The oldest fossil animals in the Arctic are some 550-600 million years old, and consist of soft-bodied invertebrates that inhabited the seafloor. Coined the Ediacaran fauna, for the most famous locality in Ediacara, Australia, these animals are found embedded in sandstones in northern Russia, Siberia, and Canada's Northwest Territories. The most diverse Ediacaran fauna is in the Arctic realm, where some 50 species of body fossils and 20 different trace fossils (that is, fossils that are not part of the actual body of the animal, but an impression, trail, burrow, or footprint) have been found in the White Sea region of northern Russia (Fedonkin, 1992). Fossils include: impressions resembling a disk, interpreted as jellyfish by some, and as burrows and mollusclike animals by others; sea pen-like, stalked animals that stood up from the seafloor; primitive members of the Phylum
Arthropoda; and the large (up to 1 m) Dickinsonia, an invertebrate resembling a flatworm. Many Ediacaran animals cannot be comfortably placed into any modern group, but their presence on all continents except Antarctica is evidence that a great radiation of animals was well under way on a global scale, including the Arctic, over 500 million years ago.
Phanerozoic Eon (The Age of Visible Life)
Earth's history from 543 million years (Ma) ago to today—the Phanerozoic Eon—is divided into three great eras: Paleozoic, Mesozoic, and Cenozoic. These which are in turn subdivided into numerous geological periods. The boundaries between eras and periods typically correspond to important changes in life on Earth, such as a major extinction event or adaptation to a changing climate. In fact, the evolution and extinction of fossil organisms form the basis of the geologic time scale. Ancient Arctic animals are discussed below for each of the three eras.
Paleozoic Era (543-248 Ma)
The Cambrian Explosion— the greatest evolutionary radiation of animal life in the oceans—took place over the relatively short time of 25 Ma from the late Proterozoic to early Cambrian, and included the first appearance of animals with skeletons and many modern phyla. Fossil evidence of this event is best preserved in the 505 million-year-old Burgess Shalelike fauna in the Canadian Rockies. However, a diverse, early Cambrian, Burgess Shale-like fauna is also known from northern Greenland. This latter fauna, termed the Sirius Passet fauna, contains about 40 species, most of which are arthropods, although worms (including armored varieties) and sponges also occur.
The earliest vertebrates, which include both jawless fishes (agnathans such as ostracoderms, anaspids, and thelodonts) and jawed fishes (gnathostomes, including sharklike placoderms and acanthodians), are well represented in Paleozoic rocks in the Canadian Arctic, Spitsbergen, northwestern Europe, and Siberia (Dineley, 1990).
One of the most exciting paleontological discoveries in the Arctic is Ichthyostega—among the oldest known tetrapods (vertebrates with four limbs) and the first undoubted amphibian—in late Devonian (~360 Ma) rocks of eastern Greenland. Traditionally considered to evidence the emergence of vertebrates onto land, Ichthyostega had a large, heavily built skeleton (>1 m in length) and scales (unlike today's amphibians, such as salamanders and frogs, which have skin), a massive ribcage, powerful limbs and feet, and a fishlike tail. Ichthyostega was more fishlike than anything else, spending most of its time in rivers and perhaps occasionally hauling itself onto land. Its eastern Greenland contemporary, Acanthostega, was even more aquatic, as evidenced by a fish's spine and tail, fishlike gills, and paddlelike legs that lacked wrists and ankles and could not have supported its weight on land. Both Ichthyostega and Acanthostega are best thought of as "fishes with legs," rather than as missing links in the vertebrate invasion of land (Clack, 2002).
Marine invertebrates, primarily crablike arthropods, echinoderms, corals, and molluscs, are known from many localities throughout the Arctic, especially northern Canada and northern Alaska, and attest to the tropical and subtropical oceans that prevailed globally in the Paleozoic. It should be noted, however, that warm-water Paleozoic faunas now preserved in the Arctic actually lived at lower latitudes, prior to the continents moving to their present locations. In addition, the several tectonic plates that now comprise the modern Arctic were parts of different continents in the early Paleozoic. It was not until just after the Paleozoic, during the earliest period of the Mesozoic Era, the Triassic, discussed below, that the Arctic Ocean assumed its present configuration as a separate northern ocean (see Geological History of the Arctic).
