Region 6: Alaska

See Chapter 2: Rocks for more information about stromatolites.

Although most of Alaska had not been assembled before the Mesozoic, the state does contain fossil-bearing rocks from the Precambrian through the Quaternary. The weakly metamorphosed Precambrian rocks in eastern Alaska contain stromatolites (Figure 3.35)—layered, mound-shaped fossils built by cyanobacteria.

Figure 3.35: Stromatolite of the <em class='sp'>Baicalia</em> type showing curved laminations, found in late Proterozoic (800 - 1000-million-year-old) rocks in east-central Alaska. About 15 centimeters (6 inches) long.

Figure 3.35: Stromatolite of the Baicalia type showing curved laminations, found in late Proterozoic (800 - 1000-million-year-old) rocks in east-central Alaska. About 15 centimeters (6 inches) long.

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 (Figure 3.36).

Jurassic marine rocks in Alaska—found around Iliamna Lake in southern Alaska, and across the Brooks Range on the North Slope—contain abundant ammonites and bivalves (Figure 3.37).

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 bivalves and gastropods; bivalves, such as Inoceramus (see Figure 3.13B) are well represented in these rocks.

Although Alaska reached its present latitude during the Cretaceous, the world as a whole was much warmer, and fossils of dinosaurs, crocodilians, palms, and other temperate to tropical species are common in Alaskan rocks. Some Cretaceous dinosaurs found near the Colville River on the North Slope are spectacularly well preserved, containing more than half of their original bone material. Alaskan dinosaurs included herbivorous ceratopsians similar to Triceratops, such as Pachyrhinosaurus (Figure 3.38); Alaskacephale, a pachycephalosaur (dome-headed dinosaurs similar to Pachycephalosaurus); and ankylosaurs, such as Edmontonia, the first dinosaur discovered in Alaska. Alaska even has its own native tyrannosaur, Nanuqsaurus (Figure 3.39), which was about half the length of Tyrannosaurus rex. Other carnivores included Albertosaurus and possibly Gorgosaurus. Plant fossils include Parataxodium, a relative of the bald cypress, as well as cycads, pines, and palms (Figure 3.40).

Crinoids

Crinoids are echinoderms, related to sea urchins and sea stars. These invertebrate animals feed by using their arms to filter food out of the water. Most are attached to the sediment by a stalk that ends in a root-like structure called the holdfast—however, some forms are free-floating. Crinoid fossils are most commonly found as “columnals,” pieces of the stalk that hold the head (calyx) above the surface. The calyx and the holdfast are only occasionally preserved as fossils.

Crown and stem, about 15 centimeters (6 inches) long.

Crown and stem, about 15 centimeters (6 inches) long.

Stem fragments.

Stem fragments.

Figure 3.36: Polished slab of limestone made up almost completely of crinoid bits and pieces, seen under a microscope.

Figure 3.36: Polished slab of limestone made up almost completely of crinoid bits and pieces, seen under a microscope.

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 Metasequoia, willows, poplars, alders, oaks, and elms.

Figure 3.37: Jurassic mollusks from northern Alaska. A) Bivalve, <em class='sp'>Aucella rugosa</em>, about 3 centimeters (1 inch). B) Ammonite, <em class='sp'>Reineckeia</em> sp., around 3 centimeters (1 inch).

Figure 3.37: Jurassic mollusks from northern Alaska. A) Bivalve, Aucella rugosa, about 3 centimeters (1 inch). B) Ammonite, Reineckeia sp., around 3 centimeters (1 inch).

Figure 3.38: <em class='sp'>Pachyrhinosaurus perotorum</em>; about 8 meters (26 feet) long from head to tail.

Figure 3.38: Pachyrhinosaurus perotorum; about 8 meters (26 feet) long from head to tail.

Figure 3.39: <em class='sp'>Nanuqsaurus</em> is a small species of Cretaceous carnivorous dinosaur from Alaska, known only from an incomplete skull. A) Drawing of the skull, with white shading showing the known fossils. B) Reconstruction of the head.

Figure 3.39: Nanuqsaurus is a small species of Cretaceous carnivorous dinosaur from Alaska, known only from an incomplete skull. A) Drawing of the skull, with white shading showing the known fossils. B) Reconstruction of the head.

Figure 3.40: A palm fossil, common in the Mesozoic and today known primarily in warm climates. About 0.7 meters (2 feet) wide.

Figure 3.40: A palm fossil, common in the Mesozoic and today known primarily in warm climates. About 0.7 meters (2 feet) wide.

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 refugia. 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.

Alaska is famously home to a number of Quaternary fossils, such as woolly mammoths, that are preserved in permafrost. The woolly mammoth was a Pleistocene and early Holocene elephant that coexisted with humans (Figure 3.41). Frozen carcasses preserving hair, skin, and even stomach contents have been found in Alaska, as well as in Siberia. The woolly mammoth was similar to extant 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 (in contrast to the pointed teeth of mastodons, which browsed trees as well; see box p. 100). Woolly mammoths survived on Wrangel Island (Russia) until 4000 years ago but became extinct in North America, along with many other megafauna, 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).

Figure 3.41: The woolly mammoth, <em class='sp'>Mammuthus primigenius</em>, was present in North America, Europe, and Asia during the Pleistocene.

Figure 3.41: The woolly mammoth, Mammuthus primigenius, was present in North America, Europe, and Asia during the Pleistocene.