PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



69 



determined is that Sphenacodon is a typical pelycosaur, very closely allied to Dimetrodon, 

 with only minor differences in skull, girdles, extremities, and vertebral centra, but with 

 very much shorter spines, which are not pointed at their extremities, but, on the other hand, 

 are dilated antero-posteriorly. The spines are long, extraordinarily long for a reptile, but 

 they could not have formed any such frill as that of Dimetrodon or the basilisk lizard. 



And this structure of the vertebra has an irnportant bearing in any disaxssion as to the 

 meaning of the spines in Dimetrodon. That Dimetrodon could have developed such extra- 

 ordinary* spines without affecting to a greater degree the characters of the skeleton proves 

 conclusively their relative physiological unimportance.* Certainly, had the enormous 

 dorsal expansion of Dimetrodon been of profound importance in the life economy of these 



Fic. 44. — Sphenacodon ferox Marsh. A, right scapula, No. 818, Yale University, X h; 

 B, right scapula of a large Dimetrodon from Coffee Creek, Wilbarger County, Texas, 

 No. 661, Yale University, X Vi. 



creatures, it must have materially afTected the structure of the skeleton elsewhere. That 

 Sphenacodon is more primitive than Dimetrodon must be admitted to be perhaps another 

 bit of evidence of the greater antiquity of the New Mexico deposits than the upper ones, 

 at least, of Texas. 



* Case would hesitate to indorse the statement that these elongated spines were physiologically unim- 

 portant. He has long considered (and frequently stated his belief) that the enormous development of the 

 spineii in Dimetrodon and probably also in Edaphosaurus (Naosatirus), imposed upon the creatures a ph\s- 

 iological burden so great in its demands upon the energies of the individual, both for their original 

 production and the repair of frequent injuries, that it was an important, if not the chief, cause of their 

 extinction. 



