in Jurassic and Cretaceous Stratigraphy
in Jurassic and Cretaceous Stratigraphy
in Jurassic and Cretaceous Stratigraphy
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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321<br />
sp. We found some molluscan fossils (the bivalves<br />
Kolymonectes mongkensis, Ryderia textulata, Palmoxy-<br />
toma cygnipes, Pleuromya sp., <strong>and</strong> Goniomya sp., <strong>and</strong><br />
the ammonoids Fontanelliceras sp. <strong>and</strong> Amaltheus? sp.)<br />
<strong>in</strong> the lower part of the Higuchi- dani Formation.<br />
Kolymonectes, Palmoxytoma, Radulonectites, <strong>and</strong><br />
Agerchlamys were bipolar <strong>in</strong> distribution dur<strong>in</strong>g the<br />
Early <strong>Jurassic</strong>, <strong>and</strong> did not extend to low palaeo-<br />
latitudes (Damborenea, 1993, 1998, 2002). Specifically,<br />
Kolymonectes was common <strong>in</strong> the northern polar region<br />
(= Boreal region; northeastern Russia <strong>and</strong> northern<br />
Canada), though Kolymonectes weaveri was excep-<br />
tionally found <strong>in</strong> western Argent<strong>in</strong>a (Damborenea,<br />
1998). The Lower <strong>Jurassic</strong> S<strong>in</strong>emurian? to Pliens-<br />
bachian Higash<strong>in</strong>agano Formation, Toyora Group,<br />
conta<strong>in</strong>s abundant corals (Chomatoseris cyclolitoides,<br />
Hispaniastraea cf. ramosa, <strong>and</strong> Act<strong>in</strong>astrea sp.) <strong>in</strong><br />
calcareous s<strong>and</strong>stone. The bivalve assemblages of this<br />
formation conta<strong>in</strong> Tethyan <strong>and</strong> Boreal species, <strong>in</strong>-<br />
clud<strong>in</strong>g Tutcheria itoi, Prosogyrotrigonia <strong>in</strong>ouyei,<br />
Sphaeriola nipponica, Card<strong>in</strong>ia toriyamai, Plagio-<br />
stoma matsumotoi, Chlamys textoria, Oxytoma<br />
<strong>in</strong>equivalve, Entolium cf. calvum, Entolium cf. lunare,<br />
Plagiostoma kobayashii, Lopha sazanami, Liostrea<br />
toyorensis, Ctenosteron japonicum, <strong>and</strong> Palmoxytoma<br />
cf. cygnipes (Hayami, 1962). Therefore, the bivalve<br />
assemblage of the Higuchi Group is typical of the<br />
Boreal biotic realm, <strong>and</strong> the benthic assemblages of the<br />
Toyora Group are probably a transitional fauna be-<br />
tween the Tethyan <strong>and</strong> Boreal biotic realms.<br />
Key words: Bivalves; Boreal fauna; Southwest<br />
Japan; Lower <strong>Jurassic</strong><br />
References:<br />
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Pect<strong>in</strong>aceans <strong>and</strong> circum Pacific palaeobiogeo-<br />
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Palaeoecology, 1993, 100: 109-123.<br />
Damborenea S.E. The bipolar bivalve Kolymonectes <strong>in</strong><br />
South America <strong>and</strong> the diversity of propeau-<br />
mussiidae <strong>in</strong> Mesozoic Time. In: Johnston P.A.,<br />
Haggart J.W. (ed). Bivalves: An Eon of Evolu-<br />
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155.<br />
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