PALEOGEOGRAPHIC SIGNIFICANCE OF LATE DEVONIAN (EARLY FAMENNIAN) OFFSHORE-MARINE CONODONT PALMATOLEPIS CREPIDA

The existence of two different forms of Palmatolepis crepida related to paleobiogeographic provincialism was elucidated by our study of three conodont samples from Sonora, northwest Mexico. The typical (s.s.) form characterizes Gondwana terranes whereas the atypical form characterizes Laurentia. Two Gondwana samples were collected from lime-grainstone turbidites of the Los Pozos Formation in the Sonora allochthon, the western end of the Ouachita-Marathon orogen. These had huge, highly diverse (>25 species) faunas. The Middle crepida Zone fauna from Arroyo Tesotitas yielded 25,000 conodonts/kg whereas the Early crepida Zone fauna from Cañón Máviro yielded 4,000 conodonts/kg. The third sample with a sparse Middle crepida Zone fauna is from the "San Miguel Formation" at Rancho Placeritos on the Laurentia carbonate shelf north of the Sonora allochthon. Palmatolepis crepida s.s. is characterized by a short, rounded platform with an obsolescent lobe, or no lobe, and a strongly sigmoidal carina. This Gondwana form was originally described from Germany and since then has been identified in Belgium, France, Poland, and herein in Morocco. The Middle crepida Zone conodont fauna on the Laurentian shelf is characterized by a slightly sigmoidal carina, an elongate platform, and a short lobe. This form occurs sparsely elsewhere in Laurentia and has been documented in Nevada, Indiana, and Alberta. We currently treat it as a new morphotype, but eventually it may be recognized as a separate subspecies.