At first sight, this is not a difficult question. Every child is able to identify an animal as a mammal. Since its earliest age it can identify what is a cat, dog, rabbit, bear, fox, wolf, monkey, deer, mouse, or pig and soon experiences that with anyone who lacks such a knowledge there would be little chance to communicate about other things as well. To identify an animal as a mammal is indeed easy. But by which characteristics? The child would perhaps explain: Mammals are hairy four-legged animals with faces.
Hair, or fur, probably the most obvious mammalian feature, is a structure unique to that group, and unlike the feathers of birds is not related to the dermal scales of reptiles. A mammal has several types of hairs that comprise the pelage. Specialized hairs, called vibrissae, mostly concentrated in the facial region of the head, perform a tactile function. Pelage is seasonally replaced in most mammals, usually once or twice a year by the process called molting. In some mammals, such as ermines, the brown summer camouflage can be changed to a white coat in winter..
The order name "Primates" (literally: "those of first rank") was introduced by Linnaeus in 1758 for a group that included man along with several non-human primates known at that time. Interestingly, Linnaeus also included bats in his order Primates, but this was soon abandoned by other taxonomists. However, some authors have questioned the proposed link between Plesiadapiformes and Primates and the principal similarities involve the molar teeth.
It is, in any case, generally agreed that the Plesiadapiformes branched away before the origin of modern primates. They are hence no more than a sister group and have accordingly been given the label "archaic primates." Modern primates and their direct fossil relatives ("primates of modern aspect" or Euprimates) can only be traced back to the basal Eocene.
Of the 4,629 species of mammals recognized by Wilson and Reeder, rodents represent 43% of species diversity within the class Mammalia. Several diagnostic characteristics associated with general morphology define rodents as a monophyletic group (e.g., group sharing a common ancestry). The primary characteristic is the pair of long incisors, resulting from the loss of canines and the creation of a diastema or gap between the incisors and cheek teeth, consisting of premolars and molars.
Other characteristics of the skull and skeleton, dentition, and basic soft anatomy tend to support monophyly. Although the monophyly of Rodentia appears well supported on the basis of examinations of morphological traits of living forms as well as fossil lineages, several studies based primarily on amino acid sequence data from nuclear genes and some limited analysis of nucleotide sequence data have suggested that the order Rodentia is not monophyletic.
Linnaeus originally assigned the name Cete to the order of mammals consisting of whales, dolphins, and porpoises. The term is derived from the classical noun cetos, meaning a large sea creature. Linnaeus conceived Cete to be the sole member of the group Mutica, one of his three primary subdivisions of placental mammals.
The term Cetacea is the plural of cetos and was coined by Brisson in 1762. The study of cetaceans has come to be known as cetology, those who practice it as cetologists. The lines of demarcation between the living cetaceans and other orders of mammals are firmly drawn, and there is no ambiguity.
Similarly, the two living suborders of Cetacea are unequivocally distinct from each other, but also monophyletic; that is, derived from a common ancestor. The Mysticeti, or baleen whales, and Odontoceti, or toothed whales, differ fundamentally in the ways that the bones of their skulls have become "telescoped."
The evolutionary history and systematics of the Carnivora are clouded in controversy, as the fossil record is patchy and incomplete. In spite of this limitation it is remarkable what paleontologists, evolutionary biologists and geneticists have managed to uncover in the way of the early history of mammals. A major breakthrough has been the development of accurate methods to date fossils.
About 65 million years ago (mya) the dinosaurs, which were the dominant animals on Earth, underwent a rapid and mass extinction. At this time the mammals were small shrew-like creatures. With the extinction of the dinosaurs many ecological vacancies, known as niches, opened up, including that of predator, and the mammals quickly filled many of them.Learn More
The timing of the origin of extant Insectivora species has been and continues to be a topic of great debate among scientists. It is widely accepted, however, that Insectivora are the most primitive of the true placental mammals existing today and the ones from which present day mammals evolved. Most of the primitive eutherian (placental) mammals were insectivores. To date, common classification practice has been to include some of the primitive eutherians with all recent Insectivora members based on their similar dentition. The earliest insectivore fossils are believed to be those of Batodon and Paranyctoides. These remains date from the mid to late Cretaceous period, approximately 100 million years ago (mya). Remains of small insect eaters from Asia, zalambdalestids, along with kennalestid remains from Central America, are believed to date back to the Late Cretaceous.Learn More
Understanding the evolutionary beginnings of the early artiodactyls, like that of the early ungulates, is hampered by an incomplete fossil record. Also, the artiodactyls appeared abruptly, along with early perissodactyls, without any clear intermediate forms between the early ungulates and the early artiodactyls. Some aspects of the evolutionary story are difficult to follow because the characteristics used to assign taxonomic position do not fossilize.
Their remains have been found only in deposits from Central and North America. The Merycoidodontidae were a very diverse group of small-and medium-sized, stocky-built herbivores, the largest of which was said to be about the size of a wild boar. They were highly successful having appeared in the late Eocene, flourished in Oligocene and Miocene, before becoming extinct in the Pliocene.Learn More
To fuel endothermy, mammals require more calories per ounce (or gram) of tissue than do ectothermic vertebrates such as reptiles. This is accomplished by more efficient digestion of food stuffs and more efficient absorption of nutrients. This efficiency begins with specialization of the teeth. Mammals have four different kinds of teeth (heterodonty) that are ideally shaped to cut, slice, grind, and crush food. An exception is the toothed whales in which all the teeth are similar (homodonty).Learn More
Mammals also regulate their body temperature within a stable range, generally between 87 and 103°F (30-39° C). This is called homeothermy. Having a constant temperature allows mammals to maintain warm muscles, which gives them the ability to react quickly, either to secure food or to escape predation. They can also maintain the optimum operating temperature for many enzymes, providing a more effective physiology. Some mammals are heterothermic (able to alter their body temperature voluntarily).Learn More
The female reproductive tract in monotremes is very much like a reptile's. A cloaca (also found in amphibians, reptiles, and birds) is a common chamber for the digestive, urinary, and reproductive system. The eggs are conveyed from the ovaries through the oviducts where fertilization occurs. After fertilization the eggs are covered with albumen and a leathery shell produced by the shell gland. In therian females the reproductive organs are separate from the urinary and digestive systems. The marsupial female has two uteri, each with its own vagina. Eutherian females may have either a single uterus or paired uteri, but always a single vagina. The placental embryo implants and develops in the uterine wall.Learn More