Pick from the Past
The Devils Corkscrew
GEOLOGIST ERWIN HINIDEY BARBOUR knew that he was looking at a spectacular new fossil, but he couldnt figure out what it was. In 1891, when he made his first expedition to the fossil-rich White River Badlands of Nebraska, the local ranchers had called his attention to the nine-foot-long, sand-filled tubes, enclosed within white fibrous material, that spiraled down into what was thought to be the remains of an ancient lake bed. Barbour was at no loss, however, for a scientific name for the weird spirals; he called them Daimonelix, the classical language equivalent of their local name, devils corkscrews.
Soon after, Barbour proposed that his Daimonelix were the remains of giant freshwater sponges. He also noted that at least one sponge had become entangled with the bones of an extinct rodent. When further research revealed that the deposits had never been associated with a lake but more likely with a semiarid grassland some twenty-two million years ago, Barbour recovered grandly by suggesting that the spirals were a new order of gigantic fossil plants. Again, a few rodent bones had turned up with the Daimonelix. While Barbour never gave up his fossil plant scenario, his fellow paleontologists had some ideas about the presence of the rodents. In 1893, Edward Drinker Cope and Theodor Fuchs independently suggested that the Daimonelix were not remains of organisms themselves, but were trace fossils of structures excavated by the rodents. In 1905, Olaf A. Peterson, of the Carnegie Museum, examined the fossils and determined that the bones were the remains of beavers and that the spirals were burrows. Like old sewer lines, the burrows were lined with roots (Barbour had been right about the plant tissue). The surrounding sediments were so rich in volcanic glass that the groundwater was charged with silica, and plant roots became embedded in a glassy matrix (the hard, white exterior of the burrows). This cast led to the preservation of the Daimonelix.
The burrowing beavers were about the size of woodchucks or smaller. Like other digging vertebrates, they had short tails and small ears and eyes. They also had long claws and superlong front teeth, or incisors, that grew rapidly to counteract the wear that results from digging. Three species are known, the large Palaeocastor magnus, middle-sized P. fossor; and the small Pseudopalaeocastor barbouri. The burrows of each species can be distinguished by the diameter within the spiral and the width of the dig marks. (North America was also home to aquatic beavers that lived at the same time as Palaeocastor; and the oldest-known beaver, Agnotocastor, was aquatic. However, the modem North American species, Castor canadensis, is descended from neither the burrowers nor Agnotocastor; it is an immigrant from Eurasia that arrived here some five million years ago.)
Not long after coming to the University of Kansas in 1970, I began a detailed examination of more than one thousand devils corkscrews. By bringing casts and actual specimens of corkscrews back to my laboratory, I discovered that the ancient beavers had left clues to their engineering strategy in the form of twenty two-million-year-old dig marks in the burrow walls.
Devils corkscrews spiraled some nine feet into the ground. Equipped with chambers and side passages, they provided beavers with safe, cool living quarters and possibly latrines and water sinks.
Instead of the narrow claw marks that I had expected, the walls were covered with broad grooves that I could match by scraping the incisors of the fossilized beaver skulls into wet sand. The beavers had used their teeth to scrape dirt off the walls. The very regular spirals were constructed by a continuous series of either right- or left-handed incisor strokes, and the burrows are divided almost fifty-fifty into right- and left-handed spirals. A burrowing beaver must have fixed its hind feet on the axis of the spiral and literally screwed itself straight down into the ground. Two or three yards underground, the burrow extended into a straight chamber slightly inclined upward where right- and left-handed incisor strokes alternate. These are the living chambers; some have low pockets that may have served as sinks for water or as latrines and side passages. This is where the skeletons of beavers and their cubs are usually found. Some burrows also contain highly inclined (about 45°) living chambers, which may have been estivation chambers, where the beavers stayed cool during hot, dry summers.
As they dug, the beavers had to dispose of the loose dirt they had scraped away with their front teeth. My investigations showed that the beavers scooped up the dirt with their paws and thrust it behind them. I think too that every so often the rodent must have used its remarkably flat head to push the accumulations out of the burrow. Burrow entrances would have been marked by high mounds of excavated soil.
I once mapped more than two hundred separate burrows that all seemed to be part of one colony. Like modem prairie dogs, these beavers may well have had extensive networks of colonies, towns covering acres. The existence of more complex social behaviors is easy to imagine but hard to prove. Did rodent guards stand on lookout on the mounds to give warning whistles of danger to other colony members? We do know that the beavers had enemies. An ancient raccoon relative, Zodiolestes daimonelixensis, as its name suggests, was found curled up in a Daimonelix looking completely at home. It may have lived within the colony and preyed predominantly on the resident beavers, much as the black-footed ferret does today in prairie dog colonies. When pursued on the surface, a Palaeocastor could attempt to escape by plunging headfirst into its burrow. The tops of burrows reveal expanded areas that would have allowed a fleeing beaver to turn around and then pop its head over the mound or to back down the hole, only a little broader than its body, then face the predator with strong jaws and formidable teeth.
The fossil record is full of examples of evolutionary developments, such as the beavers colonies of spirals, which have now disappeared. The magic is in the reappearance of many of these developments at different times. Long before Palaeocastor; and for that matter, before any true mammals existed, some members of a group called mammallike reptiles, the dicynodonts, took to burrowing and created spiral burrows so remarkably like those of Palaeocastor that they should probably be included in the same trace-fossil genus, Daimonelix. And today, while modern beavers have undertaken new engineering feats, the spirit of burrowing Palaeocastor echoes in the subterranean labyrinths of prairie dog towns.