The Rancho La Brea biota is one of the world's richest and most diverse late Pleistocene terrestrial assemblages. At the last census, in 1992, the collection exceeded 3.5 million specimens. The diversity of species (~ 600), the quality of preservation, and the large numbers of specimens makes this collection invaluable for the study and understanding of the end of the last Ice Age in North America. Rancho La Brea is perhaps best known for its extensive holdings of carnivorans, of which dire wolves (Canis dirus), saber-toothed cats (Smilodon fatalis), and coyotes (Canis latrans) predominate among the 60 plus species of mammals.
Asphalt is a superb preservative; small and delicate fossils, such as hollow bird bones or paper-thin exoskeletons of beetles are very well-preserved here. As a result, our collection of fossil birds is one of the world’s largest. Although earlier collectors focused on larger specimens, like saber-toothed cat skulls and ground sloth limb bones, the ongoing Pit 91 excavation was started with the specific goal of collecting the microfossils that early excavators passed over. Nearly all of the plants (160 species), invertebrates (205 species), fish (3 species), amphibians (5 species) and reptiles (24 species) are currently known only from Pit 91. Project 23, however, is also focusing on the recovery of all microfossils as well as the larger specimens, and thus far has produced some remarkable specimens, such as articulated millipedes and oak leaf layers. Both are new to the Rancho La Brea collections. This new project will give us environmental data from 16 separate deposits, with the potential to add new species to the Rancho La Brea faunal list.
Some of the collections are available online at http://collections.nhm.org
Mammals at Rancho La Brea
Approximately 90% of the mammals excavated at Rancho La Brea are carnivores. This proportion is due to the nature of the asphalt seeps that form a carnivore trap. When a large herbivore became mired in the asphalt, it attracted predators and scavengers to the site and these in turn became trapped. Large carnivores are represented by the dog family (Canidae), the cat family (Felidae) and the bear family (Ursidae). The most common large carnivore is the dire wolf (Canis dirus). Small carnivores include weasels and badgers (Mustelidae), skunks (Mephitidae), the elusive ringtail and racoon (Procyonidae). Other groups of animals include shrews, moles, bats, giant ground sloths, rabbits, rodents, mastodons, mammoths, horses, tapirs, peccaries, camels, deer, pronghorn, and bison. The Ancient bison (Bison antiquus) is the most common large herbivore and is represented by at least 300 individuals, many of them young.
Birds at Rancho La Brea
Rancho La Brea has one of the world’s best fossil bird collections because asphalt is a superb preservative. At least 250,000 bird specimens represent 139 species. Twenty-three bird species, or 19 percent, are extinct. Songbirds are the most common in terms of number of species, but hawks and eagles are represented by the most individuals. Some groups, such as the ducks and geese, have yet to be studied in detail. The first extinct species to be described from Rancho La Brea was Merriam's Teratorn (Teratornis merriami) in 1909. Teratorns are the largest birds found in the tar pits. New species continue to be discovered in the collection as research on the collection proceeds.
Flora at Rancho La Brea
Conservative estimates indicate more than 100,000 plant specimens have been recovered from Rancho La Brea. Most of this material comes from our Pit 91 excavation and Project 23 promises to yield many more specimens. Plants are represented by pollen, leaves, seeds, cones, and wood from approximately 158 species. In addition to the gymnosperms and angiosperms, over 80 species of diatoms (microscopic unicellular algae) and a single species of green algae have also been identified from Pit 91. Plants are often excellent indicators of climate because some species are tightly constrained to specific temperatures, humidity, and levels of shade. Some of the fossil plants we find are still represented locally, whereas others, such as the coast redwood (Sequoia sempervirens), currently grow outside the Los Angeles basin. To give visitors to Hancock Park a better insight into the vegetation that grew here during the end of the Ice Age, the Museum has created a Pleistocene Garden. Its contents are based on the fossils recovered from Pit 91.
