Maclearnia cincta (J. Sowerby, 1822)
SOWERBY, J. & J. DE C.
SOWERBY. 1812-1846. The Mineral
Conchology of Great Britain; or coloured figures and descriptions of those
remains of testaceous animals or shells. 7 vols., pls. 1-648 (and 2 bis).
London. [v. 4, p. 96, pl. 371]
1822 Pecten cinctus J. Sowerby, 1822
1839 Pecten crassitesta Roemer, 1839
1839 Pecten crassitesta Roemer, 1839
J. Sowerby, 1822, plate 371.
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«SPEC. CHAR. Orbicular, gibbose, longitudinally striated, imbricated; edges of the laminae, thin, erect; ears small; edge entire.
Remarkable for concentric, erect laminae, that are very numerous, especially towards the edge; but from their being thin, they are commonly worn off. The ears are strongly marked with close ridges: the valves are thick, especially towards the hinge, and of nearly equal convexity. The striae are sunk and rather irregular. Dawson Turner, Esq. of Yarmouth, celebrated for his botanical knowledge, was the first Friend who sent me this interesting shell. It was found in the alluvial clay of Suffolk. The specimen figured was sent me from the neighbourhood of Horncastle by Mr. Wier, both are remarkable for having grains of iron ore, arranged in one, two, or three regular rows between the concentric laminae, according to the distance of those laminae, the grains being uniform in size. It is probable that they both belong to the inferior or Ironshot Oolite, although the first is filled with indurated marl.» JAMES SOWERBY, 1822
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«Johnson (1984, p. 142, 149) considered Camptochlamys obscura to be the ancestor of C. clathrata, the former having evolved probably from Camptonectes auritus through heterochronic change associated with phyletic size increases. He further suggested (p. 136, 142) that Camptonectes cinctus (J. Sowerby, 1822) may be a phyletic descendant of Camptochlamys obscura. Camptonectes cinctus is the type species of Boreionectes Zakharov, 1965, which has been shown to be a junior synonym of Mclearnia Crickmay, 1930 (Kelly et al., 1984, 1986). The hypothesis that Mclearnia (= Boreionectes) is descended from Camptochlamys is unlikely, however, in the light of information that has been published since Johnson's monograph was completed. The oldest species of Mclearnia thus far described, Mclearnia broenlundi (Ravn, 1911), occurs in the middle Oxfordian of east Greenland (Fiirsich, 1982), and differs from later occurring Mclearnia in having a narrower umbonal angle, a less flattened right valve, and a smoother shell without well-developed commarginal flanges. None of the Mclearnia species are known to have radial costae. The earliest Camptochlamys appear in the Aalenian and Bajocian (Johnson, 1984), before the first Mclearnia, and the earliest members of the former already have a flattened right valve with well-developed radial costae. It thus appears that Camptochlamys and Mclearnia are each independently derived from Camptonectes.»
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. 220]
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«It is assumed that DENINGER's record of Pecten crassitesta ROEMER [ = Maclearnia cincta (Sowerby), restricted to cold temperate seas of Neocomian age] actually refers to C. cottaldinus, since among the rich material from the Lower Cretaceous of Eastern Sardinia no specimen resembling M. cincta was ever found, but numerous smaller camptonectids occur.»
DHONDT, A. V. & I. DIENI. 1988. Early Cretaceous bivalves of eastern Sardinia. Memorie di Scienze Geologiche, 40: 1-97, pls. 1-13, text-figs. 1-25. [p. 28]
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«Discussion.— The type species of Camptonectes (Boreionectes) cinctus is Pecten cinctus J. Sowerby (1822, p. 96, Pl. 371) (see Dhondt, 1972, p. 35-36, for full synonymy). There are two syntypes, now in the British Museum (Natural History), both originally numbered 43.300. They are refigured here (Figure 2). The original figured specimen (Figure 2A) is a right valve, which is here renumbered LL3 1441. It came 'from the neighbourhood of Horncastle, Lincs.' It was not stated whether it was found in situ or came from the drift, although the Claxby Ironstone does crop out about ten kilometers to the north east of Horncastle. It appears to have been broken since figured by Sowerby and a segment has been lost.
