Strophomena vetusta

Classification
Phylum: Brachiopoda
Class: Strophomenata
Order: Strophomenida
Family: Strophomenidae
Genus: Strophomena
Species: Strophomena vetusta (James, 1874)

Stratigraphic Occurrences

S.vetusta_strat

Geographic Occurrences

		
Map point data provided by iDigBio.

Stratigraphic Description

Sequences (Formations)

  • C5 Sequence (Lower Whitewater, Liberty, Waynesville)

Identification in Hand Sample

  • Strong posterior rugae oriented approximately perpendicular to the hingeline
  • Very fine, more or less unequal parvicostellae (6 to 8 per 1 mm) in the ventral valve
  • Much stronger costellae in the dorsal valve with unequal parvicostellae in the umbonal area becoming almost equal sized anteriorly
  • High, strongly apsacline ventral interarea
  • The shells are typically large in size, moderately convex (dorsal valve), wider than long, semi-elliptical, and in some specimens are subquadrate or have anteromedial elongation

Strophomena vetusta from the Waynesville Formation of Brookville, Indiana (OUIP 1615).

Published Description

Jin & Zhan (2001):

  • Description (southern Manitoba material): Shell medium-sized to large, semicircular to subelliptical, with length ranging from 17.5-31.9 mm. width 26.9-47.6 mm, and average length/width ratio 0.65-0.85. Lateral profile weakly resupinate, with dorsal valve much more strongly convex than ventral valve. Hingeline being widest part of shell, with pointed cardinal extremities. Ventral interarea apsacline, attaining a height of 2.7 mm; delthryium broadly triangular, covered by gently convex pseudodeltidium. Dorsal interarea much lower (<1.10 mm in height) than ventral interarea, anacline; notothyrium covered by small, arched chilidium. Entire shell covered by clearly differentiated parvicostellae, averaging 3-5 finer costellae between two adjacent coarser costellae near anterior margin. Concentric rugae irregularly spaced, being strongest towards postero-lateral margin, intersecting hingeline at near-perpendicular angle, and increasing in size (crest width) from umbonal area to cardinal extremities (pl. 6, fig. 1). Concentric growth lines fine, evenly distributed, averaging 9 per mm.
  • Microscopic shell structure: Pseudopunctate, with minute pustules best developed on both sides of shell adjacent to dorsal muscle field (pl. 5, fig. 15.).
  • Ventral interior: Teeth small, slender (pl. 5, fig. 12.); dental plates low, thick, extending anteriorly to merge with high lateral bounding ridges of muscle field. Lateral cavities present in umbonal area, largely filled by secondary shell deposits posteriorly. Muscle field sharply defined, subcircular to subrhomboidal in outline, occupying about 34% of shell width and 42% of shell length, bearing low, relatively thick median ridge (pl. 5. figs. 11-130); lateral bounding ridges thick and high, strongly ventro-medially onto valve floor, leaving narrow medial gap at anterior margin of muscle field; adductor muscle scars not clearly delimited.
  • Dorsal interior: Cardinalia small, occupying about 26% of shell width and 13% of shell length. Two lobes of cardinal process slender, discrete, with small, blade like structure in between (pl. 5, figs. 9, 10, 14, 15); crests of cardinal process lobes extending posteriorly beyond hingeline. Sockets deep, narrow, bounded by inner and outer socket ridges starting from hingeline and extending antero-laterally about 30°; outer socket ridges thin, short, bearing up to 8 crenulations on inner sides; inner socket ridges becoming thin and platy at distal ends, dipping posteriorly towards hingeline, and bearing 3 or 4 crenulations on outer sides. Adductor muscle field long, straight; posterior median ridge thick, strong, beginning from notothyrial platform, bifurcating anteriorly to merge with two central side septa (pl. 5, figs. 9, 10); anterior median ridge much weaker, not continuous with posterior median septa; central side septa long, strong, slightly divergent from each other anteriorly, extending for more than half (typically two-thirds) valve length. Pair of lateral side septa starting at posterior margin of adductor muscle field, also long and relatively straight, usually somewhat shorted than central side septa.
  • Remarks: Shells of Strophomena vetusta are characterized by large or very large size with short but consistent postero-lateral rugae developed at approximately right angles to the hingeline. Specimens from southern Manitoba attain a maximum shell width of 47.6 mm, those of the Hudson Bay Basin 40.0 mm (Jin et al., 1997), and those of Anticosti Island 38.6 mm (Dewing, 1999). Shells of Strophomena vetusta from southern Manitoba are virtually identical to those of Strophomena planocorrugata Twenhofel, 1928 from the Vauréal Formation of Anticosti Island and from the Bighorn Formation of Wyoming (Macomber, 1970). We agree with Dewing (1999) in treating S. planocorugata as a junior synonym of S. vetusta, beacuse S. planocorrugata was initially distinguished from S. vetusta only by its weak dorsal fold and somewhat smaller cardinalia, with socket ridges subparallel to the hingeline. The material of S. vetusta from the Churchill River Group of Hudson Bay Basin (Jin et al., 1997) show slightly longer dorsal transmuscle ridges but it is difficult to determine the exact range of variation in this character because of limited numbers of well-preserved dorsal interiors.

