Mammals

Mammals (Class Mammalia) appeared in the late Triassic period, about 200 Ma. Mammals were preceded by therapsids, a group of reptiles with many of the characters that were to become typical of mammals, including the jaw joint, middle ear, and tooth replacement. The fossil record of advanced therapsids, known as cynodonts, is so complete that transitional states between basal reptiles and modern mammals have been preserved, thus allowing for a historical reconstruction. Tooth replacement in cynodonts was as in living mammals: one replacement from milk teeth to adult teeth. By contrast, teeth are replaced continuously in living reptiles. Cynodonts also displayed tooth specialization, with incisors, canines, and massive cheek teeth specialized for crushing. Major changes in feeding behavior are also suggested by jaw muscles and bones that allowed for chewing.

A small, shrew-like animal called Morganucodon, from the late Triassic of Europe and China, and a variety of closely related forms, are the oldest known mammals. The body size of these animals is estimated to vary between 20 and 30 g, at least 10 times smaller than the smallest cynodonts of the same period. Morganucodon shows a brain enlargement in olfactory and auditory areas. Since olfaction and audition are distance senses most useful in nocturnal mammals, early mammals were primarily nocturnal, as are the most conservative living

Origin and Evolution of Animals 59 mammals. Some advanced cynodonts have small cavities in the snout suggesting the presence of whiskers (hair), which, in turn, suggest endothermy—the ability to maintain a constant internal temperature (as opposed to ectothermy, when internal temperature is influenced by external temperature). A small, endothermic Mesozoic animal could have exploited the nocturnal niche left open by the visually biased reptiles and birds. A switch to a nocturnal lifestyle may have selected for more extensive olfactory and auditory brain areas, thus leading to the increase in relative brain size typical of mammals (Jerison, 1973). Some Jurassic mammals, such as Hadrocodium (about 195 Ma) exhibit a degree of brain enlargement similar to that ofliving conservative mammals (Luo, Crompton, & Sun, 2001).

Three groups ofliving mammals (about 4,500 species) are distinguished:

• • Egg-laying mammals (also called prototherians or monotremes). There are only three living species of egg-laying mammals in Australia and New Guinea: the platypus and two species of echidnas. These mammals may represent what remains of Jurassic mammals (Rauhut, Martin, Ortiz-Juareguizar, & Puerta, 2002). After hatching from an egg, monotreme infants move into a pouch and feed on milk secreted by glands distributed on the ventral skin of the mother.
• • Marsupials (also called metatherians). There are approximately 260 living species of marsupials restricted to the Americas (opossums), Australia, and some islands north of Australia (kangaroos, koalas, and a variety of other species).
• • Placentals (also called eutherians). There are 4,250 species of living placentals distributed all over the world and in a variety of ecological niches: bats, dolphins, apes, and cats, among many others. Marsupials and placentals are characterized by a distinctive feature: the tribosphenic molar. This molar includes three main cusps, a variable number of small cusps, and the presence of several crests with sharp edges and valleys, all of which improve chewing and reduce the wearing of the enamel.

Mesozoic mammals remained relatively unchanged during approximately 140 Ma of evolution, but an adaptive mammalian radiation occurred during the early Paleocene. The Paleoceno adaptive radiation followed the mass extinction of reptiles at the end of the Cretaceous (Figure 2.15). The opening of ecological niches seems to have been readily occupied by new orders of mammals capable of evolving new body plans and behavioral capabilities.

Marsupials and placentals appeared in the late Cretaceous. Marsupials probably originated in the Americas, from which they dispersed to Europe and Africa, and to Australia via Antarctica during the Cretaceous, when there were land bridges. In marsupials, the embryo emerges from the uterus at an early stage and continues its development in a pouch, where it attaches itself to a teat and extracts milk.

The earliest placentals appeared in the late Cretaceous of Mongolia and the Americas. Most species were relatively small insectivores, wolf-size carnivores, and a cow-size herbivore. In about 10 million years, from the late Cretaceous to the early Eocene, the number of placental families increased from 15 to 78, and the number of genera from 40 to about 200. Factors contributing to this adaptive radiation include improved dentition, efficient feeding behavior, extensive parental care, and increased intelligence.

Most of the modern placental orders were differentiated in the Paleocene, about 60 Ma, and therefore the subsequent evolution of modern lineages has occurred in parallel. Behavioral and neural traits that changed substantially during the Cenozoic, such as social behavior and the neocortex, have evolved largely in parallel from basic mammalian features. Parallel evolution can be appreciated among convergent species of marsupials and placentals. Similar ecological pressures and similar preadaptations have favored the evolution of

Figure 2.15 A phylogenetic tree representing the adaptive radiation of the eutherian mammals during the early Cenozoic (— Tertiary) era. Monotreme, marsupial, and several lineages of extinct mammals are also included in this tree.

Source: Benton and Harper (2009); reproduced with permission from Blackwell.

striking similarities among unrelated species, including adaptations for a subterranean mode of life (marsupial and placental moles) and similar gliding structures (arboreal marsupials and placentals). There were analogous saber-toothed species of marsupials (Thylacosmilus) and placentals (Sniilodori) in the South American fauna. Because of this extensive parallel evolution, comparative psychologists must be cautious when assessing mammalian characters that underwent extensive change during the Cenozoic. For example, knowledge about social behavior or neocortical organization and function obtained from the study of rodents should be extrapolated cautiously to primates. Not only are ecological and life-history factors different, but the underlying neural processes may also differ, even when similar behaviors are observed. Parallel evolution can lead to similar behaviors based upon different neural processes.