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The earliest freshwater colonisation by marine ostracods probably took place in the early Carboniferous. The first low-salinity habitats invaded by ostracods were likely ephemeral ponds near the shoreline, or fluvial, deltaic settings influenced by marine transgression events. The dominant factor in such environments is rapid change in salinity resulting from fluctuations in sea level, evaporation and precipitation, so the first freshwater colonisers are likely to have been tolerant euryhaline species. The most obvious adaptation to such environments is tolerance to low-salinity conditions. Modern studies report that ostracods possess efficient osmoregulatory mechanisms and are characterised by wide tolerance to different salinity regimes. They also exhibit other characteristics such as mixing reproductive mode, resting and/or desiccation-resistant eggs, and brood care which may facilitate colonisation of different regimes. Ostracods possess a calcified carapace containing high concentrations of calcium carbonate but shell formation may be less effective in poorly mineralised fresh waters. However, the mineral content of early Carboniferous coastal ponds was most probably sufficient to allow the development of calcified carapaces by the first freshwater species.
We describe an asymmetrical basal delphinoid skull from the upper lower Miocene Yamato Formation of Hokkaido, northern Japan. The skull shows clear cranial asymmetry: the nasal process of the left premaxilla is longer than that of the right one; the mesethmoid and frontals are left skewed 2.9°; and the right nasal is larger than the left one. Evaluation of the deformation of the fossil based on the carbonate content of the matrix indicates that the concretion in which the skull was found formed in an early stage of diagenesis and that the present specimen was not affected by compaction during diagenesis. A cladistic analysis including the new specimen shows cranial asymmetry among Delphinoidea extends back to the late early Miocene in the fossil record, and supports the hypothesis that cranial asymmetry in basal delphinoids is more common than previously thought. On the other hand, trait analyses suggest that the common ancestor of Delphinoidea had a symmetrical skull. We hypothesize that some extinct odontocetes that had symmetrical crania were able to produce narrow-band high-frequency clicks to avoid predation, as in extant symmetrical cranial species.
Microfossils abundantly occur in the upper part of the Pliocene Kuwae Formation in the Sakai stratigraphic section, Tainai City, northern Niigata Prefecture. We clarified the biostratigraphy and reconstructed the vertical paleoenvironmental changes based on micro fossils such as ostracodes, diatoms and planktonic foraminifers. As a result, the sequence in the Sakai section was divided into three parts based on lithology and correlated with the Neodenticula koizumii-N. kamtschatica diatom zone (NPD 8, 3.5-2.7 Ma). At least the lower and middle parts of the study sequence were assigned to the horizon below the datum of the rapid increase of N. koizumii (D85, 3.1-3.0 Ma). An index fossil planktonic foraminiferal species, Globorotalia inflata (s.l.), was abundantly found only in the middle to upper part of the study interval. This interval belonged to the No. 3 G. inflata bed, which has been used as a biozone in the Sea of Japan side during the Pliocene. Four ostracode bio associations were identified using R-mode cluster analysis. Based on ostracode analysis, the Kuwae Formation in the present study section was shown to have benn deposited in the sublittoral to upper bathyal zone and at least one cycle of water depth change was found. In addition, one new ostracode species, Hemicythere sakaiensis sp. nov., was described in this study.
Calyptraphorus is a distinctive stromboidean gastropod genus with its earliest fossil record from the Campanian (Cretaceous). The genus survived the K-Pg biotic crisis and was thought to have become extinct before the end of the Eocene. Calyptraphorus sp. is described on the basis of a single specimen recovered from the mid-Pliocene Tartaro Formation in northern Luzon, the Philippines. The specimen is similar to Eocene congeners recorded from the southeastern United States and Pakistan. It is left unnamed because the single available specimen is too incomplete for comparison with other known species. The significance of this discovery is that it extends the recorded stratigraphic range of the genus from the end of the Eocene to the Pliocene, leaving a hiatus of approximately 30 Myr, which classifies Calyptraphorus sp. as a Lazarus taxon. The genus apparently survived as a refugee taxon in the tropical Western Pacific, keeping the population density very low and/or the geographic distribution limited to small areas after the Eocene biotic crisis.
Anagaudryceras compressum sp. nov., the most slender of this genus, is newly described from the Nostoceras hetonaiense Zone of earliest Maastrichtian age (Late Cretaceous), in the Hobetsu area, Hokkaido, northern Japan. Its shell is fairly small (< 80 mm in diameter) and compressed with a highly arched venter. The last whorl is ornamented with low, broad, gently flexed band-like ribs. Its ornamentation is very similar to A. matsumotoi, thus suggesting a close phylogenetic relationship with this taxon, which is probably its descendant.