A coral hotspot from a hot past: The EECO and post-EECO rich reef coral fauna from Friuli (Eocene, NE Italy)

The early Paleogene “greenhouse” is the warmest geological period of the Cenozoic and a suitable source to unlock crucial information for better understanding how ecosystems such as coral reefs reacted to a climate much warmer than the present. Herein, we analyse in detail the rich museum coral collections from Friuli region (NE Italy) focusing only on the so-called reef corals, mostly colonial, and on those localities for which the age constraints are well-defined. We grouped the localities into four time-intervals within the Ypresian-Lutetian and thus within EECO (Early Eocene Climatic Optimum) and post-EECO phase, in order to assess the main changes in the following traits: 1) richness and relative abundance at the generic and species level; 2) relative abundance of growth forms; 3) colony size. By systematic revision we recognized a total of 37 genera and 103 species. During the EECO interval (late Ypresian or Cuisian), the diversity at both genus and species level was relatively low (21 genera, 37 species). An abrupt increase is then recorded in the post-EECO 1 interval (latest Ypresian), whereas a peak is reached in the earliest Lutetian (post-EECO 2) (35 genera, 90 species). These results undoubtedly show that a coral hotspot occurred in the region during the late Ypresian-early Lutetian time and highlight that corals of Friuli reacted positively immediately after the warming event of the EECO. This coral hotspot was the most diverse at global scale during the Ypresian and formed in a deltaic depositional setting, most probably in mesophotic conditions as suggested by the dominant growth forms, where rapid sediment accumulation, enhanced nutrient delivery and stressful climatic conditions reduced frame-building capacity. We finally document that corals were resilient to climate stressors and recovered before reefs which, in contrast, required much more time to achieve their buildup potential.