Two new species of trigonoceratid nautilids from the middle Chesterian (Early Carboniferous) Pitkin Formation of Arkansas, Midcontinent North America are described. Epistroboceras pitkinense sp. nov. is most similar to E. caneyenseNiko and Mapes, 2004, but is distinguished mainly by its dorsally shifted umbilical angle. Stroboceras gordoni sp. nov. is characterized by the narrow lg 2 (second lateral groove) in the lateral grooves, the two bilaterally symmetrical lateral ridges, and the weak nodes at intersections of ridges and the growth lines. Species previously described or assigned to Stroboceras are re-examined. Similarities in the shell morphology between the trigonoceratid nautilid Stroboceras and the isochronous Early Carboniferous ammonoid Eumorphoceras probably provide an example of synchronized convergent evolution.
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