FS-1223 | June 2023

FOSSILS 2

From Our National Forests AND Grasslands

A

RCHAEOTHE RIU M

M IRACIN O NYX

ULT RASAUROS

T

YRANN OSAURUS

D

ES M ATOSUCHUS

GUNAKA D E IT

S

H

ASTASAURUS

H O P LITAS PIS

AN O M ALO CARIS

WWW.FS.USDA.GOV/SCIENCE-TECHNOLOGY/GEOLOGY/PALEONTOLOGY
FOSSILS 2

A fossil is evidence of past life. Paleontologists study fossils to learn about 

the history of life on Earth. Fossils can be traces, like footprints in stone, or 

the shells, bones, and teeth of ancient animals. 

The fossil animals shown on this poster lived in ancient environments from 

thousands to hundreds of millions of years ago in areas that today are 

national forests and grasslands of the United States.

Miracinonyx

Virginia, George Washington & Jefferson 

National Forest

A large cheetah-like cat died thousands of years ago deep in a cave on 

today’s George Washington & Jefferson National Forest. When discovered 

in 2016, the bones were covered by mineral deposits that had dripped 

from cave stalactites. Nicknamed “Petra” by its discoverers, the skeleton 

is virtually complete with all bones in place (fully articulated), even to the 

smallest toe bones.

This discovery is so new that Petra has not yet been precisely identified, 

but paleontologists believe it may be an American cheetah, called 

Miracinonyx (mih-rah-sin-ah-nix). Petra’s striking similarity to African 

cheetahs is an example of convergent evolution, when distantly related 

animals look very similar because they adapted to similar life styles. 

Petra’s long tail rules out a close relationship with other large cats like 

the Ice Age American lion or saber-toothed cat, and its long legs most 

closely resemble those of Miracinonyx.

Archaeotherium

Washington, Okanogan-Wenatchee National Forest

Geologic deposits from the middle of the Cenozoic Era (Age of Mammals, 

roughly 35 million years ago) can be found on several national forests 

and grasslands. The Okanogan-Wenatchee National Forest in Washington 

contains abundant fossilized mammals, including Archaeotherium.

Archaeotherium belongs to a group of pig-like mammals called entelodonts, 

sometimes referred to as “hell-pigs” for their fearsome appearance. 

Entelodonts had huge heads with extremely wide cheek bones, knobby 

lower jaws, and massive teeth, including large canines and crushing molars. 

Archaeotherium was among the largest mammals on the mid-Cenozoic 

landscape and was likely an active hunter, although like modern pigs it 

probably also foraged on plants and scavenged dead animals. 

Tyrannosaurus

North Dakota, Dakota Prairie Grasslands

Fossils of the most famous dinosaur of all time, Tyrannosaurus rex, have 

been found on several national forests and grasslands. T. rex is among the 

largest carnivorous dinosaurs (theropods), recognized for its small arms and 

huge skull, counter-balanced by a long, heavy tail. An adult T. rex could grow 

over 40 feet long and weigh up to 11 tons (22,000 pounds)—and could bite 

down with a force of 5 tons!

T. Rex bones and teeth have been discovered on the Dakota Prairie 

Grasslands in the late Cretaceous (68–66 million years ago) Hell Creek 

Formation. The Cretaceous climate was humid and supported subtropical 

forests and wetlands, and the rock layers contain fossils of crocodiles, 

insects, fish, flowering plants, and small mammals. Other famous dinosaurs 

found here include duck-billed hadrosaurs, horned ceratopsians, domeheaded pachycephalosaurs, and armored ankylosaurs.

Ultrasauros

Colorado, Uncompahgre National Forest

Many forests and grasslands in Colorado and Utah have multicolored rock 

exposures from the late Jurassic Period (150 million years ago) called the 

Morrison Formation. These rocks come from sediments deposited before 

the Rocky Mountains rose, when many rivers coursed through pine forests, 

leaving behind floodplain deposits rich with dinosaur bones and footprints. 

“Ultra” means extreme, and the extremely large Ultrasauros (ultra-reptile) 

is named for an enormous shoulder bone (8 feet, 10 inches long) dug up in 

1979 at the Dry Mesa dinosaur quarry on the Uncompahgre National Forest. 

Like other sauropods, Ultrasauros had a long neck with a small head and 

a massive body with a long tail—it was tall enough to have peered over a 

three-story building. Sauropods supported their great mass by powerful 

limbs positioned as vertical columns beneath their body, like elephants. 

Ultrasauros is a brachiosaurid type of sauropod dinosaur, which have longer 

front legs than rear.

Desmatosuchus

New Mexico, Santa Fe National Forest

National forests in the Southwest United States contain extensive exposures 

of bright red rocks deposited during the late Triassic Period (around 220 

million years ago). Rocks from the Triassic represent many environments: 

rivers, lakes, streams, and sand dune “seas.” Triassic rock layers deposited 

in terrestrial environments are often red due to oxidation of iron minerals 

(like rust formation).

