Capedopecten anellus Dijkstra & Janssen, 1988
DIJKSTRA, H. H. & A. W. JANSSEN. 1988. Capedopecten anellus gen. et spec. nov. (Mollusca, Bivalvia: Pectinidae)
from the Antewerp Sands (Miocene, Hemmoorian) in Belgium. Basteria, 52:
175-187, figs. 1-8. [p. 179, figs. 1a-d, 2, 3a, 3b, 5a, 5b]
1888 Capedopecten anellus Dijkstra & Janssen, 1988
H. H. Dijkstra & A. W. Janssen, 1988, figures 1a-1d.
H. H. Dijkstra & A. W. Janssen, 1988, figures 2-4.
H. H. Dijkstra & A. W. Janssen, 1988, figures 5a, 5b.
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«Description.— Shell moderately large, inaequivalve, orbicular and quite thinwalled. Height and length commonly 50 to 75 mm, transverse diameter of such specimens may reach 12 mm. Both valves slightly convex, but semidiameter of left valve about twice that of the right valve. Auricles unequal. Anterior auricles about twice as long as posterior ones. In the posterior auricle ofboth valves the transition of the dorsal margin to the posterior margin is angular. This is also the case in the anterior auricle of the left valve, whereas this transition in the anterior auricle of the right valve is distinctly more rounded. Hinge line straight, dorsal margins of anterior and posterior auricles of both valves in line. Byssal sinus in anterior auricle of right valve very shallow. No auricular denticles are visible. An inactive ctenolium is present, visible with a 10 x magnification only. The umbonal angle of the disc is about 105°.
The external surface sculpture is different on the left and the right valves. On the left valve, just below the smooth dissoconch, about twenty fine, irregular, somewhat granulated radial costae appear, that disappear at a disc diameter of about 7 mm. Smooth and irregular radial plicae persist throughout the disc, reaching the ventral margin even in completely adult specimens. Furthermore, the umbonal sculpture comprises fine concentric lamellae (visible in well-preserved specimens only), crossing the irregular radial sculpture. This concentric sculpture reaches further downwards than the radial costae and vanishes just before the central disc. In the right valve a similar radial sculpture is present in the umbonal area, sometimes better developed on the anterior side than on the middle and posterior sides. No concentric sculpture present, but sometimes the incremental lines are slightly accentuated here. Smooth radial plicae towards the ventral margin, but much weaker developed than in left valve. The entire disc surface of both valves is covered with a microscopic 'Camptonectes' sculpture. This sculpture, consisting of numerous obliquely curved, divergent striae (magnification 20 x ), is frequently invisible, as the upper shell layer is easily worn off. But, also in well-preserved specimens, this microsculpture is less developed on the central parts of the disc, sometimes virtually invisible, especially on the left valve. Irregular concentric incremental lines are present throughout the central disc and, more close-set, towards the ventral margins. The sculpture of the posterior auricle is similar to that of the adjacent disc surface, inclusive of the'Camptonectes' microsculpture. The anterior auricles demonstrate a lamellose concentric sculpture, more close-set than that on the umbonal area of the left valve. Both posterior and anterior auricles demonstrate some vague radial plicae. The internal surface of both valves is smooth, neither tadial lirae nor any other radial sculpture elements are present. Near the central part of the disc a large adductor insertion is present, situated just behind the vertical mid-line of the shell. A pallial line is not visible. The outer ligament is narrow and straight, without any cardinal crurae. The resilial pit is triangular, with a vertical angle of about 55°. Auricular denticle lines slightly pronounced, without denticles. Derivatio nominis.— anellus (Latin) = little ring, after the name of the 'Kleine Ring' motorway around Antwerp city centre, on which the type locality of this species is situated. Locus typicus.— Berchem/Borgerhout, province of Antwerp, Belgium; temporary excavations for E3 'Kleine Ring' motorway, as described above. Stratum typicum.— Basal bed of layer no. 17 (Janssen & Van der Mark, 1968), Antwerp Sands, Berchem Formation (Miocene, Hemmoorian, Oxlundian). Holotype — Figs. 1a-d, RGM 229 342. Paratypes — From the type locality the following paratypes are available: 1/1 specimen, slightly defective, leg. F. J. Janssen, RGM 229 343 (figs. 2,5); 2/2 specimens, leg. A. W.Janssen, NMR no. 6182; 1/2 specimen, leg./coll. J. Gunst; 2/2 specimens, leg. A. Haandrikman, coll. J. Gunst; 2/2 specimens, leg./coll. F. A. D. van Nieulande; 1/2 specimen, leg./coll. M. Vervoenen. From the recent excavation ofthe Borgerhout exit, as described above: 1/2 specimen, leg./coll. R. Pouwer no. 708. From the Antwerp, Ploegstraat, locality, as described above: 1/1 defective specimen and I right valve, 1 right valve (fig. 3), leg. D. van der Mark, RGM 229 344 and 229 350 respectively; 1 defective left valve, leg F. J. Janssen, RGM 229 348;3 defective left valves, 1 fragment, leg. R. E. Hamstra, RGM 229 367; 1 defective right valve, leg. M. van den Bosch, RGM 229 368; 2/2 specimens, leg. A. W. Janssen, NMR no. 5967; 2 left and 3 right valves, more or less defective, leg. A. W. Janssen, NMR no. 5812; 4/2 specimens, leg./coll. H. P. J. Keukelaar. Remarks.— Capedopecten anellus resembles the associated species Korobkovia woodi, which apparently has a much wider geographical and stratigraphical distribution. A detailed comparison is given below, but the main differences between these two taxa are the presence of a complicated macro- and microstructure on the external surface of both valves in C. anellus, whereas in K. woodi the shell surface is smooth, without any significant ornamentation. The new species reaches considerably larger dimensions and is slightly more solid (better: less fragile) than K. woodi. In the new species the internal surface of the shell is smooth, without any radial sculpture elements. In K. woodi the inside of the left valve has radial sculpture, which, by the way, is considered typical for the genus Korobkovia. The hinge line of the new species is straight, with the dorsal auricle margins of both valves in line, whereas in K. woodi the hinge line of the right valve is curved, with the dorsal auricle margins enclosing a distinct angle (most distinctly observed in juvenile specimens). Auricle denticles are absent in C. anellus, whereas such denticles are strongly developed on both valves of K. woodi. An inactive ctenolium is present in the new species, but absent in K. woodi. One of the most striking and, to the present day, unexplained features of the new species is the fact, that juvenile specimens (with a length of less than about 50 mm) have never been found. Among the much larger samples of K. woodi, incluiding hundreds of juvenile specimens, not one juvenile valve of C. anellus could be isolated. As far as we know, the new species has a very restricted distribution pattern, both in geographical and in stratigraphical respects. The species has only been observed in the base of layer 17 of the Antwerp Sands in the Antwerp city area. K. woodi, on the other hand, has a stratigraphical range including the Vierlandian, the Hemmoorian and the Reinbekian (Early to Middle Miocene), and is widely distributed all over the North Sea Basin.» HENK H. DIJKSTRA, H. H. & ARIE W. JANSSEN, 1988 |