Mesozoic Era (248-65 Ma)
Dinosaurs inhabited the Arctic for millions of years. However, they are relatively recent discoveries in Arctic regions—most have been discovered since the mid-1970s, and few Arctic dinosaurian faunas are fully studied. To date, dinosaurs are known from Jurassic and Cretaceous-age rocks in Russia and Alaska, and from Cretaceous-age rocks in northern Canada (Yukon and Northwest Territories, as well as Ellesmere and Bylot Islands) and Spitsbergen (Rich et al., 1997). The most diverse Arctic dinosaur fauna, preserved in latest Cretaceous (~69 Ma) rocks along the Colville River on Alaska's North Slope, contains Ceratopsians (horned dinosaurs), the tyrannosaur Albertosaurus, the smaller meat-eating dinosaurs Troodon and dromaeosaurs, and duck-billed dinosaurs, including Edmontosaurus. Fossils of mammals, sharks, and fishes also occur in northern Alaska. However, terrestrial, cold-blooded tetrapods—notably amphibians and nondinosaurian reptiles such as champsosaurs (crocodilelike reptiles), turtles, and crocodilians whose fossils are common in midlatitude North America—are conspicuously absent at the Colville River site. This suggests that dinosaurs tolerated the Arctic's Cretaceous environment—including lower temperatures (mean annual temperature of 2-8°C) and winter darkness—that restricted amphibians and nondinosaurian reptiles to midlatitude regions (Clemens and Nelms, 1993). However, recent discoveries of slightly older Cretaceous (~92-86 Ma) fossils of champsosaurs, turtles, and fishes on Axel Heiberg Island (Canadian Arctic) suggest a much milder climate, with mean annual temperatures greater than 14°C (Tarduno et al., 1998). Regardless of the temperature, Arctic dinosaurs must have contended with 24-h darkness during winter months, which raises a variety of questions regarding dinosaur physiology and behavior.
Other notable dinosaur discoveries in Arctic regions include: Late Jurassic stegosaurs, camarasaurs, and allosaurs from Kempenyay, Siberia; Late Cretaceous duck-billed dinosaurs and Troodon from Kakanaut, Siberia; ankylosaurs and duck-billed dinosaurs from south-central Alaska; and duck-billed dinosaurs from the Canadian Arctic Archipelago (Bylot and Ellesmere Islands) and from the Yukon and Northwest Territories. Dinosaur footprints are known from Spitsbergen (in Svalbard) and Alaska's North Slope (Rich et al., 1997).
While dinosaurs reigned over the Mesozoic landscape, giant marine reptiles ruled the oceans. Triassic-age ichthyosaurs ("fish-lizards"), which closely resemble fishes and even modern dolphins, were among the first Mesozoic marine reptiles found in the Arctic regions. In the 1850s, ichthyosaur fossils were discovered in Triassic-age rocks on Exmouth and Cameron Islands in the Canadian Arctic Archipelago by the Belcher Search Expedition, which had been sent out to search for the lost Franklin Expedition. Similarly, old ichthyosaur fossils have since been discovered on Ellesmere Island (Canadian Arctic), Alaska, and Spitsbergen (Russell, 1990). Jurassic ichthyosaur fossils are known throughout the Canadian Arctic Archipelago and from Alaska's North Slope (Davis, 1987). Other marine reptiles include fossils of Cretaceous mosasaurs (giant, seagoing Monitor lizards) and plesiosaurs (Loch Ness Monster-like marine reptiles) from the Canadian
Arctic Archipelago. During the Mesozoic, the marine reptiles inhabiting Arctic regions were similar to midlatitude forms, suggesting a distribution throughout the Northern Hemisphere (Russell, 1990).
