"Chlamys" subacuta (Lamarck, 1819)
LAMARCK,
J. B. 1819. Histoire naturelle des animaux sans vertèbres, présentant les
caractères généraux et particuliers de ces animaux, leur distribution, leurs
classes, leurs familles, leurs genres, et la citation des principales espèces
qui s'y rapportent. Tome
sixième, Premier partie, 343 p. Chez l'Auteur, au Jardin du Roi. De l'imprimerie de A. Belin. Paris, 1819. [p. 181]
1819 Pecten subacutus Lamarck, 1819
1842 Pecten acuminatus Geinitz, 1842
1819 Pecten subacutus Lamarck, 1819
1842 Pecten acuminatus Geinitz, 1842
Pecten subacutus Lamarck; A. d'Orbigny, 1844-1847, Paléontologie francaise, Description des Mollusques Rayonnés fossiles, Terrains crétacés, III Lamellibranches, plate 435, figures 5-10.
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«11. Peigne en pointe. Pecten subacutus.
JEAN BAPTISTE LAMARCK, 1819
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«Description. Valves rather flattened and covered with 19-28 well-developed, undivided ribs and numerous concentric growth lines. Spines or scales occur at the rib tops, especially on the left valve. Marginal striae occur on edge of shell. Intercostal intervals broader than the ribs. Auricles well developed; growth lines on the anterior auricle of right valve are parallel to the byssal sinus; on other auricles the radial ornament is most pronounced.
Remarks. This species is distinguished by its relatively low number of undivided, sharper, spine/scale-bearing ribs. British specimens are, on average, smaller than those from elsewhere; they are often preserved only as steinkerns, when fewer ribs are present, side ribs are lacking, spines and scales are lost and the areas appear smooth. Occurrence. Cenomanian; Devon and Isle of Wight.» CLEEVELY, R. J. & N. J. MORRIS. 2002. Introduction to molluscs and bivalves. In A. B. Smith & D. J. Batten (Eds.): Fossils of the Chalk, 99-160. The Paleontological Association. London. [p. 119, 120]
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Chlamys? subacuta (Lamarck); R. J. Cleevely & N. J. Morris, 2002, Introduction to molluscs and bivalves, plate 21, figure 6.
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«Material: One worn valve [DGP 26722: H = (20.6) mm; L = 15.2 mm; R = 32; UA = 56°)], mentioned by Parona (1908a) as Pecten sp., from Candaglia, 200 m beyond the "cantoniera" on the road towards the fossiliferous locality Col dei Schiosi (information labelled by Parona).
Discussion: The specimen lacks auricles but its general shape and rib distribution remind one strongly of Chlamys? subacuta. The small fragment of shell preserved with rib ornamentation on the anterior (?) side of the valve confirms this identification. The rib number and umbonal angle fall within the variability of ChI.? subacuta established in Dhondt (1973a).
The presence of Chl.? subacuta in the Col dei Schiosi assemblage indicates a possible connection with temperate European areas. Distribution: Rare in the Albian (southern England, Switzerland) but frequent in the Cenomanian [southern England, Belgium, western and SW France, Czechia, Germany (Saxony), NE Italy, Algeria, Tunisia, Israel, Jordan], mainly in coarse deposits (as in the western Carnic Prealps).»
DHONDT, A. V. & I. DIENI. 1993. Non-rudistid bivalves from Late Cretaceous rudist limestones of NE Italy (Col di Schiosi and Lago di S. Croce areas). Memorie di Scienze Geologiche, 43: 165-241, pls. 1-20, text-figs. 1-18. [p. 189]
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Chlamys? subacuta (Lamarck, 1819).; A. Dhondt & I. Dieni, 1993, Non-rudistid bivalves from Late Cretaceous rudist limestones of NE Italy (Col di Schiosi and Lago di S. Croce areas), plate 4, figure 8.
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«Diagnosis.— Chlamys-species with rather flattened valves covered with 19 to 28 well developed undivided ribs, which can bear spines or scales.
Both valves are covered with numerous concentric growthlines; on the top of the ribs these develop into spines or scales and particularly so on left valves. The auricles are well developed : the anterior one on the right valve is covered with very clear growthlines which are parallel to the byssal sinus; on the other auricles the radial ornamentation is more pronounced than the concentric; on left valves the radial riblets can bear spinules. Discussion
Variability:
It is noteworthy that specimens from the type-locality and from the British Cenomanian are, on average, smaller than those from the Tourtia de Tournai, Dresden and Korycany; this is also the case in Neithea (Neithea) aequicostata (LAMARCK) and in Chlamys elongata (LAMARCK). Preservation variability:
As in many species there is a marked difference between valves and Steinkernen. This difference is important because in certain areas the Steinkern preservation is the only or almost only occurrence of the fossils; if one does not realize that the differences between the two preservation types are due to the preservation alone one could consider both as belonging to different species. Steinkernen have fewer ribs: the smaller ribs near the side-margins are not present on them and the areas appear to be smooth. On Steinkernen all traces of spines and scales are gone; this is one of the reasons why many authors consider Pecten acuminatus to be different from P. subacutus: on the former the ribs are supposed to be smooth, whereas on the latter the ribs bear spines, and most authors have not understood that the « smooth» ribs are merely the result of a poor state of preservation. Synonymy:
In connection with Pecten acuminatus H. WOODS wrote in 1902 : «It seems probable that the comparison of good series of specimens might show P. acuminatus and P. subacutus to be identical». After studying a large number of specimens including the holotype of P. acuminatus, I can only confirm WOODS's hypothesis. There is no difference between both taxa except that according to most authors P. acuminatus does not bear spines; this is true of «Steinkernen», but A. FRITSCH, 1883 (p. 116) wrote : «...wohlerhaltene rechter Schale weist schuppige Verzierungen ...auf». WOODS considered Pecten brongniarti D'ARCHIAC from the Tourtia de Tournai as synonymous with Pecten subacutus. In the I. R. Sc. N. B. collection there is one specimen from Tournai which answers to the description of D'ARCHIAC: its ribs are more rounded, more numerous than on P. subacutus and its intercostal intervals narrower than or as broad as the ribs whereas on P. subacutus they are always broader than the ribs. Differentiation:
C.? subacuta can be differentiated from the other Cretaceous Chlamys and Chlamys?-species by its relatively low number of undivided, spine-bearing ribs. Generic attribution:
Thimble-microsculpture has not been found on any of the studied P. subacutus specimens. However, the ribnumber, the general shape and the auricle shape seem to make a relation with the Upper Cretaceous Chlamys-species more probable than a relation with the Mimachlamys-species. Therefore the name Pecten subacutus LAMARCK has been replaced by Chlamys? subacuta (LAMARCK).» DHONDT, A. V. 1973. Systematic revision of the Chlamydinae (Pectinidae, Bivalvia, Mollusca) of the European Cretaceous. Part 3: Chlamys and Mimachlamys. Bulletin de l’Institut Royal des Sciences Naturelles de Belgique, 49 (1): 1-134, pls. 1-9. [p. 31, 32]
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Chlamys? subacuta (J.-B. Lamarck, 1819); A. V. Dhondt, 1973, Systematic revision of the Chlamydinae (Pectinidae, Bivalvia, Mollusca) of the European Cretaceous. Part 3: Chlamys and Mimachlamys, plate 4, figures 1a-1e.
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