Placopecten magellanicus (Gmelin, 1791)
GMELIN, J. F. 1791. Caroli a Linné, Systema Naturae per Regna Tria Naturae, Secundum Classes, Ordines, Genera, Species, cum characteribus, differentiis, synonymis, locis. Tomus I. Pars VI. Classis VI. Vermes. 13th edition, Aucta, Reformata, 3021-3910 p. Impensis Georg Emanuel Beer, Lipsiae [Leipzig], 1791. [p. 3317]
1876 Ostrea grandis Solander, 1786
1791 Ostrea magellanica Gmelin, 1791
1842 Pecten tenuicostatus Mighels & Adams, 1842
1845 Pecten fuscus Linsley, 1845 [nomen nudum]
1851 Pecten brunneus Stimpson, 1851 [nomen novum pro Pecten fuscus Lindsay, 1845]
1791 Ostrea magellanica Gmelin, 1791
1842 Pecten tenuicostatus Mighels & Adams, 1842
1845 Pecten fuscus Linsley, 1845 [nomen nudum]
1851 Pecten brunneus Stimpson, 1851 [nomen novum pro Pecten fuscus Lindsay, 1845]
Amusium magnum Magellanicum; J. H. Chemnitz, 1784, Neues Systematisches Conchylien-Cabinet, Band 7, plate 62, figure 597.
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«magellanica. 9. O. testa subaequivalvi glabra: striis oblongis confertis.
Chemn. Conch. 7. t. 62. f. 597. Habitat in sreto magellanico, pleuronecti affinis, testa lateritia non radiata intus glaberrima, valva superiore magis convexa, inferiore magis plana, quam in affinibus, auriculis transversim striatis.» JOHANN FRIEDRICH GMELIN, 1791
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«DISCUSSION.— Placopecten clintonius from the Yorktown Formation of the Atlantic Coastal Plain has been considered the ancestral form of the living P. magellanicus of the North Atlantic Ocean (Dall, 1898:726). However, no fbrms transitional in morphology or age between the two species have been found in the younger Cenozoic strata of the Atlantic Coastal Plain. A single right valve found in the Lee Creek Mine appears to be an intermediate form between the two species in that lineage. The specimen was collected by Jack McLellan from spoil banks in the Lee Creek Mine. It was determined to be from the Yorktown Formation on the basis of the type of sediment adhering to small cracks in the shell: but the crucial information as to the exact stratigraphic occurrence in the Yorktown Formation is lacking. The cementation of part of an echinoid test onto the shell is a strong indication that the specimen came from unit 2 of Gibson (1967). From a phylogenetic viewpoint, the specimen should come from somewhere above the basal beds of the Yorktown, which contain the supposed ancestor, typical P. clintonius, and if it is from unit 2 this would be true. Becauuse there is only the one specimen, and particularly because the precise stratigraphic position is uncertain, the specimen will not be placed more definitely taxonomically until additional valves with stratigraphic information are found. The specimen has closer proximity to P. magellanicus morphologically, although it is presumably very close to P. clintonius stratigraphically.
