Abstract:
Three longicones (Virgilian, Upper Pennsylvanian, Texas, USA) with a ventral marginal siphuncle, a shell wall with a nacreous layer, and a long body chamber were examined. The smallest is a 25 mm long adolescent, protoconch-bearing shell with a short rostrum. The rostrum has a small post-protoconch part that is about as long as the protoconch (ca 0.25 mm), and extends along about first ten camerae. The rostrum is loosely calcified and strongly pyritized, and this suggests that it originally had a combined organic and carbonate primary composition. The first septum is mineralized, with long prismatic mural parts that extend along about ten camerae and form the shell wall. At latter growth stages a nacreous layer and then an inner prismatic layer appear. The septal necks are retrochoanitic, short dorsally and long (about 1/3 camera length) ventrally. The connecting rings are thin and were probably originally organic in composition. Thin cameral deposits are distinct in the protoconch and the first two camerae. In the latter camerae the deposits are less distinct but show pyritized membranes on their surfaces. The adolescent longicone falls within the concept of Mutveiconites Doguzhaeva, 2002, known from the Upper Carboniferous (Orenburgian) of south Urals, and is described as Mutveiconites milleri sp. n. The other two larger longicones possibly represent older growth stages of Mutveiconites, which is considered to be the earliest aulacocerid coleoid so far known. Also this occurrence represents the first report of this genus in North America.
