Plaesiomys subquadratus

Phylum: Brachiopoda
Class: Rhynchonellata
Order: Orthida
Family: Plaesiomyidae
Genus: Plaesiomys
Species: Plaesiomys subquadratus (Hall, 1847)

Taxonomic Details

Formerly: Orthis subquadrata, Orthis (Dinorthis) subquadrata, Dinorthis (Plaesiomys) subquadrata
From Wright & Stigall, 2013:Although Hall (1847) originally designated this species as Orthis subquadrata, Ross (1959) corrected the specific epithet to Plaesiomys subquadratus to facilitate gender agreement (ICZN 34.2).

Stratigraphic Occurrences


Geographic Occurrences

Map point data provided by iDigBio.

Stratigraphic Description

Sequences (Formations)

  • C5 Sequence (Lower Whitewater, Liberty)

Identification in Hand Sample

  • Wide, subquadrate shells characterized by high costal density
  • Valves are typically convexoconcave to dorsibiconvex
  • Sulcus originates proximally
  • Ventral muscle scars moderate to long, adjustor muscle scars variable
  • Ventral and dorsal umbonal angles low

Plaesiomys subquadratus from Waynesville formation of Warren County, Ohio (OUIP 1475)

Published Description

Wright & Stigall

  • Large Plaesiomys species with wide, subquadrate shells characterized by high costal density. Valve convexity and inflation are variable, but valves are typically convexoconcave to
    dorsibiconvex; sulcus originates proximally; ventral muscle scars moderate to long, adjustor muscle scars variable; ventral and dorsal umbonal angles low. Emended from Hall (1847).

Davis (1998):

  • Articulate brachiopod. The shape of the muscle impressions on the interior surface of the pedicle valve is characteristic of this genus and is very different from that of the externally similar genus Glyptorthis. This species is distinguished by its somewhat rectangular outline and the coarse, irregularly bifurcating costae that characteristically curve near the margin of the shell. Liberty and Whitewater.

Jin et al. (1997):

  • Remarks: One conjoined, largely exfoliated shell is assigned to P. subquadratus on the basis of its medium size (length 21.5 mm, width 27.0 mm, and thickness 11.0 mm), dorsibiconvex shell, weak dorsal sulcus, rectimarginate anterior commissure (pl. 1, figs. 2, 3), and relatively fine coastae with common bifurcation and intercalation to produce five to eight costae per 5 mm at the anterior margin. One of Hall’s three types illustrated by Ross (1959, pl. 55, figs. 1, 6) measures 21.3 mm long, 27.0 mm wide, and bears five to seven costae per 5 mm at the anterior margin.
  • In its fine costae, P. subquadratus is similar to P. rockymontana (Wilson, 1926) but differs in that P. rockymontana generally has a larger shall with a well developed uniplicate anterior margin (pl. 1, figs. 5-12). Within the Hudson Bay collections, P. subquadratus may be confused with the more common Dinorthis occidentalis (Okulitch, 1943). Upon careful observation, however, D. occidentalis can be distinguished by its much coarser costae (about four per 5 mm at the anterior margin), with infrequent bifurcations or intercalations, whereas P. subquadratus shows intense bifurcation and intercalation (pl. 1, figs. 1-4).