Makoiamya Grant-Mackie, 2013, n. gen.

Genus Makoiamya n. gen.

Type Species. Makoiamya cotterallae n. sp., Late Warepan (Marokopan)–Otapirian (Late Norian–Rhaetian; Late Triassic) of Murihiku Terrane, New Zealand, and Téremba Terrane, New Caledonia.

Etymology. From Maori ‘makoi’, a shell; it can also mean a tricky question, which latter emphasises my early difficulties in the classification of this fossil, plus ‘-a-mya’ to coincide with the suffixes of many generic names within this and related families (‘makoi’ also means a sharp-pointed object, not relevant here); feminine gender.

Diagnosis. Ceratomyid of moderate size, thick-shelled, oval to subtrigonal in outline, well inflated, equivalved, edentulous except for faint swelling on hinge plate below RV beak and short nymph posteriorly; beaks prosogyrous, enrolled, margins entire, not gaping; no lunule or escutcheon.

Description. Ceratomyid of moderate size, equivalve, inequilateral, well inflated, with no posterior gape or dentate margins; rounded posteroventral ridge passes from well inflated umbo to rounded-angular posteroventral margin, with slightly flattened or excavated area behind; hinge plate edentulous but with low boss posteroventral to beak and nymph parallel to hinge margin posteriorly, with ventral emargination of hinge-plate between boss and nymph; exterior unsculptured except for fine commarginal growth lines; adductor muscle scars and pallial line not certainly seen.

Remarks. Earlier records as Anodontophora (= Unionites) cannot be upheld for our fossils, given the latter’s considerable inflation, its thick shell material, and its weaker hinge details. Other genera of the family Pachycardiidae can also not accommodate these shells because of their much weaker dentition (see Cox, 1969a, p.467).

Grant-Mackie’s (1981 et seq.) postulated link with the northern Pacific ceratomyid Ochotomya took account of the inflation, suboval outline, weak dentition and lack of sculpture of the two genera, but the thick shell material of our specimens and the absence of subsidiary features anterior and posterior to the beak (see Polubotko 1966, unnumbered text figure p. 14; Fig. 12 D) prevent a close association of the two. Examination of a suite of casts provided by Dr Polubotko, including the holotypes of the type species and of his O. terechovae, shows that the hinge and umbonal area pose the greatest differences between the two groups. Polubotko’s diagnosis also refers to the presence of a weak right-angled pallial sinus, but neither on the Siberian specimens, nor in the figures in Polubotko (1966), nor on specimens from Heilongjiang Province, northeastern China, seen by me (unpublished record), can any pallial line be discerned.

Nonetheless, a familial relationship does seem likely. The Ceratomyidae are characterised (Cox, 1969b, p. 838) as ovate shells, longer than high, equivalve, moderately to strongly inflated, with prosogyrous beaks, opisthodetic subinternal ligament on the thickened posterodorsal margin of the LV, no true hinge teeth, generally no gape, and lacking lunule and escutcheon.

In all features except the shell thickness our specimens can readily fit into the Ceratomyidae. The definition of the family would need to be only slightly broadened to accommodate them. Cox (1969b, fig. F18a, b) illustrated valves of the French Kimmeridgian Ceratomya excentrica (Roemer, 1836) and Bajocian C. aalensis (Quenstedt, 1858) showing quite well thickened hinge plates, and little genetic change would have been required to produce a more extensively thickened shell. The right hinge in Makoiamya is excavated behind and slightly below the beak, with a swelling in front and a nymph behind, and in these characters it approximates Ceratomya Sandberger, 1864, (see Cox, 1969b, fig. F18c). Ceratomya is itself a cosmopolitan Jurassic genus, the only other Triassic ceratomyid, Pteromya Moore, 1861, being more elongate, less inflated, and often showing weak concentric sculpture. This family is nevertheless accepted as a suitable location for the new genus.

The only other previously described thick-shelled pholadomyoids are a few Pholadomyidae and the Megadesmidae, the latter a Permo–Carboniferous family of generally large to moderate-sized oval, equivalved burrowing bivalves. Megadesmidae are integripalliate, with subequal muscle scars, incurved beaks, often prosogyrous, and no surface ornament; they have prominent nymphs with a single blunt tooth in the RV and rarely in the LV. Some genera, e.g., Myonia Dana, 1847, Pyramus Dana, 1847, Pachymyonia Dun, 1932, are too elongate to be appropriate for the new species whereas Megadesmus J. de C. Sowerby in Mitchel, 1838, is short but too inequilateral. Megadesmids are prominent members of Australasian Permian marine faunas (e.g., Runnegar 1965; Waterhouse 1969) but none of these Permian genera can readily accommodate our Triassic form, and the somewhat more differentiated dentition does not allow its inclusion in that family.

Late Cretaceous Pachymya J. de C. Sowerby, 1826, and the Middle to Late Jurassic Machomya de Loriol, 1868, are elongate strongly inequilateral shells and thus cannot be regarded as congeneric with our material.

Other generic names have been applied to New Zealand late Triassic–Jurassic bivalves with poorly developed hinge structures and no ornament: e.g., the already mentioned Anodontophora, and Pleuromya Agassiz, 1842 (e.g., Marwick 1953), but neither of these at all closely approximate our present material, both of which are elongate shells without prominent umbones and with thin shell material, and Pleuromya in addition has a posterior gape.

The above leads to the conclusion that these Late Triassic shells are best located in a new genus, for which Makoiamya is proposed. Although so far known only from New Zealand and New Caledonia, it could be expected in nearby areas (Papua New Guinea, Antarctic Peninsula– Argentina) in similar lithologies/environments.