Late Ordovician – Middle Silurian
Rhynchotrema is an extinct genus of stationary, epifaunal suspension feeders
Identification in Hand Sample:
- Large, often semicircular
- Equibiconvex profile
- Beak can be suberect or curling in
- Dorsal fold and ventral sulcus arising at the umbones
- Uniplicate Commisure
- Ribs radiate from the Umbones
- Small pedicle opening
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- Rhynchotrema is a small rhynchonellid brachiopod that has a biconvex profile and a subtriangular outline. Ribs radiate from the umbones of both valves. A small pedicle opening is present. Specimens may resemble small specimens of Lepidocyclus, but Rhynchotrema tends to have a more pointed beak on the pedicle valve, a more pronounced fold of the commissure, and one rib within the sulcus of the pedicle valve.
New York State Museum Annual Report of the Regents (1989):
- Shells large, thick, often gibbous. In mature conditions the deltidial plates are of great size, thickened and coalesced with the bottom of the valve, their outer surface being concave. The pedicle passage encroaches upon the substance of the valve, the foramen lying behind the apex and the passage itself inclosed by the thickened deltarium.
- The teeth rest upon the thickened lateral walls of the valve, and there appears to have been no development of dental lamellae unless it was at a very early period in the life of the individual.
- In the brachial valve there is a thickened median septum which may extend for more than one-half the length of the shell; and it is upon the posterior extremity of 5this that the slender median cardinal process rests. This delicate apophysis is frequently distorted to one side or the other. The bases supporting the crura are divided by a very narrow median cleft, and are remarkably broad and stout, abruptly deflected to the deep dental sockets. The crura take their origin from the central portion of this comparatively broad hinge-plate, instead of from the margins of the dental sockets, as is usually the case in the Paleozoic rhynchonelloids. The structure of the hinge apophyses in both valves is a persistent character, while the peculiarities of the deltarium are variable with age and external conditions. The muscular impressions are usually strongly developed, there being beneath the deltidial plates a deep scar of the pedicle muscle, while the adductor impression on the pedicle-valve is often very marked. The adductors of the brachial valve and the diductors of the pedicle-valve are more or less distinctly defined.
Arthur McFarlan (1898):
- Rostrate, thick shelled, strongly plicate shells, with well-marked fold on the brachial, and sinus on the pedicle valve. Deltidial plates concave, thick. Teeth strong. Median septum of brachial valve extending more than one half the length of the shell, with the small cardinal process on its posterior
extremity. Muscular impressions usually well defined. Beak of pedicle valve closely incurved over that of the brachial valve.