Middle Ordovician – Late Ordovician
Lyrodesma is an extinct genus of facultatively mobile shallow infaunal suspension feeders
Identification in Hand Sample:
- Smooth anterior surface
- Beak-like shape of the posterior end
- Pallial sinus present
- Faint radial grooves
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Crame & Owne (2002)
- Lyrodesma has a radiating subumbonal sheaf of crenulate teeth.
Pojeta, Jr. & Runnegar (1985):
- Denticulate teeth are characteristic of the lyrodesmatid actinodontoids. Babin (1982b) has documentated the progressive reduction of the lamellar teeth from such forms as Noradonta to the denticulate pseudocardinal teeth of Lyrodesma, all of which radiate from beneath the beak.
Pojeta, Jr. (1971):
- Lyrodesma is unique in that it is the earliest known siphonate pelecypod which possesses a pallial sinus. This feature must be strongly emphasized for it indicates that infaunal siphonate pelecypods originated early in the history of the class and not in the middle or late Paleozoic as is sometimes suggested.
- Likewise, Lyrodesma in its general morphology, but especially in its hinge features is so similar to younger trigoniaceans that it seems likely the Lyrodesmatidae gave rise to the Trigoniacea.
- Both suspension feeders (Lyrodesma) and deposit feeders (Ctenodonta) are known from the late Early Ordovician onward.
- Lyrodesma and Babinka were infaunal siphonate suspension feeders. Lyrodesma is sinupalliate and elongate and therefore was probably a “normal” siphonate pelecypod.
- Lyrodesma is the oldest known siphonate pelecypod which has a pallial sinus; this feature combined with the anteriorly-posteriorly elongated shell strongly suggests that the genus was infaunal with only the siphons reaching the sediment-water interface. The rostrate nature of the posterior end of the shell is similar to such living forms as Anomalocardia and suggests a vertical life position. The oldest known specimens of Lyrodesma which show a pallial sinus are early Late Ordovician in age (Eden and Maysville); however, Middle and Early Ordovician shells with the same shape and dentition as the Late Ordovician species of Lyrodesma probably had the same mode of life.
Treatise on Invertebrate Paleontology, Part N, Mollusca 6 (1 of 3) (1969):
- Anterior surface smooth; postumbonal slope with faint radial costellae; sinupalliate.
- Equivalve, inequilateral; trigonia shaped, elongated, posterior areas striated, hinge with from five to nine diverging prominent cardinal teeth, transversely striated; ligament external.
- Character: “Equivalved, inequlateral; hinge with about eight diverging prominent cardinal teeth, transversely striated.”
- If this description, cited from Mr. Conrad, were so altered as to include shells with a greater number of cardinal teeth, it would include a species which appears closely allied to the one described as the typical form. The form of this shell, and character of the diverging cardinal teeth, or crenulations, are different from Nucula, and more nearly like Pectunculus. I prefer, however, to place it under the genus Lyrodesma, provisionally; though a better knowledge of L. plana, the typical species of the genus, may hereafter render it necessary to separate them.