LAUSR

Abstract. Nine longiconic cephalopod species of Desmoinesian (Middle Pennsylvanian; upper Carboniferous) pseudorthoceratid orthocerids are described from the Buckhorn Asphalt Lagerstätte (Boggy Formation) in Southcentral Oklahoma, Midcontinent North America. The fauna consists of Pseudorthoceras knoxense (McChesney), Arbuckleoceras tricamerae (Smith), Bitaunioceras buckhornense (Smith), Cyrtothoracoceras? sp., Dolorthoceras boggyense sp. nov., Smithorthoceras unicamera (Smith), Sueroceras oklahomense (Smith), Sulphurnites taffi sp. nov. and Unklesbayoceras striatulum sp. nov. Bitaunioceras buckhornense represents the first Pennsylvanian and therefore the oldest record of this genus. Arbuckleoceras gen. nov. differs from a comparable genus, Shikhanoceras, in possessing a weak exogastric curvature with a circular cross section of the conch and in lacking a conspicuous inflation at the embryonic shell. Smithorthoceras gen. nov. resembles orthoceratids rather than pseudorthoceratids in characters of camerae and siphuncle; however it refers to the Pseudorthoceratidae by having endosiphuncular deposits. These similarities seem to be the result of convergent evolution. Endosiphuncular deposits in Sulphurnites gen. nov. initiate at apical and adoral junctions between septal neck and connection ring, whose characters are unique for pseudorthoceratids. Unklesbayoceras gen. nov. differs from Mitorthoceras in having the endogastric conch, longer camerae and a less eccentric siphuncle. Taxonomic status of these orthoceratids was uncertain in previous biogeochemical and morphological studies. Sediments in the Buckhorn Asphalt Lagerstätte were deposited in a tropical epeiric sea (the Midcontinent Sea). Small, restricted marine basins, like that in this Oklahoma occurrence, probably provided an orthocerid refuge habitat as indicated by the high diversity and provincialism in comparison with other Middle Pennsylvanian (= Moscovian) faunas in other regions of the world.