The shape of the single right valve of Placopecten sp. aff. P. magellanicus is similar to that of the modern P. magellanicus in being equilateral with a fairly equal anterior and posterior half-length (Figure 17). This contrasts with the moderately oblique shape of the disk in a population of P. clintonius clintonius from the Lee Creek Mine. The ornamentation on the disk of Placopecten sp. aff. P. magellanicus consists of very fine radial costae. The strength of the costae is considerably less than that found in P. clintonius, especially in the population from the mine, which has moderate to coarse radial ornamentation. The single specimen from the mine has costae that fall about midway in the range of variability of P. magellanicus, not being as sharply defined as in the coarser forms, but not being so flat as to be essentially smooth as in the more finely ornamented examples. The auricles of Placopecten sp. aff. P. magellanicus are different from those of both P. clintonius and P. magellanicus, although they are more similar to the latter. In comparison to P. clintonius clintonius, the auricles in the single specimen of Placopecten sp. aff. P. magellanicus have considerably finer costae. The posterior auricle has an essentially straight free margin perpendicular to hinge line, whereas in P. clintonius this margin has a strong posterior slant away from the hinge line. In the specimen of Placopecten sp. aff. P. magellanicus the anterior end of the anterior outer ligament is about the same distance from the origin of growth as is the innermost part of the byssal notch. ln P. clintonius the bysall notch is more anterior than the anteriormost end of the ligament. The hinge line of Placopecten sp. aff. P. magellanicus is longer than most specimens of P. clintonius clintonius (Figure 18). Although there are close similarities in most characteristics of the auricles between the population of P. magellanicus and the specimen of Placopecten sp. aff. P. magellanicus (Figure 19), the latter has longer auricles than most specimens of P. magellanicus. (See Plate 16: figures 1 and 2 where the similar-sized valve of P. magellanicus has considerably shorter auricles.) The resilial insertion of Placopecten sp. aff. P. magellanicus is relatively long in relation to its height, being similar to the shape of P. clintonius but significantly wider than that of P. magellanicus. No absolute measurements are given for the resilial insertion, as it is fractured and separated.
Placopecten sp. aff. P. magellanicus is close to P. magellanicus in many characteristics, and appears to be well along in the phiylogenetic sequence from P. clintonius clintonius to that species. Unfortunately, without knowing how far above the basal Yorktown beds (containing P. clintonius clintonius) the specimen occurred, the rapidity of this transition cannot be determined. As mentioned, the nature of the small amount of sediment in crevices on the specimen and the echinoid fragrnent cemented to the shell suggest that the specimen came from the upper lower part of the Yorktown or lower middle part of the Yorktown (upper part of unit 2 to lower part of unit 5, most probably unit 2), indicating a rapid phylogenetic change. The possibility that the single specimen is an extreme variant within the population of P. clintonius clintonius cannot be dismissed entirely, but as little variation toward this form is observed in the hundreds of valves examined fronr the Lee Creek Mine and other sections in the Atlantic Coastal Plain, it seems unlikely. GIBSON, T. G. 1987. Miocene and Pliocene Pectinidae (Bivalvia) from the Lee Creek Mine and adjacent areas. Smithsonian Contributions to Paleobiology, 61: 31-112, pls. 1-31. [p. 64, 65]
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Placopecten sp. aff. P. magellanicus (Gmelin): plate 17, figure 3. Placopecten magellanicus (Gmelin): plate 16, figure 2; plate 17, figure 1. T. G. Gibson, 1987, Miocene and Pliocene Pectinidae from the Lee Creek Mine and adjacent areas.
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«This fine shell is frequently six inches in length, and is one of the largest of the genus. The superior valve is occasionally marked with pale rays, and always with concentric bands, which are either of a lighter or darker tint than the general colour of the valve, the margin of which is ochraceous.
This species constantly inhabits deep water and abounds in Passamaquoddy Bay, where it is dredged up for the table, as the animal is edible, and much esteemed in the vicinity of its location. Dr. C. Pickering found two valves on the coast of Massachussetts. That this is the shell which authors suppose to inhabit the Straits of Magellan, whence is derived the name of Magellanicus, I have no doubt, and if its name be not misapplied, it is a singular circumstance that it should inhabit seas so remote from each other, and yet be unknown on any intermediate part of the vast and continuous coast of America. Its presence, however, could not readily be detected, as dead shells are rarely cast upon the beach, even in the vicinity in which the species is common. To Mr. Titian R. Peale I am indebted for specimens, and for the information which I have obtained respecting its locality on the coast of the United States. This gentleman obtained numerous perfect specimens, and introduced into our collections in this city, the only authentic individuals by which we could claim it as a North American species. The figure is considerably reduced.» CONRAD, T. A. 1831. American Marine Conchology or descriptions and coloured figures of the shells of the Atlantic Coast of North America. 72 p., pls. 1-17. Printed for the autor. Philadelphia. [p. 7, 8]
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Pecten magellanicus; T. A. Conrad, 1831, American Marine Conchology, plate 1, figure 1.
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