Sinepecten Schein, 2006
SCHEIN, E. 2006. A new deep-sea pectinid bivalve from thermal vents of Manus back-arc Basin (south-western Pacific), Sinepecten segonzaci n.gen., n. sp. (Pectinoidea: Pectinidae), and its relationships with the genera Bathypecten and Catillopecten. Zootaxa, 1135: 1-27, figs. 1-14. [p. 4]
«Family Pectinidae Rafinesque, 1815
Subfamily Propeamussiinae Abbott, 1954
Genus Sinepecten n. gen.
Etymology: from Latin sine: without, and pecten: comb.
Diagnosis: Pectinidae deprived of radiate ornamentation, byssal notch without ctenolium, posterior auricles not delimited. Right valve showing a prismatic juvenile stage. Post-prismatic part of this valve and left valve sculptured by commarginal ridges or lamellae affecting only the external surface of the shell.
Monospecific: Sinepecten segonzaci n. sp.
Remarks: Within the subfamily Propeamussiinae, Bathypecten and Catillopecten are the genera having the strongest affinities with Sinepecten. All three genera have a juvenilelike, general shell morphology, recalling some paleozoic fossils to mind (Schein-Fatton 1988), e.g., Pterinopecten undosus Hall, 1883. The following characters are found in the post-larval shells of all Pectinidae: byssate, thin test with low convexity, no radial ornamentation, flat right valve without ctenolium, and posterior auricles poorly delimited. The arrangement of prisms of their right valve is commarginal in the three genera, as in most Propeamussiinae. Besides their resemblance, there are differences principally in the duration of the prismatic stage and in ornamentation. The outer prismatic microstructure of the right valve persists throughout ontogeny in Bathypecten and Catillopecten, whereas it is restricted to a juvenile stage in Sinepecten. The ornamentation of the adult shell, although exclusively commarginal in the three genera, is very different from one to the other. In Bathypecten, the presence of ornamentation can even be questioned, as the shell is not sculptured, but has only commarginal shallow undulations that tend to disappear through ontogeny (Schein-Fatton 1985) and can even be absent (Dijkstra & Gofas 2004). In contrast, Catillopecten and Sinepecten show a dense external ornamentation of fine ridges. In Catillopecten, the ridges of the left valve are sharp, even and regularly spaced. Moreover, the inner face is marked with grooves reflecting the ridges on the outer face. These ridges derive thus from narrow commarginal folds concerning the entire shell thickness. They become closer with growth, and sometimes the flanks of each narrow fold fuse so that the inner grooves disappear (unpublished data). In the prismatic right valve there is only a very faint and rather irregular commarginal fluting which derives from the commarginal array of prisms, and becomes more pronounced in the periphery where commarginal ridges begin to shape as in Propeamussium, Parvamussium and Cyclopecten (Schein 1989). Furthermore concentric undulations can also be present on both valves. In Sinepecten, after the prismatic stage, ornamentation and microstructure are the same on both valves. The ridges tend to turn to lamellae, and the inner face is not grooved. The ornamentation of Catillopecten and Sinepecten could be homologous if the ridges derive from narrow folds in both genera, but the absence of grooves on the inner shell face of juvenile and adult Sinepecten suggests a different origin. In the present state of knowledge (Table 1), adult Sinepecten is clearly different from Bathypecten and Catillopecten, therefore justifying the creation of a new genus. Despite the synonymy of Bathypecten and Catillopecten recently proposed without ontogenetic argumentation (Dijkstra & Gofas 2004), I consider that the present study confirms the validity of the three genera (see also the discussion below, following the description of S. segonzaci). Irregular commarginal undulations tending to disappear after the early ontogeny as in Bathypecten are typically a plesiomorphic character, common to different taxa in the Pectinidae (symplesiomorphy).
In contrast, close-set sharp ridges keeping regularly spaced throughout ontogeny constitute a derived character which separates clearly Catillopecten from Bathypecten. Moreover, ornamentation features restricted to the outer shell surface, which vary at the species level, must be distinguished from structures concerning the entire shell thickness like the typical narrow folds of the genus Catillopecten.»
ELISABETH SCHEIN, 2006
|
Sinepecten segonzaci; E. Schein, 2006, A new deep-sea pectinid bivalve from thermal vents of Manus back-arc Basin (south-western Pacific), Sinepecten segonzaci n.gen., n. sp. (Pectinoidea: Pectinidae), and its relationships with the genera Bathypecten and Catillopecten, figures 8, 9, 11.
|