Thalassonerita, MORONI, 1966

THALASSONERITA MORONI, 1966 View in CoL

Thalassonerita Moroni, 1966: 70 View in CoL . Type species: Neritina (Thalassonerita) megastoma Moroni, 1966: 71 ; pl. 1, figs 1–7 (Miocene, Italy; holotype Museo Geologico Giovanni Capellini 125), by original designation.

Bathynerita Clarke, 1989: 125 View in CoL . Type species: Bathynerita naticoidea Clarke, 1989: 125 View in CoL ; text figs A, B, pl. 2, figs 3, 4 (Louisiana slope, Gulf of Mexico; holotype MCZ 297770), by original designation.

Remarks: With their extreme conchological and ecological resemblance, there is no reason to separate the Late Miocene fossil T. megastoma ( Moroni, 1966) and extant cold-seep species ‘ Bathynerita ’ naticoidea Clarke, 1989 at generic level (see above). Feminine.

Four older fossil species from bathyal cold-seep deposits have been tentatively assigned to or compared with the type species of Thalassonerita and Bathynerita . The first is ‘ T.? ’ eocenica Squires & Goedert, 1996 from a localized cold-seep limestone of middle Eocene age in the Humptulips Formation, Washington State, western North America. This limestone, deposited 40–42.5 Mya (see Goedert et al., 2013: 79) at 1500–2000 m depth, has yielded a rich and typical seep assemblage, with such molluscan taxa as abyssochrysoid gastropods and vesicomyid and lucinid bivalves ( Squires & Goedert, 1996; Campbell, 2006). The shell of ‘ T.? ’ eocenica shows closest resemblance to that of the type species T. megastoma , although the columellar deck is not preserved in any available specimens ( Squires & Goedert, 1996; Kiel et al., 2010). With this apparent lack of the columellar deck and overall thinness of the shell, Warén & Bouchet (2001) considered this species to belong to the entirely different genus, Sahlingia , of the subclass Vetigastropoda. However, the deck area and interior of neritimorph shells are composed of aragonitic calcium carbonate ( Sasaki, 2001) and especially prone to post-mortem dissolution ( Squires & Saul, 1993; Squires & Goedert, 1996). The calcitic outer layer of the shell alone might then remain as a fossil. The fewer number of whorls and more flat apex than those of Sahlingia ( Warén & Bouchet, 2001: fig. 8a, b) also favour the position of ‘ T.? ’ eocenica in Thalassonerita . This is plausibly also the case with another middle Eocene ‘neritid’ from the cold-seep deposit of the Lomitos Cherts, northwestern Peru (see Squires & Saul, 1993; Campbell, 2006).

The third and fourth species were recovered from much older seep deposits: the Oxfordian in southern France (late Jurassic, 157–164 Mya; Kiel et al., 2010: fig. 6) and Hauterivian in southern Ukraine (early Cretaceous, 129–133 Mya; Kiel & Peckmann, 2008). These unnamed fossils have more conchological similarities to other Mesozoic neritoids than to the Cenozoic Thalassonerita , which suggests that the Neritoidea radiated at least twice into the seep environment ( Kiel et al., 2010: 36). The estimated divergence time between the Phenacolepadidae and its sister clade Neritidae is concordantly younger, ~75Mya, in the late Cretaceous (Kano et al., 2002; see also Frey & Vermeij, 2008; Uribe et al., 2016).

SUBFAMILY PHENACOLEPADINAE THIELE, 1895

Remarks: The generic classification of Phenacolepadinae is revised by taking this opportunity, based mainly on the present molecular phylogeny ( Fig. 1) and conchological examination of type specimens of type species (see Fig. 5).