Type species: Atrypa modesta Say (in Hall, 1847) (Jin et al., 1997)
Ordovician (Caradoc – Ashgill)
Zygospira is an extinct genus of stationary, epifaunal suspension feeders.
Identification in Hand Sample:
- Biconvex and subcircular
- Generally very small – (usually smaller than 1cm)
- Sulcus on the dorsal valve, Fold on the ventral valve
- Internal spires present, but hard to see
- Spiral patterns with few coils ornament the valves
- Primary lamellae are joined by a transverse jugum
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Map point data provided by iDigBio.
- Small, ventribiconvex-planoconvex; ventral valve strongly to weakly carinate; weakly sulcate dorsal valve; long hinge; small orthocline-anacline area; minute beak; apical-transapical foramen; deltidial plates minute to absent; ribs medium (coarse ribs on ventral carina), expanding distally, generally non-bifurcating; growth interruptions common, rare, slightly overlapping anterior growth lamellae; commissure sulcate; interior thinly walled; minute, medially directd teeth with dental cavities; brachidia delicate; curura small, laterally directed; spiralia dorsomedially directed with fewer than 8 whorls, filling much of shell interior; jugum posteromedial to dorsal, in mid-anterior of shell
- Distinguished from Anazyga by wider hinge line, carination, coarser ribs, dorsomedially directed spiralia
Fossils of Ohio (1996):
- Small atrypid brachiopod, generally less than 5mm in length. It has a biconvex profile and a circular to subcircular outline. Length and width are nearly equal in dimensions, although in some individuals width may be slightly greater than length. The valves are characterized by their small size and fine ribs that radiate from the beaks across both valves. There is a minute, circular pedicle opening in the beak region of the pedicle valve. Te anterior commissure of the brachial valve may be sulcate to very gently sulcate. The central rib of the brachial valve may be slightly wider than adjacent ribs at the anterior commissure. Care must be taken to distinguish Zygospira from small specimens of other ribbed brachiopod genera, such as Lepidocyclus. Specimens of Zygospira have been found in association with bryozoans, indicating that some may have attached to these colonial organisms (Richards, 1972). Zygospira is the smallest adult articulate brachiopod known from Ohio and has been recorded from the entire Cincinnatian Series.
- Zygospira is represented by numerous specimens in the present collections. All specimens in the present collections. All specimens are small, only three out of 80 exceeding 5 mm in length. The variety of form is somewhat surprising. A few can be assigned to Z. recurvirostris, a middle Ordovician species, a few to Z. putilla, a Silurian might pass for Z.aequivalvis Twenhofel or Z. kentuckiensis Nettleroth, supposedly late Ordovician species. A somewhat similar assemblage was found in a collection made by E. O. Ulrich from “Kimmswick, Pratt’s quarry, ¾ mile south of Elsberry, Mo.” (loc. 258g), but it lacks the true Z.putilla and the Z. cf. Z. circularis of the Idaho collections.
- If material in the U.S. National Museum is typical one may observe from it that most Late Ordovician Zygospira are large and coarse-ribbed, whereas Middle Ordovician species are small and fine-ribbed. In these respects, the Idaho specimens resemble most closely Middle Ordovician species.
- A small, rostrate, sub-circular, biconvex, plicated, genus. Pedicle valve with median fold, brachial with sinus. Spirals with few coils, apex directed inward. Primary lamellae united by
- Shells usually small. Outline subcircular or transversely oval. Contour subplano-convex. Surface sharply plicate. Pedicle-valve with a median plicated ridge. Umbo narrow and prominent; beak acute and incurved. Foramen elongate, rarely apical, enclosed by the deltidial plates. Hinge line long and straight; cardinal extremities rounded. A distinct false area is formed by a pair of ridges diverging from the beak toward the cardinal extremities. On the interior the teeth are moderately well developed and unsupported by dental lamelleae. The brachial valve is depressed convex in the umbonal region and bears a more or less conspicuous median sinus. The hingeplate consists of two broad, stout processes, diverging outwardly, grooved on their summits and separated from each other by a narrow, sharp cleft. They form both the socket walls and crural bases, and are supported by a low median ridge. Muscular impressions obscure in the typical species. The crura are short and straight at their union with the primary lamellae, making a rectangular curve. The first half-volution of the ribbon lies just within the margins of the valves, and the number of volutions is small. The spirals have their bases parallel to the lateral slopes of the pedicle valve and their apices directed obliquely toward the center of the opposite valve. The jugum is a continuous band, variable in position and shape. It may originate on the posterior or anterior limb of the primary lamellae, or be placed medially; its apex is always angular and directed anteriorly and the lateral curves vary in length and degree according to their position with reference to the spirals.
Geological and Natural History Survey of Minnesota (1885):
- Shells bivalve, equilateral inequivalve; surfaces plicate in the typical species; Sinus on the dorsal valve. Internal spires arranged somewhat as in Atrypa, with a broad loop passing from the outer limbs of the spiral band entirely across from side to side, near or above the center and close to the inner side of the dorsal valve,