Tuesday 6 August 2013

Insect borings in bones from the Rancho La Brea Tar Pits.

The Rancho La Brea Tar Pits are located in what is now central Los Angeles, California. 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. The La Brea Tar Pits appear to have been exposed at least intermittently at the surface for around 40 000 years, during which time a great number of animals have fallen into them.

In a paper published in the journal PLoS One on 3 July 2013, Anna Holden of the Department of Entomology at the Natural History Museum of Los Angeles County, John Harris of the George C. Page Museum and Robert Timm of the Department of Ecology and Evolutionary Biology and Biodiversity Institute at University of Kansas discuss the results of a study of insect damage to bones from the La Brea Tar Pits, and the implications of these results for the climate and conditions under which the deposits were formed.

Holden et al. compared damage on preserved bones from the Tar Pits to damage created in the lab by allowing Beetle larvae to feed on Pig and Chicken bones. They found that the damage to the ancient bones matched closely with that created by Dermestid and Tenebrionid Beetle larvae.

Damage caused to bones by Dermestid Beetle larvae. (A) Damage caused to a Pig bone in the laboratory. (B & C) Damage to the bones of Bison from the Rancho La Brea Tar Pits. Holden et al. (2013).

This has implications for the climate at the time when the bones became trapped in the tar. The types of Beetles that Holden et al. think were causing damage to bones in the La Brea Tar Pits are associated with far warmer climates than those suggested by pollen inclusions in the tar. Holden et al. suggest that this actually helps to clear up a mystery surrounding the Tar Pits; based upon pollen analysis, the temperatures at the times when the deposits were forming would have been to cool for the tar to melt, leaving a solid asphalt which animals would simply have been able to walk across.

Damage to bones from Dermestid and Tenebrionid Beetle larvae. (A) Chicken bone damaged by Dermestid Beetle larvae in the lab. (B) Camel bone from the La Brae Tar Pits showing damage by both Dermestid and Tenebrionid Beetle larvae. Holden et al. (2013).

The warmer temperatures implied by the Beetle damage suggest that the temperatures were warm enough for the tar to melt at least periodically, suggesting a fluctuating climate. Most pollen grains used in climate studies come from trees, long lived plants which can survive fairly long periods outside their optimum temperature range. A tree that lives for a hundred years or more will not die out because there is the occasional decade in which it is too warm for it to reproduce (most organisms can survive outside the conditions which they need to reproduce successfully). Thus a fluctuating climate which occasionally produced summers, or even strings of summers, would be all that was needed to produce the fossils of the La Brea Tar Pits.

Bones showing damage from Tenebrionid Beetle larvae. (A & B) A Chicken bones damaged by Tenebionid larvae in the lab. (C) Horse bone showing signs of Tenebrionid damage from the La Brae Tar Pits. (D) Bison bone from the Tar Pits showing signs of Tenebrionid damage. Holden et al. (2013).

The extent of damage can also be used to estimate how long the bones were available at the surface. Unlike a mammalian scavenger, Beetles do not simply turn up, gnaw a bone then leave, but rather lay eggs on the bone, which have to have time to hatch, and then the Beetle larvae must spend some considerable time on the bone to make holes of any size - something which can be measuered by observing Beetles in the lab.

Bones showing damage from Tenebrionid Beetle larvae. (A) Chicken bone damaged in the lab. (B) Horse bone from the La Brea Tar Pits. Holden et al. (2013).

Based upon these calculations, Holden et al. conclude that the bones examined were available at the surface for around five months, suggesting that quite long periods of warm weather occurring, i.e. that the deposits were formed in times of long warm summers, something which supports the theory that the area was subject to climatic cycles rather than occasional freak warm weather.

See also A new species of Skin Beetle from Yunnan Province, China, Insect borings in Triasic wood and Fossil Owls from the La Brea Tar Pits.

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