Mi;i;i;iA.\i — tiik TiiAi-ATT<isAri;i a 25 



in the palathu' ivuioii. tlii' lack of voiiu'riiif and ptcrvuoid teeth, ami the isodmit 

 or only slightly differentiated ehararter of the <i'nathie dentition in all the known 

 species of this epocli. 



Some of the structural differences separatini:' these two groups are doid>tless 

 to he correlated with difference in liahits; in other words, they may he 

 ascribed to somewhat different kinds of adaptation. The Ichthyosaurs were 

 largely fish eaters. They therefore had no great nee(l of a liea\-y vomerine den- 

 tition. In the same way the presence of a strong coronoid projection on the 

 thalattosaurian jaw may be correlated with somewhat increased muscular power 

 used in crushing with the broad teeth of the prevonier and of the posterior por- 

 ti(m of the dentarv. ()ther differences may be due to great increase in the size 

 of the orbits in the Ichthyosaurs, although this may be considered as due to 

 better opportunity for increase in size in that group owing to fundamental 

 difference in the original structure of the skull. 



These facts are particularly significant when we ccmsider their bearing on 

 questions concerning the origin and relationships of the Ichthyosaurs. If the 

 Thalattosaurs and Ichthyosaurs were both derived from the primitive Diaptosauria 

 and were both typical marine forms following somewhat similar lines of adapta- 

 tion, Avlw ai"e they so different? With similar environmental conditions we 

 would expect l)ut little divergence. It is evident that in this case we have one 

 of the following possibilities: 1. The Thalattosaurs and Ichthyosaurs have come 

 from the same stock but have followed quite different lines of adaptation; 

 2. They have come from the same stock and have followetl only slightly different 

 lines of adaptation, but have, one or l)oth. been in existence for a very long 

 period; 3. They have originated in different groups. 



Concerning the first possibility — there is reason to believe that the groups 

 differed somewhat though not greatly in food hal)its, but that in a broad way 

 the kind of adaptation was the same. It was such as wtuild generally tend to 

 jiroduce a certain degree of convergence. Th<' kinds of adaptation we should 

 presume to differ less than among the Cetacea, while the structural differences 

 are as great or greater than we find produced in not h^ss than four periods of 

 evolution in that more rapidly living aquatic grouj). 



Regarding the second case — we unfortunately know as yet but little con- 

 cerning the early history of either group. Thalattosaurs are known as early as 

 the beginning of the Upper Triassic, and the oldest h'hthyosaurs which we know 

 are found in the lower part of the Middle Triassic. The Ichthyosaurs certainly 

 appear to be somewhat more specialized than the Thalattosaurs and are pre- 

 sumably someAvhat older, so that we could imagine a slight difference in adapta- 

 tion acting through a long period as having resulted in llie prtxluction of these 



