Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard
Hryniewicz, Krzysztof
Little, Crispin T. S.
Nakrem, Hans Arne
Zootaxa
2014
2014-09-02
3859
1
1
66
Hryniewicz & Little & Nakrem, 2014
Hryniewicz & Little & Nakrem
2014
[238,371,1845,1871]
Bivalvia
Thyasiridae
Cretaxinus
GBIF
Animalia
Lucinida
34
35
Mollusca
genus
gen. nov.
Typespecies. Cretaxinus hurumi sp. nov.
Etymology.Refers to the Cretaceous occurrence of the typespecies, 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 AxinusJ. Sowerby, 1821, Thyasira Lamarck, 1818, Parathyasira Iredale, 1930, and Conchocele Gabb, 1866( Tab. 2). Another feature distinguishing Cretaxinusfrom 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, Axinushas a totally different shape, sulci, adductor muscle proportions and external ornament ( Tab. 2). The ligament of Cretaxinusis very distinct from that of Thyasira, Parathyasiraand Conchocele,all of which have variably long, sunken ligaments ( Tab. 2). Another major difference between Cretaxinusand Axinus, Thyasira, Parathyasiraand Conchoceleis the proportions of adductor muscle scars. In Cretaxinusthe 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 Thyasiraand Parathyasira. TABLE 2.The main characters distinguishing Cretaxinus gen. nov.from other large chemosymbiotic thyasirid genera. Genus Cretaxinus gen. nov. AxinusJ. Sowerby, 1821 Thyasira Lamarck, 1818 Parathyasira Iredale, 1930 Conchocele Gabb, 1866 Shape Subtriangular Subhexangular Ovate to ovate- polygonal Ovate to ovate diamond-shaped Subquadrangular Ornament Commarginal growth lines Commarginal wavy ridges Commarginal growth lines Commarginal growth lines Commarginal growth lines Ligament Thick and external with small internal part Partially external Variably sunken, short to medium Sunken, short to medium Thick, sunken and long Character of posterodorsal margin Ps, (sms), no auricle, wide and deep escutcheon Ps, no auricle, small escutcheon Ps, sms, auricle, variable escutcheon Ps, no auricle, ill- defined escutcheon Ps, (sms), no auricle, no escutcheon Adductor muscle scars AAMS smaller and weaker than PAMS AAMS ≈ PAMS AAMS larger than PAMS AAMS larger than PAMS AAMS much larger than PAMS Reference This study Payne & Allen 1991; Löffler et al.2006; Oliver & Holmes 2007 Payne & Allen 1991; Oliver & Killeen 2002; Oliver & Holmes 2006; Rodrigues et al.2008; Zelaya 2009 Iredale 1930; Payne & Allen 1991; Oliver & Killeen 2002; Zelaya 2009 Kamenev et al.2001; Okutani 2002 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 mmin length) with a pointed posterior margin ( Payne & Allen 1991; Oliver & Killeen 2002), unlike Cretaxinus gen. nov.Both sulci of Mendiculaare 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 Cretaxinusby 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 Thyasiridaeleads 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).