Wasp Fossils Suggest Ice Age California Similar To Modern Environment

by AMNH on

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The La Brea Tar Pits, one of the world’s richest Ice Age fossil sites, is famous for specimens of saber-toothed cats, mammoths, and giant sloths, but it also has numerous insect and plant fossils. New research on fossil galls—abnormal plant growths caused, in this case, by tiny wasps—helps reconstruct the local habitats of Southern California at the end of the last Ice Age. The work, led by Anna R. Holden of the Richard Gilder Graduate School at the American Museum of Natural History and the La Brea Tar Pits and Museum, appeared in the journal Quaternary Research.

Two-part image. On left: a two-toned, colorful bell-shaped object on a leaf. On right: a winged insect lying between two halves of a dark colored pod.
Wasp gall (Andricus kingii) attached to leaf (left) and later dissected to show inhabitant. 
© Joyce Gross

“Most people associate the Ice Age with freezing temperatures and an entirely different landscape from the present,” Holden said. “But this study shows that the environment and climate around Southern California has not drastically changed since that time.”

Cynipid wasps, which lay their eggs inside plant tissue, are one of the most common gall insects. The larvae use galls for both protection and food before they emerge. The Museum is home to one of the world’s most extensive collections of these wasps—you can get a look inside it in a recent episode of the web series Shelf Life.

Because these cynipid wasp species still live today, Holden and colleagues from the University of California, Berkeley identified and linked records of the fossil gall plant hosts (mostly oaks) and their current habitat associations to the late Ice Age at the La Brea Tar Pits. This suggests that many habitats common to modern California were present in Southern California between 33,000 and 44,000 years ago when these fossil galls were formed.

Fossil of gall nut with wasp larva holes
A fossil gall.
© Joyce Gross

“Assuming that ecological conditions required by native plants in California during the Late Pleistocene were the same as those prevailing today, the diversity of the fossil galls indicates that almost every kind of habitat existing in California at present also existed during the Late Pleistocene,” Holden said. “This suggests that a radical change or turnover in climate did not occur.”

The new paper, ”Late Pleistocene galls from the La Brea Tar Pits and their implications for cynipine wasp and native plant distribution in southern California,” can be accessed here.