The second specimen (Figure 2B) is a right valve, which is here renumbered LL3 1442. It came from 'the alluvial clay of Suffolk.' Both specimens are preserved in a limonite oolite. They were originally believed by Sowerby to have come from the Middle Jurassic 'inferior or Ironshoot Oolite,' but Judd (1867, p. 250-251) recognized the Neocomian age of the species, and discussed the early use of the name. Woods (1902, p. 155) recognized the matrix of the specimens as Claxby Ironstone from Lincolnshire. Both specimens are of Lower Cretaceous age, either Valanginian or Hauterivian. The types of P. cinctus represent a large pectinid with both valves moderately inflated, and with the dorsal portion of the flank of the right valve less inflated than that of the left valve; the commissural outline is subcircular; all auricles are severely damaged; external ornament is composed of regular spaced comarginal lamellae and of fine radiating grooves; the ligamental pit is subcentral and vertical; the pallial line is subcircular and the shell interior just outside the pallial line bears radial structures which weaken towards the smooth margin; LL3 144 1 bears an irregular growth on the interior of the shell below the anterior auricle; measurements are given in Table 2. The types of Maclearnia and Boreionectes are extremely similar. The only differences are the slightly less inflated right valve of Maclearnia and the presence of external radial ornament on Boreionectes and not on Maclearnia. However, the types of the latter are considerably exfoliated and also examination of large numbers of Boreionectes shows that this type of ornament becomes obsolete commonly in adults, and occasionally in juvenile specimens. Hertlein (1969, p. N373) described a 'Chlamys-like' ornament in Maclearnia, but no trace can be seen on the exterior of the type specimens, although there is a faint internal radial structure visible on obliquely broken shell surfaces. The name Boreionectes has been widely used in the Soviet Union (e.g., Zakharov, 1965, 1966; Zakharov and Shurygin, 1978; Paraketsov and Paraketsova, 1979; Sey and Kalatchova, 1980) and in western Europe (e.g., Dhondt, 1972; Kemper, 1976; Kent, 1980). The type specimens, like those of Maclearnia are problematical, they are abraded and broken and the original horizon is not precisely known. The subsequent figures of Woods (1902, P1. 23, text-fig. 2) are usually taken as being typical of the P. cinctus, but this may not be necessarily true. It is intended to discuss the species in more detail at a later date. It is not possible to state whether M. maclearni is a subjective synonym of Pecten cinctus or not. The reason that Maclearnia has been largely overlooked is because the type specimens are not very well preserved and are, therefore, difficult to compare with other material. Also in the original publication (Crickmay, 1930) Maclearnia was placed in the family Pteriidae without much discussion and the stratigraphic placement in the zone of 'Aucella canadiana' was not elaborated upon by the author until 1962. The authors have examined large numbers of specimens of 'Boreionectes' from Canada and from many other parts of the Mesozoic Boreal regions and would have placed Crickmay's specimens within Boreionectes. However Maclearnia has precedence nomenclaturally and therefore that name must be used. Mclearnia mclearni appears in the faunal lists of Warren and Stelck (1958, p. 60) and Crickmay (1962, p. 8). The holotype appeared refigured fortuitously next to Camptonectes sensu stricto in the Treatise (Hertlein, 1969, fig. C74.4), but the generic entry appeared much later in the text (p. N373) and was placed as 'group uncertain' within the family Pectinidae and it was commented that the genus was in need of investigation. Had the name Maclearnia not been used, it would have been desirable to have it suppressed by ICZN application, on the grounds that it was a nornen oblitum; however this is not the case.» KELLY, S. R. A., A. V. DHONDT & V. A. ZAKHAROV. 1984. Boreionectes Zakharov 1965 (Bivalvia: Pectinidae) - A synonym of Maclearnia (Crickman 1930). Journal of Paleontology, 58 (1): 109-114, figs. 1, 2. [p. 111, 113]
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Camptonectes (Maclearnia) cinctus (J. Sowerby); S. R. A. Kelly, A. V. Dhondt & V. A. Zakharov, 1984, Boreionectes Zakharov 1965, figure 2.
