No. 2300. SKELETON OF DIMETRODON GIGAS—GILMORE 527 



The one most striking feature of Dimetrodon is the high dorsal 

 fin-Uke crest along the median line of the back formed by the elonga- 

 tion of the neural spines of the vertebrae. That in life these tall 

 spmous processes were united by a thin membrane of skin there is 

 little doubt, though Professors Abel and Jaekal are disposed to think 

 the spines were covered by skin but not connected. The one living 

 lizard which throws some light on this problem is Basiliscus plumi- 

 frons from tropical America (see inset plate 73), which has the crest 

 on the back; though not so high or extensive as in Dimetrodon, is 

 nevertheless supported by the elongated spinous processes of the 

 vertebrae, and in it we have the best suggestion of the probable 

 appearance in life of the crest in Dimetrodon. 



In trying to account for some practical use for this unusual out- 

 growth, it has been suggested that it may have resembled some of the 

 ancient vegetation, and thus served to conceal the animal as it lay 

 in wait for its prey or for better concealment from its enemies. Prof. 

 E. C. Case says of these:' 



The elongate spines were useless, so far as I can imagine, and I have been puzzling 

 over them for several years. * * * It is impossible to conceive of them as useful 

 either for defense or concealment, or in any other way than as a gi-eat burden to the 

 creatures that bore them. They must have been a nuisance in getting through the 

 vegetation, and a great drain upon the creatures vitality, both to develop them and 

 keep them in repair. 



The head of Dimetrodon is enormously large in proportion to the 

 size of the body. The jaws are provided with powerful incisor and 

 maxillary tusks, the largest of which reach a length of 3 inches. The 

 cheek teeth are recurved, with sharp, serrate edges. There is no 

 animal known which has a more efficient apparatus for the capture of 

 its prey. The eyes were large and set well back and high in the head. 

 The neck is short, the limbs are strong, havuig feet with 5 digits, each 

 of which is termhiated by a sharp claw. No specimen has as yet been 

 found with a complete tail, but the rapid decrease in size of the known 

 caudal vertebrae suggests a short tail, and it has been so restored 

 in the mounted skeleton. The specimen here pictured in plate 70, 

 although a representative of the largest species of the genus, is ex- 

 ceeded in size by several known individuals. The presence of 

 the greater number of the ribs of both sides shows the great depth 

 of the body cavity and the extreme flatness of the sides of the ante- 

 rior part of the body. From a study of the habits of living reptiles 

 it is known that those with compressed bodies are usually dwellers 

 among bushes and trees. Professor Case is of the opinion that 

 Dimetrodon is descended from an aquatic ancestry, and that it ranged 

 widely over the land. It is probable that ordinarily the animal did 



« The Permo-Carboniferous, Beds of North America and Their Vertebrate Fauna. Pub. No. 207 Car- 

 negie Institution, Washington, 1915, p. 142. 



