Tropical seagrasses reached Patagonia during Miocene times

We report unequivocal evidence of seagrasses preserved in radiometrically constrained sediments (20.5 Ma) from Patagonia (Gaiman Formation), substantially extending their known palaeogeographic distribution. The new morphospecies Thalassotaenia notophyllum sp. nov. was erected based on well-preserved leaf blades that exhibit characters phylogenetically close to the genus Thalassia (Hydrocharitaceae), today distributed predominantly in tropical regions. Exquisitely preserved epibionts—diatoms, bryozoans, polychaetes, and coralline algae—occurred on these Miocene leaves, indicating that seagrass-dominated ecosystems were well established at high South American latitudes. Our finding implies that average sea surface temperatures in Patagonian coastal waters during the early Miocene were substantially warmer than today. The warm conditions are also supported by the presence of marine and terrestrial palynomorphs recovered from the seagrass-bearing sediments. Since seagrass ecosystems rank amongst the most important natural carbon sinks on earth, our discovery from Patagonia provides evidence that the wider past extension of seagrasses may have had a more important role in the Cenozoic carbon cycle than previously thought.