This view is challenged by more recent research. The shell of "Bactroceras" has important differences from those of true bactritids. For instance, the first chamber of "Bactroceras" resembles that of other Ordovician orthocerids, such as "Archigeisonoceras" and "Hedstroemoceras": it is about 10 mm in diameter, and is short, forming a spherical cap. True bactritids more strongly resemble late Silurian and Devonian orthocerids, whose first chamber is only about 5 mm across. Moreover, a large stratigraphic gap of nearly 50 million years occurs between "Bactroceras" and the next orthocones with a spherical apex and ventral siphuncle.