Tatsuhiko Yamaguchi

No place to retreat: Heavy extinction and delayed recovery on a Pacific guyot during the Paleocene–Eocene Thermal Maximum

Modern global change threatens alpine ecosystems by forcing species to migrate to higher elevations and potentially eliminating alpine habitat altogether. Here we show that an analogous restriction of suitable habitat operates on submarine mountains. During the Paleocene–Eocene Thermal Maximum (PETM, ca. 55.96 Ma), ostracodes underwent local extinction on the crest of Allison Guyot in the central Pacific Ocean, which lost 64% of its ostracode species richness (14 species reduced to three species) and as much as 94% of ostracode abundance for ∼1.1 m.y., before recolonization rebuilt biodiversity and abundance over the next 200 k.y. Biotic changes may reflect an increase in current speeds, acidification, and a decrease in food supply owing to a temperature-driven increase in metabolic rates. Notably, continental margin ostracodes also underwent extinction during the PETM (25%–38% loss) but, unlike Allison Guyot faunas, could quickly repopulate the continental slope. The absence of refugia for isolated seamounts prolonged the reduction in biodiversity initiated by the PETM, a pattern that may be expected for modern seamount faunas in an era of future global change.

Miocene Ostracodes from the Osaki Formation, Kukinaga Group, Tanegashima, Southwest Japan, and their Significance for the Biogeography of the Indo-West Pacific

Abstract. The Indo-West Pacific (IWP) benthos, which shows the highest diversity of marine organisms worldwide, is thought to have originated from the Tethys Ocean. Little is known about Miocene ostracodes in the IWP, that is, subsequent to the closure of the Tethys Ocean. To understand the paleobiogeography of Miocene ostracodes in the IWP, we examined ostracodes from the Middle Miocene—Lower Pliocene Osaki Formation, Kukinaga Group, at Tanegashima, Southwest Japan, revealing the presence of Neocytheromorpha, an IWP taxon that originated from South Australia. Neocytheromorpha occurs abundantly with Trachyleberis in Middle Miocene sublittoral deposits. This finding indicates that Neocytheromorpha migrated northward from South Australia, along the Australian coast, and settled in coastal areas of the West Pacific during the Oligocene—Middle Miocene. Because the migration coincided with diversification in the West Pacific and the invasion of corals and molluscs, it was synchronous with changes in the populations of the IWP biota. Two new species are systematically described: Acanthocythereis inouei sp. nov. and Neocytheromorpha priscipacifica sp. nov.

Early Eocene marine ostracods from the Crescent Formation, southwestern Washington State, USA

An assemblage of eleven species of Early Eocene marine intertidal and sublittoral ostracods has been found in the Crescent Formation in the Black Hills, Thurston County, Washington State, USA. The ostracods are subtropical and tropical taxa, such as Bairdoppilata, Cytherella and Neomonoceratina, which are found south of Washington today. Based on the geological age of the formation, the fauna corresponded to the Early Eocene Climatic Optimum. Five new species are recognized: Acanthocythereis olympiana sp. nov., Ambostracon irizukii sp. nov., Bairdoppilata crescentana sp. nov., Coquimba washingtonensis sp. nov. and Cytherelloidea squiresi sp. nov. This new assemblage from the Crescent Formation is the northernmost record of Early Eocene ostracods for the entire Pacific Basin.

A new late Eocene Bicornucythere species (Ostracoda, Crustacea) from Myanmar, and its significance for the evolutionary history of the genus.

The ostracode genus Bicornucythere (Ostracoda, Crustacea) is abundant in modern-day eutrophic marine bays, and is widely distributed in estuaries and inner bays throughout East Asia, including in China, Korea, Japan, and the Russian Far East. The evolutionary history of Bicornucythere is poorly understood. Here, we report on a new species of Bicornucythere (Bicornucythere concentrica sp. nov.) from the upper Eocene Yaw Formation in the Central Myanmar Basin. The oldest previously known Bicornucythere taxon, Bicornucythere secedens, was reported from lower Miocene strata in India, although a molecular phylogeny suggests that the genus first appeared in the Late Cretaceous. Bicornucythere concentrica sp. nov. is at least 10.9 million years older than the earliest known B. secedens. The new species occurs with Ammonia subgranulosa, a benthic foraminifer, an association that is representative of brackish water conditions in modern Asian bays. Our findings indicate that extant genera have inhabited Asian bays since the late Eocene. The paleobiogeography of Bicornucythere indicates that the taxon was dispersed onto Indian coasts during the collision between the Indian and Eurasian plates.

Sexual shape dimorphism and selection pressure on males in fossil ostracodes

Abstract. Sexual dimorphism is thought to have evolved via selection on both sexes. Ostracodes display sexual shape dimorphism in adult valves; however, no previous studies have addressed temporal changes on evolutionary timescales or examined the relationships between sexual shape dimorphism and selection pressure and between sexual shape dimorphism and juvenile shape. Temporal changes in sexually dimorphic traits result from responses of these traits to selection pressure. Using the Gaussian mixture model for the height/length ratio, a valve-shape parameter, we identified sexual differences in the valve shape of Krithe dolichodeira s.l. from deep-sea sediments of the Paleocene (62.6–57.6 Ma) and estimated the proportion of females in the fossil populations at 11 time intervals. Because the proportion of females in a population is altered by the mortality rate of adult males, it is reflective of selection pressure on males. We attempted to correlate the height/length ratios between the sexes with the proportion of females, taking into consideration that the valve shape was not linked with the selection pressure on males. In time-series data of the height/ length ratio, both sexes indicate no significant changes on evolutionary timescales, even though the sex ratio of the population changed from female skewed to male skewed during the late Paleocene. The sexual shape dimorphism was not driven by sexual selection. The static allometry between the height/length ratio and length indicates that the sexual shape dimorphismdid not function for sexual display. The absence of change over time in the female allometric slope suggests that the evolution of valve shape was constrained by stasis.