Friday 13 September 2013

A fossil Grasshopper from the McKittrick Asphalt Deposits of Kern County, California.

The Rancho La Brea Tar Pits of Los Angeles County are renowned worldwide for their well preserved late-Pleistocene fossils, notably large, charismatic vertebrates. However California also has a number of other tar pits producing fossils of similar age, producing a diverse assemblage of vertebrate, invertebrate and plant fossils. They are essentially oil deposits identical to those worked by oil drills in other parts of the world, but exposed at the surface. When oil deposits are exposed in this way the lighter fractions (crude oil is made up of a mixture of different oils, known as 'fractions' due to the process used to separate them, fractional distillation) such as petroleum evaporate off, leaving the heavier fractions, known as tar, or asphalt, behind. These form oily pools in which animals can become trapped.

In a paper published in the Entomological News in May 2013, Sam Heads of the Illinois Natural History Survey at the University of Illinois at Urbana-Champaign and Yinan Wang of Arlington Virginia describe a fossil Grasshopper from the McKittrick Asphalt Deposits of Kern County, California.

The specimen belongs to the species Melanoplus differentialis, the Differentiated Grasshopper, which is still extant and widely distributed across North America, but which has not previously been described from the fossil record. It is a 25.3 mm Grasshopper preserved in a chunk of asphalt containing other plant and insect remains. The species owes much of its current widespread distribution to a preference for farmland, but is naturally found living on tall herbaceous vegetation growing in wetland meadows and similar environments, which is similar to the environment in which the tar pit deposits are believed to have formed, so the discovery of the species in these deposits in unsurprising.

Specimen of Melanoplus differentialis, the Differentiated Grasshopper from the McKittrick Asphalt Deposits of Kern County, California. Scale bar is 5 mm. Heads & Wang (2013).

Heads and Wang note that Grasshoppers are not uncommon in Californian tar pit deposits, but that like other insect specimens they are easily damaged and seldom identifiable to species level. They suggest that many insect specimens may be lost due to mechanical preparation of samples, and suggest that more specimens could be recovered intact by dissolving the matrix in an organic solvent.

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