MKR1UA.M: THIASSLC IC11TII YOSACHI A. 77 



ORIGIN OF ICHTHYOSAURIA. 



Though the ichthyosaurs were throughout their whole known history con- 

 stantly specializing along the line of adaptation to their aquatic environment, 

 we find that as early as the Middle Triassie they were already in the class of 

 the highly specialized aquatic reptilian types. From what we know, we can 

 hardly suppose them to have separated themselves from the parent shore type 

 far enough to lie called ichthyosaurs later than the early part of the Trias. 

 Possibly their origin dates from an even earlier period. There seems, there- 

 fore to be good reason for considering them one of the oldest of the reptilian 

 orders. The primitive ichthyosaur was evidently a typical reptile, and consid- 

 ering the necessarily early period of its origin, antedating as it does that of most 

 of the other orders, it may be derived only from one of the very primitive mem- 

 bers of this class. Unless we consider the ichthyosaurs as themselves one of 

 the primitive divisions of the Reptilia, we have only the most primitive repre- 

 sentatives of the cotylosaurian, synapsidan and diapsidan stems with which it 

 may stand in the relation of descent. 



Woodward"" has included the ichthyosaurs in the reptilian group in which 

 the skull possesses a single temporal arcade, along with the Anomondontia, 

 Sauropterygia and Chelonia ; at the same time calling attention to the very prim- 

 itive characters of the skull. Cope 30 in one of his last phylogenic arrange- 

 ments of the reptilian orders, derived the Ichthyosauria directly from the prim- 

 itive Cotylosauria. 



Broom' 1 has separated the ichthyosaurs from both the Synapsidan and 

 Diapsidan groups. More recently Williston 32 has called attention to the fact 

 that "the presence of an original lateral temporal opening is by no means 

 proved." Baur" 3 considered the ichthyosaurs rhynchocephalian. McGregor" 

 and Osborn 30 have suggested close relationship of the Ichthyosauria with the 

 Rhynchocephalia . 



Excepting in those characters indicating adaptation to aquatic environment 

 the ichthyosaurian skeleton is undoubtedly very primitive. Whether it has 

 been diaptosaurian, synapsidan or cotylosaurian in its original form is less 

 certain. 



^ Woodward, A. Smith, "Vertebrate Palaeontology, p. 143, 1898. 



so Cope, E. D., Primary Factors of Organic Evolution, p. 115, 1896. 



si Broom, R., Proc. Zool. Soc. Lond., 1901. 



32 Williston, S. W., Biological Bulletin, vol. 7, p. 186, Sept. 1904. 



33 Baur, G., Am. Nat., vol. 21, p. 837, 1887. 



34 McGregor, J. H., Science N. S., vol. 16, p. 27, July 4, 1902. Also, Mem. Amer. Nat. Hist., vol. 9, part 2, 

 p. 91, Feb. 1906. 



35Osborn, H. F., Mem. Amer. Mus. Nat. Hist., vol. 1, No. 8, p. 456, Nov. 1903. 



