Bison Skull (IMAGE) University of California - Santa Cruz Caption The steppe bison had much larger horns than modern bison. Radiocarbon dating and DNA analysis of bison fossils enabled researchers to track the migration of Pleistocene steppe bison into an ice-free corridor that opened along the Rocky Mountains about 13,000 years ago. Credit Government of Yukon Usage Restrictions For use only with news coverage of this research. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.