In Triassic time, the earliest period of the Mesozoic Era, the Arctic Ocean assumed its present configuration as a separate ocean. This earliest Arctic Ocean basin occupied the western part of the present-day Arctic Ocean and was joined to the Pacific by a broad seaway across northeastern Asia. This precursor of the modern Arctic Ocean thereby formed a northern embayment of the Pacific Ocean, whose Late Triassic shores are now delineated by the distribution of the distinctive bivalve Monotis (Westermann, 1973; Marincovich et al., 1990). Jurassic-age ammonites (extinct relatives of today's Nautilus, squid, and octopus) and other molluscs flourished in this northernmost ocean and were largely the dominant species as in the adjacent North Pacific.
Plate tectonic movements constricted the seaway connection between the Pacific and Arctic oceans, as evidenced by progressively more distant similarities of ammonite species between these two realms, until the isolation was complete by about 105 Ma in the middle Cretaceous. However, at about the same time, the Arctic Ocean came into contact with the world ocean through two epicontinental seaways: the Western Interior Seaway in North America, and Turgai Strait in western Siberia. Sporadically during middle Cretaceous time and continuously during the Late Cretaceous, the Western Interior Seaway extended some 7000 km from the Arctic Ocean to the tropical to subtropical Gulf of Mexico (Balkwill et al., 1983), as evidenced by marine molluscs and microfaunas. This seaway sometimes contained barriers to marine faunal dispersal in the form of increased or decreased salinity, increased temperature, and reduced oxygen content. Marked deviations from normal salinity conditions affected the shelf regions, as indicated by the poor representation of many normal marine groups such as echinoderms, corals, bryozoans, sponges, brachiopods, planktonic foraminifers, and radiolarians. The close similarity of Late Cretaceous ammonite faunas between the Western Interior Seaway and central West Greenland (Birkelund, 1965) indicates that an arm of the Arctic Ocean extended into the present-day Baffin Bay region (Balkwill et al., 1983). The Western Interior Seaway was closed by latest Cretaceous time, due to a major drop in eustatic sea level and active tectonism in the rising Rocky Mountain region that caused sedimentary infilling of the seaway from the west.
In contrast to the environmental barrier of the Western Interior Seaway of North America, Turgai Strait in western Asia was alternately a physical barrier to marine and terrestrial migrations, because this seaway was not continuously present. Beginning in the late Jurassic and for much of Cretaceous and early Tertiary time, Turgai Strait extended southward from the modern Kara Sea to Tethys, the tropical marine realm of that time. Turgai Strait was fully open throughout the Late Cretaceous, but became progressively more constricted into the early Tertiary, until its closure by marine regression in the late Eocene or Oligocene (Vinogradov et al., 1967). During the Paleocene, Turgai Strait, at times as little as 120 km wide, was the narrowest part of a seaway some 5000 km long. Gradients in temperature, salinity, and oxygen content, although not yet documented, probably also hampered marine migrations, just as they did in the Western Interior Seaway. Turgai Strait Paleogene deposits are entirely subsurface and known only from marine microfossils recovered from drill holes. There are genus-level similarities between late Paleocene marine ostracods of the Prince Creek Formation in northern Alaska and ostra-cods in deposits at the northern part of the Turgai Strait region (Marincovich et al., 1990).
Cenozoic Era (65 Ma-Today)
Among the most significant contributions to vertebrate paleontology in the last century was the 1970s' discovery of early Eocene (~55 Ma) fossils of alligators, turtles, lizards, fishes, 0and a variety of mammals (including primate relatives and tapirs) in the Canadian High Arctic (~75° N) by Mary Dawson and colleagues. Their discovery was the "smoking gun" for the longstanding hypothesis, originally based upon plate tectonics and faunal data from midlatitude North America and Europe, that a land bridge once connected northern Europe to North America, allowing the migration of land mammals across the North Atlantic (see McKenna, 1975). Furthermore, the presence of alligators, giant tortoises, varanid lizards, and primate relatives at 75° N proved beyond a doubt what evidence from fossil plants had been suggesting for years—the early Eocene High Arctic was warm, temperate, and ice-free.
