Did Alaska's Dinosaurs Have Cold Feet?
No sooner had I finished writing about Earth's mid-Cretaceous hot spell recently than I got more information relevant to the subject. (The writing took place last week, but your newspaper's editor decides when, if ever, to run the column.) The journal Trends in Ecology and Evolution for January 1991 offers "Polar Dinosaurs and Ancient Climates," by British geologist Michael Benton. The article features Alaska's own giant, but long-gone reptiles.
When dinosaurs lived in Alaska, beginning in the mid-Cretaceous age, Earth's climate was warm into the high latitudes because of the greenhouse effect--volcanic activity had loaded the atmosphere with carbon dioxide: so I said in my column, following the more knowledgeable author Richard Kerr, writing in Science magazine. But I didn't say how warm, chiefly because the Science article didn't tell me.
The "Trends" article gave some probable answers to that question, and in the process told something about life in the Alaskan Cretaceous. Fossils recovered from the North Slope date from the middle and late Cretaceous (up to about 66 million years ago). They've been found bit by bit over the last 30-odd years, but--since that corresponds to middle and late Oil Exploration, so to speak, when secrecy surrounded most North Slope geologic information--the fossils have been described only recently.
Hadrosaurs, duck-billed plant-eating dinosaurs, are the best-known ones. Newspapers have run articles about these one-time denizens of our land, and the University of Alaska Museum houses some of their remains. Ceratopsian dinosaurs--the big horned and frilled herbivores, Triceratops and its kin--have left a few bits and pieces on the slope. Another plant-eating sort, the armorless and bipedal ornithopods, left only a footprint to show they were there.
The herbivores, then as now, were pursued by carnivores. Teeth from big tyrannosaurs and the similar-looking but much smaller troodonts have been found in northern Alaska.
The accompanying plant fossils hint that the resident reptiles had to cope with a real winter. Most of the vegetation was deciduous, plants that shed their leaves and stopped growing in winter. Growth rings in fossil wood also showed that winter was a time of dormancy.
Winter darkness would have stopped plant growth, and geologists estimate the North Slope then lay even further north than it does now, so the winter season would have centered on a nonstop night. But paleoclimatologists have other checks on weather, from sediment-preserved oxygen isotopes to fossil species assemblages. The consensus of these clues is that the Cretaceous-period residents of the North Slope enjoyed a cool-temperate climate, closer to that of present-day Seattle than of Prudhoe Bay.
Scientists aren't in perfect agreement about the clues, of course. Some think they have found Cretaceous-age dropstones, boulders dumped at sea as icebergs melt, which would prove that the polar seas held ice; others doubt the stones' source, but see evidence for snow and mountain glaciers in other rocks. Climate modellers think the mean annual temperature on the North Slope when the dinosaurs roamed was near freezing, but paleobiologists think it was warmer by five to ten degrees Celsius. From the plant evidence, it looks as if the late-Cretaceous winter in northern Alaska lasted some three months, and the lowest mean winter temperatures were about minus 11 degrees Celsius (near 12 above Fahrenheit).
None of those temperature ranges suit reptiles today. Speculating how dinosaurs thrived in such nontropical conditions has led to considerable creative thinking and an occasional exchange of heated comments. Benton's article implies that he favors a scenario in which little animals migrated or hibernated to avoid the chill, but big ones relied on their size. Like giant marine turtles today, the dinosaurs could have been "gigantothermic"--keeping their core temperatures high mainly because of the sheer mass that insulated them from the cool winter air of the Alaskan Cretaceous.
