[Annals N. Y. Acad. Sci., Vol. XVI., No. 9, Part III, pp. 447-482, 



March I, 1906.] 



ADAPTIVE MODIFICATIONS OF THE LIMB SKELE- 

 TON IN AQUATIC REPTILES AND MAMMALS. 



By Raymond C. Osburn. 



The history of animal hfe reveals the fact that a number of 

 widely separated groups of reptiles and mammals have taken up 

 aquatic life. That these aquatic animals were derived from ter- 

 restrial forms is now universally conceded. Some of these 

 forms seem to have attained almost to perfection in their adap- 

 tation to life in the water, but the great majority are found 

 scattered all the way along the path by which the former pro- 

 gressed. The terrestrial type of limb, with which at the be- 

 ginning all these forms were endowed, is not well fitted for use 

 in the water, either for propulsion or for balancing, hence a 

 great change in the limb is necessary before its possessor can 

 become perfectly at home in its new environment. This change 

 affects the limb throughout and finally brings about a complete 

 reorganization of the limb in which scarcely any of the origi- 

 nal elements can be recognized. Thus in later Ichthyosaurs the 

 only element of the original limb which could be identified with 

 certainty if removed from the limb, is the propodial, which by 

 virtue of its position at the junction of the limb with the girdle, 

 has retained somewhat the appearance of a propodial bone. 

 While, at the beginning, there must necessarily have been dif- 

 ferences in size, shape and arrangement, and even in the num- 

 ber of bones present in the animals belonging to groups of such 

 widely different ancestry as these here dealt with, yet in the at- 

 tempt to produce a perfect swimming organ the amount of par- 

 allelism exhibited is remarkable. 



In most cases it is safe to say that the limbs were at first used 

 both as organs of propulsion and of equilibration, and, indeed, 

 certain groups have never passed this condition (Chelonia, 

 Plesiosauria and Pinnipedia), though it is quite possible that 



447 



