A Late Pleistocene coastal ecosystem in French Guiana was hyperdiverse relative to today

Warmer temperatures and higher sea level than today characterized the Last Interglacial interval [Pleistocene, 128 to 116 thousand years ago (ka)]. This period is a remarkable deep-time analog for temperature and sea-level conditions as projected for 2100 AD, yet there has been no evidence of fossil assemblages in the equatorial Atlantic. Here, we report foraminifer, metazoan (mollusks, bony fish, bryozoans, decapods, and sharks among others), and plant communities of coastal tropical marine and mangrove affinities, dating precisely from a ca. 130 to 115 ka time interval near the Equator, at Kourou, in French Guiana. These communities include ca. 230 recent species, some being endangered today and/or first recorded as fossils. The hyperdiverse Kourou mollusk assemblage suggests stronger affinities between Guianese and Caribbean coastal waters by the Last Interglacial than today, questioning the structuring role of the Amazon Plume on tropical Western Atlantic communities at the time. Grassland- dominated pollen, phytoliths, and charcoals from younger deposits in the same sections attest to a marine retreat and dryer conditions during the onset of the last glacial (ca. 110 to 50 ka), with a savanna- dominated landscape and episodes of fire. Charcoals from the last millennia suggest human presence in a mosaic of modern-like continental habitats. Our results provide key information about the ecology and biogeography of pristine Pleistocene tropical coastal ecosystems, especially relevant regarding the-widely anthropogenic- ongoing global warming. Significance The Last Interglacial interval (128 to 116 ka) is a remarkable deep- time analog for temperature and sea- level conditions as projected for 2100, that had not been documented in the equatorial Atlantic thus far. Here, we report hyperdiverse fossil communities of coastal marine and mangrove affinities, dating back from this interval and unearthed at the Europe's Spaceport in Kourou, French Guiana. Mollusk assemblages suggest stronger ecological affinities between Guianas and the Caribbean than today. Grassland- dominated pollen, phytoliths, and charcoals from younger deposits in the same sections attest to a marine retreat and dryer conditions during the Last Glacial Period (100 to 50 ka). These records provide key ecological and biogeographic information about Late Pleistocene tropical coastal ecosystems prior to human influence.