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Middle Devonian Tabulate Corals from the Kamiarisu Formation, Iwate Prefecture, Japan

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Middle Devonian Tabulate Corals from the Kamiarisu Formation, Iwate Prefecture, Japan Bull. Natl. Mus. Nat. Sci., Ser. C, 45, pp. 13–18, December 27, 2019 Middle Devonian tabulate corals from the Kamiarisu Formation, Iwate Prefecture, Japan Shuji Niko Department of Environmental Studies, Faculty of Integrated Arts and Sciences, Hiroshima University, 1–7–1 Kagamiyama, Higashihiroshima, Hiroshima 739–8521, Japan Abstract A coral fauna was discovered from the lowest part of the Kamiarisu Formation in Iwate pre- fecture, Northeast Japan. Among the faunal constituents, two species of pachyporid tabulate corals, Gracilopora? sp. indet. and Thamnopora sumitaensis sp. nov., are described herein. The new species is diagnosed by its relatively slender corallites, abundant mural pores, well-developed septal spines in peripheral stereozone, and commonly developed tabulae. On the basis of similarities between the spe- cies and such previously known taxa as Thamnopora itoae Niko, Ibaraki and Tazawa, 2014, T. nichol- soni (Frech, 1885), and T. siavis Dubatolov in Dubatolov et al., 1959, the fauna is considered to be of Middle Devonian (probably Givetian) age. Co-occurrence of Gracilopora? sp. indet. will not contradict this conclusion. Although the Kamiarisu Formation has been assigned to the Silurian, its age should be changed upwardly by this new finding. Key words: pachyporid tabulate corals, Gracilopora, Thamnopora, Givetian, Southern Kitakami Belt, Northeast Japan Introduction locality. Judging from north-south to N 5°E strikes with nearly vertical dips of the shales and the During the field work in May, 2017, an abundant absence of disturbed stratification by fault between coral fauna was discovered from the Kamiarisu For- locality 1 and 2, these two localities probably situate mation (Tazawa et al., 1984) belonging within the the same stratigraphic horizon that represents the Southern Kitakami Belt, Northeast Japan. This dis- lowest part of the Kamiarisu Formation (Figure 2). covery represents the first record of fossil corals All specimens examined in this study are prefixed from the formation. Among the constituents of this NSMS and housed in National Museum of Nature fauna, the present paper focuses on relatively well- and Science, Tokyo. preserved tabulate corals and discusses their strati- graphic significance. Two localities from which the Systematic Paleontology material was taken are in the Kamiarisu area of Sumita-cho, Kesen-gun, Iwate Prefecture (Figure 1). Subclass Tabulata Milne-Edwards and Haime, 1850 Locality 1 occurs at latitude 39°15′9″N and longi- Order Favositida Wedekind, 1937 tude 141°41′43″E, where a forest road cuts fossilif- Suborder Favositina Wedekind, 1937 erous shale. Except for tabulates, fossils in this out- Superfamily Pachyporidae Gerth, 1921 crop are rugose corals, brachiopods, and bryozoans. Locality 2 occurs at latitude 39°15′17″N and longi- Family Pachyporidae Gerth, 1921 tude 141°41′44″E, where shale and intercalates of Genus Gracilopora Chudinova, 1964 thin sandstone layers are exposed on stream bank Type species: Gracilopora acuta Chudinova, 1964. near the junction of the Tuchikura-zawa and Kanno- zawa Valleys. The both rocks of Locality 2 yielded Gracilopora? sp. indet. rich fossils including brachiopods, bryozoans and (Figs. 3-A, B) crinoids, but tabulates ware obtained only from sandstone. Rugose corals were not detected in this Material examined: NMNS PA18684. Description: A single internal mold was available © 2019 National Museum of Nature and Science for study; it is ramose corallum formed by cylindri- 14 S. Niko Fig. 1. Index map showing fossil localities in the Kamiarisu area, Sumita-cho, Iwate prefecture, Northeast Japan, on the “Digital Japan Basic Map” published by Geospatial Information Authority of Japan. Fig. 2. Stratigraphic column of the Kamiarisu Forma- tion (alter Tazawa et al., 1984) showing position of the present tabulate corals. cal branches; branching frequent and probably bifurcate; branches slender, approximately 2–3 mm in diameter, and composed by numerous and gradu- phology also is suggestive of Egosiella Dubatolov in ally inflated corallites; diameters of corallites are Sokolov, 1955, but the Kamiarisu specimen does not very small, up to 0.5 mm; each corallite differenti- show anastomoses of adjoining branches that are the ated into proximal narrowly divergent portion and most diagnostic features of the latter genus. distal outwardly curved one; calical opening is per- pendicular to branch surface. Genus Thamnopora Steininger, 1831 Occurrence: Locality 1. Type species: Thamnopora madreporacea Steininger, 1831. Discussion: The present corallum is tentatively placed in Gracilopora on the basis of the possessions of slender branches