Added Later: Proponnents of the 40,000 year old date have responed here:
It is clear that the dates reported by Renne’s group need to be replicated and independently confirmed. This is important because we applied the Ar-Ar method as well and had no good results, and concluded that they were not reliable. Also, it is not clear from where exactly they took their samples and which fraction was dated. We took our samples directly from the footprint horizons.
Even if we are wrong and the Xalnene ash is indeed 1.3 million years old, as suggested by Renne’s et al, that is not automatically a reason to disregard interpretation of the features reported as “footprints”, simple because they are not in agreement with the established models for the settlement of the Americas.
This highlights the desperate need of further dating with different methods on the deposits in the Valsequillo Basin to make sure they make sense and to be able to establish a reliable chronology, considering the palaeontological and archaeological evidence included in them. This is exactly what we are doing at the moment with a grant from NERC (Natural Environment Research Council), during the next three years.
What we have ahead is a new era in the scientific research of the archaeological evidence found in the Valsequillo Basin and that is what Science is all about.
Our own dating efforts up to now and arguments explaining why we interpret the trace fossils preserved on the ash as human footprints will appear in the January issue of the Quaternary Science Reviews.