Abstract
Massive biotic change occurred during the Eocene as the climate shifted from warm and equable to seasonal and latitudinally stratified. Mild winter temperatures across Arctic intercontinental land bridges permitted dispersal of frost-intolerant groups until the Eocene-Oligocene boundary, while trans-Arctic dispersal in thermophilic groups may have been limited to the early Eocene, especially during short-lived hyperthermals. Some of these lineages are now disjunct between continents of the northern hemisphere. Although Eocene climate change may have been one of the most important drivers of these ancient patterns in modern animal and plant distributions, its particular events are rarely implicated or correlated with group-specific climatic requirements. Here we explored the climatic and geological drivers of a particularly striking Neotropical-Oriental disjunct distribution in the rove beetle Bolitogyrus, a suspected Eocene relict. We integrated evidence from Eocene fossils, distributional and climate data, paleoclimate, paleogeography, and phylogenetic divergence dating to show that intercontinental dispersal of Bolitogyrus ceased in the early Eocene, consistent with the termination of conditions required by thermophilic lineages. These results provide new insight into the poorly known and short-lived Arctic forest community of the Early Eocene and its surviving lineages.
Highlights
Following the rise of angiosperm plants during the Cretaceous Terrestrial Revolution[1], the most biologically significant event during the Cenozoic was arguably the transition of a widespread and equable ‘hothouse’ climate to a largely seasonal ‘icehouse’ climate’, varying strongly with latitude[2]
As the climate polarized and the Arctic rainforests transitioned to temperate over the late Eocene and Oligocene[2], the thermophilic and frost intolerant lineages went extinct over much or all of their Eocene distribution but some retreated toward the equator and survive today in single or multiple refugia, disjunct between continents of the northern hemisphere[10]
Based on the divergence time estimate for its Neotropical and Oriental clades, intercontinental dispersal of Bolitogyrus ceased during the early Eocene, far before the end of mild winters (CMMT > 5 °C)[2] on Arctic land bridges during the late Eocene[2,12]
Summary
Following the rise of angiosperm plants during the Cretaceous Terrestrial Revolution[1], the most biologically significant event during the Cenozoic was arguably the transition of a widespread and equable ‘hothouse’ climate to a largely seasonal ‘icehouse’ climate’, varying strongly with latitude[2] Just before this transition, mild winters during the Eocene and the presence of high-latitude land bridges allowed an Arctic rainforest and its associated fauna to extend across continents, up to 76–78°N paleolatitude[3,4]. Mild winters during the Eocene and the presence of high-latitude land bridges allowed an Arctic rainforest and its associated fauna to extend across continents, up to 76–78°N paleolatitude[3,4] This community, known widely as the ‘boreotropics’, was a unique mixture of frost-intolerant groups such as palms and crocodilians that occur only in the paratropics and tropics, and frost-tolerant lineages such as the alders that today form elements of temperate forests[3,5,6]. We assess the role of major Eocene geological and climatological events in the intercontinental dispersal of and subsequent divergence within Bolitogyrus using a novel integration of evidence from Eocene fossils, a comprehensive occurrence dataset, paleoclimate reconstructions and fossil-calibrated divergence dating
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