of the lower jaw (in nonmammalian synapsids, the jaw joint is between the quadrate and articular bones or, in a few cases, all four of these bones participate); and (3) each half of the lower jaw consisting of a single bone, the dentary (in primitive mammals and nonmammalian synapsids, each half includes more than one bone).Modifications leading to the typical mammalian condition occurred in several advanced cynodont lineages, and the point at which the “mammalian threshold” was crossed has traditionally been defined by possession of a jaw joint between the squamosal and dentary. It is not clear whether all the other features that are usually considered “mammalian” were acquired simultaneously. Evidence from fossils suggests that they were not. Thus, use of the jaw joint criterion to define the Mammalia is arbitrary—any of several other features could equally well be employed. For this reason, paleontologists have moved away from character-based definitions of the Mammalia (or any higher-level taxon) and have come to rely on ancestry as the criterion for defining a group.
Mammalia are now usually restricted to the group that includes the living groups of mammals and their common ancestor, as well as all the extinct descendants of this ancestor. It is true that application of the name Mammalia to this particular group is also arbitrary, but this system does not rely on characters and promotes a more stable system of grouping organisms.There are two major subdivisions of living mammals, the Monotremata and the much more diverse Theria or “higher mammals.” The monotremes are restricted to Australia, New Guinea, and Tasmania and include the duckbill platypus and two species of echidnas. Although they have hair and mammary glands, they retain the general amniote reproductive strategy of laying and incubating eggs. They possess several derived characters, however. For example, adult platypuses lack teeth and the rostrum is covered by a leathery bill or beak.
The Theria is subdivided into the Metatheria (marsupials) and the Eutheria (placental mammals). The metatherians were much more widespread and diverse earlier in their history. Living metatherians, while still fairly diverse, are restricted primarily to Australia and South America, with a fairly recent incursion, the opossum, into North America. Metatherians are also known as marsupials or pouched mammals due to the presence of an abdominal marsupium (pouch) or fold in the female. Typically, the young are born in an immature condition and make their way from the vulva to the marsupium, where they latch on to a nipple and complete their development. This is the reproductive strategy of the more familiar marsupials such as the kangaroo, wallaby, and koala. However, not all marsupials follow this strategy. Some, such as the caenolestids or shrew-opossums, lack a pouch, and others (the bandicoot) even have a placenta similar to that of the placental mammals.
The eutherians are the most diverse and widespread of mammals, present on all continents save Antarctica (with modern humans as the exception, of course).
They have undergone several major radiations during their history and are currently among the more dominant or conspicuous elements of most terrestrial and aquatic habitats. The degree of habit and structural modifications are truly remarkable. Eutherians include the relatively generalized shrews, the fossorial moles, and the armored armadillos. Several groups of herbivorous types have evolved, including rodents, the most numer-
FIGURE 7.1 Skeleton of the cat in left lateral view.
ous of mammals; the bovids, such as the common cattle, as well as bison and sheep; the swift and graceful gazelles; the sleek and powerful equids, such as horses and zebras; and the huge rhinoceroses, hippopotami, and elephants, the largest of living land animals. The chiropterans, or bats, have mastered the skies and are the second most numerous group of mammals, while several groups have invaded the seas. Among the latter are the cetaceans (whales, dolphins, and porpoises) and sirenians (manatees and dugongs), which are completely aquatic, and the pinnipeds (walrus and seals), which live both on land and in the water. The primates, a group that includes humans, are generally arboreal specialists.
Another great group of placental mammals includes the carnivores, to which the cat belongs. Living carnivores include ursids (bears), canids (dogs and their kin), mustelids (weasels and kin), procyonids (raccoons), hyaenids, felids (cats), and viverrids (genets), as well as the pinnipeds mentioned above. The felids include some of the fiercest and largest mammalian predators, such as tigers and lions. The domestic cat is among the smaller members of the group.