composed by numerous coral- lites having very small diameters. Its corallite mor- Fig. 3. A, B, Gracilopora? sp. indet., NMNS PA18684. A, weathered surface, showing longitudinal section of branches, ×5. B, partial enlargements of Fig. 3-A, ×10. C–K, Thamnopora sumitaensis sp. nov. C, paratype, NMNS PA18649, external view of branch, silicon rubber replica, ×2. D, paratype, NMNS PA18653, external views of branches, silicon rubber replica, ×2. E, paratype, NMNS PA18655, longitudinal thin section, ×10. F, G, holo- type, NMNS PA18654. F, transverse polished section, ×5. G, longitudinal polished section, ×5. H, I, paratype, NMNS PA18650. H, external view of branch, silicon rubber replica, ×5. I, weathered surface, showing longitudi- nal and oblique sections of branches, ×2.5. J, paratype, NMNS PA18660, longitudinal polished section, ×5. K, paratype, NMNS PA18652, weathered surface, showing longitudinal sections of corallites, arrow indicates vestiges of septal spines, ×10. Devonian Corals from Iwate 15 16 S. Niko Thamnopora sumitaensis sp. nov. type series consisting of fifteen specimens. Except (Figs. 3-C–K) for a paratype (NMNS PA18655, Fig. 3-E), skele- tons of these specimens are dissolved and their Material examined: Holotype, NMNS PA18654. internal structures are preserved as “steinkern”. Paratypes, NMNS PA18649–18653, 18655–18663. Accordingly, darker parts in branches of the holo- In addition, 20 specimens, NMNS PA18664–18683, type (Figs. 3-F, G) and a paratype (NMNS were also assigned to this species. PA18660, Fig. 3-J) are acrylic resins injected into Diagnosis: Species of Thamnopora with traces of intercorallite walls and tabulae. 5–12 mm in usual diameters of branches and rela- The most closely related species to Thamnopora tively slender corallites having diameters of 0.2– sumitaensis sp. nov. is T. itoae Niko, Ibaraki and 2.1 mm; intercorallite walls attain approximately Tazawa (2014, pp. 61, 63, figs. 2-1–3, 4-1–6). They 0.8 mm in thickness; mural pores abundant; short show the similarities in the measurements of branch conical septal spines well-developed in peripheral diameters, intercorallite wall thickness, the mode of stereozone; tabulae common. occurrence of mural pores, and the possession of Description: Coralla ramose with cylindrical to well-developed septal spines at their peripheral rarely foliose branches, cerioid; branching common, zones. The principal differences between the two bifurcate or trifurcate; diameters of branches are rel- species are slightly smaller corallite diameters atively slender for the genus, 5–12 mm in cylindri- (approximately 1.4 mm in distal parts) and some- cal one; corallum diameter and growth form are what fewer development of tabulae in T. itoae. The unknown owing to fragile nature. Corallites pris- new species rather closely resembles T. nicholsoni matic, mostly 4–6 sided; there are approximately (Frech, 1885, pp. 104, 105) and T. siavis Dubatolov 50–70 corallites in transverse section of branch; in Dubatolov et al. (1959, pp. 22, 23, pl. 7, figs. each corallites differentiated into proximal portion 2a–e), but it differs from the latter two species prin- that runs nearly parallel with branch axis and out- cipally in having well-developed septal spines. wardly curved distal one; inflation of corallite is gradual in proximal and relatively rapid in distal Significance portions; diameters of corallites 0.2–2.1 mm, with 1.7 mm mean at calical rim; calices open obliquely Although Thamnopora ranges from Silurian to upward with approximately 40°–75° in angle for Devonian, most species of the genus have been branch surface; calical pits very deep; lateral recorded from Devonian strata (Hill, 1981; May, increases of new corallites frequently occur in axial 1997). Exceptional late Silurian species, such as T. zone of branch. Intercorallite walls weakly thick- senzaii Niko, 2003, and T. suberidaniensis Niko, ened, 0.13–0.21 mm in axial zone, then they 2003, possess slender branches, small diameters of increase in thickness up to approximately 0.8 mm corallites and thin intercorallite walls, whose primi- and form indistinct peripheral stereozone; walls tive characters are not recognized in T. sumitaensis consist of median dark line and stereoplasm; micro- sp. nov. from the Kamiarisu Formation. The new structure of stereoplasm is not preserved; tabularia species is closely related with T. itoae, T. nichol- (lumina) subcylindrical; mural pores abundant on soni, and T. siavis. The stratigraphic position of T. corallite faces, subcircular in profile, approximately itoae is unclear per se, because the species occurs 0.1–0.2 mm in diameter; septal spines common in from limestone pebbles belonging to Mesozoic con- axial zone and well-developed in peripheral ste- glomerate. However, some index species, such as reozone, short conical; tabulae common, complete. Thamnoptychia mana Niko and Senzai, 2010,
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