Part of a series of articles titled Park Paleontology News—Vol. 18, No. 2, Fall 2026.
Article
Paleontology of Anza-Borrego Desert State Park National Natural Landmark
NPS / Laurie Jenkins
Inland from the Pacific Coast, the southern end of California is known for its deserts, mountains, and badlands, inhabited by a specialized community of plants and animals. It may be hard to picture this area as part of the Gulf of California, or inhabited by camels, horses, mammoths, and sloths, but the sedimentary rocks of the badlands preserve records of these times. Vast tectonic forces acting on the western margin of North America in the past few million years led to rapidly changing geography and environments here, with their own special lifeforms.
Part of this record is preserved in Anza-Borrego Desert State Park and studied and conserved by the park’s paleontology program. This park is the largest of California’s state parks. At 670,000 acres, it is the third largest state park in the United States. Its outstanding geologic record contributed to its designation as a National Natural Landmark in 1974. Much of the following is summarized from chapters in Jefferson and Lindsay (2006).
Part of this record is preserved in Anza-Borrego Desert State Park and studied and conserved by the park’s paleontology program. This park is the largest of California’s state parks. At 670,000 acres, it is the third largest state park in the United States. Its outstanding geologic record contributed to its designation as a National Natural Landmark in 1974. Much of the following is summarized from chapters in Jefferson and Lindsay (2006).
NPS / Laurie Jenkins
Fossils, especially the marine mollusk shells that are so unexpected in a desert, have long been observed in the area of the state park. The invertebrates were the first aspect of the park’s paleontology to attract interest, with extensive collecting and description in the first few decades of the 20th century.
The first serious investigations of fossil vertebrates began in the 1930s, around the same time as the establishment of the park in 1932. The Los Angeles County Museum of Natural History began making significant investigations in the 1950s, followed by Imperial Valley College in the 1970s. Scientists and volunteers from these two institutions and the state park amassed major collections of vertebrate fossils, now reposited at the park’s Colorado Desert District Stout Research Center.
Anza-Borrego Desert State Park sits in the Salton Trough, a depression in the earth’s crust formed by continental rifting. The trough leads south into the Gulf of California, formed by the same process. The gulf opened about 13 million years ago, and water moved north, flooding into the area of the state park around 6 million years ago.
At the same time, the newly separated fragment of crust including coastal California and Baja California began moving northwest along the San Andreas fault zone, bringing the future area of the park north. Deposition from the Colorado River began filling the north end of the gulf, pushing the sea back to the south. By about 3.5 million years ago, the area of the state park was a delta on its way south to its present location. Throughout these events, a thick layer of sediment was deposited.
The first serious investigations of fossil vertebrates began in the 1930s, around the same time as the establishment of the park in 1932. The Los Angeles County Museum of Natural History began making significant investigations in the 1950s, followed by Imperial Valley College in the 1970s. Scientists and volunteers from these two institutions and the state park amassed major collections of vertebrate fossils, now reposited at the park’s Colorado Desert District Stout Research Center.
Anza-Borrego Desert State Park sits in the Salton Trough, a depression in the earth’s crust formed by continental rifting. The trough leads south into the Gulf of California, formed by the same process. The gulf opened about 13 million years ago, and water moved north, flooding into the area of the state park around 6 million years ago.
At the same time, the newly separated fragment of crust including coastal California and Baja California began moving northwest along the San Andreas fault zone, bringing the future area of the park north. Deposition from the Colorado River began filling the north end of the gulf, pushing the sea back to the south. By about 3.5 million years ago, the area of the state park was a delta on its way south to its present location. Throughout these events, a thick layer of sediment was deposited.
NPS / Laurie Jenkins
The marine interval is documented by abundant and diverse fossils of marine life. Invertebrates include oysters, scallops, mussels and other bivalves, snails, corals, barnacles, bryozoans, and abundant sea urchins. Marine vertebrates are also represented, including sharks and rays, bony fishes, sea turtles, and marine mammals—including a walrus!
Together, these fossils document a rich and diverse marine ecosystem that occupied this region as the Salton Trough was in its earliest stages of formation. The marine invertebrates show that there was still a connection between the Pacific Ocean and Caribbean Sea at this time. The Caribbean relationship was recognized early on by scientists studying these fossils, before there were geologic explanations for how this could be possible.
