Jin and Holmer—Karlsorus new genus from the Silurian of Gotland
diversifications in shell morphology during the Wenlock and Ludlow:
(1) Drastic increase in shell size, sometimes with highly arched ventral umbo to give the ventral valve a somewhat conical appearance (e.g., Harpidium; see Boucot et al., 2002). Large shells of Rhipidium Schuchert and Cooper, 1931 from Gotland may exceed 135mmin length (e.g., specimen B5578 in the NHM collection), and large forms of Karlsorus n. gen. are nearly 130mm long.
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dorsal umbo; trilobation of dorsal valve inconspicuous. Spon- dylium and ventral median septum same as for Pentamerus. Inner hinge plates low, subparallel to each other; outer hinge plates inclined ventromedially, discrete at posterior and anterior parts, commonly fused into single plate in middle part to form brachiophorium. Crura attached to medial sides of junctions between inner and outer hinge plates.
(2) Inversed biconvexity, with a flattened ventral valve and reduced ventral beak, in contrast to a strongly convex dorsal valve with its umbo and beak arched over the ventral posterior, as is typical of Brooksina and Capelliniella, which are common in North America and the Ural Mountains.
(3) Increased length of the spondylium and hinge plates, sometimes extending very close to the anterior margin (e.g., Brooksina; Harpidium; Kirkidium Amsden, Boucot, and Johnson, 1967; and Pentamerifera). These extravagant internal structures are rarely seen in the pentamerids of Llandovery age.
(4) Development of the brachiophorium, as discussed earlier in this study, can be viewed as a dorso-ventrally inversed cruralium, which seems to have first appeared in late Wenlock time, corresponding to the Cyrtograptus lund- greni graptolite Biozone.
large-shelled Pentameridae is a reflection of the success of this family in their ecological expansion in the level-bottom regime, as well as their invasion into the reefal environments during
The great range of morphological diversifications of
the Silurian. Pentamerid shells with a reversed biconvexity, a flattened ventral valve, and an orthocline ventral beak/palintrope had the delthyrium well exposed, and thus may have retained a functional pedicle to attach to a variety of hard surfaces (e.g., walls of large cavities or other sheltered areas) in reefal environments.
Systematic paleontology
Order Pentamerida Schuchert and Cooper, 1931 Suborder Pentameridina Schuchert and Cooper, 1931 Superfamily Pentameroidea M’Coy, 1844 Family Pentameridae M’Coy, 1844 Genus Karlsorus new genus
Type species.—Pentamerus gothlandicus Lebedev, 1892, upper Slite beds, upper Wenlock, Cyrtograptus lundgreni Biozone, Gotland, Sweden (Bassett, 1977; Calner et al., 2004).
Species included.—Type species only. The species has been reported from Wenlock strata of Gotland (Slite beds; Bassett and Cocks, 1974), England (Much Wenlock Limestone; Bassett, 1977), and Podolia (Kitaigorod horizon; Nikiforova, 1978).
Diagnosis.—Shell very large, elongate, nearly equibiconvex with moderate convexity; ventral valve strongly trilobate, longer than dorsal valve, with prominent umbo and beak arched over
Etymology.—Named after the island Stora and Lilla Karlsö, Gotland, where the type species is most abundant and best preserved.
Remarks.—In external shell morphology, the new genus is most similar to Pentamerus in its subpentagonal shell outline and subequibiconvexity (Fig. 1.1–1.5), with the ventral valve slightly deeper than the dorsal valve, especially in the posterior half, and the strongly convex ventral umbo and a beak arched over the dorsal umbo. Internally, Karlsorus n. gen. resembles Pentameridfera Khodalevich, 1939 (see Sapelnikov, 1972) in having a tent-like brachiophorium formed by the inner and outer hinge plates (Fig. 1.7, 1.10, 1.11). Pentamerifera, from Ludlow strata of the eastern slope of the Ural Mountains, differs in having an extremely long spondylium and hinge plates that extend for more than three-fourths of the total shell length, whereas in Karlsorus n. gen. those internal structures do not exceed one half of the shell length (Figs. 1.1, 1.2, 2.8). A few other predominantly Ludlow pentameroids, such as Brooksina Kirk, 1922 from Alaska, Nevada, and the Ural Mountains, may also have developed a brachiophorium-like structure inconsistently (Boucot and Johnson, 1979, p. 107). Several dorsal valves of Brooksina from the C Fauna (Ludlow) of the Roberts Mountains Formation, central Nevada, illustrated by Johnson et al. (1976, pl. 19, figs. 3, 8, 15), possess a brachio- phium similar to that of Karlsorus n. gen., in that only the middle part of the outer hinge plates are fused together. How- ever, Brooksina has a distinctly ribbed shell and tends to have a reversed lateral shell profile compared to the new genus and Pentamerifera and has the dorsal valve larger and more convex than the ventral valve, and has the spondylium and hinge plates extending near the shell anterior margin (see Sapelnikov, 1972, 1985).
Karlsorus gothlandicus (Lebedev, 1892) (new combination) Figures 1–3; Table 1
1892 Pentamerus gothlandicus Lebedev, p. 22, pl. 2, figs. 11a, 11b, 12.
non 1938 Pentamerus (Pentameroides) cf. gotlandicus; [sic]
St. Joseph, pl. 5, figs. 7, 8, pl. 6, figs. 13, 15, text- figs. 1, 8. St. Joseph’s material is from the Oslo region, assignable to Pentameroides subrectus (Hall and Clarke, 1893)
1974 Pentamerus gothlandicus; Bassett and Cocks, p. 23. No illustrations; localities and stratigraphic occurrences discussed.
1977 Pentamerus gothlandicus; Bassett, p. 170, pl. 47, figs. 1–3, text-fig. 19. Lectotype selected.
1978 Pentamerus gothlandicus; Nikiforova, p. 170, pl. 1, figs. 1–5.
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