Vesicomyidae Dall and Simpson, 1901

Hybertsen, Frida, Goedert, James L. & Kiel, Steffen, 2022, A new genus of chemosymbiotic vesicomyid bivalves from the Oligocene of western North America, Acta Palaeontologica Polonica 67 (3), pp. 703-709 : 704-705

Family Vesicomyidae Dall and Simpson, 1901

Genus Squiresica nov.

Zoobank LSID: urn:lsid:zoobank.org:act:551BF127-9A67-49DA-80E1-B2FCB1A25354

Type species: Archivesica knapptonensis Amano and Kiel, 2007 , Oligocene part of the Lincoln Creek Formation, western Washington State, USA .

Species included: The type species and Archivesica marincovichi Kiel and Amano, 2010 , from the Oligocene Kulthieth Formation in Alaska, USA .

Etymology: In honor of Richard L. Squires (Northridge, USA), who pioneered work on fossil vesicomyids from the US West Coast, and the ending of the related vesicomyid genus Archivesica .

Diagnosis.—Shell small (L up to 50 mm), elongate, weak to moderately inflated; escutcheon narrow, lunular incision indistinct to well incised, narrow to moderately broad; umbo elevated. Pallial line impressed anteriorly; pallial sinus small and shallow, or even just a slight forward sloping of the pallial line before meeting the posterior adductor muscle scar; hinge plate moderately broad, with thick cardinal 1 radiating anteriorly, cardinal 3a thin, short and parallel to shell margin or reduced, cardinal 3b small, pointing posteroventrally, with parallel or somewhat diverging raised edges; subumbonal pit elongate or deep.

Remarks.—The new genus Squiresica has an Archivesica - like hinge dentition, but differs from Archivesica by (i) being much smaller, (ii) having an elevated umbo with strongly coiled beaks, (iii) its pallial sinus, which is only a small indentation, whereas Archivesica in general, and the type species A. gigas in particular, has a large and broad pallial sinus, (iv) possessing an escutcheon and typically a lunular incision, both of which are absent from Archivesica gigas . Archivesica redwoodia Kiel and Amano, 2010 , a small species from Alaska, shares the small pallial sinus with the species of Squiresica , but differs by having a long anterior cardinal 3a parallel to the dorsal shell margin, a more ventrally pointing cardinal 3b, and a much broader and deeper pallial sinus (see Table 1 for comparison to other pliocardiin genera).

Austrogena nerudai Krylova, Sellanes, Valdés, and D’Elía, 2014 , type species of Austrogena from the Southeastern Pacific, shares the presence of an escutcheon, a lunular incision, a subumbonal pit and an elongate cardinal 4b with species of Squiresica ; however, it can be distinguished from these by the complete lack of a pallial sinus ( Krylova et al. 2014). The oldest vesicomyid, “ Archivesica cf. tschudi ” from the middle Eocene of western Washington ( Amano and Kiel 2007) is very small and Archivesica - like, but it has a low umbo and lacks a pallial sinus. Hubertschenckia Takeda, 1953 , reaches 90 mm in length and is thus much larger than Squiresica . Further differences include the cardinal 3b, which points downwards or slightly anteriorly in Hubertschenckia , but posteriorly in Squiresica , and Hubertschenckia has a posterior nymphal ridge, absent in Squiresica ( Takeda 1953; Amano and Kiel 2007). Other pliocardiins sharing the small sized shell with Squiresica are Pliocardia Woodring, 1925 , and Isorropodon Sturany, 1896 . Both genera have an elevated umbo and a lunular incision, but Isorropodon lacks a pallial sinus and Pliocardia has a posterior ridge ( Sturany 1896; Woodring 1925; Cosel and Salas 2001; Krylova and Janssen 2006; Amano and Kiel 2007). Isorropodon is less elongate than Squiresica , and all cardinal teeth of Isorropodon are thin and more or less parallel to the dorsal shell margin (e.g., Cosel and Salas 2001), in contrast to the radiating teeth of Squiresica . Pliocardia is more rounded than Squiresica , it features a posterior ridge on the outside of the shell, lacking in Squiresica , and its cardinal 3b is shorter and more compressed than that of Squiresica . The Pleistocene Arctic species Archivesica arctica Hansen, Hoff, Sztybor, and Rasmussen, 2017 , has stronger and more radiating teeth than the species of Squiresica , a very distinct, pointed pallial sinus, and reaches much larger size (55–67 mm compared to 40 mm in the species of Squiresica ).

A further species that could potentially belong to Squiresica is Pleurophopsis lithophagoides Olsson, 1931 , from the lower Oligocene of northern Peru ( Olsson 1931). That species is notably smaller than most other species of Pleurophopsis Van Winkle, 1919 (see Kiel et al. 2020: table 1) but is in the size range of the species of Squiresica . It also has a similar elongate shape and is rather flat like Squiresica knapptonensis . The cardinal 3a of Pleurophopsis lithophagoides remains unknown, as well as the presence or absence of a pallial sinus ( Olsson 1931; Kiel et al. 2020), and hence a potential assignment to Squiresica remains unresolved.

Stratigraphic and geographic range.— Oligocene, Pacific Coast of North America , from Alaska to Washington State.