Ardmosteges orchamus, new genus, new species, from the Early Pennsylvanian Morrowan sequence in southern Oklahoma, is characterized by a well-developed early aulostegid stage, followed by a fully developed richthofenoid stage. It differs markedly from any previously described genus. The nature of the early development, with a rounded aulostegid shell and a prominent interarea in the ventral valve, suggests that it evolved from the Aulostegidae. The most unique feature of Ardmosteges is that it shows for the first time, a documented mechanism for the addition of new shell material above the hinge area in the ventral valve in shells that proceeded to develop a richthofenoid cone. Shell layers were extended posteriorly from the margins of the ventral valve onto the interarea of that valve indicating that the mantle grew onto the hinge area from each side. This irregular and asymmetrical process gradually filled the space above the hinge and was followed by the upward growth of a typical richthofenoid cone. In the proposed development of the Permian richthofenoids from Ardmosteges it is postulated that the characteristic interarea of Ardmosteges was suppressed during extreme development of a distinctly cylindrical cone.
Ardmosteges provides the missing link between the Aulostegidae and the Richthofenoidea. Its characters are primitive compared to those of the Permian richthofenoids but it is similar in having a deeply recessed opercular dorsal valve, protective spines in the vestibule above the dorsal valve, and in being attached by rhizoid spines.