Lyriochlamys cretosa (Defrance in Brongniart, 1822)
BRONGNIART, A. 1822. Description géologique des couches des environs de Paris, parmi lesquelles se trouvent les gypses à ossemens. In: G. Cuvier: Les Ossemens fossiles, vol. II, 2: 229-648. [p. 251, 598, pl. 3, fig. 7 a-b]
1822 Pecten cretosus Defrance in Brongniart, 1822
1822 Pecten arachnoides Defrance in Brongniart, 1822
1822 Pecten nitida Mantell, 1822
1842 Pecten nodoso-costatus F. von Hagenow, 1842
1847 Pecten barbesillensis Orbigny, 1847
1850 Pecten zeiszneri Alth, 1850
1892 Pecten irnichensis Vogel, 1892
1901 Pecten barthi Wolleman, 1901
1940 Pecten densicostatus Tzankov, 1940
1993 Mimachlamys catulloi Dhondt & Dieni, 1993
1822 Pecten arachnoides Defrance in Brongniart, 1822
1822 Pecten nitida Mantell, 1822
1842 Pecten nodoso-costatus F. von Hagenow, 1842
1847 Pecten barbesillensis Orbigny, 1847
1850 Pecten zeiszneri Alth, 1850
1892 Pecten irnichensis Vogel, 1892
1901 Pecten barthi Wolleman, 1901
1940 Pecten densicostatus Tzankov, 1940
1993 Mimachlamys catulloi Dhondt & Dieni, 1993
A. Brongniart, 1822, plate 3.
|
«Il est plat, les deux valves sont égales ainsi que les oreilles, il est marqué de côtes très-nombreuses, très-fines, un peu ondulées, qui portent vers leur extrémité marginale des espèces de petites lames relevées, imbriquées, comme l'indique, mais avec peu de netteté, la fig. 7, B. L'interstice entre les côtes n'offre aucun travail particulier.
Ce peigne paroît très-voisin de celui qui se trouve dans la craie tufau à Maestricht, et qui est figuré dans l'ouvrage de M. Faujas, sur la montagne de Saint-Pierre, pl. 25, fig. 8. L'individu figuré vient de Meudon et de la collection de M. Defrance.» JACQUES LOUIS MARIN DEFRANCE, 1922
|
«Description. Medium to large species with flattened ribs; its valves covered with a very variable number of ribs, which may bear small scales (often little more than small ridges) at their intersection with the concentric growth lines. New ribs formed by intercalation. The auricles are well developed; the anterior auricle is large with a broad deep sinus.
Remarks. Ornament varies from coarse- to fine-ribbed depending upon the nature of the sediment (Dhondt 1973). A subspecies, M. cretosa denticulata (von Hagenow), occurs in the Maastrichtian of Europe and is identified by its 'chlamys' shape and very spiny ornament. M. robinaldina (Barremian-Cenomanian) is distinguished from M. cretosa in being more convex, bearing spinules instead of scales on its fewer ribs, and covered by striae. Occurrence. Turonian, S. plana Zone, to Campanian, O. lunata Zone; occurs at many localities in the counties around London and particularly in Norfolk and Suffolk.» 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. 120, 121]
|
Mimachlamys cretosa (Defrance); R. J. Cleevely & N. J. Morris, 2002, Introduction to molluscs and bivalves, plate 20, figures 7, 8.
|
«Among the Cretaceous Chlamys - Mimachlamys species known, the ornamentation of M. catulloi is closest to that of M. cretosa (Defrance in Brongniart, 1822), Turonian to latest Maastrichtian, as figured in Woods, 1902 (under Pecten cretosus, p. 174, pI. 32, figs 4-6, pI. 33, figs 1-12), and discussed by Dhondt (1973 a). It remains possible that specimens collected later shall prove that the types of M. catulloi represent an unusual morphotype of M. cretosa. At present we have to consider it as belonging to a separate species, since its shape falls out of the known variability of M. cretosa.»
