. Bulletin of the Geological Society of America. Geology. DEVONIAN 329 the Ithaca fanna. Associated with these Upper Devonian forms we find several species characteristic of Middle Devonian horizons. Among these are Pholidostrophia cf. iowensis, Cyrtina cf. hamiltonensis, SchucherteUa chemungensis var. arctostriatus, Reticularia fimhriata, Nucleospira cf. concinna, and species resembling the European forms Gypidula cf. bipUcatus and G. cf. galeatus. Two possible explanations of this asso- ciation of Middle and Upper Devonian species in the same fauna present themselves. It has been shown by Wi
. Bulletin of the Geological Society of America. Geology. DEVONIAN 329 the Ithaca fanna. Associated with these Upper Devonian forms we find several species characteristic of Middle Devonian horizons. Among these are Pholidostrophia cf. iowensis, Cyrtina cf. hamiltonensis, SchucherteUa chemungensis var. arctostriatus, Reticularia fimhriata, Nucleospira cf. concinna, and species resembling the European forms Gypidula cf. bipUcatus and G. cf. galeatus. Two possible explanations of this asso- ciation of Middle and Upper Devonian species in the same fauna present themselves. It has been shown by Williams and Kindle that Middle Devonian species sometimes persist till late Devonian time and appear in certain New York sections associated with Upper Devonian species.^^ It appears most probable, however, from what we know of the relations of Pugnax pugnus and its associated fauna to the Upper Devonian of New York, that the occiirrence of Upper and Middle Devonian species in the same fauna at Old Eampart is not the result of late persistence of the earlier fauna. This species evidently migrated into the New York prov- ince from the northwest in Upper Devonian time. The t^vo significant facts of its association with Middle Devonian fossils in an Alaskan fauna and its abrupt appearance in an Upper Devonian faima in the United States, taken together, point very strongly to the probability that inter- communication between the eastern Alaska province and the interior American province M^as cut off during Middle Devonian, but became free about the beginning of Upper Devonian time, when conditions became favorable for the dispersal and migration of such forms as were adapted to it. Some of the species which had during Middle Devonian time been confined to this northern basin spread southward and helped to give a distinctive character to the Upper Devonian fauna as we know it in the United States. This appears to be the probable explanation of an associa- tion of species which upon c
Size: 1636px × 1528px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1890
