A few more sweeps with the whisk broom, and the bone at my knees suddenly came into focus. I was looking at the snout of a Pachyrhinosaurus, a particularly odd horned dinosaur, a rare relative of Triceratops. It was not the first, or even the second, fossil of this creature found in Alaska, but I could already see parts of this skull that were not preserved on the other specimens. Continued excavation at the site—with picks and shovels supplementing our whisk brooms—yielded the bones and teeth of at least three other dinosaur species. It would take another trip for me to realize we were also crawling over 10 more skulls of Pachyrhinosaurus. Most individuals were close in age and had probably died together in a flood or other catastrophe. This grouping was the first evidence that horned dinosaurs north of the Arctic Circle behaved gregariously.
I had come to this remote spot on a bluff overlooking the Colville River in the summer of 2006 with colleagues from the Perot Museum of Nature and Science and Southern Methodist University to excavate the skull of a Pachyrhinosaurus I had found in 2002. We undertook a massive excavation from 2005 to 2007.
The genus name Pachyrhinosaurus means “thick-nosed lizard.” After years of careful laboratory work and study of the new specimens gathered during our excavations, we discovered that our horned dinosaur was a new species of the genus. We named the Arctic horned dinosaur Pachyrhinosaurus perotorum, after the primary benefactors of our museum, the Ross and Margot Perot family.
No one has yet excavated a complete dinosaur skeleton from this site or anywhere else in Alaska. We have been able to identify from partial skeletons, isolated bones, teeth and fossil footprints several types of dinosaurs (including birds) and other fossil vertebrates that lived as contemporaries in the far north [see illustration on pages 58 and 59]. All these animals are from the Cretaceous period, which lasted from 145 million to 66 million years ago, but our animals date from just 70 million to 69 million years ago, some three to four million years before the famous mass demise of the planet's dinosaurs. Our work is ongoing, but we have begun to fill in some details about what kind of dinosaurs lived at the top of the world millions of years ago and how they survived there.
The Cast of Characters
Some species ate plants, whereas others, called theropods, preyed on the plant eaters and other creatures. By far the most abundant dinosaurs of Alaska were the duck-billed dinosaurs, which are known from both fossil bones and footprints. Hadrosaurs—large, plant-eating dinosaurs—also go by the name “duckbills” because they had broad, flat mouths; in contrast to ducks, however, they had hundreds of teeth that could grind the tough plants they fed on. They could stand on their back legs to reach overhead foliage, although they traveled on all fours, probably in a rocking gait, because their rear legs were longer than their front. Edmontosaurus earns the prize for the hadrosaur that is best known from bones and so is the best characterized. Weighing in at between 1,400 and 1,800 kilograms, Edmontosaurus ranks among the largest of the hadrosaurs found in North America. These hadrosaurs comprise about 80 percent of the herbivorous dinosaurs found in Alaska. Like other hadrosaurs, Edmontosaurus were social animals that gathered in herds, as evinced by their bones, which have been found in piles at various places in northern Alaska as though groups of them had died in a flash flood.
Since 2002, Alaska has proved to be a paleontological candy store; it seems as though there are new discoveries in new places every year. Thanks to continued support from the National Park Service, we now have a robust record of dinosaurs from parks around Alaska, including from Aniakchak National Monument, Wrangell–St. Elias National Park, Yukon–Charley Rivers National Preserve and, most notably, Denali National Park—where the fossil record includes a remarkable diversity of birds. Together the finds in this region give us insights into biodiversity and biological adaptation in an ancient, high-latitude terrestrial ecosystem.
Alaska was not the only surprising home to dinosaurs. Other researchers, such as Patricia Vickers-Rich of Monash University and Thomas Hewitt Rich of Museum Victoria, have uncovered fragmentary remains of dinosaurs that lived near the South Pole during a much earlier period [see “Dinosaurs of Polar Australia,” on page 46].
How Did They Get There?
How did dinosaurs find themselves at the planet's northern extreme? More than likely they came from Asia because ancestral forms of almost all the Cretaceous dinosaur families found in North America existed in Asia. Most paleontologists believe that some of these dinosaurs migrated across a land bridge exposed by a drop in sea level where the Bering Sea sits today [see box on preceding page]. The configuration of continental plates during the Cretaceous suggests that the earliest these plates were in position to serve as a land bridge was approximately 120 million years ago. Some of the immigrants probably just stayed in the far north because the environment there supplied their needs; others headed south. One species, though, seems to have taken a different route. Alamosaurus, a plant-eating dinosaur roughly 20 meters in length, apparently arrived by a southerly migration path—remains of its ancestors are found on the continents of South America and Africa.
Alaska is built of enormous geologic blocks, some of which originated far from their present location. During the Cretaceous, however, many of these blocks of land were near their current latitudinal position or higher. Thus, the dinosaur fossils found in Alaska were not posthumously hijacked from distant climes and brought there on moving plates: they lived in the high latitudes during the Cretaceous. Did they stay there all year? And if so, how did they manage it?
Coping with Cold
Answering that question requires knowledge of the climate in Alaska some 70 million years ago. True, the world was warmer then, but the climate in the high-latitude lands was still challenging, with cold, snowy winters and several months of darkness. Climatological data from fossil pollen, leaves and wood indicate that the Cretaceous forests of northern Alaska consisted of a mixed canopy that included deciduous conifers with an understory of flowering plants, ferns and horsetails. Today mixed coniferous forests occupy a wide but well-defined range of climates with mean annual temperatures from three to 13 degrees Celsius (37 to 55 degrees Fahrenheit), suggesting that the average yearly temperature for northern Alaska during the Cretaceous was similar.
