608


Journal of Paleontology 92(4):596–610


Judging from the descriptions and figures of Leanza (1940) and Griffin and Pastorino (2006) and preliminary observations by RGJ and AK,M. neuquena possesses an edentulous hinge and a caspiconchiid process, suggesting affiliation to the genus


Caspiconcha. However, we need further investigations to con- firm its taxonomy. Several bivalves found at even older vent and seep deposits were considered as belonging to the mod- iomorphids and may thus be related to Caspiconcha as well. These include a species from a Late Triassic seep deposit in Oregon (Peckmann et al., 2011), large modiomorphid bivalves from Late Triassic seeps in Turkey (Kiel et al., 2017), the large and elongate Ataviaconcha wendti Hryniewicz et al., 2017a found in Devonian and Silurian seep deposits in Morocco (Aitken et al., 2002; Hryniewicz et al., 2017a; Jakubowicz et al., 2017), and Sibaya ivanovi Little et al., 1999 from a Devonian vent locality in the Ural mountains, Russia (Little et al., 1999). This recurrence of modiomorphid/kalenterid–like bivalves at vent and seep deposits from Silurian through Cretaceous time may imply either that modiomorphid/kalenterid bivalves have repeatedly colonized these environments through Earth’s his- tory, or that Caspiconcha is the last member of a long-lived vent- and seep-inhabiting lineage among the kalenterids. Whatever the exact relationships among these bivalves are, it remains intriguing that the dimerelloid brachiopods colonized seep (and vent) environments throughout roughly the same time interval, and disappeared just slightly earlier, in the mid- Hauterivian (Early Cretaceous) (Kiel et al., 2014). If Caspi- concha and its relatives were indeed chemosymbiotic (cf. Kelly et al., 2000; Jenkins et al., 2013), the apparent long coexistence of dimerelloid brachiopods and chemosymbiotic bivalves con- tradicts the hypothesis that brachiopods were pushed out of the vent/seep environment by chemosymbiotic bivalves through competitive replacement (cf. Campbell and Bottjer, 1995b).


Figure 10. Range chart of Caspiconcha species and relatives; note the highest diversity in the Albian.


decrease in sulfide release at methane seeps after the mid- Cretaceous drop in marine sulfate concentrations; fewer sulfides would mean less food for the presumed sulfur-oxidizing sym- bionts of Caspiconcha (Kiel, 2015). An additional or alternative explanation is offered by Hryniewicz et al. (2017b): the decline of Caspiconcha coincides with the appearance of large thyasirid bivalves at methane seeps. These infaunal bivalves could potentially have mined the available sulfide produced deep in the sediment so that very little sulfide reached the upper sediment layers that Caspiconcha was able to penetrate (Hryniewicz et al., 2017b).


History of seep-inhabiting kalenterids/modiomorphids.—The apparent origin of Caspiconcha in the latest Jurassic should be treated cautiously because the fossil record of seep deposits before the late Jurassic is scarce (Campbell, 2006). A possible Early Jurassic species belonging to Caspiconcha is Myoconcha neuquena Leanza 1940 from the Early Jurassic of Argentina.


Implications for the modern chemosynthetic fauna.—There was a considerable confusion about the origin of the vesicomyid bivalves because molecular age estimates indicated an early Cenozoic origin, although three vesicomyid species from Cretaceous sediments in Hokkaido, Japan, were known from the paleontological literature (e.g., Little and Vrijenhoek, 2003; Kiel and Little, 2006). The first to question the vesicomyid affinities of these species were Amano and Kiel (2007), and two of the species were subsequently identified as members of dif- ferent bivalve families: ‘Calyptogena’ sp. from the Albian Ponbetsu seep deposit (Kanie et al., 1993) was described by Kiel et al. (2008) as the solemyid Acharax mikasaensis Kiel, Amano, and Jenkins, 2008, and Vesicomya inflata Kanie and Nishida, 2000 from large Cenomanian concretions in the Horokonai area (Kanie and Nishida, 2000; Kanie et al., 2000) was redescribed as type species of the new lucinid genus Ezolucina Amano et al., 2008. The last of the three alleged Cretaceous vesicomyids, ‘Calyptogena’ sp. from the Omagari seep deposit (Hikida et al., 2003) is here described as Caspiconcha lastsamurai n. sp. Thus, all three Cretaceous ‘vesicomyids’ were clearly misidentified and the oldest confirmed fossil occurrence of the Vesicomyidae is Archivesica cf. A. tschudi (Olsson, 1931) from the Eocene of Washington State, USA (Amano and Kiel, 2007), although subsequent work suggested that it belongs to a different vesi- comyid genus (Amano and Kiel, 2012; Amano et al., 2014).


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