480 University of California Publications in Geology [ VoL - 7 



Tentative Suggestions as to Evolution of Dentition. — Accord- 

 ing- to H. W. Elliott, 2S C. M. Scammon 29 and others, the sea-otter 

 feeds to some extent on fish, but for the most part on sea-urchins, 

 crabs, mussels, and clams. This would seem to account for its 

 highly specialized crushing dentition. It is of interest to note in 

 this connection that a primitive ichthyosaur from the middle 

 Triassic (Phalarodon fraasi) 30 has a crushing dentition, the teeth 

 being of a thick, low-crowned type. This is in marked contrast 

 with the dentition of later Jurassic ichthyosaurs 31 which have 

 teeth of a simple subcorneal type. It would seem possible that in 

 ichthyosaurs the crushing dentition was one of the early steps in 

 aquatic adaptation. 



In the course of the evolution of any pelagic mammal which 

 is descended from a land-living species, there may be three 

 habitat stages, namely. (1) one in which the animal is terrestrial 

 only (and would ordinarily possess molars with conical or with 

 compressed tubercles for securing and killing active land ani- 

 mals) ; (2) one in which the animal preys upon organisms of 

 the littoral zone (and might have a crushing dentition) ; (3) one 

 in which the animal depends upon pelagic forms and fishes which 

 it does not crush or masticate (and might have slender teeth). 



It might be expected that when a terrestrial species takes to 

 aquatic life it would first become gradually accustomed to feed- 

 ing on animals of the littoral zone, probably using a few land 

 or river forms at the same time. As it progressed in adaptation 

 to life in the water and became more specialized, it would depend 

 to a continually greater degree on littoral animals for food. At 

 this stage of evolution such a species would best be served by a 



- s Elliott, H. W., ' ' Report upon the condition of affairs in the Terri- 

 tory of Alaska." House Exec. Doc. 83, 44th Congress, 1st Session (Wash- 

 ington, Government Printing Office, 1875), pp. 54-62. 



29 Scammon, C. M., Marine mammals (1874), pp. 168-174. Snow (In 

 forbidden seas, London, Edward Arnold), 1910, pp. 279-280, asserts that 

 remains of clams, limpets or mussels were not found in a single one of 

 hundreds of stomachs examined by him, the food consisting principally 

 of crabs, sea-urchins, sea-squirts, and what looked like fish-spawn (see 

 p. 481). 



so Merriam, J. C, ' ' The skull and dentition of a primitive ichthyosaurian 

 from the middle Triassic," Univ. Calif. Publ. bull. Dept. Geol., vol. 5 

 (1910), p. 383. 



3i Merriam, J. C, "Triassic ichthyosauria, " etc., Mem. Univ. Calif., 

 vol. 1 (1908), p. 74. 



