Using Fossil Beetle Assemblages to Determine Temperature Ranges During the Late Pleistocene

Man’s scientific investigation of the natural world will always remain incomplete.  The amount of potential knowledge seems infinite while the number of scientists available for study will always be quite finite.  Some topics have never even been explored or they’ve barely been studied.  So if a single scientist takes a sincere lifelong interest in one obscure topic, they can make a big difference in the state of mankind’s knowledge of at least that singular subject.  Dr. S.A. Elias is an example of a difference-maker.  He took an interest in Pleistocene-age fossil beetle assemblages, and how ancient climate could be reconstructed based on species composition.  Without just this one scientist, we would know next to nothing about this obscure but fascinating topic.

Dung beetle moving a beagle turd.  Photo from www.nativeplantwildlifegarden.com There must have been a lot of dung beetles  pushing megafauna dung around in the Pleistocene environment.

Past climates can be derived by comparing the current mutual range of extant beetle species with that of fossil beetle assemblages found in various Pleistocene-age sites across North America and Eurasia.  This is known as mutual climate range reconstruction or MCR.  Species of beetles have specific maximum temperatures in summer (TMAX) and minumum temperatures in winter (TMIN) within which they or their eggs and larva can survive.  So by cataloguing the species of beetles and their present day temperature requirements, scientists can determined the approximate maximum and mininum temperatures that occurred at a certain site during the time of deposition.  And they can determine what type of environment prevailed because most species of beetles are associated with specific habitat types.  The geographical ranges of different species of beetles have shifted over time in correlation with climate change.  Surprisingly, the geographical ranges of slow moving flightless beetles and even cave beetles have also shifted in correlation with climate change. Very few, if any, Pleistocene beetle species, became extinct at the end of that era.  They survived changes in climate by shifting their geographical ranges.

Chart from one of the below referenced studies comparing modern day average annual maximum temperatures in Alaska and the Yukon with those from different periods during the last Ice Age based on data from beetle species composition.  Not included on this chart is information about the Sangamonian interglacial in Alaska.  TMAX temperatures during the last interglacial were 5 degrees F warmer than those of today.  Paradoxically, TMIN temperatures in January were slightly warmer than those of today during the Last Glacial Maximum when TMAX temperatures during the summer were as much as 10 degrees F cooler.  This indicates less seasonality then.

Pleistocene beetle fossils have mostly been found in 3  regions of North America.  In the midwest and east they’re found in bogs.  In the west they’re found in ancient packrat middens.  And in Alaska they’re found in permafrost.

In the east and midwest, Pleistocene-age beetle fossils have been found in Canada, New York, Pennsylvania, Michigan, Minnesota, Wisconsin, Illinois, Missouri, and Iowa.  Six beetle assemblages excavated from a site in Titusville, Pennsylvania date to between 48,000 BP-43,000 BP.  Interstadial conditions occurred here ~47,500 BP with average annual temperature maximums only about 3.5 degrees F cooler than those of today (I converted the celsius temperatures in the scientific study to Fahrenheit).  However, the fossil beetle assemblages suggest average annual minimum temperatures were almost 11 degrees F cooler than those of today.  By~43,200 BP temperatures here cooled even more drastically.  Fossil beetle assemblages at a site in St. Charles Iowa also indicate decisive environmental change.  ~38,500 BP, a prairie/savannah environment prevailed here with some mixed conifer/hardwood forests, but by 32,900 BP  a cool spruce forest predominated.  Boreal and arctic species of beetles lived in the midwest during the Last Glacial Maximum beginning ~30,000 BP.

Beetle species compositions from ancient packrat middens (packrats collect shiny objects including beetle exoskeletons) in Texas, Colorado, New Mexico, Utah, Montana, North Dakota, and California indicate alternating wet and dry climate phases.  Paradoxically, during stadials when the rest of the continent endured cold dry conditions, the southwestern region of the continent was wet with lush vegetative growth and many lakes.  As recently as 13,500 BP, the Great Basin in Utah, today a hot dry desert, hosted a composition of beetles presently found in the Pacific northwest.

Permafrost in Alaska preserves beetle fossils from as old as 150,000 years ago.  Evidence from changing beetle compositions here demonstrate alternating cycles of dry steppe grassland and wet tundra.  During the grassland phase, ground beetles from the genuses Amara and Harpalus, pill beetles, dung beetles, and sagebrush weevils compose the assemblage, while during the wet tundra phase ground beetles from the Cryobius and Pterostichus genuses, rove beetles, and dwarf birch weevils dominate.  The Cordilleran Glacier expanded south of Alaska during the Ice Age.  This ice sheet locked up atmospheric moisture, creating a frigid grassland environment with little snow here.  During interstadials and interglacials, more snow and rain fall on Alaska, fostering growth of spruce forests and wet bogs.

Sagebrush weevil.  Fossils of this species indicate a dry grassy sagebrush environment.  There is little sagebrush in Alaska today, but it was a dominant component during stadial phases of the Ice Age.

Ground beetle from the Harpalus genus.  This genus of beetles predominated in Alaska during stadials when dry steppe grasslands were widespread in this region.

The La Brea Tar Pits in California also trapped beetles.  Many of the species found at this fossil site are the same as those found in the area today, but there is a slight difference.  The types of beetles are more representative of those found in San Francisco rather than present day Los Angeles, indicating slightly cooler climatic conditions.  The only beetle fossil site in the southeast that I could find in the literature is Vero Beach, Florida.  The fossil beetles found here are the exact same as those found at the present time.  Other localities in the south probably would show changing compositions of beetles, but perhaps to a lesser degree than those from other parts of the continent because the Atlantic Ocean had a moderating effect on climate in this region.  Currently, there isnt’ enough data here to know for sure.

References:

Elias, Scott A.

“Mutual Climate Range Reconstruction of Seasonal Temperatures Based on Late Pleistocene Fossil Beetle Assemblages in Arctic Beringia”

Quaternary Science Review 20 (1) Jan 2001

Elias, Scott A.

Encyclopedia of Quaternary Science

Elsevier Science Publisher 2007

Tags: Alaska permafrost, dung beetle, Harpalus, MCR, mutual climate reconstruction, Pa., packrate middens, Pleistocene beetle assemblage, sagebrush weevil, Titusville