The First Mammals



The first mammals appear at the boundary between the Triassic and the Jurassic, most likely just before the end of the former period (see Table 3.1). Before describing them we must consider exactly what we mean by the term ‘mammal’. Quite clearly no single character by itself expresses the concept of a mammal. To most people a mammal means a warm-blooded furry animal, whose mother nourished it from birth with her milk, a secretion of the modified sebaceous glands called mammae. Such a definition is of little value to a palaeontologist with only fossils to study. Skeletal characters are clearly needed, since, except in exceptional circumstances, nothing else remains in a fossil. In modern mammals there is a chain of three bones or auditory ossicles (the malleus, incus and stapes), which conducts sounds across the middle ear from the tympanic membrane to the fenestra ovalis of the inner ear. In all other tetrapods there is but a single auditory ossicle — the stapes. The mammalian jaw joint is between the condyle on the dentary and the glenoid on the squamosal, which is in contrast to the articular-quadrate joint of the Amphibia and reptiles. The reptilian articular and quadrate have become the malleus and incus respectively of the Mammalia, where the lower jaw consists of but a single bone, the dentary, as against the many-boned lower jaw of the other tetrapods. There are other skeletal characters, such as the three phalanges in the digits of a mammal, which had already been acquired by some of the later mammal-like reptiles. So the palaeontologist is left with a choice of just two characters, which are to some extent correlated with each other.


Molar Tooth Cheek Tooth Main Cusp Auditory Ossicle Small Cusp 
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Copyright information

© Doris M. Kermack and Kenneth A. Kermack 1984

Authors and Affiliations

  1. 1.Department of Pure & Applied BiologyImperial CollegeLondonUK
  2. 2.Department of ZoologyUniversity College, LondonLondonUK

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