Řehor, Bailey, 2021

Bailey, Jack Bowman, 2021, The genera that never were: The impact of Janeia and Janacekia on phyletic and taxonomic relations within the Solemyidae (Bivalvia: Protobranchia), Palaeontologia Electronica (a 12) 24 (1), pp. 1-47 : 23-24

JANEIA VERSUS JANACEKIA

There is a significant complication to the Janeia story. Růžička and Řehor (1978) were critical of King’s (1850) withdrawal of the name Janeia in favor of Solemya , arguing that King had failed to provide objective evidence that Janeia was related to Solemya . Determination of the anterior and posterior ends of the shell, placement of the ligament, and organization of the internal anatomy, they asserted, were King’s personal opinions. Yet, because King’s original diagnosis of Janeia is both ambiguous and subjective, they were also critical of subsequent authors who adopted the name. Curiously, however, Růžička and Řehor (1978) agreed with King’s erroneous opinion that the long end of the shell is posterior. In their study of an assemblage of Janeia -like shells from the Carboniferous of the Czech Republic, Růžička and Řehor (1978) concluded that the ligament was external, rather than internal, as King (1850) and subsequent authors had alleged. By comparing their shells to those of extant Solemya togata, Poli 1795 , they also affirmed that King (1850) was wrong in asserting that the ligament of Janeia , like that of Solemya , was internal. Unfortunately, the authors did not compare their material with extant examples of Acharax , comparing them instead to the solecurtid genus Tagelus Gray, 1847 , which they selected as an archetype. Because their interpretations agreed neither with Solemya nor Janeia sensu King , they proposed a new genus, Janacekia Růžička and Řehoř, 1978 , herein accepted as a junior synonym of Acharax . Thus, there is neither justification for recognition of Růžička and Řehoř’s family Janacekiidae nor any basis for regarding it as synonym of the Solemyinae as opposed to the Acharachinae (contra Bieler, et al., 2010, p. 115).

Both Acharax and Janacekia share the same shell profile, the same thick periostracum marked by radial ribs and lirae, and the same primary ligament (external, parivincular, opisthodetic). Significantly, the ligament of Janacekia as originally described by the authors is ironically in agreement with Acharax but not Solemya : “Ligament ist ein äusseres, kurz und opisthodet.” (Růžička and Řehoř , 1978, p. 37). However, because their understanding of anterior and posterior is reversed, the term “opisthodet” was incorrectly applied. Thus, this portion of their description becomes essentially correct but for the wrong reason.

Of critical importance in diagnosing Janacekia is a unique feature Růžička and Řehor termed the “pseudolunula”, consisting of paired external lamellae located on the brevidorsm. Having observed an equivalent feature in “ Solemya” radiata Meek and Worthen, 1866 (Pennsylvanian of Illinois), they suggested that “ S.” radiata would more reasonably placed in Janackeia rather than Solemya . However, based on examples with well-preserved ligaments from the Mazon Creek Lagerstätte (Pennsylvanian of Illinois), Bailey (2011) transferred “ S.” radiata to Acharax . Visual comparison of the brevidorsum of Janacekia (e.g., fig. 64 of Růžička and Řehoř , 1978) with that of Acharax (e.g., pl. 2, fig. 2 of Pojeta, 1988) demonstrates that the “pseudolunula” and external nymphae are synonymous.

Janacekia and Acharax show additional similarities that further secure their identity. As shown in figure 61 of Růžička and Řehoř (1978), the buttress in Janacekia , as in Acharax , is simple, extending dorsally from the anterior limit of the posterior adductor scar to the underside of the umbo without any suggestion of attachment to nymphae, and the posterior adductor muscle shows no indication of dorsal occlusion.

Although the analysis of Janacekia by Růžička and Řehor (1978) is comprehensive, it contains numerous errors and internal contradictions. For example, in their extensive biometric section (p. 18-27) the anteriorly elongate shell orientation is understood correctly, whereas, in the systematic section, the incorrect orientation (posteriorly elongate) is applied beginning on their page 37: “Der Vorderteil der Schalen ist kurz, enger als die Hinterzeite. Die hintere Teil ist stets länger als die Vordere.” In addition, the authors provided a detailed reconstruction (their fig. 65) of the foot and pedal accessory musculature (pedal protractor, retractors, and elevators) placed at the short end of the shell, which they regarded incorrectly as the anterior.

Interpretative errors notwithstanding, the deltoid outline of the distinctive external, convexupward ligament of Acharax is documented in those authors’ photos of Janacekia herberti (pl. 1, fig. 2, showing collapsed ligament with nymph termini), and Janacekia leosi (pl. 7, fig. 9). Moreover, deltoid traces (or possible traces) of the external ligament of Janacekia herberti and Janacekia leosi seem evident in several of their other photos (pl. 3, fig. 12; pl. 8, fig. 9; pl. 11, fig. 4; and especially both pl. 7, fig. 9 and pl. 14, fig. 6). Traces of the anterior ligamental extension like Acharax radiata (Meek and Worthen, 1860) as figured by Bailey (2011, pl. 3, 4) are evident in their pl. 6, fig. 8. The type species, Janacekia herberti from the Carboniferous (Namurian) Ostrava Schichtengruppe, Upper Silesian Coal Basin, Czech Republic, is so remarkably similar to Acharax radiata (Meek and Worthen) from the Mazon Creek Lagerstätte that, aside from differences in the relative relief of the radial ribs, the two are nearly indistinguishable. For example, compare Růžička and Řehoř (1978, pl. 7, fig. 9) with Bailey (2011, pl. 3, figs. 1-9; pl. 4, figs. 1-7). Interestingly, Růžička and Řehoř listed numerous specimens referred by various authors to Solemya primaeva Phillips (= type species of Janeia King, 1850 ) as synonyms of both Janacekia herberti and Janacekia leosi . However, as noted above and by both Bailey (2011, 2016) and Logan (1967), S. primaeva is a probable Acharax .

In addition to J. herberti , three additional species of Janacekia (i.e., J. leosi , J. mariae , and J. talboti ) were described by Růžička and Řehor (1978) (see Appendix). However, the apparent differences in shell profile and prosopon, which both seem fairly minor, are possibly due to variable taphonomic effects on the thin shells and compliant periostracum.