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 fos- 

 sils of structures excavated by the rodents. 

 In 1905, Olaf A. Peterson, of the Carnegie 

 Museum, examined the fossils and deter- 

 mined 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 sil- 

 ica, and plant roots became embedded in a 

 glassy matrix (the hard, white exterior of 

 the burrows). This "cast" led to the preser- 

 vation 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 in- 

 cisors, that grew rapidly to counteract the 

 wear that results from digging. Three spe- 

 cies are known, the large Palaeocastor 

 magnus, middle-sized P. fossor, and the 

 small Pseudopalaeocastor barbouri. The 

 burrows of each species can be distin- 

 guished by the diameter within the spiral 

 and the width of the dig marks. (North 

 America was also home to aquatic beavers 

 that Uved at the same time as Palaeocas- 

 tor, and the oldest-known beaver, Agnoto- 

 castor, was aquatic. However, the modem 

 North American species. Castor canaden- 

 sis, is descended from neither the burrow- 

 ers nor Agnotocastor; it is an immigrant 

 from Eurasia that arrived here some five 

 milUon years ago.) 



Not long after coming to the University 

 of Kansas in 1970, 1 began a detailed ex- 

 amination of more than one thousand 

 devil's corkscrews. By bringing casts and 

 actual specimens of corkscrews back to 

 my laboratory, I discovered that the an- 

 cient beavers had left clues to their engi- 

 neering strategy in the form of twenty- 

 two-million-year-old dig marks in the 

 burrow walls. 



Devil's 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." 



Drawing by Ed Heck 



Instead of the narrow claw marks that I 

 had expected, the walls were covered with 

 broad grooves that I could match by scrap- 

 ing 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 it- 

 self straight down into the ground. Two or 

 three yards underground, the burrow ex- 

 tended into a straight chamber slightly in- 

 clined upward where right- and left- 

 handed incisor strokes alternate. These are 

 the living chambers; some have low pock- 

 ets that may have served as sinks for water 

 or as latrines and side passages. This is 

 where the skeletons of beavers and their 



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cubs are usually found. Some burrows also 

 contain highly inclined (about 45°) living 

 chambers, which may have been esfiva- 

 tion 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. 1 think too that every so often the ro- 

 dent 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 exca- 

 vated 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 so- 

 cial behaviors is easy to imagine but hard 

 to prove. Did rodent guards stand on look- 

 out on the mounds to give waming whis- 

 fles 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 predomi- 

 nantly on the resident beavers, much as the 

 black-footed ferret does today in prairie 

 dog colonies. When pursued on the sur- 

 face, a Palaeocastor could attempt to es- 

 cape 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 reap- 

 pearance of many of these developments 

 at different times. Long before Palaeocas- 

 tor, and for that matter, before any tme 

 mammals existed, some members of a 

 group called mammallike reptiles, the di- 

 cynodonts, 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 modem 

 beavers have undertaken new engineering 

 feats, the spirit of burrowing Palaeocastor 

 echoes in the subterranean labyrinths of 

 prairie dog towns. 



60 Natural History 4/94 



