EMBRYONIC ORTHOCERATID NAUTILOIDS OF THE IMO FORMATION (LOWER CARBONIFEROUS - UPPER CHESTERIAN) OF ARKANSAS (USA)

The taxonomy of the Orthocerida is highly problematic. The mature shell parts provide very few useful characters, and these characters are subject to repeated homeomorphy and iterative evolution. Thus, a usage of Orthocerida for studies on phylogeny, biodiversity, paleoecology is nearly impossible at this time because of excessive overestimation of taxon ranges.

In order to improve the current taxonomy of a specific orthocerid fauna, more than 500 specimens of embryonic shells of orthocerid nautiloids from the Imo Formation were investigated. Although the material is recrystallized, the external and internal features of the early growth stages are exceptionally well preserved. The material comprises eight species of Pseudorthoceratidae.

Our analysis indicates that the morphologic diversity of the early growth stages of the shells of these species is much more diverse than expected. The species vary strongly in embryonic shell size, cicatrix position and shape, numbers of septa in the embryonic shell at the time of hatching, embryonic shell ornamentation, and the outline of the first segment of the siphuncle and its position in cross section. This study shows that the shape and position of the cicatrix is a morphologic feature that has been under utilized in previous investigations. The high morphologic variance of the embryonic shells in these Imo orthocerids requires a revision of the concept of the Pseudorthoceratidae. In addition, this analysis strongly supports using the morphology of the embryonic shell, and especially the cicatrix, in all future orthocerid systematic and phylogenetic analyses because this feature provides an important tool for detecting homeomorphic evolutionary relationships that are not discernable in mature specimens.