One amazing fossil from these rocks is Desmatosuchus, a large reptilian 

herbivore up to 15 feet long. Desmatosuchus belongs to a group of armored 

reptiles called aetosaurs (a-eat-o-sores). Aetosaurs are more closely related 

to crocodiles than dinosaurs, and, like crocodiles, have rows of large, 

interlocking bony plates (osteoderms) covering the head, neck, back, 

and tail. The large shoulder spikes of Desmatosuchus are extensions of 

its shoulder bones. Armor plates of Desmatosuchus are known from the 

Santa Fe National Forest in New Mexico. 

Gunakadeit

Alaska, Tongass National Forest

The only known fossil of Gunakadeit (goo-na-ka-date) was discovered by 

chance on Hound Island on the Tongass National Forest. Rocks along the 

island’s shore are only exposed for a few days each year during low tide. 

The name Gunakadeit comes from a traditional Alaskan Tlingit story about 

a sea monster that brings good luck to those who see it.

Gunakadeit belongs to a group of Triassic crocodile-like marine reptiles 

called thalattosaurs. Thalattosaurs were only partly adapted to life in the 

water, because their small limbs were not modified into paddle-like shapes 

to assist in maneuvering underwater. Gunakadeit’s pincer-like jaws may 

have been best suited to capture soft-bodied animals by probing in cavities 

and crevices.

Shastasaurus

California, Shasta-Trinity National Forest

One of the world’s largest marine reptiles, Shastasaurus is named for Mount 

Shasta on the Shasta-Trinity National Forest in California. Many national 

forests in the Western United States have marine rock layers of Triassic 

age (approximately 230 million years ago) that contain fossil skeletons of 

ichthyosaurs (ick-thee-o-sores). The shastasaur family includes the largest 

known ichthyosaurs. Some species grew to lengths of 70 feet—almost the 

size of a blue whale!

“Ichthys” means fish in Greek, and the name ichthyosaur means “fish-lizard.” 

Ichthyosaurs were dolphin-shaped swimmers with long snouts bearing full 

rows of teeth and huge eyes to locate prey. Early Triassic ichthyosaurs like 

Shastasaurus have longer, more eel-like bodies and tails than geologically 

younger ichthyosaurs, which evolved more fish-like appearances, including 

dorsal fins and tail flukes.

Hoplitaspis

Michigan, Hiawatha National Forest

Limestone beds of Ordovician age (430 million years ago) on the Hiawatha 

National Forest were deposited in a marine lagoon and contain fossilized 

impressions of soft-bodied creatures, including rarely preserved jellyfish. 

The bizarre looking Hoplitaspis hiawathai is represented by over 50 complete 

specimens collected from these limestones. Hoplitaspis belongs to a group 

of marine arthropods called chasmataspids (chaz-ma-taz-pid), which are 

related to eurypterids (“sea scorpions”) and modern horseshoe crabs. 

The Hoplitaspis fossils from the Hiawatha National Forest are the only 

complete chasmataspid fossils known anywhere in the world. Growing up 

to 1 foot in length with large compound eyes and a pair of large grasping 

claws, Hoplitaspis was probably the dominant predator in its environment. 

Anomalocaris

Utah, Uinta-Wasatch-Cache National Forest

National forests in southern Idaho and northern Utah have rock layers 

from the Cambrian Period (about 520 million years ago), called the Spence 

Shale. These rocks contain amazing fossils of primitive animals in the 

world’s oceans. The Spence Shale formed on an ocean floor with very 

low oxygen, so soft tissue often did not decay and remarkable details of 

fossils are preserved.

Anomalocaris was a 2-foot-long apex predator of the Cambrian oceans, 

with large compound eyes and two large, hook-like grasping organs on the 

sides of a doughnut-shaped mouth. The abundant trilobites of the Spence 

Shale, scouring the ocean floor sediments for food, were common prey for 

the looming Anomalocaris. The anatomy of Anomalocaris is so strange that 

before discovery of complete fossils, isolated body parts were thought to be 

different animals.

AGE PERIOD FOSSIL RECORD

MILLIONS 

OF YEARS 

AGO

Cenozoic

Neogene Modern humans

23

Paleogene Abundant mammals

66

Mezozoic

Cretaceous

Flowering plants, 

ends with mass 

extinction of 

dinosaurs 145

Jurassic

First birds and 

mammals, 

abundant dinosaurs

201

Triassic Abundant 

coniferous trees

252

Paleozoic

Permian Extinction of nearly 

90% of all species

299

Pennsylvanian

Fern forests, 

abundant insects, 

first reptiles 323

Mississippian Sharks, large 

primitive trees

359

Devonian

Fish and sea life 

diversify, first 

amphibians 419

Silurian Early plants and 

animals on land

444

Ordovician First fish

486

Cambrian

Abundant hardshelled marine 

invertebrates

541

Precambrian

Proterozoic

Primitive aquatic 

plants, soft-bodied 

multicellular 

organisms 2,500

Archean Oldest fossils, 

bacteria and algae

4,000

MAJOR DIVISIONS OF 

GEOLOGIC TIME

Archaeotherium

Anomalocaris 

Tyrannosaurus

Hoplitaspis

Gunakadeit

Shastasaurus

Ultrasauros

Desmatosuchus

Miracinonyx

Illustrations by Karen Carr USDA is an equal opportunity provider, employer, and lender.