Phylum: Brachiopoda
Class: Rhynchonellata
Order: Orthida
Family: Plaesiomyidae
Genus: Plaesiomys Hall & Clarke, 1892
Cincinnatian Species: Plaesiomys subquadratus

Taxonomic Details

Type species: Plaesiomys subquadratus (Hall, 1847) (Jin et al., 1997)

Geologic Range
Late Ordovician (Caradoc – Ashgill)

Common Paleoecology
Plaesiomys is an extinct genus of stationary epifaunal suspension feeders

Identification in Hand Sample:

  • Cardinal extremities obtuse, convexoconcave, broadly unisulcate, multicostellate, sporadically lamellose
  • Ventral interarea short, curved
  • Ventral muscle scar without median ridge
  • Posterior pair of quadripartite dorsal adductor scars larger than anterior
  • Subquadrate outline and a straight hinge line

Geographic Occurrences

Published Description

Wright & Stigall (2014):

  • Genus of Plaesiominae with abundant bifurcating costellae and densely spaced epipunctae microstructure. Outline subquadrate to suboval, convexoconcave to dorsibiconvex;
    multicostellate, costae emplaced by bifurcation or intercalation, aditicules on costae and costellae, uniplicate to rectimarginate, epipunctae microstructure, densely spaced; ventral interarea
    short, apsacline, dorsal interarea orthocline to anacline; ventral muscle field subcordate, diductor scars large, adductor scars oval shaped, elongate, and enclosed within diductor muscle field,
    adjustor scars large with variable outline; posterior scars typically larger than anterior scars, divided longitudinally by a medial myophragm; myophore bilobate to trilobate, crenulated; brachiophores divergent. Emended from Williams and Harper (2000). Occurrence.—Late Ordovician (late Sandbian to Hirnantian) of North America with possible occurrences in Kazakhstan, Estonia, Siberia, and the British Isles (see remarks below).

Treatise on Invertebrate Paleontology, Part H, Vol. 3 (2000):

  • Cardinal extremities obtuse, convexoconcave, broadly unisulcate, multicostellate, sporadically lamellose; ventral interarea short, curved, anacline dorsal interarea vestigial; ventral muscle scar without median ridge; posterior pair of quadripartite dorsal adductor scars larger than anterior.

Fossils of Ohio (1996):

  • A medium –sized orthid brachiopod that ranges in profile from convexi-concave to unequally biconvex. It has a subquadrate outline and a straight hinge line. Width is greater than length. The delthyrium is open. Numerous branching ribs radiate from the umbones of both valves. The interior of the pedicle valve shows the teeth and a muscle-scar area that has two broad diductor muscle scars enclosing a smaller, central adductor muscle scar. Plaesiomys has a more rectangular outline than Glyptorthis. Both have irregularly branching ribs, but in Plaesiomys the ribs curve noticeably toward the commissure. Plaesiomys has been recorded from the Liberty and Whitewater Formations.

P. subquadratus