Dinosaurs under the Aurora

6. The Arctic during the Cretaceous

86 6 THE ARCTIC DURING THE CRETACEOUS North America and the western Arctic as experienced by dinosaurs and their kin were much different places than today. Today we find the planet dominated by large continents with interiors that are relatively far removed from the ocean margins. The oceans are now at what is called a low stand, because significant amounts of water are locked up as ice caps at both poles. Oceans act as great moderators of temperature and moisture. They are conveyors of heat energy, transferring energy to the colder land masses during winters and acting as coolers when the continents heat up during the summer.1 The oceans act as reservoirs of moisture by recycling water to the continental surfaces as precipitation. The pattern of precipitation is directly linked to the flow of heat energy and to the patterns of high and low pressure. Temperature and precipitation directly control the distribution and types of vegetation that are available to terrestrial animals such as dinosaurs. To understand the ancient world that dinosaurs of Alaska and the rest of the western Arctic dealt with, one must go back to the Cretaceous of one hundred to sixty-eight million years ago and look at the distribution of land and sea. Today, it is hard to envision the large continent of North America as composed of two to three subcontinents—a smaller and skinny western landmass, and at various times during the Cretaceous, one or two broader landmasses to the east. These landmasses were separated by shallow interior seas. Alaska during most of the Cretaceous was at the northern end of an island-like subcontinent that was thousands of miles long and hundreds of miles wide along most of its length.2 It was bordered on the west by what is now the Pacific Ocean. This western ocean reached depths over 10,000 feet (3,000 meters). The eastern side of this long narrow island was washed by a shallow interior sea that was less than a 1,000 feet (300 meters) deep at its deepest points. This North American interior seaway has been given a variety of names in the geologic literature that reflect the rich marine record of fossil life that it nurtured, but I refer to it as the Western Interior Seaway. It stretched from the paleo-Arctic southward to what is now the Gulf of Mexico and reached 1,000 miles (1,600 kilometers) at its widest part.3 Alaska and the Western Interior Seaway The Arctic during the Cretaceous 87 Computer models indicate that seaway was often subject to vertical mixing over the year, and this helps to explain the rich marine faunas and floras that fossil collectors have detailed for over a hundred years. The southern end of this seaway was part of an even more extensive warm, life-rich, mega-seaway, called the Tethyan Seaway, that stretched almost halfway around the waist of the earth.4 The proximity of the ocean and its warm currents reduced the harshness of the paleo-Arctic winters and provided abundant precipitation throughout the year—a set of conditions supportive of overwintering for at least some dinosaurs. Several computer models for the Western Interior Seaway have indicated that it was affected by large gyres that brought warm southern waters northward.5 The presence of a connection to tropical warm waters at the its extreme southern end and the natural moderating effect of water masses on three sides gave Cretaceous Arctic Alaska a climate similar to that found today some 2000–3,000 miles (3,200–4,800 kilometers) to the south along the coasts of northern California, Oregon, and Washington. Mean annual temperatures ranged from 7.0 to 13° C (42 to 52° F) in the mid-Cretaceous to 2.0 to 8.0° C (33 to 43° F) at the end of the Late Cretaceous. This compares with mean annual temperatures of −12 to −8.0° C (10 to 18° F) at similar latitudes in the Arctic today.6 Unlike now, the Arctic Ocean of the mid- to Late Cretaceous was largely ice free.7 It is important to note that in the Late Cretaceous, the North Slope dinosaur localities of Alaska were at least 70–75° N and may have been near 85° N, some 15° farther north than today.8 The western edge of this great seaway provided long stretches of shoreline and beaches with warm, sun-bathed offshore shallows that in places extended for miles offshore...