14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 149 



area, both above and below the Tipton tongue, as well as the red beds 

 of the Big Horn Basin are barren of Meniscotherium. 



A more nearly paludal facies is represented near the base of the 

 Wasatch at Bitter Creek, where colorless or drab sandy shales alter- 

 nate with coal or peaty layers. Meniscotherium, though sparse, 

 is represented here by small M. tapiacitis, which may have been less 

 adapted to a savanna environment or was possibly less discriminating 

 in this respect than the larger, later species. It seems possible, more- 

 over, that M. tapiacitis may be represented nearly 700 feet above the 

 base of the Wasatch not far from Bitter Creek. Roehler (unpublished 

 charts) has designated the beds at this higher level as in general 

 semifluviatile, although at the fossil site they are indicated as shaly. 



The reported occurrence of Meniscotherium in the Tipton tongue 

 on Table Mountain might appear disconcerting and contrary to the 

 supposition that the later Meniscotherium preferred a savanna envi- 

 ronment, but I find that the rock involved is a massive sandstone 

 in a more general facies described by Roehler as semifluviatile, which 

 he has shown as interfingering with typical lacustral sediments of the 

 Tipton tongue. 



While supposing a savannalike environment for later M. robustum 

 and M. chamense, close proximity of a large body of water, the 

 earlier stages of Bradley's Gosiute Lake in southwestern Wyoming, 

 in no way inhibited their range as represented in fluviatile facies 

 interfingering with lacustral. Nevertheless, coincident with the wide- 

 spread extension of the Green River lacustral facies, the Tipton or 

 Fontenelle tongue — Gosiute Lake encroaching on the marginal low- 

 lands — Meniscotherium robustum evidently became extinct. Fol- 

 lowing retreat of this tongue the form was replaced, as represented 

 in the overlying fluviatile sequence, by a somewhat smaller, possibly 

 more widely ranging species believed to be the Largo M. chamense. 



The disappearance of M. chamense at or near the close of Wasat- 

 chian time would seem unrelated to features of the complex of 

 Green River lakes in Wyoming, Utah, and Colorado, because of its 

 known more widespread distribution, such as in the Wind River 

 Basin. The advent, however, of the more extensively distributed 

 Laney member would suggest climatic change of a regional nature 

 coincident with the extinction. Moreover, Bridgerian time in this 

 region is characterized by voluminous ash falls, so clearly demon- 

 strated in the fluviatile sequences interfingering with the Laney, which 

 may have been detrimental to Meniscotherium either directly or by 

 altering the environment. 



