Modern and fossil insects body size as a possible proxy to understand environments of the past

The fossil records of Insects is quite rich and abundant, telling a story of the group’s rise through the Paleozoic, with the subsequent conquest of sea, land, freshwater and finally, for the first time in history of animals – air. Fossil insects also can tell us about the environment they lived in. It is relatively common to use insect remnants, especially head capsules of  non-biting midges (Diptera, Chironomidae) preserved in the sediments from the period including several last Ice ages the Holocene (11650 Years BC – Present) to reconstruct temperatures and the climate patterns of the past. Most of the midges in the Holocene are representatives of modern species, which allows us to extrapolate their ecology from the modern representatives of the same species. Based on our knowledge of the temperature preference of this modern species we can quite easily reconstruct and model their temperature preferences in the past.

Reconstruction of the temperature optimums of all the taxa in the community, together with analyses of the other paleoecological proxies (i.e. plant pollen profiles) enables us to assess the range of the temperatures experienced by the area in which midge samples of Chironomidae was obtained in the Holocene and latest Pleistocene. We cannot rely on such ecological extrapolation from the modern animals' ecology for the animal’s fossil records from the deep past, for example from Cretaceous or Triassic periods.

Therefore, we are proposing a more universally applicable climate proxy, independent of our knowledge of the fossil organism’s ecology. Animal size is one of the best candidates for such proxy. It is well known that the body size of the homoeothermic (“warm-blooded”) animals follows (roughly) so-called Bergman rule when size within the group of organisms is increasing from South to North ( i.e. polar bear and Amur tiger are both the northernmost and the largest representatives of their respective groups). We hypothesized that flies (Diptera) are suitable candidates for a quantitative paleoclimate proxy. Flies are very abundant in the fossil records from the mid Triassic (245 Mya) up until modern time. Their size is appears directly negatively correlated with temperature, i.e. representatives occurring further North are larger than the ones from the equatorial regions. This relationship allows us to use the relationships between the insect size and the geographic latitude at which they occur and the temperature at which these insects occur. Here we present a first results from analysis of > 2000 species of Chironomidae from around the globe, in a phylogenetic-constrained framework. First results are showing that non-biting midge’s wing and body size is growing by about 0.02 mm per one degree of geographical latitude, as one moves from the equator, mostly regardless of the phylogenetic position of the species analysed. This first results are showing that Insect size might be a promising proxy for reconstructing the palaeotemperature.