THRACIIDAE Stoliczka, 1870

Carole, Hickman, S. & Ca, 2014, Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, part IV: The anomalodesmatans, PaleoBios 31 (3), pp. 1-21 : 7-8

THRACIIDAE Stoliczka, 1870 (1839)

Thraciidae , along with Laternulidae Hedley, 1918 and Periplomatidae Dall, 1895 , traditionally have been classified in a superfamily Thracioidea ( Newell 1965, Boss 1978, Yonge and Morton 1980, Morton 1981, 1982, 1985, Prezant 1998, Bieler et al. 2010). Cladistic analyses combining morphological and molecular sequence data initially suggested that these three families form a deepbranching monophyletic group ( Harper et al. 2000). However, thraciids and periplomatids and laternulids also

have been treated along with five other extant families under superfamily Pandoroidea (Newell 1969, Runnegar 1974). It is therefore significant that the first sequence data for a periplomatid ( Bieler et al. 2014) add this family to a monophyletic “thraciid lineage.”

Living thraciid species are known primarily from shells, which display a great variety of external form and may be morphologically similar to some periplomatids. However, the granular homogeneous aragonite microstructure of the outer and inner shell layers of four species examined to date ( Taylor et al. 1973, Sartori and Domaneschi 2005) clearly distinguish them from the prismato-nacreous shells of periplomatids. Eastern Pacific Cenozoic thraciid fossils all lack nacreous inner layers, apparently a derived condition because Runnegar (1974) reports that Mesozoic thraciids in collections at the Natural History Museum (London) have nacreous shells. Eroded beaks of Cenozoic fossil specimens sometimes show a peculiar iridescence, although the microstructural basis has not been determined.

Thraciids are capable of deep burrowing and prolonged maintenance of siphonal tube connections to the sediment-water interface.Long, highly extensible siphons and mucus agglutination of the separate inhalant and exhalant siphonal tubes are hypothesized to facilitate deeper burrowing in some species ( Yonge 1937). Detailed functional anatomy has been documented for two living thraciids ( Morton 1995, Sartori and Domaneschi 2005). Animals removed from the sediment are slow to re-burrow, but capable of adjusting their position vertically if burrow openings are covered with sediment. Inequivalved species rest on the flatter left valve, with the more inflated valve uppermost and the commissural plane horizontal. This is consistent with the typical preservation of the Eastern Pacific fossil thraciids as double-valved specimens that lack alterations from post-mortem exposure or transport. The primary taphonomic characteristic is diagenetic crushing of the thin, brittle shells by compressional forces perpendicular to the commissural plane.

Stratigraphic range— Upper Triassic (Rhaetian) to Holocene.