Permian and Eotriassic bivalves of the Middle Rockies. Bulletin of the AMNH ; v. 125, article 1

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New York : [American Museum of Natural History]
"The Permian-Triassic succession in the Middle Rockies of North America records one of the major gaps in the history of fossil invertebrates. Bivalve mollusks are the only invertebrates that are major components of both the Permian and the Eotriassic faunas. Seventy taxa, including two new genera and seven new species, are described from Permian rocks of the Park City group. Twenty-one taxa, including one new species, are described from Lower Triassic rocks. A proper interpretation of the magnitude of the local faunal break depends on the accurate dating of both the Permian and the Triassic sequences. Rocks of Park City age are subdivided into two age groups on the basis of bivalve correlations. Age group I, consisting of the basal members of the Park City and Phosphoria formations, is correlated with rocks of Upper Leonardian-Lower Wordian age in the West Texas standard Permian marine section. Age group II, including the remaining nine members of the Park City, Phosphoria, and Shedhorn formations, is correlated with rocks of Upper Wordian-Capitanian age in West Texas. This suggests an upper age limit of Kazanian (Capitanian) for the Park City group and indicates that the youngest Permian marine strata (Tartarian, Ochoan) are not locally represented. The Eotriassic Dinwoody formation is of early Scythian age. The nature of the Permian-Triassic contact indicates that at least locally, weathering and erosion took place and suggests a withdrawal of Permian seas and a period of emergence prior to transgression of the early Eotriassic sea. Generic and specific diversification in the Permian bivalves suggests that major evolutionary changes occurred in normal marine, shallow-water, carbonate facies. Latest Permian and earliest Triassic marine deposits are very restricted over the world and are notable for a general lack of calcareous facies. It seems that a major regression of the seas closed the Permian, which was followed by an early Triassic transgression in all of the known sequences. Perhaps the ecological changes recorded by these events were responsible for the revolutionary paleontological changes at this time rather than a very long time hiatus. Although bivalves are abundant in both the Permian and the Eotriassic faunas, only two species, out of 91 noted in the two local sequences, are closely comparable. Lower Eotriassic rocks contain an impoverished, intermediate fauna, which does not closely resemble younger Triassic faunas. The latter do not become established until the return of normal marine, clastic-free, carbonate facies in later Eotriassic time"--P. 5.
100 p., 16 p. of plates : ill. ; 27 cm.
Thesis (Ph. D.)--Columbia University.
Includes bibliographical references (p. 98-100).