One of the striking aspects of the modern Arctic is the angle of sunlight and the length of the day—commonly, though mistakenly, referred to as six months of daylight and six months of night. In actuality, north of the Arctic Circle, darkness occupies a longer and longer part of each day until the late-December winter solstice, when the sun does not rise. During the Cretaceous, northern Alaska was even farther north than it is today, and so the dinosaurs that lived there would have needed mechanisms to cope with both the cold and the dark.
We do not know the full explanation of how they survived. It seems unlikely that a 10-meter-long (35-foot-long) hadrosaur dug a hole in the ground and went into hibernation. But during times of environmental stress, some animals can lower their metabolic rates enough to reduce their need for food; perhaps Arctic dinosaurs did something similar without reaching an official state of hibernation.
In trying to explain how hadrosaurs survived the harsh climate, Nicholas Hotton III of the Smithsonian Institution suggested that in winter they migrated thousands of kilometers to find forage, warmer temperatures and better light conditions. Subsequently, other researchers used modern caribou as an example of a long-distance migrant to support such theories about the migration of Arctic dinosaurs.
To investigate the likelihood that hadrosaurs migrated, Roland Gangloff, now a professor emeritus at the University of Alaska, and I decided to see how well caribou work as an analogy for hadrosaurs. First we compared the body sizes of adults and juveniles in three Arctic herds of caribou. We discovered that juvenile caribou reach 80 to 85 percent of adult length and 53 to 74 percent of adult mass by the onset of migration.
Then we looked at the hadrosaur fossils. The cell structure of the bones clearly shows that the small hadrosaurs were juveniles at least one year old—not some form of a dwarf, high-latitude population. The lengths of the bones of the juveniles indicate that they were 27 to 37 percent of adult size and an estimated 11 percent of adult mass. The juvenile hadrosaurs, then, were relatively much smaller at one year of age than juvenile caribou are at the time of their seasonal migration. Therefore, on simple biomechanical grounds, it seems unlikely that hadrosaurs of the Arctic migrated great distances. We have not, however, discovered nests or eggs, which would of course clinch the argument that dinosaurs remained year-round at these high latitudes.
But if they stayed year-round, what did they eat during the cold winter months? We presume that the predators continued to eat meat because the patterns of wear on their teeth suggest no change in diet during the year.
We do not know exactly what the herbivorous dinosaurs ate. Edmontosaurus offers an opportunity to speculate, however, because it had a range from northern Alaska all the way south to West Texas. Today another herbivorous vertebrate—the mountain sheep of North America—has a comparable range. The diets of southern-latitude sheep are more restricted than those of northern-latitude sheep, probably because they have more resources available to them and can afford to be more particular. Analogously, northern individuals of Edmontosaurus may well have had a more generalized diet than their southern counterparts.
If the dinosaurs did not migrate, then presumably they would show adaptations supportive of year-round life in the high latitudes. Troodon provides the clearest example at the moment. This small, meat-eating dinosaur, known mainly from its teeth, is rare in more southern places, such as Alberta, Montana and Texas. In contrast, isolated teeth of this animal are very common in Alaska, which suggests that the population was large and widespread. What sets Troodon—at any latitude—apart from other predatory dinosaurs is its exceptionally large eyes. Among modern animals, proportionately large eyes tend to be an adaptation for living in low light conditions. Troodon may have been preadapted to the physical constraints of the high-latitude environment, which gave it a competitive advantage and set it on the path to become the most abundant predator of the northern ecosystem.
If Troodon was well adapted to the low light of Arctic winters, one might wonder how it could function during the long periods of daylight in warmer months. The forest might have provided refuge then. Anyone who has walked through even a modestly dense forest can appreciate how much lower the light level is in the woods than it is in an open field. In such an environment Troodon's large eyes would have continued to make it a fearsome predator.
We cannot confirm the size of the eyes of the other dinosaurs that lived in Alaska, because those parts of the skulls are only fragmentary or the bones are still being prepared for study. Although the dinosaurs described from southern Australia by Tom and Pat Rich are much older and very different, the Riches did note a pattern of increased orbital diameters for some dinosaurs.
Interestingly, Troodon in the Arctic were nearly twice the size of Troodon found in more southern locations. This difference contrasts sharply with the pattern discerned from measuring the bones of herbivorous dinosaurs of the North Slope, which fall well within the size range of those found in lower latitudes. It could be that Troodon's large eyes gave it a competitive advantage, eventually allowing it to become the top predator and to increase in size. Observers have reported a similar phenomenon in modern ecosystems: where wolves have been removed, coyotes have sometimes achieved larger body sizes.
Clearly, many questions remain to be answered about these extraordinary creatures. One of the most fascinating is whether they might have survived beyond the catastrophe that killed off the dinosaurs in other parts of the world at the end of the Cretaceous period.
Most paleontologists believe that a collision between the earth and a large asteroid drove the dinosaurs to extinction. The most likely location for the impact is the Chicxulub crater in Mexico. To study the far-reaching effects of such an impact, ideally one would look to a place far removed from the site, a place such as Alaska. Unfortunately, we have not found any fossils of dinosaurs in Alaska at just the right time to have a direct bearing on the question of whether dinosaurs died out abruptly or gradually. Fossil pollen data, however, offer tantalizing evidence that sections of rocks on the North Slope and elsewhere in Alaska are the right age to shed light on the extinction if they prove to contain fossils of fauna in addition to pollen. The possibility adds even more impetus to our search for ancient bones. We have just scratched the surface.