The climate in the Los Angeles basin during the end of the last Ice Age appears to have been a coastal maritime climate similar to that currently prevailing on the Monterey peninsula, approximately 300 miles north of Los Angeles. Evidence from pollen in deep sea cores suggests that the climate was both cooler and wetter than it is today in Southern California. However, isotopic evidence from Rancho La Brea mammals and plants indicates that that the rainfall was restricted to the winter. As the climate became cooler, so the amount of carbon dioxide in the atmosphere became smaller. Carbon dioxide is important for plant photosynthesis. Evidence from the Rancho La Brea plant material, including the 14,000 year-old fossil juniper tree from Pit 3, indicates that the late Pleistocene plants were severely stressed and plant productivity was very low. Low plant productivity would have adversely affected food supplies for herbivores. This would in turn explain why the teeth of carnivorous mammals recovered from Rancho La Brea show more wear and damage than those of modern carnivores. Because the numbers of their herbivorous prey were reduced due to low plant productivity, the carnivores had to eat more of the carcasses, including the bones, and hence their teeth became damaged and broken.
On the basis of recovered fossils, there were at least four different plant associations present in the area during the late Pleistocene: coastal sage scrub, riparian (stream margin), chaparral, and deep canyon.
Invertebrates at Rancho La Brea
Large numbers of invertebrates have been recovered from Rancho La Brea, in particular from Pit 91. These include more than 45,000 specimens of mollusks (clams and snails) representing 56 species and well over 100,000 arthropods. The presence of both freshwater clams and snails suggest derivation from several different aquatic habitats ranging from shallow fast-flowing streams to stagnant ponds. Today, the land dwelling snail species live at higher elevations than the Los Angeles basin, indicating that the climate was cooler and moister than it is today. Arthropods include Scorpionida (scorpions), Araneida (spiders), Ostracoda (water fleas), Isopoda (pill bugs) and Diplopoda (millipedes) as well as seven orders of insects: Odonta (dragonflies), Orthoptera (grasshoppers and crickets), Isoptera (termites), Hemiptera (true bugs and cicadas), Coleoptera (beetles), Diptera (flies), and Hymenoptera (ants and wasps). Thus far, beetles are the most commonly recovered insects, representing 25 families. Many of the invertebrate fossils recovered are land dwellers and would not be preserved under normal conditions. A large number of the beetles and flies were carrion feeders and became trapped while feeding on carcasses of the larger animals that got stuck in the asphalt. The diversity and the different life-cycle stages of these insects are evidence indicating that decaying large animals lay on the surface of the asphalt for up to five months. Most of the fossil invertebrates found here represent living species although they may not live in Los Angeles today.
Fish, Amphibians, and Reptiles at Rancho La Brea
The large mammals of Rancho La Brea are significant but are by no means the only animals found here. Early excavations were focused on the recovery of large fossils and many of the smaller animals were unfortunately missed. Until recently, many of our fish (three species), amphibians (five species), and reptiles (24 species) were only known from Pit 91 matrix that was sorted under a magnifier in the Fishbowl Lab. It is expected that Project 23 matrix will yield many new species. These smaller animals are important keys to understanding this region during the Pleistocene because they probably lived their whole lives in the Hancock Park area. Many of these species are still living today, but are found in other areas.
The most common fish found thus far is the three-spined stickleback (Gasterosteus aculeatus). More than 50 spines have been identified although there is a backlog of unsorted matrix that may contain other elements. Sixteen species of snake, 14 of which belong to the Colubridae family, have been identified. The most common is the garter snake (genus Thamnophis). This semiaquatic snake feeds on frogs, tadpoles, and other aquatic or semiaquatic animals, suggesting that the area must have had either permanent or seasonal water sources. Garter snakes are represented by well over 900 vertebrae, accounting for nearly 70% of all our snake species. Arboreal salamanders (Aneides lugubris), toads (Bufo sp.), and tree frogs (Hyla sp.) have also been found, corroborating the idea that water sources existed in the vicinity of the park during the end of the Pleistocene.