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Strophomenata - Strophomenida - Strophomenidae
It was corrected as Oepikina by Alikhova (1953), Macomber (1970), Mitchell (1977), Williams et al. (2000), Sepkoski (2002), Jakobsen et al. (2014) and Congreve et al. (2015).
It was assigned to Strophomenidae by Salmon (1942), Alikhova (1953) and Macomber (1970); to Oepikinae by Mitchell (1977); to Strophomenida by Sepkoski (2002); and to Furcitellinae by Williams et al. (2000), Jakobsen et al. (2014) and Congreve et al. (2015).
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If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.
|E. S. Salmon 1942||Concavo-convex; surface covered with fine radial costellae of several sizes, the smaller ones tending to disappear toward the anterior margin, leaving a space of nearly 1 mm. between the primaries. Concentric lines very fine, only visible on extremely well preserved specimens. Cardinal margins not denticulate.
The pedicle interior shows a very large muscle area in comparison to the size of the entire shell; it usually extends about halfway from the beak to the anterior margin of the shell and may extend farther. The diductor scars are circular to broadly oval, shallow, and not usually well defined. The adductor track is a rather broad ridge, highest in the anterior portion, extending almost to the anterior border of the diductor scars. The dental plates are strong, diverging at an angle of 90?-120?. The visceral area of the pedicle valve is marked off by a low ridge, strongest in the posterior part of the shell at the sides of the delthyrial cavity, and continuing, though fainter, around the entire anterior part of the shell, near the margin.
The brachial interior is more distinctive than that of the pedicle valve. (See text fig. 6.) The cardinal process is stout, with two triangular branches; the myophores make an angle of 90? or more with the plane of the interior surface. They do not extend anteriorly, but in some cases posteriorly, from the seat of attachment.
From each side of the base of the cardinal process extends a curving ridge of adventitious shell material bearing the brachiophores, minute processes on the posterior side of the ridge. There is also a ridge around the entire visceral area of the interior, less strong on the sides but becoming sharper on the anterior, and having a fluted appearance due to the strongly marked vascular system. The ridges near the cardinal margin, bearing the brachiophores, are apparently homologous with the short, curving ridges of adventitious shell material in the brachial interior of Rafinesquina.
The median line of the brachial interior of Opikina is marked by a ridge starting at the base of the cardinal process, where it is low and broad, which becomes narrower and sharper toward the anterior. In some species it becomes exceedingly sharp and prominent. There are, in addition, two pairs of lateral septa: the posterior pair divide the posterior adductor scars into two more or less unequal areas; the anterior pair are nearly parallel to the median septum and mark the lateral margins of the anterior adductor scars.
The posterior adductor scars are bounded on the sides and separated from the anterior adductor scars by a prominent broad swelling, which is usually highly pustulose and fluted with vascular markings.
The surface of the visceral area outside the muscle scars is very finely pustulose; it appears smooth to the unaided eye, but with a hand lens or under the microscope is seen to be minutely granular.
The shell structure is very finely pseudopunctate; the pseudopunctae lie close together and are apparently not arranged in any definite order. In this character, as well as in the distinctive brachial interior, Opikina differs markedly from Rafinesquina, since the latter genus shows much larger pseudopunctae, easily visible with the unaided eye on weathered specimens, and arranged in definite radial rows between the costellae.