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, 190]
|
Mimachlamys catulloi sp. novo; 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, figures 13, 14.
|
«Most species of Late Cretaceous Lyriochlamys, such as Pecten dentatus Nilsson, 1827, from the Campanian and Senonian of Europe (Dhondt, 1973) and P. faujasi Defrance, 1825, from the Turonian to Maastrichtian of Europe (Dhondt, 1973), still retain evidence of an extensive inner aragonitic layer. Among poorly preserved specimens such as those found in chalk facies, this evidence commonly consists of a dissolution line that parallels the disk and auricular margins. Dissolution also obliterates any trace of the originally aragonitic hinge teeth and resilifer.
There is one species of Lyriochlamys, however, that forms an important exception to the generalization that the inner aragonitic layer approaches the shell margins in this genus. It is Lyriochlamys cretosa (Defrance in Brongniart, 1822), which is found in the Turonian to Maastrichtian of Europe (Dhondt, 1973). This species appears to vary rather widely in the extent of aragonite. Although many specimens from the British and Belgian Cretaceous show a good "aragonite line" near the margins of disk and auricles, specimens from the Maastrichtian of Denmark examined by one of us (Waller) show no such line and furthermore preserve calcitic dorsal hinge teeth. It would appear that in these northern forms of the species the aragonitic inner shell layer faded out gradually outside of the pallial line and was absent from at least the dorsal and terminal portions of the hinge plate. In anticipation of the following species description, it is important to note that Lyriochlamys cretosa also differs from its congeners in having intercalated costae on its right valve, with branching costae rare or absent even early in ontogeny. The concept of Lyriochlamys employed in the present study encompasses species placed by Dhondt (1973) in Chlamys (Mimachlamys) as well as Pecten semptemplicatus and Pecten ternatus Muenster in Goldfuss, 1833, placed by Dhondt (1972b) in "Lyropecten (Aequipecten?)." Mimachlamys Iredale, 1929 (type species, Pecten asperrimus Lamarck, 1819, Holocene, Indo-Pacific), is an extant genus that originated in the Eocene with characters not present in the Mesozoic taxa (Waller, 1991). True Chlamys, as distinguished by Waller (1991) and in the present study, differs from both Praechlamys and Lyriochlamys in having a hinge plate and auricles composed entirely of foliated calcite (except for minor deposits of aragonite in ligament insertion areas). On the inner surface of the disk, aragonite rarely extends more than a very short distance beyond the pallial line and is usually entirely confined to an area inside the pallial line. In boreal species, such as Chlamys islandica (Müller, 1776) living in the North Atlantic, a calcitic inner layer transgresses the aragonitic ostracum from the dorsum ventralward inside the pallial line early in ontogeny (Waller, 1991). The range of radial sculpture of Chlamys broadly ovarlaps that of Lyriochlamys. Branching ribs commonly occur on the right valve and intercalated ribs on the left, but branching may be absent al- together or intercalation may sometimes occur on both valves without branching. Antimarginal microsculpture is present but is commonly masked on the disk flanks by radial costation. Defined in this way, true Chlamys is a Cenozoic genus, and Maastrichtian Lyriochlamys cretosa, which shows a Chlamys-like microstructure in the northern part of its range, may well be the species from which Chlamys originated. Although pectinid history in the Paleocene is still very obscure, true Chlamys was already well represented in the middle Eocene of Europe (Waller, 1991) and is well represented in the later Cenozoic and in the modem ocean.» WALLER, T. R. & L. MARINCOVICH JR. 1992. New species of Camptochlamys and Chlamys (Mollusca: Bivalvia: Pectinidae) from near the Cretaceous/Tertiary boundary at Ocean Point, North Slope, Alaska. Journal of Paleontology, 66 (2): 215-227, figs. 1-5. [p. 223, 224]
|
«Diagnosis.— Medium to large Mimachlamys species with flattened ribs; valves are covered with a very variable number of ribs which can bear small scales at the intersection with the concentric growthlines. The auricles are well developed.
