LAUSR

Significance With geological sea-level fluctuations driven by climate change, the distribution of mangrove forests has expanded and contracted through time. We studied an inland, isolated mangrove forest located 170 km away from the nearest coastline in the interior of the rainforests of the Yucatan Peninsula (Mexico). Combining multiple lines of evidence, we demonstrate that this extant forest is a relict from a past, warmer world when relative sea levels were 6 to 9 m higher than at the present. Our finding highlights the extensive landscape impacts of past climate change on the world’s coastline and opens opportunities to better understand future scenarios of relative sea level rise. Climatic oscillations during the Pleistocene played a major role in shaping the spatial distribution and demographic dynamics of Earth's biota, including our own species. The Last Interglacial (LIG) or Eemian Period (ca. 130 to 115 thousand years B.P.) was particularly influential because this period of peak warmth led to the retreat of all ice sheets with concomitant changes in global sea level. The impact of these strong environmental changes on the spatial distribution of marine and terrestrial ecosystems was severe as revealed by fossil data and paleogeographic modeling. Here, we report the occurrence of an extant, inland mangrove ecosystem and demonstrate that it is a relict of the LIG. This ecosystem is currently confined to the banks of the freshwater San Pedro Mártir River in the interior of the Mexico–Guatemala El Petén rainforests, 170 km away from the nearest ocean coast but showing the plant composition and physiognomy typical of a coastal lagoon ecosystem. Integrating genomic, geologic, and floristic data with sea level modeling, we present evidence that this inland ecosystem reached its current location during the LIG and has persisted there in isolation ever since the oceans receded during the Wisconsin glaciation. Our study provides a snapshot of the Pleistocene peak warmth and reveals biotic evidence that sea levels substantially influenced landscapes and species ranges in the tropics during this period.