Fish of Anza-Borrego Desert State Park
Ancient Freshwater Fish of Southern California

Fish are among the oldest vertebrates preserved in the fossil record of Anza-Borrego Desert State Park. Although their fossils are relatively uncommon, they provide some of the strongest evidence that rivers and lakes once occupied what is now one of the driest regions of North America. The fossil fish of Anza-Borrego record the evolution of the ancient Colorado River system and help scientists reconstruct the changing landscapes of southern California during the Pliocene and Pleistocene.

Unlike many fossil mammals, the freshwater fish found in Anza-Borrego belong to living groups that still survive in the Colorado River drainage today. Their fossils provide an important link between ancient and modern aquatic ecosystems.

Fish in Ancient Anza-Borrego
During the Pliocene and early Pleistocene, much of the Anza-Borrego region was crossed by rivers and floodplains that flowed into lakes and deltas within the developing Salton Trough. These waterways supported a variety of freshwater fish that lived alongside turtles, amphibians, waterfowl, and numerous mammals.

The presence of freshwater fish demonstrates that permanent bodies of water existed across the region for long periods, supporting aquatic ecosystems very different from today's desert washes.

One of the most significant discoveries is a nearly complete fossil skeleton of the razorback sucker, recovered from deposits of the Palm Spring Group. This remarkable specimen is one of the finest fossil fish ever discovered in Anza-Borrego and provides valuable information about the history of the Colorado River drainage.

The Razorback Sucker
The razorback sucker (Xyrauchen texanus) is one of the largest native fishes of the Colorado River system. Adults may approach one meter (three feet) in length and are recognized by the distinctive raised ridge behind the head that gives the species its common name.

A nearly complete fossil skeleton discovered in the San Felipe Hills demonstrates that this species inhabited rivers flowing through ancient Anza-Borrego during the Pliocene. The fossil is among the earliest well-documented records of the species and helps establish the antiquity of the Colorado River fish fauna.

Although the razorback sucker still survives today, it has declined dramatically because of dams, water diversion, and introduced fish species, making its fossil history even more significant.

What Fish Fossils Tell Us
Fish fossils help scientists reconstruct:

• The evolution of the Colorado River drainage
• Ancient rivers, lakes, and wetlands
• Climate and environmental change
• Connections between prehistoric and modern aquatic ecosystems
• The history of native southwestern freshwater fishes

The fossil fish assemblage suggests that drainage systems flowing through the Salton Trough had already become isolated from those of coastal southern California and the Mojave Desert by the latest Miocene. This isolation helped shape the distinctive fish communities that characterize the Colorado River today.

Why Anza-Borrego Matters
Anza-Borrego Desert State Park preserves the earliest well-documented freshwater fish assemblage known from the Colorado River drainage. These fossils provide a rare glimpse into aquatic ecosystems that flourished millions of years before the modern desert formed.

Together with amphibians, reptiles, birds, and mammals, fossil fish help reconstruct the complex environments that once existed across the ancient floodplains, lakes, and river systems of southern California.

The Fossil Record
Family Salmonidae

• Pacific salmon and trouts (Oncorhynchus)

Family Catostomidae (Suckers)

• Razorback sucker (Xyrauchen texanus)

Family Cyprinidae

• Chub (Gila)
• Pike minnow (Ptychochelius lucius)

Quick Facts
Class: Actinopterygii (Ray-finned fishes)
Includes: Native freshwater fishes of the Colorado River system
Famous species: Razorback sucker (Xyrauchen texanus)
Age: Pliocene and Pleistocene
Role: Freshwater aquatic vertebrates inhabiting rivers and lakes
Significance: Earliest well-documented fossil freshwater fish assemblage from the Colorado River drainage
Location: Anza-Borrego Desert State Park, California

Learn More
Read Chapter 8, "The Fossil Lower Vertebrates: Fish, Amphibians, and Reptiles," in Fossil Treasures of the Anza-Borrego Desert for a detailed discussion of the fossil fish of Anza-Borrego Desert State Park and what they reveal about the history of the Colorado River and the ancient waterways of southern California.