Praeexogyra Charles & Maubeuge, 1953

Koppka, Jens, 2015, Revision of the Bivalvia from the Upper Jurassic Reuchenette Formation, Northwest Switzerland — Ostreoidea, Zootaxa 3927 (1), pp. 1-117 : 42-43

Genus Praeexogyra Charles & Maubeuge, 1953

Type species. Ostrea acuminata J. Sowerby, 1816 (p. 82, pl. 135, fig. 2) (left and right figure); non Ostrea acuminata J. Sowerby, 1816 , pl. 135, fig. 3, = Praeexogyra hebridica ( Forbes, 1851) . "Lower Fuller’s Earth", Bathonian, Middle Jurassic, Bath in Somerset, England; coordinates: 51°22'26.40''N, 2°22'35.70''W (centre of Bath).

Lectotype (designated herein). BM L.78248, J. Sowerby Collection of the Natural History Museum UK (London), refigured in Arkell (1934, pl. 2, fig. 29, 29a) and Stenzel (1971, Fig. J79.2a–b); see also Cox & Arkell (1948, p. 18).

Paralectotype—BM L.77830, figured in J. Sowerby (1816, pl. 135, fig. 2, middle figure), also in Arkell (1934, pl. 2, fig. 28, 28a) and Stenzel (1971, fig. J79.1a–b).

Diagnosis. Shell tiny to small (up to 3 cm), inequivalve, outline crescentic, reniform or ovate, little higher than long (H/L ratio 1.2–1.6); umbo usually pointed, opisthogyrate; attachment area small- to medium-sized; LV capacious, slightly to moderate convex in anteroposterior direction, less curved dorsoventrally, posterior margin concave to a variable degree. RV flat with a concave centre, anteroventral margin occasionally upturned. Surface of LV with regularly spaced commarginal swellings or weakly squamous growth welts, with depressed and relative smooth interspaces of variable distance, antimarginal riblets and furrows rare and when developed than only weak and with restriction to the attachment area or the umbo. RV relatively smooth, with weak commarginal depressions and ventrally always with fine, antimarginal riblets. Ligament area short and opisthogyrate (rarely prosogyrate); resilifer broad and slightly recurved, with posterior bourrelet narrower than anterior. PAM of lunate shape, dorsally convex or slightly biconcave (dorsocentrally convex and both sides slightly concave); position posterocentral, muscle scar not ventrally elevated (buttressed). Umbonal cavity (LV) usually absent or very weak (ca. 1 mm depth), both valves thin-shelled (1–2 mm thick).

According to Carter (1990, p. 358), P. acuminata has an outer layer of nearly vertical simple prisms of 80–90 Μm (RV) and 20 Μm (LV) thickness, a middle shell layer ranging from homogeneous „mosaic“ (LV) or regular foliated (RV) to irregular CCF, and an inner layer with regular foliated structure. Siewert (1972, p. 22) described a compact (regular) foliated structure, occasionally with few small lenses, filled with sparitic cement.

Remarks. The present diagnosis is based on P. acuminata , P. dubiensis and P. monsbeliardensis . The phylogenetic position and composition of the genus is as yet unresolved. Douvillé (1904) considered P. acuminata to be a typical example of his newly established Liostrea , whereas Rollier (1911) and Dutertre (1931) assigned the type species to Exogyra . Arkell (1934) noted that the hinge has nothing “exogyrine” about it and described it as Ostrea (“ Liostrea ”). Charles and Maubeuge (1953) created Praeexogyra as a new subgenus of Catinula assuming that it is transitional between Catinula and “true” Jurassic Exogyrinae (Gryphaeidae) . Stenzel (1971, p. N1100, N1104) considered all three taxa, Catinula , Praeexogyra , and Liostrea as genera within Gryphaeinae rather than Exogyrinae .

Charles & Maubeuge (1953) also included Ostrea sowerbyi Morris & Lycett, 1853 , and O. subrugulosa Morris & Lycett, 1853 , in their new genus Praeexogyra , ignoring the fact that both are junior synonyms of “ Ostrea ” hebridica Forbes, 1851 ( Arkell 1934) . However, whereas P. acuminata is Liostrea -like in many shell characters, “ Ostrea ” hebridica is crassostreine in these aspects (ostreine according to Hudson & Palmer 1976) thus sharing more characters with the Japanese Jurassic “ Crassostrea ” tetoriensis Komatsu & Chinzei in Komatsu et al., 2002, than with Praeexogyra or Liostrea . In addition, both species seem to prefer euryhaline environments. These latter two species are therefore thought to belong to the same, but as yet unnamed, independent lineage within the Crassostreinae .