[B-CG05] Frontier in diversity and ecology of protists and microfossils

[BCG05-P05] Characteristics on Cretaceous radiolarian fauna and shell structure from a high-latitudinal area

SYUNKI Kawano², *Rie Hori, S.¹, Chris J. Hollis³ (1.Department of Earth Science, Faculty of Science, Ehime University, 2.Natural Science programe, Graduate School of Science and Engineering, Ehime University, 3.Victoria University of Wellington, NZ)

Keywords:Radiolaria, Cretaceous, high-latitude, Thanarla brouweri

High-latitude radiolarians have been shown to differ in assemblage composition and shell morphology between low and high latitudes based on studies of radiolarian fossils from Cretaceous strata which had been previously studied mostly in samples from the Northern Hemisphere (ex. Empson-Morin 1984), and there are fewer documents of radiolarian fossils from sedimentary rocks on land in the Southern Hemisphere. In order to clarify the differences in shell morphology between latitudes, this study compared the radiolarian fauna from pelagic sedimentary rocks in Cape Palliser, North Island, New Zealand, which is possibly considered to represent the relatively higher latitudes of the Mesozoic Cretaceous strata in the Southern Hemisphere based on geologic constrains (ex. Mortimer 2019), with those from the lower latitudes in Paleo-Pacific. The age of radiolarian fossils obtained from the Cape Palliser is considered as early Cretaceous (Aptian?) however, it is not yet fixed because of numerous unidentified species. We focused on differences in shell morphology among latitudinal zones based on measurements of shell thickness and size. Thanarla brouweri, which a cosmopolitan Cretaceous index species. We examined shell size and thickness of T. brouweri from the Cape Palliser sample and compared with those of species obtained from the low latitudinal areas. Based on these studies, the following results are obtained, 1) high-latitude individuals have thicker shells than low-latitude individuals, 2) high-latitude individuals often have larger shells than low-latitude individuals, and 3) low-latitude fauna have small individual differences in shell size, while high-latitude one have significant individual differences in shell size. These results are inferred to be general feature between low-latitude radiolarian species and high-latitude ones among nassellarian shells.