Vol.20 No.4

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One of the commonest explanations for the evolutionary radiation of animals during the Cambrian and Ordovician periods (about 541 to 443 million years ago) is the predation hypothesis. According to this widely cited but untested idea, the first rise of predatory animals would have accelerated evolution, and so diversification, by increasing natural selection on preys and starting predator-prey coevolutionary arms races. This predation-diversification hypothesis is here tested for the first time for the Cambrian—Ordovician radiation, by comparing the pace of origination of marine invertebrates and the dynamics of the predator ratio in fossil communities, at the local scale and genus level, focusing on the open shelf habitat (reefs excluded). Overall, origination rates decreased whereas the median predator ratio first increased up to a maximum at the middle—late Cambrian and then decreased more or less gradually during the Ordovician. Time-series analysis, using data differencing and detrending, shows that there is no positive correlation between origination rate and synchronic predator ratio over the Cambrian and Ordovician. Thus, the results do not provide evidence of any acceleration of origination rate driven by the rise of predation. Predatory animals might have contributed to the start of the Cambrian—Ordovician radiation by promoting defensive exoskeletons and infaunal lifestyles, but the results suggest they did not facilitate diversification in any other way.

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Twelve species of middle and late Campanian (Late Cretaceous) ammonoids, of which one is a newly described species, are reported from the Chinomigawa Formation of the Yezo Group in the Urakawa area, southern central Hokkaido, northern Japan. Furthermore, two ammonoid biozones, the Metaplacenticeras subtilistriatum and Baculites subanceps in ascending order, are recognized. Zircon radiometric ages reveal that the ages of tuffs immediately below the B. subanceps Zone are 75.1±0.9 Ma and 76.0±1.3 Ma respectively, which infer an early late Campanian age. Therefore, the Metaplacenticeras subtilistriatum and Baculites subanceps zones correlate with the upper middle Campanian and lower upper Campanian, respectively. The latter zone is probably a correlative of the lower part of the Didymoceras sp. Zone of the Izumi Group in Southwest Japan because of the discovery of the herein newly described species, Didymoceras hidakense Shigeta sp. nov. Since Didymoceras and Baculites subanceps flourished in other regions during late middle Campanian time, their occurrence in the Urakawa area suggests that they extended their geographic distribution from other areas to the Northwest Pacific region in early late Campanian time.

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This paper describes a Kungurian (early Permian) brachiopod fauna consisting of 16 species in 16 genera from the lower part of the Nabeyama Formation of Ogama in the Kuzu area, Ashio Mountains (Mino Belt), central Japan. The fauna includes a new species, Acritosia ogamensis. The Ogama fauna is a tropical Panthalassan fauna showing strong affinity with the Wolfcampian—Leonardian (early Permian) fauna of West Texas, USA.

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Pleurotomaria yokoyamai Hayasaka is a remarkably large gastropod commonly found in the middle Permian allochthonous Akasaka Limestone of central Japan. Its supraspecific systematics have however been unsettled. Examination of a museum collection and newly obtained material reveal previously unreported shell characteristics such as the presence of a long labral slit in the outer lip and a slit-like umbilicus surrounded by an accretionally growing callus pad. On the basis of these and previously known shell characters, a new genus Nipponomaria is proposed. We include Pleurotomaria? anatolica Enderle from the Permian anthracolithic limestone of northwestern Turkey as a member of this new genus.