Geisinidae Sohn 1961

Family Geisinidae Sohn, 1961

Preliminary remark

This family has been revised by Adamczak (2006). Following these recommendations, the family name Knoxitidae Egorov, 1951 is here not adopted because “no generotype was explicitly designated” (Adamczak 2006: 277) and we chose to use Geisinidae Sohn, 1961.

Several genera belonging to this family have very similar morphologies (Adamczak 2006): Geisina Johnson, 1936, Hypotetragona Morey, 1935, Jonesina Ulrich & Bassler, 1908, and Knoxiella Egorov, 1950. This observation has led some authors to consider these genera as synonyms (e.g., Sohn in Moore 1961; Adamczak 2006; Jones 2011). Pollard (1966) considered that Geisina and Jonesina are both valid genera and indicated that Jonesina differed from Geisina by valves strongly and deeply bisulcate, S2 deep and ending by a pit, hinge not depressed in the posterior half and overlap absent along the hinge except at cardinal angles. Lethiers (1981) distinguished Knoxiella from Hypotetragona essentially by the invagination of the hinge and weak variations of the reticulation (Lethiers 1981: fig. 17). Here, we consider that these characters are not sufficient for generic discrimination, more specifically owing that the weak reticulation variations may be related to the preservation state. Thus, the morphologic differences observed by Lethiers (1981) could rather be differences at specific level.

Adamczak considered Knoxiella and Jonesina as “waste-basket taxon(s)” (Adamczak 2006: 299) and indicated that Hypotetragonais is preferred over Knoxiella in recent work. For Jonesina, it seems that “most of the species referred to this genus belong to Geisina and Hypotetragona ” (Moore 1961: 413). However, Hypotetragona and Knoxiella are still being used (e.g., Casier 2017 for Knoxiella; Nazik et al. 2021 for Hypotetragona; Olempska 1999 used both genera) and an in–depth revision of all species is necessary to clarify their generic status.

As this revision is out of the scope of the present contribution and material:

– We here consider Knoxiella and Hypotetragona as synonyms, Knoxiella being a junior synonym of Hypotetragona. We therefore attribute all the present species to Hypotetragona, for which we can fully appreciate the morphological characters;

– We keep the genus name Knoxiella for the species that have already been attributed to this genus until they are thoroughly revised (Table 1).