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Jin and Holmer—Karlsorus new genus from the Silurian of Gotland


Table 1. Measurements (mm) of shell dimensions for Karlsorus gothlandicus (Lebedev, 1892) n. comb., based on collections at the Swedish Museum of Natural History and the Gotland Museum, Sweden. Abbreviations: dv, dorsal valve; Ms/Ohp, length of median septum or outer hinge plates; sh, shell with conjoined valves; vv, ventral valve; *value estimated from damaged shell;— measurement could not be obtained from broken shell.


Br 105773, sh Br 105774, dv Br 108382, dv Br 108383, dv Br 108385, dv Br 108386, dv Br 109995, vv GM 5939, sh GM 5943, vv


Length 128.5


>94.0* 123.2 105.5 106.8


101.2


>70.0* 99.0 93.8


118.4 —— 58.7 99.3 —


101.3 87.4


102.0 —— 92.1 50.6


56.0 52.0 49.6


42.5


Width 93.0*


Depth/Thickness Ms/Ohp 66.5


— — — — —


56.5 44.7


917 Interior of ventral valve characterized by very small and


week teeth; spondylium narrow, deep, extending for less than one-half of shell length; supported by relatively long and high median septum for about one-half of spondylium length (Fig. 2.6, 2.8); thick prismatic layers of median septum forming wedge-shaped junction with prismatic layer of valve floor. Interior of dorsal valve with correspondingly minute hinge


Types.—VSEGEI 102/324, selected by Bassett (1977; see also Nikiforova, 1978), exact locality and stratum unknown. On Got- land, the species is largely confined to the upper Slite beds, corre- sponding to the Cyrtograptus lundgreni Biozone of the upper Wenlock (Bassett and Cocks, 1974; Calner et al., 2004, fig. 2).


Occurrences.—On Gotland, K. gothlandicus is most abundant and best preserved in the Slite beds on Lilla Karlsö (see Hede, 1927), a tiny island about 5 km off the southwest coast near Djupvik. Other specimens examined are from coeval strata at Tjeldersholm 1 (57°37'56.4''N, 180°46'34.9''E; Laufeld, 1974), Blåhäll, Fröjel (57°18'53.6''N, 18°09'40.5''E; Laufeld, 1974), and Valle 1 and 2 (see Laufeld, 1974 for detailed locality information). There are many other localities in Gotland where the species has been reported, but such material was not exam- ined in this study. More detailed information on K. gothlandicus localities can be found in Laufeld (1974) and Bassett and Cocks (1974).


Description.—Shell very large, with maximum observed length of 128mm and width 102mm (Table 1); subpentagonal in outline, with maximum width located at about two-thirds length from apex; nearly equibiconvex, or with ventral valve slightly deeper than dorsal valve in umbonal area. Anterior commissure rectimarginate to gently sulcate (Fig. 1.1–1.5). Hinge line varying from short to relatively long, about one-third to two- thirds of shell width. Ventral umbo strongly convex, narrow, with apical angles ~ 90°, relatively low for shell size, raised ~15mmabove hinge line in shells over 100mmlong; palintrope defined by fairly clear ridge; beak strongly incurved, arched over but not touching dorsal umbo; pseudodeltidium present, strongly concave, almost appressed to floor of spondylium in apical area (Fig. 1.9). Ventral valve trilobate, with medial lobe broadening rapidly from valve apex to anterior margin, approximately twice as wide as each lateral lobe, delimited by gentle to prominent furrow on each side to separate from lateral lobes (Figs. 1.2, 2.4, 2.7), forming slightly protruding tongue at anterior margin. Dorsal umbo moderately and evenly convex, not marked by sulcus or fold, with small beak buried in delthyrial area of ventral valve; antero-medial portion of valve gently and faintly depressed in some shells, or marked with faint development of trilobation. Shell essentially smooth, except for randomly developed, concentric growth lines, especially close to anterior margin.


sockets; inner hinge plates extending for about two-fifths of valve length from valve apex, discrete along entire length (Figs. 1.1, 2.2, 2.6), very low, attaining about 1/4 height of outer hinge plates (Figs. 1.7, 1.8, 2.6, 2.8, 3.1); outer hinge plates about same length as inner hinge plates, discrete in posterior and anterior portions, fused in medial portion to form tent-like brachiophorium, with ventro-medial portion forming crest-like structure (Figs. 1.7, 1.8, 1.10, 1.11, 2.1, 2.2); crura slender, rod-like, fused to inner surface of junction between inner and outer hinge plates (Fig. 3.1, 3.3), becoming free only at distal ends.Muscle scars not observed in either ventral or dorsal valves.


Remarks.—The internal structures of K. gothlandicus are partially similar to those of Pentamerus, especially in the relative size and shape of the spondylium, and the configuration of the posterior part of the hinge plates. In addition to the brachiophorium, however, the inner hinge plates of the Gotland species are proportionally much lower than in Pentamerus,in which the inner hinge plates are about half as high as the outer hinge plate (see Jin and Copper, 2000). In the Gotland species, the inner hinge plates are only about one-quarter of the height of the outer hinge plates, with the outer-inner plate junction, marked by the crura, being close to the valve floor. The unusually large shell size ofK. gothlandicus, typically exceeding 100mmin length, is very rarely achieved by various Pentamerus species of Llandovery age (Jin and Copper, 2000). From the limited number of exposed shell interiors and


sectioned specimens available for study, it appears that the fusion of outer hinge plates in the new genus occurs only in the middle portion of the plates. In their anterior and posterior potions, the configuration of the hinge plates is very similar to that in Pentamerus, except for the notably low inner hinge plates (Figs. 1.10, 2.6). There has been some doubt that the medially coalesced hinge plates may have been the result of preservation artifact, which explains why the brachiophorium was not considered an important diagnostic character in the revised brachiopod volumes of the Treatise on Invertebrate Paleontol- ogy (Boucot et al., 2002). In this study, however, a thin section of a well-preserved dorsal valve clearly shows that the lamellar layers of the two outer hinge plates are fused into a single lamellar layer in the coalesced part (Fig. 3.1, 3.2), which strongly suggests that the fusion was not a random compression of the two plates together during post-mortem taphonomic or diagenetic processes.


Acknowledgments


S. Eliason of the Gotland Museum (Visby, Sweden), C. Skovsted of the Swedish Museum of Natural History (Stockholm), and Z. Hughes of the Natural History Museum (London, UK) kindly arranged loans of specimens for this study. Z. Zhang’s assistance in fieldwork on Gotland is much


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