Uppermost Devonian ammonoids from Oklahoma and their palaeobiogeographic significance

Abstract

The upper part of theWoodward Shale of southern Oklahoma has yielded the first moderately diverse NorthAmerican ammonoid fauna from the uppermost Famennian (Upper Devonian VI). It includes six species from three clymeniid and one goniatite family: Kielcensia vagabunda sp. nov., Riphaeoclymenia polygona sp. nov., R. pontotocensis sp. nov., Cyrtoclymenia cf. procera Czarnocki, 1989, Spirosporadoceras overi gen. nov. sp. nov., and a poorly preserved different juvenile sporadoceratid that may represent a second new genus. For comparison, the related Spirosporadoceras delicatum sp. nov. from Germany is described. Kielcensia specimens from Oklahoma represent the first uncontested record of triangularly coiled wocklumeriids from NorthAmerica. Together with Riphaeoclymenia, the Oklahoma fauna has similarities and strong biogeographical ties with the far distant Holy Cross Mountains of Poland. Kielcensia and Riphaeoclymenia are missing from the diverse contemporaneous ammonoid faunas of Middle and Southern Europe, which were located between the Oklahoma and the Polish occurrences. Geographically intermediate contemporaneousMoroccan faunas also show a fundamentally different composition but the Afro-Appalachian migration route must have been viable in the uppermost Famennian. Migrations through regions without leaving a trace in available very rich fossil records (“ghost distributions”) create a bias for the palaeobiogeographical analysis of nektonic organisms. The faunal composition of theWoodford Shale suggests a control of ammonoid distribution patterns by palaeoecological factors that are not recognizable in the lithofacies.