Davis (1998):

  • Articulate brachiopod. Differs from similar species by having the edge of the shell along the hinge line marked by wrinkles and by having the interior of the brachial valve marked by four parallel ridges. Liberty and Whitewater.

Jin et al. (1997):

  • Remarks: Specimens of S. vetusta from the Hudson Bay Lowlands are comparable to those from the Richmondian of Ohio (Foerste, 1912; Caster et al., 1961) where James’s types probably came from the Liberty Formation or the Whitewater Formation (Foerste, 1912, p. 100). Diagonistic features include: strong posterior rugae oriented approximately perpendicular to the hingeline; very fine, more or less unequal parvicostellae (6 to 8 per 1 mm) in the ventral valve (pl. 8, figs. 1-4), and much stronger costellae in the dorsal valve with unequal parvicostellae in the umbonal area becoming almost equal sized anteriorly, 2 to 3 per 1 mm near the anterior margin (pl. 8, figs. 7-10); high, strongly apsacline ventral interarea (4.3 mm high compared to 4 to 5 mm as recorded by Foerste, 1912); internally, a strong median ridge in the posterior of the dorsal valve, passing into two strong median transmuscle septa extending for more than two-thirds of the valve length, flanked by a shorter, major lateral septum on each side (pl. 9, figs. 1-3). Similar deveopment of the transmuscle septa is evident in specimens from Richmondian rocks of Michigan (Hussey, 1926). The shells are typically large in size, moderately convex (dorsal valve), wider than long, semi-elliptical, and in some specimens are subquadrate or have anteromedial elongation. Resupination begins at about 10 to 15 mm length, with greatest curvature near the anterior margin but no clear geniculation. Naturally exposed ventral interiors are rare in the Hudson Bay collections; a ventral valve shows the posterior half of its muscle field with a strong median ridge beginning from the apex (pl. 9, figs. 4, 5), much like that illustrated by Foerste (1912, pl. 6, fig. 2F).
  • There is considerable variation in the development of posterior rugae. Although their size generally increases toward the cardinal extremities, the rugaeon each side of a specimen may vary from 4 or 5 that are uniformly strong, distinct, and regularly spaced, to up to 11 that are thin, unequal in size, and irregularly spaced. Even in a single specimen, the rugae may be delicate on one side, robust on the other.
  • Strophomena vetusta has typical Strophomena-type, fine pseudopunctae, which are several times smaller than those of Tetraphalerella and irregularly arranged (pl. 8, fig 3.). In his study of Ordovician and Silurian strophomenids, Dewing (1995) showed similar development of pseudopunctae in specimens of Strophomena vetusta from Anticosti Island, Québec.

McFarlan (1931):

  • A large species often attaining a width of 40 mm., length, 0.7 width, with rectangular or slightly extended cardinal angles, and high cardinal area. Radiating striae very fine on the pedicle (ventral) valve (18-21 in 5 mm.), much coarser (8-10 in 5 mm.) on the brachial (dorsal) valve. The presence on the pedicle valve of concentric wrinkles, growth lines, and the tendency of the striae to change their direction anteriorly and laterally gives a characteristic irregularity to the surface markings. Shell vertically wrinkled along the hinge line.
  • Middle Liberty to the top of the Whitewater (also Saluda). It is known in Kentucky as far south as Madison County in the east, and Nelson County in the west.

Foerste (1912):

  • The muscular area of the pedicel valve equals in width about one-third to two-fifths of the width of the shell. Poster-laterally, the muscular area is abruptly limited by a sharply ascending border. Antero-laterally, the surface of the area rises more gradually toward the border, and here the limiting border may rise sharply above the general surface of the interior of the valve, or only the outer slope of the border may be sharply defined. The surface of the muscular area is not conspicuously striated in a radiate or flabellate manner, as in Strophomena neglecta. Anteriorly, the lateral border is deflected forward, leaving a rather broad median gap, which is one of the characteristics of this species. A rather narrow median ridge, widening only slightly posteriorly, indicated the line of attachment of the adductor muscles. The deflected anterior terminations of the lateral borders of the muscular area are extended anteriorly as ridges, between which one or two additional ridges may appear, the various ridges and intermediate grooves are much less distinct than those on the median parts of the brachial valve, and usually extend only about half way from the muscular area toward the anterior margin of the valve. The shell usually is only moderately thickened along the anterior and lateral edges. The interior of the brachial valve is marked by four strong parallel ridges with the intermediate vascular sinuses. Posteriorly, they terminate within the diductor area, the two innter ridges having a tendency to unite with the median elevation extending forward from the thickened portion of the shell beneath the cardinal process. The interior of the brachial valve is marked by irregular, elongated papillae or short ridges, which become coarser toward the vascular marking and toward the posterior parts of the shell, but are finer toward the lateral and anterior margins.