Ordovician (Caradoc – Ashgill)
Strophomena is a genus of extinct stationary epifaunal suspension feeders
Identification in Hand Sample:
- Medium to large size
- semicircular outline
- Fine ribs radiate from umbones of both valves toward commisure
- Strong teeth
- If a triangular calcareous plate, the symphytium, can be distinguished along the flat interarea of the concave (ventral) valve, then the specimen is Strophomena, not Rafinesquina.
- Squatter and more triangular cardinal process lobes
- Changes convexity along profile
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- Very similar to Tetraphalerella. On weathered or etched specimens, pseudopunctae in Strophomena can be seen to be irregular distributed, whereas pseudopunctae in Tetraphalerella are arranged in radial rows. Some Cincinnatian Strophomena have been tentatively referred to by some workers as species of Trigrammaria, which is distinguished by a subtriangular folded shape
Jin & Zhan (2001):
- Remarks: Cocks (1990) proposed Leptanea planumbona as the type species to replace the poorly known Strophomena rugosa de Blainville, 1824, which is probably conspecific with L. planumbona. This proposal was formalized by ICZN Opinion 1671 (1992). Despite the detailed study of Rong and Cocks (1994), confusion remains about the true identity of Strophomena planumbona, especially with regards to its dorsal internal structures and its type lcoality and type stratum. Hall’s (1847) broad statement of the type stratum as “a position equivalent to that of the Trenton Limestone… in Cincinnati and Oxford (Ohio), Madison (Indiana), and Maysville (Kentucky)” was taken at face value by Rong and Cocks (1994), who assigned Caradoc age to the type species. In the late nineteeth century, Trenton Limestone was used for a much wider range of Ordovician strata than in modern usage, and could include rocks of late Caradoc to mid Ashgill age. In the American midcontinent, Strophomena planumbona was largely confined to Maysvillian and Richmondian strata. In the Cincinnati type area, in particular, Strophomena planumbona occurs only in Richmondian strata of the Arnheim, Waynesville, and Liberty formations (Davis, 1985). In other areas of North America, the species is known from the following strata: (1) Brainard Member (Richmondian) of the Maquoketa Formation of Iowa (Wang, 1949); (2) Viola Formation (lower Maysvillian) of Oklahoma (Alberstadt, 1973); (3) Arnheim of Tennessee (Howe, 1988); (4) Surprise Creek and Caution Creek formations (late Maysvillian- early Richmondian), Hudson Bay Lowlands (Jin et al., 1997); (5) Vauréal Formation (Richmondian), Anticosti Island, Québec (Dewing, 1999). Available information indicates that Strophomena planumbona is largely of Ashgill age and most likely confined to Richmondian strata in the Cincinnati type area.
- As pointed out by Rong and Cocks (1994) and Dewing (1999), there has been a great deal of confusion over the dorsal internal structures of Strophomena planumbona with those of S. vetusta (James, 1847) and S. filitexta (Hall, 1847), mainly because Hall (1847) did not illustrate any dorsal interiors in his initial or subsequent descriptions of the species. Muir-Wood and Williams (1965) illustrated a dorsal interior of S. planumbona (from Ohio) with four long, strong, and straight transmuscle septa, extending for over two-thirds of the valve length; these septa are similar to those of S. vetusta. This seems to agree with Pope’s (1976, p. 176) definition of the Strophomena-type transmuscle septa. The two dorsal valves illustrated by Rong and Cocks (1994), however, are drastically different, showing short and unusually weak transmuscle septa confined to the posterior half of the valve; the two central side septa are arched laterally, enclosing a suboval area, as is characteristic of Holtedahlina, Pentlandina, and Katastrophomena (Cocks, 1968; Pope, 1976). Serial sections of S. planumbona from Ohio and Anticosti Island confirm the presence of four high, sharp, closely spaced transmuscle septa (Dewing, 1999). The dorsal interior of true Strophomena, therefore, should have two pairs of long, strong, relatively straight transmuscle septa, as illustrated by Muir-Wood and Williams (1965) and typified in S. vetusta. The two dorsal valves illustrated by Rong and Cocks (1994) as S. planumbona require further study to determine their identity.
Treatise on Invertebrate Paleontology, Part H, Vol. 2 (2000):
- Profile gently resupinate; ornament variable from parvicostellate to costellate; prominent pseudodeltidium; small chilidium; teeth strong, sometimes with irregular denticles, crenulations; dental plate extending into elevated bounding ridges largely surrounding the subcircular to rhomboidal ventral muscle field; adductor scars not enclosed by diductor scars; ventral myophragm sometimes present; socket ridges sometimes crenulated; strong, short median ridge coming from the posterior edge of notothyrium; ridge sometimes forked anteriorly; dorsal muscle field gently impressed, with weak bounding ridges sometimes present laterally; occasionally weak transmuscle ridges sometimes present, often absent.
- Squatter more triangular cardinal process lobes and dorsal side septa often weakly developed (compared to Keilamena); similar to Tetraphalerella but generally larger, with flaring although small ventral adductor muscle scars and more anterolaterally directed socket plates rather than recurving back to the hinge line
Jin et al. (1997):
- Cocks (1990) proposed Leptaena planumbona as the type species to replace the poorly known Strophomena rugosa de Blainville, 1825, which is probably conspecific with L. planumbona. This proposal was formalized by ICZN Opinion 1671 (1992), and S. rugosa was thereby suppressed (for further discussion, see Rong and Cocks, 1994)
Fossils of Ohio (1996):
- A medium sized to large, resupinate, strophomenid brachiopod. It has a semicircular outline truncated by a straight hinge line. Width and length are approximately equal. Ornamentation consists of numerous fine ribs radiating from the umbones of both valves toward the commissure. If a triangular calcareous plate, the symphytium, can be distinguished along the flat interarea of the concave (pedicle) valve, then the specimen is Strophomena, not Rafinesquina. The brachial-valve interior of Strophomena has a bilobed cardinal process, typically smaller than that in Rafinesquina, and low septa that may be traced for more than half the length of the valve interior. The pedicle-valve interior has wedge-shaped teeth and a subcircular muscle scar that may be bordered by a ridge. Strophomena is recorded from the Cincinnatian Series.
- Resupinate concavo-convex shells, essentially biconvex in the sinuate species.