ANCESTRY OF HOMOIOTHERMIC ANIMALS 13 



Mammals 



There is a general agreement among vertebrate 

 palaeontologists that mammals originated from the order 

 of Triassic reptiles known as the Cynodontia. During 

 Permian and Triassic times the cynodonts had developed 

 temporal bony arches, a secondary palate, paired occipi- 

 tal condyles, heterodont dentition in which incisors, 

 canines, premolars, and molars were differentiated, and 

 a functionally important dentary. These reptiles de- 

 veloped a new type of jaw articulation, and the quadrate 

 bone, which plays such an important role in lizards, be- 

 came degenerate. 



Hairs are believed to have been derived from reptil- 

 ian scales and there is evidence which supports this view. 

 The genital organs of primitive modern mammals, mono- 

 tremes and marsupials, show transitional stages between 

 those of reptiles and more specialized placentate mam- 

 mals. The marsupium probably enabled early mammals 

 to carry their eggs away from the nest. The eggs were 

 thus better protected. Later the young mammal prob- 

 ably took the place of the egg in the pouch. Lactation 

 probably came in with the acquirement of the homoio- 

 thermous condition. The oily fluid which perhaps at first 

 served to lubricate the pouch and help keep the young 

 warm later came to serve as a regular means for nourish- 

 ment. The young at first licked milk from hairs ; sucking 

 succeeded licking and the young became in a sense ex- 

 ternal parasites on the mother. The placenta developed 

 and the young were then able to obtain nourishment 

 w^ithin the body of the mother and could thus grow to 

 larger size before birth (Gregory, 1910). 



In Cretaceous times the dinosaurs and other reptiles 

 lost their dominant position among the animals of the 



