PRE-CAMBRIAN AND PALEOZOIC ERAS 171 



"finback" because of the enormous elongation of the neural spines pro- 

 jecting up from its backbone (Fig. 9.3, pelycosaurs, p. 177). Too slender 

 to serve for protection, these spines apparently supported a membrane that 

 stretched down the back like an enormous fin. But of what use to a land 

 animal is a fin? Speculations are as varied as they are ingenious; perhaps 

 the best idea is that the membrane served in connection with control of 

 body temperature, since it presented a considerable area of skin to the 

 surrounding air. In another species each spine was provided with a series 

 of transverse "yardarms" somewhat after the manner of a mast on a square- 

 rigged sailing vessel, or of a telegraph pole. 



Among the Permian reptiles a group of particular interest are the 

 therapsids, or mammal-like reptiles (Fig. 8.25). Although they formed a 



FIG. 8.25. Skull of a therapsid reptile, Bauria, lateral view, a, angular; 

 d, dentary; /, lacrimal; m, maxilla; n, nasal; p, parietal; pm, premaxilla; 

 po, postorbital; prf, prefrontal; sa, surongular; sm, septomaxillary; sq, 

 squamosal. (After Broom and Boonstra; from Romer, Verfebrafe Paleon- 

 fology, University of Chicago Press, 1945, p. 289.) 



diversified group, collectively the therapsids foreshadowed most of the dis- 

 tinguishing characteristics of the skeleton of mammals. While reptilian 

 teeth are typically simple cones, therapsids developed teeth resembling the 

 incisors, canines, premolars, and molars of mammals (Fig. 8.25). Like 

 mammals they developed two occipital condyles in place of the single con- 

 dyle possessed by other reptiles. Compared to other reptiles, they had a 

 smaller number of bones in the skull, approaching the mammalian num- 

 ber. The pineal opening, the site of the third or pineal eye in many rep- 

 tiles, had disappeared. The opening in the temporal region of the skull 

 posterior to the eye (temporal fossa) was arranged as it is in mammals, 

 and frequently the bony bar separating it from the orbit of the eye had 

 disappeared (Fig. 8.25), as it has in many mammals. 



