Sisonia, Kiel, Fernando, Magtoto & Kase, 2022

Kiel, Steffen, Fernando, Allan Gil S., Magtoto, Clarence Y. & Kase, Tomoki, 2022, Mollusks from Miocene hydrocarbon-seep deposits in the Ilocos-Central Luzon Basin, Luzon Island, Philippines, Acta Palaeontologica Polonica 67 (4), pp. 917-947 : 933

Genus Sisonia nov.

Type species: Sisonia frijellanae sp. nov., see below.

Species included: Possibly belonging to Sisonia are three specimens identified as “ Solemyidae gen. indet.” from the Miocene Moonlight North seep deposit in New Zealand ( Saether et al. 2016: fig. 3B). The figured specimen shows a similar overall outline and muscle scar pattern, though it is much less inflated than Sisonia frijellanae . Further specimens most likely belonging to Sisonia have been found in a Pleistocene seep deposit in Taiwan (SK unpublished data).

Etymology: For the Municipality of Sison, where the type locality is located in Pangasinan province.

Diagnosis.—As for the type species.

Remarks.—This genus is difficult to place among the heterodont bivalves. Its overall outline and hinge resembles some vesicomyids, though the strong posterior inflation is uncommon in that family. The most distinctive difference of Sisonia frijellanae from vesicomyids is the rather indistinct and elongate, pear-shaped anterior adductor muscle scar. Vesicomyids of this size typically have a distinct, deeply impressed anterior adductor muscle scar bound by a strong ridge on the posterior side. Trapeziidae have a somewhat similar outline and also lack a pallial sinus, but their umbones is more prosogyrate, and they possess posterior lateral teeth, absent in Sisonia frijellanae . Remarkably similar shell shapes, both internally and externally, can be found among the galeommatoids ( Morton and Scott 1989), for example the lasaeid Neaeromya rugifera ( Carpenter, 1864) or Bornia sebetia ( Costa, 1830). However, galeommatoids rarely reach sizes beyond 10.0 mm and are thus much smaller than the almost 70.0 mm long Sisonia frijellanae . Interestingly, at least one galeommatoid species, the saxicavellin Atopomya dolobrata Oliver, 2013 , harbors symbiotic bacteria in its gills ( Oliver 2013).