Small beetles provide insights about ancient arctic rainforests
An interdisciplinary study of rove beetles suggest extensive rainforest in the Arctic in the early Eocene. The rainforest was short-lived but essential for an exchange of species between tropical biotas of Americas and Eurasia that ceased with the climate cooling.
Eocene palaeoclimate, especially its warmer 'hothouse' periods, is one of the unresolved scientific puzzles. Much effort is spent in reconstructing Earth climate and landscapes that existed 50-30 million years ago because this helps to understand conditions today. Many ‘strange’ features of the mountains or plains, forests or steppes, rivers or lakes, birds, flowers and insects that we see today, can be explained by the past warmer or colder periods in Earth history where these nature elements stem from.
In particular geographic distributions of some organismal groups widely separated between remote areas of different continents are hinting to something puzzling in their history. Evolutionary theory says that an organismal group must have a certain geographically restricted point of origin. In this light, disjunct distributions like in the rove beetle genus Bolitogyrus, where one chunk of species is restricted to the tropics of Central and South America and another to the tropics of South-East Asia (Fig. 1), needs an explanation. Did the genus originate in Central America and then some species somehow ‘jumped’ to South-East Asia and further evolved there, or vice versa? Something like this is difficult to imagine for small beetles living in dead logs hidden in the shade of a rainforest.
Adam Brunke, a former PhD student of the Natural History Museum of Denmark, and his supervisor and beetle collection curator Alexey Solodovnikov explored this puzzling case in detail together with international collaborators. Brunke et al. studied beetle collections in numerous museums to find and identify every existing species of this genus. They travelled to the American and South-East Asian tropics to sample more specimens, especially for the DNA-analysis.
It turned out that the genus is much more species-rich than known before and many species new to science have been discovered. Using DNA sequences, the researchers conducted a phylogenetic analysis that robustly showed that both American and Asian stocks of Bolitogyrus indeed evolved from a common ancestor and surely were geographically connected in the past.
Even fossil specimens of stem Bolitogyrus from North America and Europe were discovered (Fig. 1) and thoroughly examined with the µCT scanner. The identity of these fossils spoke even more for a geographic connection in the past. Using a molecular clock calibrated by these and other rove beetle fossils, Brunke et al. further established that the connection between both lineages of Bolitogyrus existed for several million years until the early Eocene climate cooling (Fig. 2).
By analyzing climate requirements of modern Bolitogyrus species and palaeogeography of land bridges connecting Eurasia and North America, they concluded that rich tropical rainforest must have existed in the early Eocene in the Arctic. Only such forests could provide a dispersal corridor for ancient thermophilic Bolitogyrus that, following subsequent climate cooling and polarization, contracted their distributions to their present areas.
The study of Brunke et al. shows how much better we will understand Earth and natural history in general when the tens of thousands of beetle species still awaiting their discovery and systematic study are properly known, which is an enormous effort which is exactly what a museum-based biosystematic science must aspire for.
For the paper by Brunke et al. (2017) click here
For more details about rove beetles of the genus Bolitogyrus, see:
A revision of the Oriental species of Bolitogyrus Chevrolat (Coleoptera, Staphylinidae, Staphylininae)
A revision of the Neotropical species of Bolitogyrus Chevrolat, a geographically disjunct lineage of Staphylinini (Coleoptera, Staphylinidae)
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