Middle Ordovician – Upper Ordovician
Modiolopsis is an extinct genus of stationary semi-infaunal suspension feeders
Identification in Hand Sample:
- Shell thin, obliquely elongate very inequilateral, with small anterior and large posterior end.
- Valves crossed by oblique depression extending backward from anterior portion of umbo and marked by concentric lines of growth
- Characteristic anterior adductor scars large and deep and often visible in the shell, forming a little-circumscribed elevation.
- Lacking teeth
- The lateral shape of Fordilla is readily compared with that of various Ordovician modiomorphid isofilibranchs such as Modiolodon, Corallidomus, and Modiolopsis.
- Modiomorphidae (Modiolopsidae) either lack teeth as in Modiolopsis or have small cardinal teeth on a hinge plate, as in Modiolodon.
Pojeta, Jr. (1971):
- Based on the type species, the name Modiolopsis is applicable to modioliform shells which expand noticeably posteriorly so that the maximum height of the shell is significantly more than the height measure down from the beaks and which also have concentric sculpture, an entire pallial line, anisomyarian musculature, and multiple accessory muscle scars in front of the beaks.
- Ulrich (1894, p. 521) illustrated the hinge lines of two species of Modiolopsis which generally fit the definition of the genus based on the type species. Although the hinges of Ulrich’s specimens are not so well preserved as indicated on his figures, they do suggest that the genus was edentulous.
- Based on shell shape and general hard-part morphology such Ordovician modiomorphids as Modiolopsis, Modiolodon, Whiteavesia, and less well-known genera such as Pholadomorpha are strongly reminiscent of the geologically younger modioliform mytilaceans.
- In one Ordovician species of Modiolopsis, it was possible to document that it was a nestler on trepostome bryozoan colonies. The trepostome was identified as probably a species of Hallopora by O.L. Karklins, U.S. Geological Survey.
Treatise on Invertebrate Paleontology, Part N, Mollusca 6 (1 of 3) (1969):
- Edentualous; without marked lateral sulcus or ventral sinus; radial ornamentation lacking
- Shell thin, obliquely elongate very inequilateral, with small anterior and large posterior end. Beaks not prominent, near anterior end. Valves crossed by oblique depression extending backward from anterior portion of umbo and marked by concentric lines of growth. Teeth absent. Anterior adductor scar large and deep, posterior one large but faint. Ligament chiefly external.
- Generic Diagnosis. Shells with a black epidermis, subovate, widest posteriorly, the dorsal edge very gently convex, the ventral side slightly sinuate. Umbones, though small and rather low, projecting distinctly above the hinge-line, situated near the anterior end and over the inner third of the deeply impressed anterior adductor scar; umbonal ridge rounded, low, notable mainly because of an undefined but wide depression over the antero-ventral slope; cardinal slope gently concave. Surface usually with concentric lines and narrow ribs, the latter strongest, rounded, and most regularly spaced on the posterior half. Hinge-plate thin, with a single, undefined subrostral tooth and socket in each valve; ligament external but lying partly in a very narrow area. Pallial line simple; posterior adductor scar large but very faintly outlined.
- Stratigraphic Range About twenty-five species, ranging from Lower Stones River to Medina and possibly Clinton (of Ontario and Quebec).
- Character. Equivalve, inequilateral, elongated, becoming broader posteriorly; umbones near the anterior extremity, which is marked by a single strong muscular impression as in Modiola. A sinus often extends from the anterior side of the umbones, obliquely backwards, leaving the anterior portion separated as a kind of lobe. Surface marked by fine, concentric striae; shell thin.
- This genus, as defined, includes a very natural group of shells found in the older Silurian strata, some of which have been referred to Cypricardia, Modiola, Pterinea, and other genera. One of the most prominent characters is the strong muscular impression, which is close to the anterior margin: this is often visible in the shell, forming a little-circumscribed elevation, and more conspicuous in the cast, where it is usually well preserved. There is often a slight contraction or sinus below, or posterior to, the umbones, but this is not always conspicuous. The shells of this genus are, for the most part, smooth, or marked only by fine concentric lines, indicating the laminae of the shell, and they are generally free from angular ridges.