Middle Ordovician – Late Ordovician
Orthodesma is an extinct genus of stationary epifaunal suspension feeders
Identification in Hand Sample:
- Considered to have burrowing and non burrowing forms
- Have a general soleniform shell shape
- Radial ornamentation lacking
Pojeta, Jr. (1971):
- For various reasons, several Ordovician genera have been considered to be burrowing forms including: Cymatonota, Psiloconcha, Orthodesma, Rhytimya, and Cuneamya.
- Orthodesma as defined by Ulrich (1894) was supposed to have anterior and posterior shell gapes. None of the specimens of this genus seen by me show this in an unequivocal manner and some species placed in the genus have a modioliform aspect. However, most species of Orthodesma have a general soleniform shell shape.
- It may be that Orthodesma as presently defined includes both burrowing and nonburrowing forms.
Treatise on Invertebrate Paleontology, Part N, Mollusca 6(1 of 3) (1969):
- Edentualous; without marked lateral sulcus or ventral sinus; radial ornamentation lacking.
- The genus Orthodesma was founded in 1875 by Hall and Whitfield to include transversely elongated shells having typically the length equal to more than twice the height and resembling Orthonota Conrad and Palaeosolen Hall in the subparallel ventral and dorsal margins. In the latter genera, the hinge is continued in a straight line both behind and in front of the beaks, whereas in Orthodesma the hinge line is more or less uninterrupted and deflected in front of the beaks by a lunule. The original diagnosis of the genus follows:
- More or less elongate bivalve shells, having the hinge line straight and generally extended posterior to the beaks but contracted or bent beneath or anterior to them; hinge plate apparently edentulous; valves united by an external ligament extending to a greater or less distance along the posterior cardinal margin. Posterior muscular scar elongate ovate; anterior scar smaller; both faintly marked; pallial line simple. Shells thin marked externally with irregular concentric plicae. Type, Orthodemsa recta.
- In 1894 the genus was redefined and restricted by Ulrich, who added as one of the generic characters a very slight gape anteriorly and posteriorly. The Chapman material is not well enough preserved to add to our knowledge of the structure of the genus but the present species is the first recorded above the Ordovician in America.
Winchell & Ulrich (1897):
- Shell elongate, usually increasing slightly in height (sp?) posteriorly. Anterior end comparatively long, contracted in front of the beaks. Valves moderately convex, usually with a strong umbonal ridge and a broad mesial depression in front of it, their edges fitting tightly along the straight or sinuate ventral margin, but leaving a narrow gape at each end. Umbones prominent, wide, compressed, often extending posteriorly as low cardinal ridges between which the hinge is sunken. Hinge plate edentulous, very thin, long, extending in almost a straight line from the posterior cardinal angle, past the beaks, nearly to the anterior extremity of the shell. Ligament linear, internal and external, the latter chiefly. Posterior muscular scar large, very faint, elongate ovate; anterior scar large, though scarcely half the size of the posterior, well defined, ovate or approaching semicircular in shape, the vertical diameter the longest. Pallial line simple. Shells thin, marked externally with more or less distinct concentric striae and wrinkles.
- Type: Orthodesma rectum Hall and Whitfield
- The above diagnosis does not agree exactly with Hall and Whitefield’s original description of the genus, but as it corresponds with the fossils no apology is necessary. They make, for instance, the erroneous statement that the hinge plane is bent down in front of the beaks; and the fictitious feature has become so well established in literature that it stands as the most important peculiarity of the genus, indeed, as the only one separating it from Orthonota, Conrad. Now, despite the fact that the hinge plate is nearly or quite as straight in Orthodesma as in Orthonota, I am fully satisfied that there is little affinity between the two genera. The Lower Silurian genus, doubtless, is closely related to Modiolopsis and Actinomya. Not so, however, with the Devonian genus, which seems to me to be totally different and nearer Solen than Modiolopsis.
- Species have been placed under Orthodesma that are very different from the types, some of them belonging, I believe, to other families. Thus, O. byrnesi S.A. Miller, and O. mickleboroughi Whitfield, belong to Rhytimya, a new genus that obviously belongs to the same family as Pholadella, Hall, and Allorisma, King. O. cuneiforme Miller, has recently been made the type of his new genus Sphenolium. This genus seems to be related to Cuneamya and therefore cannot belong to the Modiolopsidae. O. subovale Ulrich, together with a number of undescribed species, belongs to the new genus Psiloconcha, while O. saffordi Ulrich, should be referred to Actinomya.
- In the Minnesota report above indicated, I have restricted and redefined this genus and shown that it is related to Modiolopsis rather than to Orthonota. So far as known, the genus is restricted to the Lower Silurian rocks in which it is represented by about fourteen species, three of them occurring in the Trenton, the rest in the various beds of the Cincinnati group. The greater number of the latter are new to science and as I found room for only two of them in the present work, the remainder will have to await some other opportunity for publication.