EEVEMIBLE AND IRREVEKSIBLE EVOLUTION. 1045 



51* Reversible and Irreversible Evolution ; a Study based 

 on Heptiles. By Dr. FiiANCis, Baron Nopcsa. 



[Keceived October 10, 1923 : Read November 20, 1923.J 



(Text-figures 8 «k 9.) 



The great amount of information that we have about tlie 

 evohition of some groups of reptiles, the great amount of varia- 

 bility that these animals show, the long span of time that the 

 history of their evolution covers, seem to make it advisable to 

 base an investigation of the laws of evolution on the history of 

 these groups. Fejervtiry was one of the first who worked on 

 these lines (6). 



As is well known, in the skeletal structure of Reptilia many 

 characters occur that show what has been called the irreversi- 

 bility of evolution. Such characters are : the development of a 

 secondary armour in Dermochelys (4), the changes in the pelvis 

 of tlie ovthopodous Dinosaurs (5), the secondary growth of the 

 plastron in the CinosternidfB (9), and the development of a new 

 element (pra)pubis) functioning as pubis in the Crocodilia. 

 Apart from these changes, some of which have been well studied, 

 one can detect other less well-known changes tending to prove 

 that .sometimes a reversal of evolution can take place.' Changes 

 of this sort are : the secondary elongation of the anterior limbs 

 in Dinosaurs, the development of the postorbital bar in Mammals 

 and theromorphous reptiles, the redevelopment of more or less 

 plate-like ventral pelvic elements in difierent reptiles, the occur- 

 rence of polygonal flat carpal and tarsal bones in highly specialised 

 reptiles, and the relationship of the frontal to the orbit in 

 different groups. 



The aim of this paper is to give a description of the different 

 changes of this second type and to draw conclusions. 



§ (1) 2'he elongation of the anterior limbs in S2)ecialised Dino- 

 saurs, 



In all primitive diaptosaurian reptiles, such as Rhyncho- 

 saurians and Parasuchians, and in a less marked degree in the 

 true Rhynchocephalians, the anterior limbs ai-e generally only a 

 little shorter than the posterior. These animals are exclusively 

 quadrupedal. In the short-necked Ornithosuchians and in the 

 long-necked Proterosaurians, which were partly bipedal, a 

 marked shortening of the anterior limbs can be detected. This 

 shortening is stronger in the originally bipedal Dinosaurs. It 

 is very noticeivble in the lightly-built triassic carnivorous Dino- 

 saurs {Podoheosaur^is, Ilallopus, Procompsognathus) but less 

 marked in the Jurassic and cretaceous representatives of this 

 group. In Procompsognathns (and Podokeosanrus'i) the ratio 



Piioc. ZooL. Soc— 1923, Xo. LXVIII. 68 



