The Ordovician and Silurian conodonts of Japan: Their biostratigraphical and paleobiogeographical significance

Abstract

Conodont‐bearing Ordovician and Silurian rocks have a wide geographical distribution in Japan. They are exposed on Honshu, Shikoku, and Kyushu islands. Two recently studied samples from Gionyama, in Kyushu, yield conodont assemblages of Telychian (Distomodus sp., Apsidognathus sp. (cf. A. tuberculatus arcticus), Oulodus cf. rectangulus, Ozarkodina cf. waugoolaensis, Panderodus cf. amplicostatus) and probably the earliest Sheinwoodian (?Distomodus sp. (cf. D. staurognathoides), Ozarkodina sp., Oulodus sp., Walliserodus sp. (cf. Walliserodus sp. nov. c Männik), Panderodus sp.) ages, providing further biostratigraphical constraints for the age of the Gionyama Formation in the Kurosegawa Terrane. Most of the conodont taxa identified have wide paleobiogeographical distribution, but probable occurrence of Oulodus rectangulus, Ozarkodina waugoolaensis, and Panderodus amplicostatus suggests a relationship between Australia, the South China paleocontinent, and the Kurosegawa Terrane of ‘Proto‐Japan’ during the Silurian. These paleogeographic interpretations agree with the existing notions based on rugose corals.

Japan's earliest ostracods

“…One sample obtained from a ‘loose boulder’ (sample GY‐1) yielded conodonts indicative of the Telychian Stage, Llandovery Series. A probable earliest Sheinwoodian Stage, Wenlock Series age is suggested from conodonts recovered from another sample from outcrop (GY‐2; see below and Männik et al, ) that also contained ostracods featured herein.…”

“…Each sample yielded several tens of mostly fragmentary and also some complete ostracod valves. Sparse conodonts and scolecodonts were also recovered (see Männik et al, ; Vandenbroucke et al, ). The specimens are extremely fragile and in almost all cases the ostracods still retain some matrix on the valve surfaces that is impossible to remove.…”

“…Walliserodus sp. nov. C of Männik; see Männik, , Figure ) suggests a lower or middle Wenlock age (Männik et al, ) for at least part of the Middle Member (sensu Kido & Sugiyama, ) of the Gionyama Formation. In the Baltic region Walliserodus sp.…”

“…(cf. D. staurognathoides ), suggests that the sample might be as old as earliest Sheinwoodian (Männik et al, ).…”

“…Moreover, scolecodonts present in the Gionyama Formation of Kyushu, including Pistoprion (sample GY‐2 herein), suggest similarity between Paleozoic polychaete faunas of Japanese terranes and those of Baltica–Laurentia and a mid‐Silurian chitinozoan species from the Yoshiki Formation of the Hida‐Gaien Terrane is also present in the microcontinent of Avalonia (Vandenbroucke et al, ). Conodonts from the Silurian of Japan variously hint at contacts with Laurentia, Baltica, Severnaya Zemlya, and south China (Männik et al, ). In the Silurian Baltica, Laurentia and the closely adjacent Avalonia are positioned many thousands of kilometers distant from the terranes of Japan, across the Rheic ocean to the west (Cocks & Torsvik, ; Torsvik & Cocks, , ); in this case a strategy (e.g.…”