Ground Sloths and Other Xenarthrans of Anza-Borrego Desert State Park
Ancient Visitors from South America

Among the most unusual mammals preserved in the fossil record of Anza-Borrego Desert State Park are members of the Xenarthra, a group that today includes sloths, armadillos, and anteaters. During the Pliocene and Pleistocene, several extinct xenarthrans inhabited southern California and became important members of the region's mammal communities.

The appearance of these animals in North America was part of one of the most significant biological events in Earth's history: the Great American Biotic Interchange. Following the formation of the Isthmus of Panama, animals from North and South America began moving between the continents. Xenarthrans were among the most successful immigrants from South America.

Ground Sloths of Ancient Anza-Borrego
Ground sloths were among the largest and most distinctive mammals living in Anza-Borrego during the late Cenozoic. Unlike the tree-dwelling sloths of Central and South America today, these extinct relatives were large terrestrial herbivores.

Ground sloths possessed powerful limbs, large claws, and massive bodies that allowed them to browse vegetation unavailable to most other mammals. Some species may have stood upright on their hind limbs while feeding on shrubs and low tree branches.

Their fossils occur alongside horses, camels, mammoths, antelope, and saber-toothed cats, providing evidence of the rich ecosystems that once occupied the region.

More Than Just Sloths
The Xenarthra included more than ground sloths. Relatives of modern armadillos also participated in the Great American Biotic Interchange and expanded into North America.

The Anza-Borrego fossil record preserves evidence of this broader group of South American immigrants. Their presence helps document the exchange of animal species that reshaped mammalian communities on both continents.

These animals represent an important chapter in the evolutionary history of North America and demonstrate how changing geography can influence the movement and diversification of species.

The Great American Biotic Interchange
The arrival of xenarthrans in North America occurred after the emergence of the land bridge connecting North and South America.

This event allowed mammals to move between previously isolated continents. While horses, camels, and bears migrated southward, ground sloths, armadillos, and other xenarthrans expanded northward.

The fossil deposits of Anza-Borrego preserve evidence of this remarkable migration and provide scientists with important clues about the timing and success of these dispersal events.

What Xenarthran Fossils Tell Us
Fossils of ground sloths and their relatives help scientists reconstruct:
  • Mammal migrations between continents
  • The Great American Biotic Interchange
  • Ancient vegetation and habitats
  • Climate and environmental change
  • Evolutionary relationships among extinct mammals
Because many xenarthrans occupied ecological roles different from those of modern North American mammals, they provide a unique perspective on prehistoric ecosystems.
Explore a Fossil in 3D
Rotate, zoom, and examine this specimen from the Anza-Borrego Desert State Park collections.
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Interactive 3D Model: Megalonyx wheatleyi mandible
Specimen Information
  • Catalog Number: IVCM1027 V 3406
  • Element: Skull
  • Formation: Ocotillo
  • Epoch: Pleistocene
  • Age: Irvingtonian
  • Locality: Borrego Badlands
  • Scanned using Agisoft Metashape
  • Specimen © California State Parks. 3D model produced by the Anza-Borrego Desert State Park Paleontology Society.
Why Anza-Borrego Matters
Anza-Borrego Desert State Park preserves one of the most important records of late Cenozoic mammals in North America. The xenarthran fossils found here help document the arrival and success of South American mammals in the northern continent.

These discoveries contribute to a broader understanding of continental-scale migration, evolution, and environmental change during the Pliocene and Pleistocene.

Today, the fossils of ground sloths and other xenarthrans remain among the most fascinating reminders of a time when southern California supported a remarkably different fauna than exists today.

The Fossil Record
  • Family Megalonychidae
    • Jefferson's ground sloth (Megalonyx jeffersonii)
    • Wheatley's ground sloth (Megalonyx wheatleyi)
  • Family Megatheriidae
    • Shasta ground sloth (Nothrotheriops shastaensis)
  • Family Mylontidae
    • Harlan's ground sloth (Paramylodon harlani)

Quick Facts
  • Group: Xenarthra
  • Includes: Ground sloths, armadillo relatives, and other South American immigrants
  • Age: Pliocene and Pleistocene
  • Significance: Evidence of the Great American Biotic Interchange
  • Lifestyle: Large herbivores and specialized mammals
  • Location: Anza-Borrego Desert State Park, California

Learn More
Read Chapter 10, "The Ground Sloths: Invaders from South America" in Fossil Treasures of the Anza-Borrego Desert for a detailed discussion of ground sloths and other members of this remarkable group of mammals that migrated from South America into North America.