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: Pseudorthoceras knoxense (McChesney, 1860); Ristedtoceras teriliratum n. sp.; Mooreoceras imoense n. sp.; Mooreoceras striatulum n. sp.; Reticycloceras peytonense Gordon, 1965; Dolorthoceras tenuifilosum Gordon, 1965; Mitorthoceras girtyi Gordon, 1965; and Euloxoceras angustinus Gordon, 1965. A new genus, Ristedtoceras, is erected and the genus Pseudorthoceras Girty, 1911 is emended.
The analysis indicates that the morphologic diversity of the early growth stages of the shells of these species are much more diverse than expected. The different species vary strongly in the 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 our understanding of the Pseudorthoceratidae. In addition, the implication of this analysis strongly supports using the morphology of the embryonic shell, and especially the cicatrix, in all future orthocerid systematic and phylogenetic analyses because it is proving to be an important set of characters in detecting homeomorphic evolutionary relationships that are not discernable in mature specimens.