Journal of Paleontology, 91(5), 2017, p. 911–918 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.46
Pentameroid brachiopod Karlsorus new genus from the upper Wenlock (Silurian) Slite Beds, Gotland, Sweden
Jisuo Jin1 and Lars E. Holmer2
1Department of Earth Sciences, University of Western Ontario, London, Canada, N6A 5B7 ⟨
jjin@uwo.ca⟩ 2Department of Earth Sciences, Palaeobiology, Uppsala University, SE-752 36, Uppsala, Sweden ⟨
lars.holmer@pal.uu.se⟩
Abstract.—Karlsorus n. gen. is proposed in this study as a large, smooth-shelled pentameride brachiopod of the family Pentameridae, based on Pentamerus gothlandicus Lebedev, 1892, from the Wenlock (Silurian) Slite beds of Gotland, Sweden. This species is transferred from Pentamerus to the new genus because of the combination of a Pentamerus-like shell shape and the development of a brachiophorium through fusion of the outer hinge plates in the middle portion, like a dorso-ventrally inversed cruralium. The first appearance of brachiophorium in pentamerids is in the late Wenlock, known also in Brooksina, Pentamerifera, and other related pentamerid genera, marking a significant stage in morphological transformation of dorsal internal structures, as part of the Silurian pentameride diversification in both level-bottom and reefal depositional environments.
Introduction
Pentamerus Sowerby, 1813 is one of the most widely reported early Silurian pentameride brachiopods worldwide, often regarded as a good paleobiogeographic indicator for relatively cosmopolitan marine benthic shelly faunas in the early Silurian, in contrast to the strong endemism of Late Ordovician shelly benthos. True Pentamerus, however, occurs predominantly in middle–upper Llandovery strata, especially in Laurentia, Baltica, Avalonia, Siberia, and Kazakhstan. In South China, Pentamerus occurs only rarely in Aeronian rocks, whereas the common forms previously reported as Pentamerus have been assigned to a different genus, Sulcipentamerus Zeng, 1987 (see Rong et al., 2007). Thus, the occurrence of ‘Pentamerus’ gothlandicus Lebedev, 1892 in the upper Wenlock Slite beds of Gotland (Bassett and Cocks, 1974; Bassett, 1977) has been one of the well-known exceptions outside the common stratigraphic range of the genus, and on Gotland this species forms a widely distributed marker bed known as the Pentamerus gothlandicus layer (e.g., Hede, 1927; Calner et al., 2004; Bassett, 2005) Lebedev (1892) initially proposed the species Pentamerus
gothlandicus based only on ventral valves from Gotland (see Bassett, 1977). Among the large number of genera of the family Pentameridae, the ventral valve usually has a rather con- servative morphology, with a consistently developed median septum and spondylium. As a result, the differentiation of pen- tamerid genera often relies on the differences in internal struc- tures of the dorsal valve, such as the various configurations of hinge plates and crura, skeletal structures that supported lophophores for suspension filter feeding and respiration. The lack of information from the original description of P. gothlandicus, in addition to its unusual stratigraphic position, cast some doubt on the affinity of this species to Pentamerus.
In terms of shell shape and outline, relative convexity of the ventral and dorsal valves, and the configuration of ventral and dorsal umbo and beak, P. gothlandicus is nearly identical to the type species Pentamerus oblongus Sowerby, 1839, although the Gotland species generally has considerably larger shells than other known species of the genus on average (Fig. 1.1–1.5). In subsequent studies, Bassett and Cocks (1974) and Bassett
(1977) provided two important clarifications on this species: first, P. gothlandicus is confined to the upper Wenlock Slite beds or its coeval strata on Gotland; second, the semi- transparent shell reveals two discrete inner hinge plates (i.e., outer brachial plates in old usage) at their junctions with the dorsal valve floor. The latter point, in particular, was used by Bassett (1977) as a convincing basis for confirming the identity of P. gothlandicus as Pentamerus. After several recent revisions of Pentamerus (for a
summary, see Boucot and Johnson, 1979; Sapelnikov, 1985; Jin and Copper, 2000), most species now seem to occur in strata from mid-Aeronian to Telychian; only a small number of species are found in the Wenlock–Ludlow, notably in the Ural Mountains and in the Racine Dolomite of North America. In many cases, the preservation of purported Wenlock–Ludlow species of Pentamerus is rather poor, with shells commonly showing distortion, disarticulation, or break- age. This is true also for P. gothlandicus, which occurs predominantly as disarticulated or broken valves in skeletal packstone or grainstone facies, although conjoined or even complete shells can be found. Such preservation bias has undoubtedly contributed to the difficulty of taxonomic identifi- cations of these pentamerids. In this study, all the relatively well-preserved shells available from museum collections, supplemented by newly collected specimens, are assembled in an attempt to elucidate the morphology and taxonomic affinity of the Gotland species.
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