POSITION AND RELATIONS OF THE PELYCOSAURIA. 



The Pelycosauria constitutes a highly specialized and short-lived branch 

 from the beginning of the Rhynchocephalian stem. Its history is a most 

 striking example of rapid evolution to extreme specialization from very 

 primitive and generalized conditions — illustrating the fact that high special- 

 ization of individual parts may arise as modifications of structures that are 

 in general on a very low plane of development as well as of structures that 

 have reached a very high plane of development. Extreme specialization may 

 occur at the beginning of a phylum as well as in the more advanced stages. 



The most primitive of the suborder, the Poliosanrida?^ are not far from 

 the Proterosani'ia and Proganosauria; perhaps rather more primitive, because 

 of the persistent notochordal condition of the centra, but revealing no more 

 of the origin of the reptiles. The Pelycosauria in general differ from the 

 Proterosauria in the higher degree of ossification, especially of the pelvis 

 and pectoral girdles, and from the Proganosauria in the lack of adaptation 

 to water life. The Therocephalia of South Africa approach the Pelycosaurs 

 in many particulai's of structure, but the fundamental difference in the 

 temporal arches prevents any assumption of genetic relationship. 



The most striking feature of the group is, of course, the enormous 

 neural spines, but the specialization of the group is shown in other points 

 not to be overlooked. The form of the alveolar edge of the upper and lower 

 jaws ; the secondary closing of the superior temporal vacuity ; the method 

 of rib attachment ; the enormous claws ; these are departures from the normal 

 fully as gi-eat as the condition of the neural spines. The use of the spines is 

 the first consideration presented in the study of the animals and it seems 

 impossible to assign to them any utilitarian value. The animals were 

 fiercely carnivorous, developing enormous tusks, perhaps in correlation with 

 the developing armor of the Amphibia^ Cotylosauria, and Chelydosanria 

 which formed their prey. Easily masters of their world, they may have 

 developed the spines as a mere exuberance of growth from some possible 

 utilitarian beginning, but finally they served no more useful purpose than 

 the plates of Stegosaiirus or the spines of the modern Phrynosoma or Basi- 

 liscus. It is perhaps an illustration of Beecher's law that the development 

 of spines and excrescences accompanies the approaching extinction of a 

 group. 



There is no doubt that the Pelycosauria existed beyond the limits of 

 North America, as evidenced by the Naosaurns of the Permian of Bohemia. 

 The forms described by v. Huene are not true Pelycosaurs, but it is very 

 possible that they are fonns derived from the beginning of the Pelycosaurian 



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