Fossils of Alaska

Precambrian and Paleozoic fossils

Precambrian fossils

Although most of Alaska had not been assembled before the Mesozoic, the state does contain fossil-bearing rocks dating as far back as the Precambrian. The weakly metamorphosed Precambrian rocks in eastern Alaska contain stromatolites, layered, mound-shaped fossils built by cyanobacteria.

Paleozoic fossils

Carboniferous rocks found in parts of northern Alaska, including the Brooks Range, Point Hope, and Gates of the Arctic National Park, contain corals, brachiopods, gastropods, and crinoids.

Mesozoic fossils

Mesozoic marine fossils

Triassic marine reptiles

Triassic marine reptiles are rare in Alaska. An unnamed ichthyosaur from the Brooks Range in northern Alaska is similar to Shonisaurus, a genus of large Triassic ichthyosaurs from Nevada (read more about Shonisaurus at in Earth Science of the Western United States: Fossils of the Basin and Range). A fairly complete specimen of a thalattosaurid, another type of marine reptile, was found in the Keku Islands of southeastern Alaska.

Jurassic marine fossils

Jurassic marine rocks in Alaska, which are found around Iliamna Lake in southern Alaska and across the Brooks Range on the North Slope, contain abundant ammonites and bivalves.

Cretaceous marine fossils

Shallow marine rocks from the Cretaceous are well represented on Alaska’s Northern Coastal Plain, around Norton Sound, the Kuskokwim Mountains, the Kenai Peninsula, and Kodiak Island. The Brooks Range and other mountains arose during the Cretaceous, which led to extensive erosion and deposition of sediment in shallow marine environments and coastal swamps. Alaska’s Cretaceous marine fossils are dominated by modern groups such as gastropods and bivalves, although ammonoids are also found.

Cretaceous terrestrial fossils

Cretaceous fossils of Denali National Park

Alaska reached its present latitude during the Cretaceous, when the world as a whole was much warmer. Fossils of dinosaurs, dawn redwoods, ginkgoes, and other species are common in Alaskan rocks. Fossil dinosaur tracks that are about 70 million years old have been discovered in the Cantwell Formation in Denali National Park.

Cretaceous Colville River dinosaurs

Some Late Cretaceous dinosaurs found near the Colville River on the North Slope (northern Alaska) are spectacularly well preserved, containing more than half of their original bone material. Alaskan dinosaurs included herbivorous ceratopsians (horned dinosaurs) such as Pachyrhinosaurus; Alaskacephale, a pachycephalosaur (dome-headed dinosaurs with thick skulls); hadrosaurs (duck-billed dinosaurs) like Ugrunaaluk; ankylosaurs (armored dinosaurs), such as Edmontonia, the first dinosaur discovered in Alaska. Alaska even has its own tyrannosaur, Nanuqsaurus, which was about half the length of Tyrannosaurus rex. Other carnivores included Albertosaurus and possibly Gorgosaurus. These dinosaurs lived in conifer forests where ginkgoes and angiosperms (flowering plants) also grew.

Cenozoic fossils

Paleogene fossils

Paleogene fossils indicate that Alaska's climate was much warmer than it is today. In addition to plants that may occur in Alaska today, like alder (Alnus), plants characteristic of warmer climates have been found. Most notable among these are palm leaves, which have been found in southeastern Alaska in sites near the Gulf of Alaska and on Kupreanof Island. The occurrence of palms in southern Alaska is consistent with the high global temperatures that occurred in the Paleocene and Eocene.

Neogene fossils

In Alaska’s Neogene rocks, which occur mostly along the southern coast and onto the peninsula, gastropods and bivalves are the most common marine fossils.

Land plants preserved in these rocks include many familiar tree species, like willows, poplars, alders, birches, oaks, beeches, and elms. The flora also includes some types of trees native only in Eurasia today, like dawn redwood (Metasequoia), ginkgo (Ginkgo), katsura (Cercidiphyllum), and zelkova (Zelkova). 

Desmostylians

Desmostylians are a group of enigmatic Oligocene to Miocene mammals that looked like and probably lived in similar habitats to modern hippos. Desmostylians are found only around the northern edges of the Pacific Ocean. In the eastern Pacific, they are found on the West Coast of North America from the Aleutian Islands in Alaska to Mexico. The modern relatives of these animals are unknown.

Desmostylian fossils are rare in Alaska. Only two species are known, both collected from Unalaska Island in the Aleutian Islands. The older is the Oligcene species Cornwallius sookensis, which ranged all the way south to Baja California Sur, Mexico. The younger is the Miocene species Ounalashastylus tomidai, which has been found only on Unalaska Island.

Quaternary fossils

Much of Alaska was covered in ice during the Pleistocene ice age, but some refuges did exist where terrestrial animals were able to persist. Beringia, the land bridge that allowed humans and other animals to pass into North America from Asia, was likely one of these refuges. Numerous mammal fossils can be found throughout the state. The area around Fairbanks has many Quaternary deposits that yield mammoth, mastodon, bison, and elk bones.

Pleistocene mummies

Rarely, frozen carcasses preserving hair and skin have been found in Alaskan permafrost, although such carcasses are more frequently found in Russia. The two most famous such mummies are "Effie," a baby woolly mammoth discovered in 1948 that is over 21,000 years old, and "Blue Babe," a steppe bison discovered in 1979 that is about 36,000 years old. Both were discovered during gold mining.

In 2012, another frozen steppe bison called "Bison Bob" was discovered in a riverbank. While Bison Bob's skeleton is nearly complete, Bison Bob is not completely mummified like Blue Babe. Nevertheless, some of Bison Bob's connective tissue and hair is preserved. Bison Bob is 43,000 years old.

The woolly mammoth

The woolly mammoth (Mammuthus primigenius) was a Pleistocene and early Holocene elephant that coexisted with humans. Woolly mammoths lived in both Eurasia and North America; in North America, their range extended from Alaska southeast to the northeastern United States. They were one of three species of mammoths that lived in North American during the Pleistocene. The other two were the larger Columbian mammoth (Mammuthus columbi), which ranged throughout most of the lower 48 U.S. states and into Mexico, and the small pygmy mammoth (Mammuthus exilis), which was found only on the Channel Islands in southern California. 

Woolly mammoths were similar to modern African elephants in size but had a heavy coat of fur, small ears, and a short tail to minimize heat loss. Mammoths primarily ate grasses and sedges and had flat teeth similar to those of modern elephants. 

Most woolly mammoths, along with many other megafauna, went extinct during the Quaternary extinction event as a result of climate change and exploitation by human hunters; the relative contribution of each of these factors is still disputed. The youngest woolly mammoths from the United States come from St. Paul Island (a remote volcanic island located to the southwest of mainland Alaska), where they survived until about 5600 years ago. The last known woolly mammoths in the world survived on Wrangel Island (a Russian island to the north of eastern Russia and northwest of Alaska) until about 4000 years ago.