Bactroceras Holm 1898

Genus Bactroceras Holm, 1898 Type species

Bactroceras avus Holm, 1898; Middle Ordovician, late Darriwilian Stage, Seby Limestone Formation; Öland, Sweden; by original designation.

Slender, orthocones or weakly curved cyrtocones with sub-circular or circular conch cross section; apical angle low, typically below 10°; surface sculpture with straight, transverse growth lines or annulations, and/or striae; hyponomic sinus broad and shallow; phragmocone chambers rather deep, concavity at least 0.3 of conch height; sutures straight and transverse; distance between two sutures between 0.25 and 0.5 of corresponding conch cross section; siphuncle marginal or slightly removed from shell margin; diameter of the siphuncle between ¹/7 and ¹/20 of corresponding conch cross section; septal necks orthochoanitic to hemichoanitic; siphuncular segments tubular to slightly expanded; connecting rings thin and homogeneous (adopted from Aubrechtová 2015).

One of the diagnostic characters of Bactroceras, given in Aubrechtová (2015), is the absence of endosiphuncular deposits. This is a slight emendation of previous diagnoses of the genus by Aubrechtová (2015) adopted by King & Evans (2019). In Kröger & Evans (2011) the diagnosis reads “endosiphuncular deposits unknown” and in the original diagnosis of Bactroceras of Holm (1898) no statement about endosiphuncular deposits is given.

With the discovery of endosiphuncular deposits, herein, in a species which is very similar to the type species of Bactroceras, the subtle emendation by Aubrechtová (2015) becomes crucial. Accepting it would require the erection of a new genus, which would be identical to Bactroceras, but which would differ in the known presence of endosiphuncular deposits. Alternatively, the original genus diagnosis could be revived again, so that potentially species with endosiphuncular deposits can be included. Here the latter solution of the problem is suggested, because it is very difficult, and often impossible, to definitively make a statement on the absence of endosiphuncular deposits. A sufficiently large sample of apical fragments to contain individuals that grew large enough to have developed endosiphuncular deposits is required. This character (the absence of deposits) would render the genus very difficult to practically apply to species, or it would increase the risk that it becomes a wastebasket taxon for inadequately known fragments (e.g., Ormoceras Stokes, 1840). And more generally, the absence of a feature, such as endosiphuncular deposits, whose presence theoretically cannot be ruled out, is a diagnostic character that can be falsified, even if the chances of falsification are low. Therefore, the original diagnosis of Holm (1898) is here taken as a reference. Thoraloceras Kröger & Evans, 2011, which is a troedsonellid with a marginal siphuncle with endosiphocones, similar to Bactroceras, and which occurs in Tremadocian strata of the Montagne Noir, differs from the latter mainly in having a wider siphuncle (¼ of the conch cross section) and a smooth conch surface.