Monday, September 26, 2005

Mexican Footprints: An Update

Readers may remember the footprints found in Mexico that have been dated to about 40,000 years BP. Geotimes has an interesting story on the subject:

In summer 2003, researchers Matthew Bennett of Bournemouth University and Silvia Gonzalez and David Huddart of Liverpool John Moores University in the United Kingdom were dating and mapping the geology of the Valsequillo Basin in Mexico, about 130 kilometers south of Mexico City, when they came across what appeared to be footprints on the floor of an abandoned quarry. Examining the site further, they found the site littered with footprints, Bennett says — 269 individual prints of humans and animals intermingled.

Sixty percent of the footprints appear to be human, with telltale arches and impressions of the heels, balls and toes, and 36 percent of those appear to be child-sized, according to the researchers, whose work is in press in Quaternary Science Reviews. The remaining 40 percent of the prints were from a variety of animals, Bennett says, including dogs, big cats and animals with cloven hooves, such as deer and camels. The researchers also found mastodon and mammoth teeth.

Previously, in the 1960s and 1970s, archaeologists found megafaunal remains, including bones that had been “worked” with tools, scattered throughout the basin. Those remains had been unreliably dated to be between 20,000 and 40,000 years old, Bennett says, so the sites have been somewhat ignored since then.

The footprints are preserved in a layer of volcanic ash from the eruption of Cerro Toluquilla beneath a shallow lake in the Valsequillo Basin just over 40,000 years ago. “Volcanic ash lithifies quickly, like cement,” Bennett says, so when the inhabitants of the lake shores wandered across the mucky ash, their footprints were captured. When lake levels later rose, water washed over the footprints, burying them in lake sediments, he says. “So we have this great stratigraphic sequence” of lake sediments, topped by ash, which is then topped again by lake sediments, Bennett says, that can be dated.

The dissenting opinion:

But Michael Waters, a geoarchaeologist at Texas A&M University in College Station, is not convinced. He says that the ash layer is likely much older than 40,000 years, and should be retested using different methods. Furthermore, says Waters, who has visited the site, “I have serious reservations as to whether or not these are even footprints, human or animal.” The site has been so extensively quarried over the years, being chopped with axes and picks, that these imprints could just be tool marks that have weathered.

The team, Waters says, needs to find tracks in outcrops or areas that have not been quarried — “look for them like you would look for dinosaur or other trackways.” Bennett says that he and his colleagues are planning to begin just such excavations soon.

Even more interesting, there is a link to the Mexican Footprints Website Which contains a wide variety of information on the geology, dating methods, etc. Based on what I saw at the site I am a little less skeptical - although I still have some reservations.