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VERTEBRATE LIFE AND ORGANIZATION 





Figure 23.17. Two mammal-like reptiles. A, Dimetrodon, an early member of the 

 group; B. I.yaienops, a later mammal-like reptile similar to those that gave rise to 

 mammals. (Courtesy American Museum of Natural History.) 



by the lower jaw and transmitted via the articular and quadrate to the 

 stapes. This point is uncertain, for a soft part such as a tympanic mem- 

 brane would not be preserved in the fossils. It is known that a process 

 of the stapes did connect with the quadrate. 



Some pelycosaurs, such as Dimetrodon (Fig. 23.17 A) were quite 

 active, and may have been warm-blooded. Dimetrodon had a peculiar 

 sail along its back supported by long neural spines of the vertebrae. This 

 sail may have been a device to radiate heat, for it considerably increased 

 the body surface relative to mass. Active animals produce a large amount 

 of heat as a by-product of their metabolic activity, and need special 

 means of dissipating it. 



Later mammal-like reptiles (Lycaenops, order Therapsida, Fig. 

 23.17 B) came to resemble mammals more closely. Their limbs were be- 

 neath the body where they could provide better support and move more 

 rapidly back and forth. Their teeth were specialized, like those of mam- 

 mals, into ones suited for cropping, stabbing, cutting and grinding. The 

 major osteologic character that separated them from mammals was 

 the reptilian nature of the jaw joint and the sound-transmitting ap- 

 paratus. The mammalian jaw joint is between two dermal bones (the 

 dentary of the lower jaw and squamosal of the upper jaw) that lie just 

 anterior to the quadrate and articular. The mammalian homologues of 

 the quadrate and articular (the incus and malleus respectively) are 

 covered by a tympanic membrane, and form with the stapes a chain of 

 three delicate auditory ossicles that transmit air-borne vibrations from 

 the tympanic membrane to the inner ear (Fig. 29.6). This character had 

 not been achieved by the late therapsids, but the dentary and squamosal 

 were very close together, and the quadrate and articular were small. The 

 change to the mammalian condition was made by the middle of the 

 Mesozoic; shortly afterward the mammal-like reptiles became extinct. 



