What Have Scientists Learned From Pit 91?

The annual excavation of Pit 91 at the La Brea Tar Pits has provided scientists with extensive data on the climate and ecology of the Los Angeles Basin 28,000 years ago. Most of the plant species and many of the insect and molluscan species are known either mainly or exclusively due to the ongoing excavation.

Summer 2005

The 2005 Excavation Season yielded over 2,700 fossil specimens.  These remains included nearly 350 dire wolf bones (including 3 skulls), 327 saber-tooth cat bones (including 5 skulls), over 120 ground sloth bones, over 150 bird fossils, and many, many more.  Check back soon for a complete list of recovered fossils.

Congratulations are in order for the 25 volunteers who gave up 1,003 hours of their time to make the 2005 Excavation Season such a success!  Thanks!

What have they found during the 2003 Pit 91 Excavation?
During the 2003 excavation, more than 1,321 fossil specimens were recovered, including those from a saber tooth cat, dire wolf, bison, sloth, horse, rodents, turtles, and some fossilized plant remains.

What was found during Summer 2002?
During the 2002 excavation, more than 1,300 fossil specimens were recovered, including: a sloth vertebra, a dire wolf vertebra, a saber-toothed cat vertebra and shoulder blade, and the humerus (upper arm) of a teratorn (the
largest Ice-Age bird). Other fossils included tiny bones that made up the " body armor" of a ground sloth, deer vertebrae, toes from a dire wolf and saber-tooth cat, a tibia (shin bone) of a dire wolf, bison teeth, and some fossilized plant remains.

What have they found during the 2001 Pit 91 Excavation?
They have completed two mostly sterile grids. In these 3 ft x 3 ft grids, they went down one foot, six inches at a time. (Note: After 6 inches, you have to make the floor and walls flat and even, do geology drawings, and take photos.) One grid did have some fossil wood in it, and a tooth (premolar) from a horse. The excavation team have opened another grid, and about a quarter of it is fossiliferous. So far, they have found: (updated August 28, 2001)

-Sabertoothed cat throat bone, chest bone, vertebrae, upper arm bone, lower
arm bones, wrist bones, ankle bones, toes, claws - Smilodon fatalis
-Dire wolf incisor, throat bone, wrist bone, hand bones, ankle bone, claw -
Canis dirus
-Rodent jaw, upper leg bone
-Bird vertebra, leg bones, toes, claw
-Rabbit ankle bone, jaw, arm bone
-Fossil seeds
-Bison toe - Bison antiquus
-Sloth tooth, dermal ossicles - Glossotherium harlani
-Turtle shell - Clemmys marmorata

During the 2000 excavation, more than 1,600 fossils were recovered, mostly
mammals and birds. Also discovered were 90 plant fossils, six seeds, and 62
rocks.

The 1998 excavation had a total of 1,016 new fossils that were recovered and
measured which included 807 vertebrates (mostly mammals, 21 birds and 1
reptile), 55 wood fossils, an insect fossil and 141 rocks.  Vertebrate
fossils included:

-Three skulls of Smilodon (saber-toothed cat) and four skulls of Canis dirus
(dire wolves)
-Lower jaws from an Equus (horse) and bison
-Several bones from the largest carnivore from Rancho La Brea ? The rare
Arctodus simus (short-faced bear)
-Bones from Canis latrans (coyote)
-Clemmys marmorata (turtle shell)
-Gymnogyps amplus (extinct condor, wing bone)
-Nine bones from Paramylodon harlani (Harlan's ground sloth)

The plant and molluscan fossils found in Pit 91 indicate that the climate
was wetter and cooler 28,000 years ago, when the fossil deposit was forming
than it is today.  The types of plants fossilized in Pit 91 would now be
found closer to San Francisco than to Los Angeles. Isotopic studies, a
method of chemical analysis, were conducted on herbivore teeth by the Page
Museum's Dr. John Harris and Dr. Thure Cerling of the University of Utah.
Their findings indicate that the horses, bison and camels of Rancho La Brea
were eating what is known as C4 grasses - a type of vegetation that is no
longer present in the region because it is dependent on summer rain.

Dr. Harris, along with Dr. Blaire Van Valkenburgh of UCLA and Dr. Lillian
Spencer of Duke University, have been awarded a grant from the National
Science Foundation to study the formation of the Pit 91 fossil deposit. This
three year research project will examine each excavated bone to determine
how long it was exposed before being buried in the asphalt. The fossils will
also be inspected to see if there is evidence of damage that occurred at or
just after an animal's death that may have been caused by carnivores,
rodents, insects or trampling by large mammals.  These factors will help
explain exactly how the fossil deposit accumulated.