How long ago did ancient ‘beavers’ start ‘engineering’ Earths landscape?
-lecture report by Pauline Donaldson for an unpublished 2011 Whip-poor-will
More than 50 members of MVFN and the public were treated to a fascinating story packed with modern science details and enthusiastically told at our March lecture. Speaker Dr. Natalia Rybczynski, Canadian Museum of Nature paleobiologist, came to talk about our national animal, the beaver. When and why did this unique creature evolve? “How did evolution come up with this?” These are interesting questions from the perspective of the structure of our environment. How long have beavers been around cutting down trees and most significantly, re-engineering the very landscape they inhabit with their incredible dams?
Natalia explained that our Canadian beaver Castor canadensis and European beavers (Castor fiber) are the only surviving members of a diverse group of 18 genera of beaver ancestors which died out millions of years ago. These include ones with skulls much larger and much smaller than Castor ranging from the ‘infamous’ very large Castoroides, to the non-swimming burrowing beavers which dug with their teeth (forming, for the fossil record, mysterious, grooved tunnel structures known as devils corkscrews whose origins were earlier attributed to worms or thought to be channels from plant roots), to Agnotocastor which had a rather ‘boring’ rat-like tail.
To find out how long ago ‘beavers’ have been engineering the landscape, Dr. Rybczynski considered what features make a beaver the engineer it is. Swimming ability, wood-cutting talents and, of course, the ability to build dams; but how far back in the fossil record can one find evidence of these features, and which of Castors ancient relatives possessed them? As a paleobiologist Natalia studies the fossil record to better understand animals that live and that once lived. A key site Rybczynski and colleagues visit several times a year is ‘Beaver Pond’ on Ellesmere Island, an Early Pilocene peat deposit rich in fossils of bear, beaver and beaver-cut wood from a time when that arctic area enjoyed a lush, greener, warm period.
Fossil evidence suggests that beginning ~ 23 million years ago some of Castor’s ancestors could swim. Modern-day beaver are the most swimming adapted of rodents: closed ears, webbed feet, a streamlined body, and, last but not least, the characteristic flattened tail, an improvement which seems to have come about ~ 5-10 million years ago, and which is unique to Castor. This amazing tail (as we saw clearly in an underwater video clip) is not just a rudder for steering but, as the flapping rate showed, clearly contributes to the animal’s powerful underwater propulsion. The familiar flattened part is only part of the tail; there is a furry, muscular part closer to the body.
The next feature to consider is wood cutting. Once one knows how a beaver’s teeth cut wood, one can predict which of beavers ancestors had teeth that might work the same way. So, Dr. Rybczynski went undercover with some zoo beavers and made a movie of them cutting wood. They use their lower teeth only, and, turning their heads, they use only one lower tooth at a time! The cutting produces a characteristic gnawing pattern on two pieces of wood, an ‘A’ and a ‘B’ piece. Castor uses about 60% of the cutting edge of its incisor for cutting. At the Ellesmere Island site fossil evidence of beaver ancestor Dipoides was discovered with what could be wood-cutting teeth, although it likely used only 45% of its incisor surface. It was exciting to learn that there was fossilized Dipoides-cut wood to match. No fossil cut wood has yet been found to match the dentition of the mighty Castoroides ohioensis (shown in this early painting by O.M. Highley). When asked about the likelihood of it ever being found, Dr. Rybczynski says, yes, she thinks it will eventually be found.
So 23 million years ago there were creatures which could swim and cut wood. Perhaps this means our planets landscape has been engineered by beaver-like creatures for 23 million years. The final piece of the puzzle will, of course, be to determine when dam building actually began. The hope now is to find a fossil of an ancient beaver dam. Dr. Rybczynski speculates that ‘Beaver Pond’ on Ellesmere Island may actually be one. However, they will need to do more research to look at the sequence of deposition of materials there before they can be sure.
