Past warming climates promoted expansion of seagrasses to high latitudes

Seagrasses form one of the most productive ecosystems in the world’s oceans. Here, we evaluated whether the distribution of seagrass fossils was geographically constrained by past global temperatures. We hypothesized that ancient seagrasses reached higher latitudes during warm epochs compared to their extant relatives. Fossils of both the Cymodoceaceae ‘complex’ and Hydrocharitaceae occur at latitudes similar to the maximum latitudinal limits of extant seagrasses, and significantly higher than their latitudinal midpoints. For Zosteraceae, however, the latitudinal midpoints of extant species are similar to those of fossils, since this is a relatively young lineage that originated in cooler waters. For both the Cymodoceaceae ‘complex’ and Zosteraceae, global temperatures over time had a positive effect on the latitudinal distribution of fossils, such that warmer epochs facilitated poleward expansion. In conclusion, the paleobiogeography of seagrasses was coupled to long-term climate fluctuations, supporting the roles of niche conservatism and climate tracking in their distribution. Seagrass paleobiogeography closely tracked long-term climate warming, with warmer epochs enabling poleward expansion consistent with niche conservatism, based on latitudinal analyses of seagrass fossil distributions across paleoclimate belts.