Cretaxinus, Hryniewicz & Little & Nakrem, 2014

Hryniewicz, Krzysztof, Little, Crispin T. S. & Nakrem, Hans Arne, 2014, Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard, Zootaxa 3859 (1), pp. 1-66 : 35-36

Genus Cretaxinus gen. nov.

Type species. Cretaxinus hurumi sp. nov.

Etymology. Refers to the Cretaceous occurrence of the type species, and to the genus Axinus .

Diagnosis. Shell inequilateral, triangular in outline, thin. Ornament of commarginal growth lines. Beaks weakly prosogyrate, not very prominent. Cardinal area edentulous, with small ligament groove. No lateral teeth. Lunule long and deep. Posterior sulcus shallow, posterior fold not very prominent. Submarginal sulcus shallow; no auricle. Ligament short, thick, external, with possible small internal portion. Anterior adductor muscle scar elongated, very weak; size difficult to estimate, but relatively small. Posterior adductor muscle scar larger than anterior one, circular, well impressed. Pallial line entire.

Remarks. Comparison of Cretaxinus gen. nov. with other large, chemosymbiotic thyasirids ( Dufour 2005) suggest it can be clearly differentiated from all of them ( Tab. 2 ). The most noticeable feature is its subtriangular shape, which is very distinct from the shapes of the genera Axinus J. Sowerby, 1821 , Thyasira Lamarck, 1818 , Parathyasira Iredale, 1930 , and Conchocele Gabb, 1866 ( Tab. 2 ). Another feature distinguishing Cretaxinus from other thyasirids is its short, thick and external ligament set in a deep escutcheon, accompanied by a possible small internal portion. A similar feature is seen only in Axinus . However, Axinus has a totally different shape, sulci, adductor muscle proportions and external ornament ( Tab. 2 ). The ligament of Cretaxinus is very distinct from that of Thyasira , Parathyasira and Conchocele , all of which have variably long, sunken ligaments ( Tab. 2 ). Another major difference between Cretaxinus and Axinus , Thyasira , Parathyasira and Conchocele is the proportions of adductor muscle scars. In Cretaxinus the anterior adductor muscle scar is small and weak, and the posterior adductor muscle scar is large and well impressed; in the other genera the proportions of the muscle scars are reversed. The sulci are less distinct and somehow anteroposteriorly flattened in comparison to the sulci of the other discussed genera. The escutcheon is wider and deeper than that of Thyasira and Parathyasira .

Knowledge of small thyasirid species is far less complete than of their large relatives ( Payne & Allen 1991; Oliver & Killeen 2002) and therefore their direct comparison with Cretaxinus gen. nov. is more problematic. Mendicula Iredale, 1924 , has a very small, oval shell (<3 mm in length) with a pointed posterior margin ( Payne & Allen 1991; Oliver & Killeen 2002), unlike Cretaxinus gen. nov. Both sulci of Mendicula are shallow and the ligament is entirely internal ( Zelaya 2010), also unlike in Cretaxinus gen. nov., which has weak sulci but largely external ligament. Adontorhina Berry, 1947 , apart from small, oval shells with an internal ligament, differs from Cretaxinus by characteristic granules on the hinge plate of the former ( Scott 1986; Barry & McCormick 2007). Axinulus Verrill & Bush, 1898 , has a small oval shell which is higher than long ( Payne & Allen 1991) and a largely internal ligament, also unlike Cretaxinus gen. nov.

The current lack of taxonomically robust characters in the shells of the Thyasiridae leads to problems of generic separation within the family (e.g. Payne & Allen 1991; Oliver & Killeen 2002; Oliver & Sellanes 2005; Oliver & Holmes 2006; Rodrigues et al. 2008). Molecular data suggest that current thyasirid generic definitions might be inadequate and additional morphological studies may result in redefinition of the genera ( Taylor et al. 2007). Further, the same data show only a weak separation between larger, chemosymbiotic thyasirids and small (<10 mm) thyasirid genera without symbionts, like Mendicula Iredale, 1924 , Leptaxinus Verrill & Bush, 1898 , Adontorhina Berry, 1947 , and Axinulus Verrill & Bush, 1898 ( Dufour 2005) .