Changing surface ocean circulation caused the local demise of echinoid Scaphechinus mirabilis in Taiwan during the Pleistocene–Holocene transition

Abstract

Abundant fossil specimens of Scaphechinus mirabilis, now occurring mostly in temperate waters, have been found in the Toukoshan Formation (Pleistocene) in Miaoli County, Taiwan. Environmental changes leading to its extirpation (local extinction) have thus far been elusive. Here, we reconstruct past environmental and oceanic conditions off northwest Taiwan by analyzing clumped isotopes, as well as stable oxygen isotopes, of well-preserved fossil echinoid tests collected from the Toukoshan Formation. Radiocarbon dates suggest that these samples are from Marine Isotope Stage 3 (MIS 3). Paleotemperature estimates based on clumped isotopes indicate that fossil echinoids were living in oceanic conditions that range from 9 to 14 °C on average, comparable with the estimate derived for a modern sample from Mutsu Bay, Japan. Notably, this temperature range is ~ 10 °C colder than today’s conditions off northwest Taiwan. The substantially lower temperatures during ~ 30 ka (MIS 3) compared to the modern conditions might be due to the rerouting of surface currents off northwest Taiwan when the sea level was ~ 60 m lower than today, in addition to the cooling caused by a lower atmospheric CO2 level during the Last Glacial Period. Colder waters brought here by the China Coastal Current (CCC) and the existence of shallow subtidal zones termed “Miaoli Bay” (mainly located in the present-day Miaoli county) during MIS 3 plausibly sustained generations of S. mirabilis, yielding tens of thousands of fossil specimens in the well-preserved fossil beds. The likely extirpation driver is the drastic change from a temperate climate to much warmer conditions in the shallow sea during the Pleistocene–Holocene transition.

Phylogeny, ancestral ranges and reclassification of sand dollars

“…Regarding the distribution of modern Clypeasteroida and Scutelloida, the seas around Taiwan are unique due to mixing of surface ocean currents. The warm, Kuroshio Branch Current coming from the tropical zone and the cold, China Coastal Current coming from the temperate region meet in the Taiwan Strait 23 . Consequently, a mixture of warm- (e.g., Arachnoides placenta 24 ) and cold-water (e.g., Scaphechinus mirabilis 23 and Astriclypeus mannii ; Table 1 ) species occur in Taiwan.…”

“…The warm, Kuroshio Branch Current coming from the tropical zone and the cold, China Coastal Current coming from the temperate region meet in the Taiwan Strait 23 . Consequently, a mixture of warm- (e.g., Arachnoides placenta 24 ) and cold-water (e.g., Scaphechinus mirabilis 23 and Astriclypeus mannii ; Table 1 ) species occur in Taiwan. Based on the global distributions of sand dollars sensu lato, three biogeographic patterns seem to intersect at this area (Fig.…”