Mammals are taxonomically separated from other animals at the class level (kingdom Animalia, phylum Chordata, subphylum Vertebrata, class Mammalia). Modern mammals are readily differentiated from other animals by the following characteristics: hair; a four-chambered heart with the aorta descending on the left; red blood cells that lack a nucleus (allowing for increased surface area for oxygen transport); a muscular diaphragm separating the abdominal and thoracic cavities that aids in breathing; descent of the testes into a scrotum to achieve a temperature environment amenable to sperm development; a variety of skin glands including sebaceous, sweat, and milk or mammary (the characteristic giving mammals their name); and elaborate dermal musculature (controlling the skin), particularly in the face (associated with suckling in young). All of these characters relate to the high metabolic rate of mammals. Mammals and birds are the only vertebrates that maintain a consistent body temperature through physiological (endothermy) rather than behavioral means (ectothermy).
None of these characters are readily apparent in fossils. There are, however, a number of skeletal and dental traits that are unique to mammals. The characters most useful in tracing the origin of mammals are: a bony secondary palate in the skull; a jaw joint between the dentary (jaw bone) and the squamosal bone of the skull (other terrestrial vertebrates have a quadrate-articular jaw joint); three bony ossicles (malleus, incus, and stapes) in the middle ear for sound transport rather than just one (stapes); teeth that are specialized for a variety of functions, including stabbing, nipping, shearing, and grinding; and a limb skeleton that can passively support the body off the ground (versus the reptilian posture of legs to the side). Most of these traits also relate to the high metabolic demands of endotherms. The reptile-to-mammal transition is one of the best documented in the fossil record. The first mammals appeared over 200 million years ago, about the same time as the first dinosaurs.
William P. Wall
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