Syncyclonema Meek, 1864
MEEK, F. B. 1864. Check list of the invertebrate fossils of North America: Cretaceous and Jurassic. Smithsonian Miscellaneous Collections, 177: 1-40. [p. 31]
«195 = SYNCYCLONEMA, MEEK. Type Pecten rigida, HALL & MEEK, Mem. Am. Acad. Arts and Sci. Boston, V, new ser. 381, ii, 4, a, b, c (not SOWERBY, 1818). The type of this group is a very small, nearly equivalve, ovate-suborbicular, compressed shell, with small, flat, slightly unequal ears, and closed margins. Hinge short; surface with fine, obscure concentric striae, and sometimes on the right valve, small rounded concentric ridges.
Some of the larger smooth Cretaceous and Jurassic species may possibly also belong to this group. None of the so-called Pectens, of the Cretaceous or older rocks, belong properly to the genus Pecten, Müller, as typified by the recent P. maximus, Linn.» FIELDING BRADFORD MEEK, 1864
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Pecten rigida n. sp.; J. Hall & F. B. Meek, 1856, Descriptions of New Species of Fossils, from the Cretaceous Formations of Nebraska with Observations upon Baculites ovatus and B. compressus, and the Progressive Development of the Septa in Baculites, Ammonites, and Scaphites, plate 1, figures 4a-4c.
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Plate 1.
Plate 2.
Plate 3.
Syncyclonema halli (Gabb); I. G. Speden, 1967, Revision of Syncyclonema (Upper Cretaceous) and comparison with other small pectinid bivalves and Entolium, plate 1, figures 1-9; plate 2, figures 1-5; plate 3, figures 1, 2, 4-6.
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«GENUS SYNCYCLONEMA
AUTHOR. Meek, 1864a, p. 31, 7.
TYPE SPECIES. By original designation, Pecten rigida Hall & Meek (1856, p. 31, pl. 1, figs. 4a-c; non Pecten rigida Sowerby, 1818, p. 5, pl. 205, fig. 8 ; = Pecten halli Gabb, 1861, p. 214), Sage Creek, South Dakota, Pierre Shale, Upper Campanian-Maastrichtian.
EMENDED DIAGNOSIS OF SYNCYCLONEMA. The following diagnosis is based on study of the type species halli (see below):
Small, subequivalve, subequilateral. Shell of vitreous appearance, thin, of three layers: thin outer homogeneous or prismatic layer, middle layer of zigzag lamellar structure, and thin inner complex cross-lamellar layer. Ornament sub-macroscopic, except on ears, of fine non-punctate diverging radial striae, fine growth striae, and coarser irregular growth lamellae tending to give reticulate pattern. Radial striae strongest at ventral margin and on dorsal flanks of disc of shell, sometimes absent on center of disc. Concentric lamellae strong on ears. Interior of shell sometimes with low faint rounded concentric ridges and fine radial striae. Concentric undulations sometimes evident on external surface, particularly on the right valve. Anterior auricle equal to or significantly larger than the posterior. Anterior auricle of the right valve distinctly separated from disc of shell, fasciole very narrow, and byssal sinus deep, open V-shaped. No ctenolium. Dorsal margin of right valve overlaps that of the left valve. Each auricle with one thin cardinal crus below the ligament band, that on the posterior auricle of each valve extending usually only about two-thirds of the distance towards the posterior margin (pl. 3, figs. 1, 5). One relatively long strong tooth-like process on either side of the resilifer pit of the right valve (pl. 2, fig. 5; pl. 3, figs. 2, 5). No auricular crura. Adductor muscle impression small, subcircular, sited above twothirds the height of shell adjacent to basal part of posterior auricle. Pallial line continuous. DISCUSSION. Several morphological features of the type species, S. halli, require discussion before the status of Syncyclonema is fully clarified.
1. Byssal sinus. Syncylonema halli has a deep byssal sinus on the right valve (pl. 2, figs. 1, 5). Careful examination of the internal and external surfaces of right valves provided no evidence of a ctenolium at any stage of growth. S. halli was probably a free swimming pectinid. Stewart (1930, p. 120) thought the small specimens of S. halli described by Hall & Meek (1856) and Meek (1876) might be juveniles of a species of Entolium, but growth lines show the presence of a relatively large byssal auricle at all stages of growth, and for this reason S. halli cannot be classed in Entolium (see below). The normal occurrence of specimens 10 to 13 mm long, but none larger than this, suggests that the largest specimens are adults of a small species. 2. Musculation. The adductor muscle impression is weakly impressed and was observed on only a few specimens. It is small, subcircular, sited postero-dorsally, with its base at or above twothirds of the height of the shell adjacent to the basal part of the posterior ear (pl. 2, fig. 3; pl. 3, fig. 6). Whitfield (1880, pl. 7, fig. 1) illustrated the adductor scar which, as he noted, occurs in an unusual position. Examination of his specimen (USNM 12272; pl. 1, fig. 8) shows that the impression is partially masked by a pattern of vermiculated markings. The pallial line sometimes coinicides with the contact of the inner and outer shell layers and has a small oval inflated impression near its postero-ventral extremity (pl. 3, fig. 6). 3. Auricular crura. No auricular crura are developed, although a few right and left valves have the internal surface of the anterior ear slightly thickened along the junction with the main disc of the shell. 4. Ornament. Meek (1864a, p. 31), in his original diagnosis of the genus, described the ornament of the right and left valves as "surface with fine obscure concentric striae and sometimes on the right valve, small rounded concentric ridges." In his (Meek, 1876, p. 26) later, more detailed diagnosis he stated "surface showing only concentric striae, and sometimes stronger, regularly defined concentric ridges on the right valve." Although he uses the word "sometimes" in this description, on p. 27 he stressed the discrepancy of ornament between the valves. In their original description of S. halli Hall & Meek (1856) described but did not emphasize the discrepancy in ornament. Clearly the emphasis should have been on the similarity of the ornament on each valve, and on the uncommon occurrence of weak concentric plicae. Misinterpretation of Meek's diagnosis and discussion has undoubtedly contributed to the uncertain status of the genus. Most of the specimens from the Fox Hills Formation, admittedly younger stratigraphically than those of Hall & Meek (1856) from Sage Creek, have similar ornament on each valve. Weak concentric ridges are sometimes present on the inner surface of the shell of either valve (pl. 3, fig. 4), and tend to show up more strongly on steinkerns. Strong concentric lamellae are rarely developed. The lamellate ornament, like the internal concentric ridges, tends to be accentuated by the removal of the thin outer prismatic layer of the shell. The concentric ridges are more regular on the steinkern illustrated by Whitfield (1880, pl. 7, fig. 1; see pl. 1, fig. 8), and on the right valves in AMNH 9347 (pl. 1, figs. 1, 3), than on any specimen collected from the type area of the Fox Hills Formation. Compared with the overall similarity of the other morphological features, the difference in this one character is not considered of species rank, particularly as the concentric ridges are stronger on small than large specimens (compare pl. 1, figs. 1, 3, 8, with pl. 3, fig. 4).» SPEDEN, I. G. 1967. Revision of Syncyclonema (Upper Cretaceous) and comparison with other small pectinid bivalves and Entolium. Postilla, 110: 1-36, pls. 1-7. [p. 7-9]
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