DAKOTA AND SUNDANCE FAUNAS 347 



DAKOTA FAUNA 



The Dakota fauna, as described'* and as represented in collections, is 

 small but complex. Eliminating the Comanche species from the neigh- 

 borhood of Salina, Kansas, which were described as Dakota by Meek, 

 there remains a number of fresh-water species, some of which are related 

 to those of the Bear Eiver, and several brackish-water and marine mol- 

 iiisks which show relationship with the succeeding Benton fauna. None 

 of the fresh-water species suggests derivation from Morrison forms. 



Time Limits of tiik Morrison Formation as determined by asso- 

 ciated MARINE Faunas 



GENERAL DISCUSSION 



The Morrison fauna tben stands by itself, distinct from the few fresh- 

 water invertebrates that preceded it in the Triassic and distinct from the 

 non-marine Cretaceous faunas which followed it. Of course, there is no 

 basis for comparison with the marine faunas which are nearest to it in 

 time, but the study of these marine faunas serves to fix definitely the 

 time limits within which the Morrison formation must fall. 



SUNDANCE FAUNA 



The lower limit is fixed by the age of the marine Jurassic Sundance 

 formation on which the Morrison rests in southern Wyoming. The 

 Sundance is classified as Upper Jurassic, and for that reason it lias been 

 assumed by some geologists that everything above the Sundance must be 

 post-Jurassic. But a study of the fauna of the Sundance shows that it 

 is liy no means the latest Jurassic, but that it belongs in the hjircr ])art 

 (if llic Upper Jurassic. It is characterized by Cardioceras conlifoniir 

 jiikI (ilhcr iiiM'rtebrates, which indicate approximate correlation with llie 

 Oxfoidian of the European Jurassic. In Europe the Oxfordian is fol- 

 lowed by hitiT Jurassic sediments, classified by De Lapparent as Se- 

 <|uanian, i\ immeridgian, and Portlandian (including Purbeckian). In 

 Ahiska. also the fauna most closely related to the Sundance fauna occui-s 

 in l)('ds which arc o\ci-lain by several thousand feet of lalci- Jurassic 

 sli'ata. 'J'here is ample i(join, therefore, for tlie Morrison within the 

 Jurassic. 



The incursion of the sea which resulted in the deposition of the 

 Sundance formation and its e(|ui\alcnts in the Koeky Mountain region 



» F. B. Mepk : U. S. Geol. Survey Terr., vol. 9, 187G. 

 ('. A. WliU(> : I'roc. U. S. National ^f^ls^Mlm, vol. 17, 1894, pp. 131-138. 



