THE APPARATUS OF EARLY DEVONIAN ICRIODUS SPECIES (CONODONTA) FROM SUBSURFACE WEST TEXAS

Lower Devonian carbonates from the subsurface of West Texas have yielded low diversity conodont faunas that include relatively abundant elements of Early Devonian species of Icriodus. Elements of the Lochkovian species Icriodus postwoschmidtiMashkova occur in samples from the upper part of the Frame Formation from the Amoco Three-Bar 74 well in southern Andrews County, Texas. Although only about 120 Icrioduselements occur in addition to 23 I. postwoschmidtiPa elements, sufficient elements are present to permit a complete reconstruction of the apparatus. The apparatus is octimembrate, with Pa, Pb, Pc, M, Sa?, Sa/Sb, Sb, and Sc elements. The Pb-Pc-M positions are occupied by coniform elements that form a symmetry transition series. A second symmetry transition series (S elements) comprises coniform elements with lateral costae except for the ramiform Sc element. The I. postwoschmidtiapparatus is similar to that of the older species, I. woschmidtiZiegler, as reconstructed by Serpagli (1983), in which the Sa, Sa/Sb and Sc elements still possess processes. Except for the Pa element, the I. postwoschmidtiapparatus is remarkably like that of the Silurian distomodontid Coryssognathus dubius(Rhodes), as reconstructed by Miller and Aldridge (1993). Although the non-Pa elements of C. dubius possess denticulated processes, they have the same general morphology and changes in symmetry as those of I. postwoschmidti. The nearly identical apparatus structures show that Icriodusdeveloped from Coryssognathusduring the late Silurian and upon gaining a robust Pa element, quickly reduced the non-Pa elements to coniform morphologies by the end of the Lochkovian.

Reconstruction of the apparatus of the Pragian species Icriodus claudiaeKlapper is based on the recovery of hundreds of Icrioduselements that occur in association with this Pa element in numerous samples of Thirtyone Formation from several wells in West Texas. Although many of the Pa elements have the lateral process broken from the spindle, the other elements of the Icriodusapparatus are well preserved and abundant. The elements of I. claudiae retain the same morphological gradation among elements as seen in the apparatus of I. postwoschmidti,but differ slightly in morphology in the Pb, Sb, and Sc positions.