Monday 13 February 2012

Fossil Owls from the La Brea Tar Pits.

The 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.

Owls are not well represented in the La Brea Tar Pit deposits for a number of reasons, though they are some of the most abundant predators in the sort of open woodland terrain in which the deposits formed. Firstly, like all birds, owls have lightweight, fragile skeletons, ideal for flight, but giving them poor preservational potential. Secondly there is collection bias; the deposits have been heavily excavated for fossils since their discovery in 1875, however for much of that time collectors concentrated on big, prestigious, fossils such as mammoths and saber-toothed cats; many small fossils were simply discarded and lost. Finally there is the behavior of owls themselves. Predators are well represented in the Tar Pits deposits, it is thought because many became trapped while scavenging on the bodies of other trapped animals. Owls, however do not forage for carrion, they hunt living prey, mostly insects and small mammals. Such animals, if court in tar, will expire rapidly, and owls will ignore non-moving prey items. In addition owls live primarily in trees and spend little time on the ground, making them less likely to become trapped in tar pits.

Despite all this a number of owls have been described from the La Brea Tar Pit deposits. Prior to now these have been divided into nine species, eight still extant and one unique to the Tar Pits. In a review of the known owl fossils from the La Brea deposits, Kenneth Campbell of Vertebrate Paleontology at the Natural History Museum of Los Angeles County, and Zbigniew Bocheński of the Institute of Systematics and Evolution of Animals at the Polish Academy of Sciences, to be published in a forthcoming paper in the journal Acta Palaeontologica Polonica, conclude that two previously undescribed species of miniature owls have are present in the collections, and formerly describe those species.

Both the new species are described from their leg bones; not unusual with bird fossils, since the leg bones are generally those that survive as fossils best, which makes it easy to see why the fossils were overlooked by non-owl specialists.

The first new owl was assigned to the genus Glaucidium (pygmy owls) and named as Glaucidium kurochkini; Kurochkin's Pygmy Owl, in honor of the late Russian palaeornothologist Evgeny Kurochkin. Campbell and Bocheńska calculate the living owl would have weighed about 71.4 g, and assign 12 specimens to the new species.

Tarsometatarsals from (A) Glaucidium kurochkini and (B) the extant Glaucidium californicum. From Campbell and Bocheńska (2012).

The second owl was placed in the new genus Asphaltoglaux (Asphalt Owls, in reference to the Tar Pits) and named Asphaltoglaux cecileae, or Cécile's Asphalt Owl, in honor of the French palaeornithologist Cécile Mourer-Chauviré, of the Université Claude Bernard in Lyon. Campbell and Bocheńska estimate Asphaltoglaux cecileae would have been a robust bird, weighing about 78.2 g.

Tarsometatarsals from (A) Asphaltoglaux cecileae and (B) the extant Aegolius acadicus. From Campbell and Bocheńska (2012).