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Variability:
As already stated by H. WOODS the proportion U. P. D./W can vary greatly in C. cinctus and results in two different shapes of the discs. H. WOODS figured and described the extremes in the variation. The number of known specimens is small; hence it has been impossible to check whether the shape variation can be correlated with stratigraphical origin. It is surprising that no small specimens of C. cinctus have been recorded; it could be that eventual small specimens have been considered as belonging to another species such as C. cottaldinus (D'ORBIGNY), for instance or otherwise, small specimens were very brittle and did not fossilize. Synonymy: Pecten circularis GOLDFUSS seems to be lost; consequently, the problem as to whether it is synonymous with C. cinctus cannot be solved. Pecten crassitesta ROEMER is identical with C. cinctus; ROEMER said this in 1841 but other German authors continued to use the ROEMER-name. The identity of both taxa can easily be checked on the topotypical specimens in the Museum of the von Humboldt University of Berlin. D'EICHWALD's description leaves some doubt as to the complete identity of his material with C. cinctus because the auricles he describes are Entolium (Amusiidae) auricles and not Camptonectes-auricles. If he only possessed left valves, however, then the description could be understandable. Differentiation: In the genus Camptonectes, as represented in Cretaceous deposits, C. cinctus is unusually large. In Jurassic deposits, however, large Camptonectes species are known: C. giganteus ARKELL, 1926 from the Berkshire Oolite Series and C. sandfootensis ARKELL, 1930 from the Upper Calcareous Grit. C. cinctus is easily differentiated from other Cretaceous Camptonectes species: C. dubrisiensis (WOODS) is less convex, has a less developed concentric macrosculpture, larger auricles and U. P. D./W. is always more than 1. C. cottaldinus (D'ORBIGNY) has larger auricles, less convex and less circular valves, and a narrower umbonal angle. The concentric ornamentation is less developed. Because of the last three characteristics it is possible to differentiate even incomplete (i.e. auricle-missing) specimens. C. virgatus (NILSSON) is very much smaller and, apart from a strongly and rather coarsely developed Camptonectes-ornamentation, it is smooth; the auricles are broader and the umbonal angle is narrower. C. striatopunctatus (ROEMER) has larger auricles and appears smooth: its only ornamentation is a very fine, almost microscopical Camptonectes-ornamentation. C. gaultinus (WOODS) is very small, and less convex; it does not have a concentric ornamentation and U. P. D./W. is always more than 1. C. ? milleri (SOWERBY) has a radial ornamentation consisting of "ribs" and the diverging punctate striae are straight. Some badly preserved auricle-missing C. cinctus can be differentiated from Entolium orbiculare (SOWERBY) by : — the absence of Camptonectes-microsculpture on E. orbiculare (a microsculpture of diverging radial striae is found on all Pectinidae, but not on Amusiidae); — the almost flat shell-shape in E. orbiculare (can be used even if the preservation state is a "Steinkern"); — the very brittle and thin shells in E. orbiculare. Generic attribution: Pecten cinctus SOWERBY, 1822, is the type-species of Camptonectes (Boreionectes) and the correct name automatically becomes Camptonectes (Boreionectes) cinctus (SOWERBY, 1822). DHONDT, A. V. 1972. Systematic revision of the Chlamydinae (Pectinidae, Bivalvia, Mollusca) of the European Cretaceous. Part 1: Camponectes. Bulletin de l’Institut Royal des Sciences Naturelles de Belgique, 48 (3): 1-60, pls. 1, 2.
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Camptonectes (Boreionectes) cinctus (J. Sowerby, 1822); A. V. Dhondt, 1972, Systematic revision of the Chlamydinae of the European Cretaceous, Part 1: Camponectes, plate 1, figure 1.
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