Why we count by tens

Natural History, Feb, 2005 by Stephan Reebs

Digging for fossils on Nova Scotia's Bay of Fundy can be unnerving. Twice a day the highest tides in the world sweep up the beach, threatening to pin unwary visitors at the foot of a bluff. But the rewards of digging there are worth the risk. One site, dated to between 345 million and 359 million years ago, has just yielded the oldest extensive collection of tracks ever found of four-legged terrestrial animals.

Spencer G. Lucas, a paleontologist at the New Mexico Museum of Natural History and Science in Albuquerque, and his colleagues report that six kinds of tracks, all left by different species and now carved into the rock, have been discovered by a local collector, Chris Mansky. The footprints range from three-quarters of an inch to four inches long, all of them made by feet with five digits. There's little evidence of dragging tails or bellies, suggesting that most of the animals were walking, not sliding or slithering,

Before the find, no one could be sure about the number of toes on the first terrestrial tetrapods. Five is a common number for fossil feet, but some with fewer digits had been found, and some with more. But the newfound tracks make it clear that pentadactyls were prominent among the earliest conquerors of land. (gsa.confex.com/ gsa/2004AM/finalprogram/ abstract_77934.htm)