WHAT IS A MAMMAL? 



Mammals are warm-blooded vertebrates possessing twelve 

 pairs of cranial nerves, a four-chambered heart, double circu- 

 latory system, thoracic cavity separated from abdominal cavity 

 by a muscular diaphragm, skull articulating with atlas at two 

 occipital condyles, lower jaw (a single bone on each side) 

 articulating directly with skull, bodies covered by hair (if not 

 in the adult stage at least during some part of the embryonic 

 development) and two pairs of limbs (hind limbs lost or 

 vestigial in Cetacea and Sirenia); they bear the young alive 

 and nurse them at the breast. The name mammal is derived 

 from this method of feeding the young at the mammas or 

 breasts. The chief distinctions between mammals and birds 

 are the hairy covering, non-nucleate red blood corpuscles, and 

 the bearing of live young nursed at the breast. 



Early Mammals 



The earliest known mammal has been found as a fossil in the 

 depositsof the Triassic epoch, about 150,000,000 to 180,000,000 

 years ago.^ Although comparatively few specimens of mam- 

 mals have been discovered as far back as the Triassic, Jurassic, 

 and Cretaceous, a large number are known from the Eocene, 

 the following geological epoch, some 50,000,000 to 60,000,000 

 years ago, and scientists are able to trace the major develop- 

 ment of most of our present-day mammals from these ancient 

 ancestors. 



Most of these ancestral forms seem to have been quite well- 

 developed mammals and already very distinct from other 

 quadrupeds. The mammals are believed to have been derived 

 from reptile-like ancestors and the very first mammals are 

 thought to have evolved from Cynodont reptiles early in the 

 Triassic epoch. This places the birth of the class Mammalia 

 so far back, roughly 200,000,000 years, that the time element 



^ Figures taken from Arthur Holmes, The Age of the Earth, 1927; and 

 Joseph Barrel, Rhythms and the Measurements of Geologic Time, Bull. 

 Geol. Soc. Amer., 1917. 



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