日本における板皮類化石の発見

抄録

Placodermi remains were discovered from the Fukuji Formation (Lower Devonian) in the Fukuji district, Gifu Prefecture, Central Japan. One of the authors, Mr. Okura discovered some dermal armours of an acanthothoracid placodermi from the D_4 Bed of the Fukuji Formation. Two specimens were identified from their morphological features as the right shoulder girdles of Romundina sp. indet., Palaeacanthaspidae. The locality is situated in the Hida Marginal Belt (Fig. 1), and the stage of the Fukuji Formation is Lower Gedinian to Upper Emsian. The specimen A is composed of a spinal plate and a part of the anterior ventro-lateral plate (Fig. 2 , Plate I ), and the specimen B is composed of a spinal plate, an anterior ventro-lateral plate, a part of the anterior lateral plate, antero-ventral plate and an intero-lateral plate (Fig.3, Plate II). In the shoulder girdle, the spinal plate, the anterior lateral plate and the anterior ventro-lateral plate are fused together, so that the sutures between them were obscured. The spine is one third of the total length of the spinal plate, and it curves inward at the tip. The anterior ventro-lateral plate is narrower than that in arthrodires. The dermal armour is ornamented with many tiny stellate tubercles. These tubercles become somewhat long towards the apex of the spine, but the row of small spines on the lateral-margin of the spinal plate as in Palaeacanthaspis (Brotzen,1934) was not observed. These features are almost in accordance with those of acanthothoracids. Specially the form of the spinal plate and the stellate tubercles bear a close resemblance to that of the genus Romundina (0rvig, 1975) among the seven genera of Palaeacanthaspidae. However, the tubercles aremore numerous than those of R. stellina and the spines are longer and curved inward. Therefore, we described them as Romundina sp. indet., but further discoveries of the other parts of the dermal armour may make it clear whether they should be classified as a new species or new genus. Histological structure was observed in a transverse ground section of the dermal armour (Plate III). It is composed of central cartilaginous tissue, osseous tissue like the spongy bone and superficial dark coloured tissue of the tuberculum. The distribution of acanthothoracids has been limited to the Lower Devonian (Gedinian-Emsian) in central and eastern Europe, central Asia, north Siberia, Arctic Canada and southeast Australia. Therefore, the authors consider that this discovery is very important for the investigation not only of the evolution of acanthothoracid placodermi but also of the paleobiogeographic relationships between the Hida Marginal Belt of Japan and other areas of the world in the age of Early Devonian. This paper is the first report of Placodermi found in Japan, it is possible therefore that many kinds of primitive fishes may be found from the Paleozoic era of the Japanese Islands in the future.