We used authentication tests developed for ancient DNA to evaluate claims by 
Asara et al. (Reports, 13 April 2007, p. 280) of collagen peptide sequences 
recovered from mastodon and Tyrannosaurus rex fossils. Although the mastodon 
samples pass these tests, absence of amino acid composition data, lack of 
evidence for peptide deamidation, and association of alpha1(I) collagen 
sequences with amphibians rather than birds suggest that T. rex does not.