Anomalodonta gigantea

Phylum: Mollusca
Class: Bivalvia
Order: Pterioida
Family: Ambonychiidae
Genus: Anomalodonta
Species: Anomalodonta gigantea (Miller, 1847)

Stratigraphic Occurrences


Geographic Occurrences

Map point data provided by iDigBio.

Stratigraphic Description

Sequences (Formations)

  • C5 Sequence (Waynesville)
  • C4 Sequence (Arnheim)

Identification in Hand Sample

  • Large pelecypod 8-10 cm. in length, 8-10 cm in width
  • Radial plications broad, deep
  • The beak is sharp, incurved, and situated at the end of the cardinal margin
  • Byssal opening immediately below the beak
  • Lack auricles

Anomalodonta gigantea from Waynesville formation of Montgomery County, Ohio (OUIP 1306)

Published Description

Davis (1998):

  • Pelecypod. Characterized by its outline, large size, broad, deep plications, and tooth structure Richmondian.

Fossils of Ohio (1996) :

  • The Upper Ordovician genus Anomalodonta is represented by A. gigantean Miller, a large species that lacks auricles and has broad, radial plications.

McFarlan (1931):

  • An unusually large pelecypod 8-10 cm. in length and almost as wide, alate posteriorly and marked by 30-40 strong radiating ribs. Anterior edge about at right angles to hinge line. Beaks sharp, slightly incurved, and situated at the anterior end of then cardinal margin. Byssal opening immediately below the beak, 6-7 mm. in diameter. In a specimen 8 cm. long the hinge has a length of about 5 cm. From 4-14 cartilage grooves extend from the end of the wing to the byssal opening. A characteristic Richmond species known from the Arnheim to the Whitewater. In the region around Louisville it seems to be restricted to the Arnheim.

Miller (1874)

  • Shell equivalve, inequilateral, alate posteriorly and compressed, more convex toward the umbones and the anterior side, anterior side abruptly declining, beaks rather sharp and slightly incurved. Surface marked by 30 to 40 strong radii, same width as the intermediate spaces, which are concave grooves marked with concentric striae, giving much the same external appearance as those of an Ambonychia radiata. Shell marked exteriorly, toward the margin, with lines of growth and concentric striae, which cross the radii, rendering it likely that concentric striae crossed the radii over the whole surface of the shell, though they appear now to be smooth. Byssal sinus immediately below the beak anteriorly, about ¼ of an inch in diameter in a large specimen. Height of the shell from 2 to 4 inches, greatest breadth about 1/5 less. Shell quite thick about the umbones and wing, but thinner towards the base. Large cardinal tooth or elevation beneath the umbones and sloping posteriorly from the beak. The cardinal elevation on the left valve having a depression to receive a corresponding elevation, though slight, on the right valve. From the cardinal tooth there are from 4 to 18 lateral cartilage grooves extending posteriorly to the end of the wing, and terminating with the shell, and there are the same number of cartilage grooves on the anterior side of the cardinal tooth that immediately run together as the pass into the byssal sinus. The cartilage grooves vary in number with the age and size of the shell. The shell is thickened on the anterior side, and appears to show lines of growth passing through the sinus to the base. A large muscular impression is found near the anterior margin, half way from the sinus to the base of the shell, and there are appearances that indicate another muscular impression on the posterior wing of the shell near its termination. Greatest depth of a valve in a large specimen, ½ and inch.
  • This is the largest bivalve yet known in the Cincinnati group. It may readily be distinguished from Anomalodonta alata by the surface markings, though the general outline form of the two shells are nearly the same.
  • I found this species near Versailles, Indiana, about 40 miles west of Cincinnati, and about 300 feet below Upper Silurian rocks; and I also found what I believe to be a cast of the same at Richmond, Indiana. I do not know that it can be found elsewhere, but the probabilities are that it can be found in the upper part of the Cincinnati Group, from Madison to Richmond, and at other places.