As the sea was overtaken by the delta, a floodplain spread over the former marine beds. Woodlands took root, now represented by petrified wood in accumulations as large as petrified forests. A complex flora grew here, with elements similar to the modern mixed-evergreen and hardwoods of the central California coastline, the modern semi-arid and arid lands of the U.S. Southwest, and conifer woodlands. These plant fossils show an environment that was wetter, cooler in the summer, and warmer in the winter than present. Although much of what we know of Anza-Borrego Desert State Park’s paleobotany comes from wood, there are fossil leaves as well, including palm fronds.
Anza-Borrego Desert State Park and its surroundings are famous in vertebrate paleontology. Not only do they have diverse vertebrate fossils, but they can be placed in a stratigraphic sequence several million years long that includes two significant interchanges with other continents. The first was a sustained connection with South America across Panama, after North America and South America had been separated for tens of millions of years. The new connection brought animals such as ground sloths and porcupines to the Anza-Borrego Desert State Park area.
After that, Ice Age glacial periods lowered sea level and put North America and Asia in occasional contact through the Bering Land Bridge, and animals such as mammoths arrived. This is the same route that bison and humans eventually used to travel to North America.
Together, these fossils document a rich and diverse marine ecosystem that occupied this region as the Salton Trough was in its earliest stages of formation. The marine invertebrates show that there was still a connection between the Pacific Ocean and Caribbean Sea at this time. The Caribbean relationship was recognized early on by scientists studying these fossils, before there were geologic explanations for how this could be possible.
As the sea was overtaken by the delta, a floodplain spread over the former marine beds. Woodlands took root, now represented by petrified wood in accumulations as large as petrified forests. A complex flora grew here, with elements similar to the modern mixed-evergreen and hardwoods of the central California coastline, the modern semi-arid and arid lands of the U.S. Southwest, and conifer woodlands. These plant fossils show an environment that was wetter, cooler in the summer, and warmer in the winter than present. Although much of what we know of Anza-Borrego Desert State Park’s paleobotany comes from wood, there are fossil leaves as well, including palm fronds.
Anza-Borrego Desert State Park and its surroundings are famous in vertebrate paleontology. Not only do they have diverse vertebrate fossils, but they can be placed in a stratigraphic sequence several million years long that includes two significant interchanges with other continents. The first was a sustained connection with South America across Panama, after North America and South America had been separated for tens of millions of years. The new connection brought animals such as ground sloths and porcupines to the Anza-Borrego Desert State Park area.
After that, Ice Age glacial periods lowered sea level and put North America and Asia in occasional contact through the Bering Land Bridge, and animals such as mammoths arrived. This is the same route that bison and humans eventually used to travel to North America.
NPS / Laurie Jenkins
Most of the terrestrial fossils come from between about 3.5 million years ago and 750,000 years, or the later part of the Pliocene Epoch and most of the Pleistocene Epoch. Almost all of the famous North American mammals of the Ice Age are found here. The presence of older Pleistocene fossils is important because fossils of this age are uncommon in North America compared to fossils from the end of the Pleistocene.
A rich assemblage of the fossil vertebrates has been found at the state park and its immediate surroundings. Some are now extinct, such as mammoths and ground sloths; some are no longer found here but still live elsewhere, such as porcupines; and some are still present, such as coyotes. Both bones and footprints are known. The assemblage includes several species of bony fish, two dozen species of reptiles and amphibians, nearly three dozen bird species, and more than one hundred mammal species.
Twenty of the species known from body fossils and nine of the species known from footprints were named from fossils found at the state park or nearby. It is not only large, well-known animals that have been found here, but also many small animals such as rodents. Because small animals evolve quickly, their species only exist for short times, so they are important for relative dating of strata. Also, they may have very specific ecological needs, so they are useful for interpreting ancient environments.
Courtesy of Benji Paysnoe
Camels, horses, sloths, and proboscideans (mammoths and their relatives) are among the most frequently found fossil mammals at Anza-Borrego Desert State Park. Two of the most notable finds are one of the best skeletons of an early mammoth from North America and a patch of ground sloth skin larger than a square meter. This patch, from over the hips, includes articulated bits of pebbly dermal ossicles. More than 80 proboscidean sites have been found at the state park, making it one of the most productive areas for this group in North America.
NPS / Laurie Jenkins
To date, significant paleontological resources have contributed, in whole or in part, to the designation of 58 National Natural Landmark sites across the country, and paleontological resources are known at many others as well. These landmarks represent the range of geologic history, from very early marine life 510 million years ago to more recent (40,000 years ago) fossil mammals.
NPS / Laurie Jenkins
Further Reading
Jefferson, G.T., and L. Lindsay, editors. 2006. Fossil treasures of the Anza-Borrego Desert. Sunbelt Publications, Inc., San Diego, California. 394 p.Last updated: October 28, 2026