Macrosculpture: the radial ribs on the discs are very variable in number, thickness and ornamentation: sometimes they bear rounded scales, exceptionally spinules. Right valve: numerous ribs of varying thickness : all transitions between very narrow ribs with wide intervals to broad ribs with intervals which are narrower than the ribs themselves. In the intercostal intervals are concentric striae which sometimes have become concentric ridges present on the whole disc-surface but are most clearly visible near the umbo. Their distribution is not even: in the umbonal region they are much nearer to each other than at the pallial margin. In the intercostal intervals new ribs are formed by intercalation. Near the side margins the rib density is lower but there are fine striae which are perpendicular to the margin. Anterior auricle: large, with broad and deep byssal sinus; on the lower auricle part there are 2 to 5 spinule bearing ribs. Poste¡ior auricle : smaller and obtuse-angled; 6 to 8 radial ribs with small scales. Left valve: more convex than the right valve, macrosculpture as on the right valve, but with fewer ribs, and the whole ornamentation has a cearser aspect. The concentric striae are less clearly visible. Anterior auricle: curved outer margin; 7 to 12 spinule bearing ribs. Posterior auricle: as on the right valve. Discussion
Variability:
The variability in number and thickness of the ribs is very great and this had led several authors to consider varieties such as Pecten cretosus var. nitida SCHROEDER, P. cretosus t. typica HÄGG, Chlamys (Chlamys) cretosa zeiszneri PASTERNAK; the differences between the "varieties" are in the ribs: some specimens have very thin, almost linear, ribs (var. nitida) or broader ribs with narrower intervals and well developed scales on the ribs (f. typica and var. zeiszneri). The first "variety" can be seen in WOODS, pl. 33, fig. 6a, b (there are specimens with even thinner and fewer ribs) and the second on pl. 33, fig. 1a, b, c. I agree with WOODS (1902, p. 178) that these varieties do not have any specific importance: they are based on the relative width of the intercostal intervals as compared with the ribs, and this, as stated in the description, changes according to the valve, even when the variability is not taken into account. Synonymy:
To WOODS' considerations I would like to add the following points: — The type-specimen of Pecten zeiszneri ALTH in the Naturhistorisch Museum in Vienna is for all the details of sculpture a normal M. cretosa with a high rib number. — Pecten nodosocostatus VON HAGENOW is described as if it were a M. cretosa with many ribs and well developed scales ("elliptische Knoten"). — Pecten undulatus NILSSON: it is generally accepted that pl. 9, fig. 10, in NILSSON, 1827 is a figure of the species described by NILSSON, whereas pl. 10, fig. 10a, b, b (sic) is in fact a Pecten cretosus. — Pecten irnichensis VOGEL has been created for very small specimens (18/15 mm) which have a macrosculpture thar can be compared with what DEFRANCE described as "Pecten arachnoides". There does not seem to be any difference with M. cretosa: the prominent concentric striae noted by VOGEL for his specimens are also visible on specimens from Hainaut and Limburg Maastrichtian. — Pecten barthi WOLLEMANN has been erected for a fragment with a slightly unusual macrosculpture: broad ribs with very narrow intercostal intervals. An identical specimen is in the I.R.Sc.N.B. collections from the Maastrichtian of Hainaut and this macrosculpture though at the extremes of the variability still belongs to M. cretosa. Differentiation:
Mimachlamys cretosa is closely related to M. robinaldina (D'ORBIGNY). Some Upper Cenomanian specimens can be considered as transitory between the species. The older of the two species is more convex, bears spines instead of scales, has generally fewer ribs, and the whole disc is covered with striae which are clearly visible near the side margins as oblique striae. Most M. robinaldina specimens have a narrower umbo than in M. cretosa) but this cannot be used as a differentiating characteristic. The other Upper Cretaceous Mimachlamys-species have fewer ribs and do not bear scales. Pecten serratus NILSSON, which is often considered as belonging to M. cretosa, cannot be definitely said to do so: the type-specimen is lost, but what HENNING states as P. serratus are left valves of Chlamys dentata NILSSON (see p. 17). Generic attribution:
There is sufficient similarity between Pecten asperrimus LAMARCK, type species of Mimachlamys, and Pecten cretosus DEFRANCE to enable us to be certain that the latter species also belongs to Mimachlamys; thus its correct name becomes Mimachlamys cretosa (DEFRANCE).» 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. 86, 87]
|
Mimachlamys cretosa (M. Defrance, 1922); A. V. Dhont, 1973, Systematic revision of the Chlamydinae of the European Cretaceous: Chlamys and Mimachlamys, plate 6 figures 2a, 2b, 2c. (above); plate 7, figures 1a, 1b (bellow).
|