Warm, dark-brown, sticky tar oozes out of the ground at Rancho La Brea in California, creating deep lakes of asphalt belching forth bubbles of methane. These asphalt lakes, or tar seeps, are particularly hazardous to animals passing by, trapping and swallowing up carcasses whole. And the tar seeps at La Brea have been trapping animals and luring predators to their deaths for 50, 000 years.
Rancho La Brea has produced around 3 million Pleistocene and Holocene fossils belonging to hundreds of vertebrate, invertebrae, and plant species, including dire wolves, sabertooth cats, mammoths, ground sloths, hawks, geese, owls, snakes, frogs, scorpions, spiders, ants, beetles, poison oak, juniper, red cedar, and thistle. The majority of fossils belong to mammalian predators that probably attempted to eat rotting carcasses stuck in tar, then found themselves similarly stuck in the asphalt ooze, then died and decayed thus becoming new lures for passing predators and scavengers. But no-one is quite sure how long decay might take before the carcass becomes a less appetising jumble of asphalt-soaked bones, and if those bones separate from each other and are pushed along by currents while floating at the surface, or disarticulate after sinking to the bottom of the tar seep.
A new paper by Brown et al. (2017) explores these questions by using actualistic taphonomic experiments. The authors took limbs from carcasses of a modern mammalian predator, the bobcat (Lynx rufus), and placed them in wire cages that were then lowered into tar seeps in Chivo Canyon, California. Over 10 weeks, they removed a limb from the tar seep every 2 weeks and noted how much soft tissue had decayed, as well as the types of microbes feeding on the flesh and living in the tar.
Rancho La Brea has produced around 3 million Pleistocene and Holocene fossils belonging to hundreds of vertebrate, invertebrae, and plant species, including dire wolves, sabertooth cats, mammoths, ground sloths, hawks, geese, owls, snakes, frogs, scorpions, spiders, ants, beetles, poison oak, juniper, red cedar, and thistle. The majority of fossils belong to mammalian predators that probably attempted to eat rotting carcasses stuck in tar, then found themselves similarly stuck in the asphalt ooze, then died and decayed thus becoming new lures for passing predators and scavengers. But no-one is quite sure how long decay might take before the carcass becomes a less appetising jumble of asphalt-soaked bones, and if those bones separate from each other and are pushed along by currents while floating at the surface, or disarticulate after sinking to the bottom of the tar seep.
A new paper by Brown et al. (2017) explores these questions by using actualistic taphonomic experiments. The authors took limbs from carcasses of a modern mammalian predator, the bobcat (Lynx rufus), and placed them in wire cages that were then lowered into tar seeps in Chivo Canyon, California. Over 10 weeks, they removed a limb from the tar seep every 2 weeks and noted how much soft tissue had decayed, as well as the types of microbes feeding on the flesh and living in the tar.
Figure from Brown et al. (2017) showing the stages of soft tissue decay in tar pits. The severed bobcat legs were lowered in to the tar in wire mesh cages. After 7 days some bone is already visible, and after 40 days most of the tissue has been eaten away.
They found that decay occurred surprisingly quickly and surmised that a rich bacterial community exists in the tar seep, ready and waiting to consume flesh. Their microbial tests showed the bacteria did not hitch-hike into the tar seep on the bobcat carcasses. We know that hundreds of petroleum-eating bacteria species already exist in the tar seeps - they produce the methane bubbles - but it now seems that there are bacteria living in these tar seeps that specialise in eating organic material.
The authors conclude that modern bobcat limbs take around 2-3 months to fully decay in tar seeps. It appears that without the presence of the experimental wire cage, their bones would disarticulate (separate) after only a few weeks. The authors propose that the majority of decay and disarticulation therefore occurs at or just below the surface of tar seep, with single bones then being buoyed along by wind or water currents. The authors admitted that while these smaller limbs portions immediately sank into the tar, larger bodies may float at the surface while decaying with parts of the body exposed to the elements, and plan to conduct more actualistic experiments using whole carcasses in the future. I'd like to see more data on the temperatures of the tar seep, and how that might speed up or slow down decay depending on the types of bacteria present. Overall, this is a very thoughtful and interesting paper, so check it out (if you can get past the paywall).
The authors conclude that modern bobcat limbs take around 2-3 months to fully decay in tar seeps. It appears that without the presence of the experimental wire cage, their bones would disarticulate (separate) after only a few weeks. The authors propose that the majority of decay and disarticulation therefore occurs at or just below the surface of tar seep, with single bones then being buoyed along by wind or water currents. The authors admitted that while these smaller limbs portions immediately sank into the tar, larger bodies may float at the surface while decaying with parts of the body exposed to the elements, and plan to conduct more actualistic experiments using whole carcasses in the future. I'd like to see more data on the temperatures of the tar seep, and how that might speed up or slow down decay depending on the types of bacteria present. Overall, this is a very thoughtful and interesting paper, so check it out (if you can get past the paywall).
References
Brown, C., Curd, E., Friscia, A. 2017. An actualistic experiment to determine skeletonization and disarticulation in the La Brea tar seeps. PALAIOS, 32:119-124.