Other notable fossil vertebrate discoveries in the Canadian High Arctic include early Miocene (~22-23 Ma) fossils of rhinoceros, rabbit, shrew, and swan from Haughton Astrobleme, a meteorite impact-crater on Devon Island (Dawson, 1990), and a 3.5 million-year-old beaver pond on Ellesmere Island, under study by Richard Harington and colleagues. The beaver pond site contains fossils of beaver, rabbit, badger, ancestral wolverine, three-toed horse, shrew, fish, and frog, as well as beaver-cut sticks, seeds, leaves, mosses, freshwater sponges, molluscs, and beetles, suggesting a warmer, forested Arctic environment 3.5 million years ago, similar to today's treeline environment (Harington, 2001).
Fossils of Ice Age (Pleistocene) mammals are known from Siberia, Alaska, and northern Canada (including the Canadian Arctic Islands, Yukon, and Northwest Territories). Among the most famous are the frozen, virtually complete, carcasses of mummified mammoths from permafrost in Siberia and the Russian Far East, including the Berezovka mammoth (discovered in 1900 by a Lamut tribesman) and a baby mammoth named Dima (discovered in 1977 by a placer gold miner). Mummies of woolly rhinoceros, horse, and bison are also known from Siberia, while the frozen mummy of a steppe bison coined Blue Babe was discovered in 1979 at a placer gold mine in Alaska (Guthrie, 1990). Roughly 10,000 years BP, mainland mammoths became extinct, alongside many other large, Ice Age mammals, including mastodonts, giant ground sloths, and sabertooth cats. However, a relict population of small woolly mammoths (and the youngest recorded mammoths) survived into historic times (~3700 years BP) on Wrangel Island, off the north coast of Siberia (Vartanyan et al., 1993).
The Arctic Ocean was almost completely isolated from the world ocean during the early Paleogene (Marincovich et al., 1990; Marincovich, 1993), owing to the absence of the Western Interior Seaway and the intermittent presence of a progressively constricted Turgai Strait. However, a new Paleocene seaway began to form between the Arctic Ocean and the North Sea Basin of northwestern Europe. In turn, the presence of a few identical species of marine molluscs, ostracods, coccoliths, planktonic and benthic foraminifers, corals and fishes in West Greenland and the North Sea Basin is evidence for an indirect Paleocene marine connection between the Arctic Ocean and the world ocean. This marine link has been best evidenced by late Paleocene marine molluscs in the Prince Creek Formation of northern Alaska and the Mount Moore Formation of northern Canada (Marincovich et al., 1990).
There is a conspicuous absence of marine deposits in the Arctic Ocean from the Paleocene to the late Miocene or Pliocene, owing to nondeposition or, more likely, to subsequent erosion and submersion beneath the modern sea. The oldest post-Paleocene Arctic Ocean marine fauna is in the upper Miocene or lower Pliocene Nuwok Member of the Sagavanirktok Formation in northeastern Alaska, in which the marine fauna of molluscs, ostracods, and foraminifers shows a strong North Atlantic influence. The most significant change in the Arctic Ocean marine fauna was caused by the initial submergence of Bering Strait in the late Miocene at 5.4-5.5 Ma (Marincovich and Gladenkov, 1999), owing to the combination of a eustatic sea-level rise and a tectonically related depression of the Bering Strait region. This new gateway permitted the first connection of North Pacific and Arctic Ocean marine faunas since the middle Cretaceous, a span of some 100 million years. When the strait first opened, the Arctic and North Atlantic bivalve Astarte migrated into the North Pacific and is found in southwestern Alaska. However, a major shift in Arctic Ocean oceanography was directly caused by the emergence of the Isthmus of Panama and the ensuing reorganization of Northern Hemisphere ocean circulation (Marincovich, 2000). As a result, the flow of water through Bering Strait shifted from southward to its modern northward direction. As a consequence of this, North Pacific molluscs and marine vertebrates (walruses and seals) came to dominate the modern Arctic Ocean fauna.
The onset of the Ice Ages at 1.6 Ma resulted in the Bering Strait being closed off by glacial ice at least four times, separated by interglacial episodes of marine flow. This waxing and waning of the Ice Ages is clearly evidenced in Pleistocene molluscan faunas of the Gubik Formation in northern Alaska and unnamed deposits in the Bering Strait region.
Jaelyn J. Eberle and Louie Marincovich Jr.
See also Beringia; Geological History of the Arctic; Mammoth; Pleistocene Megafauna
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