Mammalian evolutionary history goes back 230 million years. The earliest mammals occupied a nocturnal niche and developed a suite of traits that allowed them to adapt to the cooler temperatures of the night. Mammals are endothermic (able to produce their own heat) and most are homeothermic (able to maintain their body temperature within a particular range). Many mammalian structures and their functions are involved with maintaining body temperature, which requires efficient generation and conservation of heat.
Modern mammals evolved from early mammal-like reptiles called therapsids, which had a mosaic of reptile and mammalian traits. In defining what a mammal is, we must utilize characteristics that are preserved in the fossil record, which are largely skeletal. When a transitional fossil species has a mammal trait it is usually accompanied by a reptile trait. Consequently, there is controversy as to when the actual mammal-reptile division occurred.
The key identifier in mammalian fossils is found in the dentary-squamosal articulation because the mammalian lower jaw (the mandible) is unique. It consists of two bones, the dentaries, which articulate directly with the cranium. This is the key criterion that defines mammals. And there is a relationship between the repitian jaw and the mammalian ear. The reptilian jaw consists of two bones, the articular and the quadrate. In mammals, these were modified to become the malleus and the incus bones of the ear, which, along with the stapes (called the columella in reptiles), form the auditory ossicles in the mammalian skull. Thus, mammals have three bones in their middle ears (malleus, incus, and stapes), and reptiles have only one (the columella).
Living mammals also have many soft anatomy traits that further define them as mammals. For example, at some stage of their lives, all mammals have hair. (However, it has been suggested that the reptilian flying pterosaurs had fur, so we must be cautious about saying that hair is exclusive to mammals). On the other hand, mammary glands are unique to mammals among the living vertebrates. Another structure unique to mammals is the respiratory diaphragm that separates the thoracic and abdominal cavities. Only mammals possess this structure as a muscle, whereas other vertebrates have either a membranous diaphragm or no diaphragm at all. The mature red blood cells of mammals are enucleated (without a nucleus), whereas the red blood cells of other vertebrates contain a nucleus. The mammalian heart differs from other vertebrates in that only the left aortic arch is developed in adult mammals. In mammal brains, the neopallium (neocortex) is elaborated and expanded compared to reptile brains. Each of these unique mammal structures are discussed in context in the rest of this entry.
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