Conodont biostratigraphy of Devonian strata of West Texas and eastern New Mexico and the apparatus of early Devonian Icriodus species
Meyer, Beverly Diane
Devonian strata in the Permian Basin reflect eperrogenic events and eustatic fluctuations in the southern midcontintent North America Changes in relative sea level demonstrated by conodont faunas and facies changes within the Frame, Thirtyone and Woodford Formations are compared to the Devonian qualitative eustatic curve. The lower Devonian (Lochkovian, woschmidit zone through omoalpha zone) conodont species Icriodus postwoschmidti Mashkova, 1968 occurs in carbonates of the Frame Formation in the Permian Basin and within the pre-Ia cycle of the eustatic curve. Icriodus claudiea Klapper, 1980 occurs in overlying carbonates and cherts of the Thirtyone Formation providing a Pragian (sulcatus to kindlei zone) age for the upper part of the formation and placing the Thirtyone in cycle Ia of the eustatic curve. Above the Thirtyone, green and gray clay shales at the base of the Woodford Formation produced an abundant Middle-Upper Devonian (Givetian-Frasnian) varcus to falsiovalis zone faima that ranges through cycle IIa into IIb of the sea-level curve. Typical Woodford black shales (Upper Devonian, Famennian) contain a crepida through expansa zone fauna of cycles IIe and IIf of the sea-level curve. No Mississippian conodont species were recovered from the Woodford Formation in the Permian Basin. Carbonate turbidites of the Dunple Limestone document headward erosion and transport of Woodford Formation deposits from the southern midcontment into a closing Ouachita ocean basm to the south or east during the Early Pennsylvanian. Four conodont faunas are described from reworked conodont elements recovered from the Dunple in the Marathon Uplift. Elements rangmg in age from Middle varcus Zone (Middle Devonian) to Lower crenulata Zone (Lower Mississippian) were admixed throughout the samples, with no original stratigraphic superposition preserved. The recovery of abundant elements of the Early Devonian species Icriodus postwoschmidti and I. claudiea from the Frame and Thutyone formations allows apparatus reconstmction and comparison of the morphological characteristics of the two species. Coryssognathus dubius (Rhodes, 1953) has been suggested as the most likely ancestor of Icriodus based on similar morphology of the non-Pa elements to those of the oldest species of the genus, I. woschmidti Ziegler, 1960. The apparatus reconstruction of I. postwoschmidti allows interpretation of changes in the apparatus that occurred between I. postwoschmidti and its ancestor species I. woschmidti, and suggests a relationship to C. dubius. Reconstruction of the apparatus of I claudiae allows comparison to both I. postwoschmidti and younger species and demonstrates continued morphological changes through time.