Evolution

Mammals originated from members of Therapsida reptilian order in the Triassic period (252 million to 201 million years ago). Therapsids belonged to subclass Synapsida regarded as mammal like reptiles. Therapsids were unimpressive compared to other reptiles of their time. Synapsids are regarded as earliest known reptiles and existed during carboniferous period (359-299 million years ago). They dominated Permian period (299-252 million years ago). Primarily they were predatory but some herbivorous species also existed. In the Mesozoic period ruling reptiles archosaurs were the most important synapsids and Therapsids were small carnivores. Therapsids evolved heterodont dentition found in mammals. Just like mammals dentition of evolved Therapsids included molars, incisors and canines.

As different features distinguishing modern reptiles from modern mammals evolved at different rates and in response to various interrelated conditions, during the period of transition from reptiles to mammals, forms that combined features of both groups also developed. Mosaic is the term used for such evolutionary pattern. This is common phenomenon in transitions resulting in origin of major new species. Point to be noted here is that if advanced Therapsids existed today then categorization of each species as reptilia or mammalia would have been difficult.

Classification

Consideration of a very broad array of characteristics is basis for the higher classification of the class Mammalia. Conventionally, evidence from comparative anatomy was of principal importance, but, lately, information from such disciplines as genetics, physiology and serology (the study of immune reactions in body fluids) has proved valuable in considering relationships. Comparative study of living creatures is supplemented by the findings of palaeontology. A historical dimension is added to knowledge of mammalian relationships by study of the fossil record. The fossil record has been found to be sufficient to allow lineages to be traced in great detail in some cases, for example horse.

Compared to other major vertebrate groups, the fossil record of mammals is far better. Fossilization depends upon various factors, the most vital of which are the structure of the organism, its habitat, and circumstances at the time of death. Teeth and the associated bones of the jaw and skull are without doubt the most common remains of mammals. Enamel covering the typical mammalian tooth is made up of prismatic rods of crystalline apatite and is inarguably the hardest tissue in the mammalian body. It is highly resistant to physical and chemical weathering. Because of the huge abundance of teeth in deposits of fossil mammals, dental features have been extensively used in the interpretation of mammalian phylogeny and relationships. Dental characteristics are particularly well suited for this vital role in classification because they mirror the broad radiation of mammalian feeding specializations from the primitive predatory habit.