The megathermal ant genus Gesomyrmex (Formicidae: Formicinae), palaeoindicator of wide latitudinal biome homogeneity during the PETM

The Palaeocene-Eocene Thermal Maximum (PETM) is one of the most important Cenozoic climatic events shaping modern biodiversity, yet reconstructions of its palaeobiomes remain controversial. Here we describe Gesomyrmex gallicus sp. nov., a new, extinct species of the ant genus Gesomyrmex Mayr, 1868, based on minor and major workers preserved in the early Eocene amber of Oise, France. Although there are only seven known extant species in the genus, all confined to warm and humid forests of SE Asia, the fossil record of Gesomyrmex indicates that the genus once radiated across Eurasia. The new species therefore provides direct evidence that this radiation likely co-occurred with the PETM. This observation constrains palaeoclimatic reconstructions of the Early Eocene by requiring the presence of an extensive, homogeneous and interconnected rainforest-like biome across palaeo-Eurasia, a scenario otherwise corroborated by pollen assemblages. The phases of regression of warm and humid forests and the widening of dry biomes probably occurred later during the Eocene, in between hyperthermals, and were probably less extensive than some computer models predicted.

Automated summary

This study describes a new extinct species of the ant genus Gesomyrmex, based on preserved workers in early Eocene amber from Oise, France. The fossil record suggests that the genus once had a wide distribution across Eurasia and likely co-occurred with the Palaeocene-Eocene Thermal Maximum (PETM) event. This observation provides evidence for the presence of an extensive rainforest-like biome in palaeo-Eurasia during the Early Eocene, which is supported by pollen assemblages. The regression of warm and humid forests and the expansion of dry biomes likely occurred later during the Eocene and may have been less extensive than predicted by computer models.