[M-IS13] New frontiers in geology

[MIS13-05] Possibilities of Ostracoda for reconstruction of palaeogeography and palaeoenvironment

*Gengo Tanaka¹ (1.Kumamoto University)

Keywords:Phanelozoic, Ostracoda, Functional Morphology, Palaeoenvironment, Palaeogeography

Ostracoda is a kind of crustaceans with two strongly calcified valves. They have been reported from many sedimentary rocks around the world since the Ordovician as microfossils. In particular, marine and brackish-water species are considered to be one of the important taxonomic groups or the reconstruction of palaeogeography because they are endemic to each region because they live benthic life throughout their lives. Our laboratory is conducting (palaeo-) biogeographical research in Japan from the Palaeozoic to Recent in collaboration with international researchers. Ostracoda have many characters on their shells, some of which are highly genetically constrained, some of which show great plasticity due to environmental changes, and others of which are subjected to both genetic and environmental effects. The characters with strong genetic constraints include the position and number of sensory pores. In marine taxa, this character is stable within the population of a species, so it can be used for species identification, and it is possible to estimate the place of origin of the species and its migration or dispersal route using extant and fossil species. In this session, I will introduce the origin and dispersal route of a shallow marine taxa, Loxoconcha japonica species group, which has been abundant as a fossil since the late Cenozoic and inhabit around East Asia. By focusing on the stable characters, it is possible to estimate the dispersal paths and geographical distributions of even extinct taxa. As such examples, I report on the study of Palaeogene and Devonian shallow-water and estuarine species in Japan. Palaeoenvironmental reconstruction, on the other hand, relies heavily on extant species-based analyses using Recent analogue methods and shell trace elements / stable isotope ratios. In this session, I will discuss methods for estimating palaeoenvironment that can be applied to fossil species. Functional morphological approach of some characters preserved as fossils are convenient and useful. In particular, ostracod eyes have excellent light-gathering organs called naupliar eyes that sence only light. The quantification of the shape of the naupliar eye, especially its eye tubercle which is a part of naupliar eye and is preserved as fossil on shell, is making it possible to quantify the light intensity and water depth under the seafloor. The position and number of the sensory pores in ostracods are genetically stable, but some sensory pores are surrounded by small pore openings called ‘sieve’, these ‘sieve-type pore canals’ have been known from Carboniferous onward, and previous researchers have interpreted the sieve-type pore canals’ accepting some kind of environment. In our laboratory, we focused on the homologous ‘sieve-type pore canals’ in the right shells of a shallow living species Cytheromorpha acupunctata, widely distributed in the Ariake Sea, and preliminarily investigated whether the porosity of ‘sieve-type pore canals’ correlated with environments. As a result, it was found that the porosity of ‘sieve-type pore canals’ is related to temperature, dissolved oxygen and pH. By conducting culturing experiments which changing these three environmental parameters, it is expected that the ‘sieve-type pore canals’ will serve as indicators of the bottom water environments. In the near future, it will be possible to reconstruct the Palaeozoic and Mesozoic marine bottom environments with high accuracy by comparing and examining these morphologically highly dependent characters with environmental factors.