Chitinozoans and scolecodonts from the Silurian and Devonian of Japan

Abstract

Silurian and Devonian chitinozoans and scolecodonts are recorded from strata of the Hida‐Gaien Terrane, central Honshu. Silurian chitinozoans include Eisenackitina, Bursachitina, and the species Angochitina elongata. The latter provides a precise biostratigraphical tie between the Japanese succession and the Type Ludlow Series of the Welsh Borderland, UK, and indicates a Ludlow age (Gorstian or early Ludfordian) for the upper member of the Yoshiki Formation in the Ichinotani Valley. Chitinozoans from other Yoshiki Formation localities contain other specimens of Lagenochitinidae. Scolecodonts are more common than chitinozoans in the palynological residues, but are mostly represented by fragments or minor apparatus elements with a low biostratigraphical value. However, material from the Yoshiki and Fukuji Formations includes several species of Mochtyella as well as representatives of Oenonites, Kettnerites, Lunoprionella, Vistulella? and possibly other placognath taxa. Scolecodonts are also present in the Silurian Middle Member of the Gionyama Formation, Kyushu, including a well‐preserved jaw of Pistoprion, and fragments of putative Kettnerites and Oenonites. These finds of scolecodonts suggest close similarity in assemblages between Paleozoic polychaete faunas of Japanese terranes and those of Baltica‐Laurentia.

Japan's earliest ostracods

“…This age determination is supported by recent evidence from chitinozoans recovered from the Yoshiki Formation in the Ichinotani Valley, Fukuji area of the Hida‐Gaien Terrane, which represents a laterally equivalent unit to the ‘Upper’ Hitoegane Formation at Hitoegane. The presence of Angochitina elongata in the Upper Member of the Yoshiki Formation correlates the Japanese succession to Ludlow Series (Gorstian or lower Ludfordian) successions of the Welsh Borderland, UK (Vandenbroucke et al, ).…”

“…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.…”

“…It was previously noted that a pan‐tropical signature is known from other Silurian macrofossil and microfossil groups from Japan (see Williams et al, ), groups that have relatively wide dispersal capacity, such as radiolarians (plankton) and corals (which have a pelagic larval stage). 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, ).…”

“…Similarly the scolecodont associates (Figure m) are mostly represented by fragments or minor elements with low biostratigraphical value. The scolecodont Pistoprion is common from the mid‐Ordovician to Ludlow, and a few questionable specimens are also found in Devonian rocks (Vandenbroucke et al, ).…”

The paleobiogeographical significance of the Silurian and Devonian trilobites of Japan

“…Steeman et al, ). However, the Japanese succession is so far devoid of graptolites, and biostratigraphically significant chitinozoans have only now been reported (Vandenbroucke et al, 2018). The biostratigraphical assignment of different formations also depends on the interpretation of shelly faunas, or on the intermittent and serendipitous occurrence of biostratigraphically significant conodonts and radiolarians (e.g.…”