Cyrtodonta

Classification
Phylum: Mollusca
Class: Bivalvia
Order: Arcoida
Family: Cyrtodontidae
Genus: Cyrtodonta Billings, 1858
Cincinnatian Species: Cyrtodonta cuneata

Geologic Range
Middle Ordovician – Silurian

Common Paleoecology
Cyrtodonta is an extinct genus of facultatively mobile semi-infaunal suspension feeders

Identification in Hand Sample:

Geographic Occurrences

		

Species Differentiation

Published Description

Pojeta, Jr. & Runnegar (1985) (in reference to Morris, 1978):

  • The cycloconchacean Glyptarca Hicks (Early Ordovician) has dentition approaching that of Cyrtodonta. In both genera, the anterior teeth are small, hook-shaped, and mostly anterior to the beak. In Cyrtodonta, the lamellar posterior lateral teeth are relatively short and separated from the anterior teeth by a large edentulous space. In Glyptarca, the posterior teeth are long and separated from the anterior ones by a small edentulous space. The dentition of Cyrtodonta could have evolved from that of an actinodontoid like Glyptarca.

K.R. Walker (1972) (in reference to Pojeta, Jr. 1971):

  • Cyrtodonta is interpreted as a bysally attached, semi-infaunal bivalve.

K.R. Walker (1972) (in reference to Yonge, 1953):

  • The musculature of Cyrtodonta is heteromyarian (large posterior, small anterior abductor muscles), a condition typical of byssate bivalves.

K.R. Walker (1972):

  • Also, Cyrtodonta is frequently found in living position, and its orientation is commensurate with the semi-infaunally attached habit. It is always oriented with plane of commissure vertical, ventral margin horizontal, and some distance below the associated bedding surface.

Pojeta, Jr. (1971):

  • Shells of the Vanuxemia type were showing a stabilization in position of the dental elements, whereas, shells of the Cyrtodonta type are highly variable in this regard especially in the number and placement of the anterior teeth.
  • Cyrtodontids of the Cyrtodonta type lack an umbonal shelf or shell thickening, and some or all of the anterior teeth are not placed immediately below the beaks. In this group, an accessory muscle scar is present on the lateral surface of the hinge plate opposite the anterior teeth.

Treatise on Invertebrate Paleontology, Part N, Mollusca 6(1 of 3) (1969):

  • Cardinal teeth 3, commonly only 2, rarely 4, directed backward, horizontal or nearly so, and not essentially radial.

L.R. Cox (1959):

  • In Cyrtodonta, an early actinodont, the umbones lie near one end of the hinge margin and the outline of the shell is obliquely oval. There is a tendency for the anterior adductor scar to be smaller than the posterior scar. We know nothing of the mode of life of these early actinodonts, but, since their valve margins were completely closed, they do not appear to have been able to anchor themselves by a byssus to other objects. Their distinctive features may not, therefore, have resulted from adaptation to a sedentary existence. Along the various branches thought to be descended from them, there was, however, a strong potentiality for byssal attachment.

C. cuneata