We use authentication tests developed for ancient DNA to evaluate claims by Asara et al. of collagen peptide sequences recovered from mastodon and Tyrannosaurus rex fossils. Although the mastodon passes, absence of amino acid composition data, lack of evidence for peptide deamidation, and association of the α1(I) peptide sequences with amphibians not birds, suggests that T. rex does not. CommentEarly reports of DNA preservation in multi-million year old bones (i.e. dinosaurs) have been largely dismissed (see 1 and SOM T1, 2) but reports of protein recovery are persistent (see 3 for review). Most of these studies used secondary methods of detection, but protein sequence, arguably the gold standard for molecular palaeontology, has now been claimed for the first time (2). Following initial optimism generated by reports of dinosaur DNA, there arose a gradual awareness of the problems and pitfalls which bedevil analysis of ancient samples (1), leading to a series of recommendations for future analysis (1, 4). As yet, there are no equivalent standards for fossil protein, so here we apply the recommended tests for DNA (4) to the authentication of the reported protein sequences (2) ( Table 1).* To whom correspondence should be addressed. E-mail: mc80@york.ac.uk. Europe PMC Funders Group Likelihood of collagen survivalThe extremely hierarchical structure of collagen results in unusual, catastrophic degradation (5) as a consequence of fibril collapse. The rate of collagen degradation in bone is slow because the mineral 'locks' the components of the matrix together, preventing helical expansion which is a pre-requisite of fibril collapse (6). The packing which stabilises collagen fibrils (6) also increases the temperature sensitivity of degradation (E a 173 kJ mol -1 ; Fig. 1). Collagen decomposition would be much faster in the T. rex buried in the then megathermal (>20 °C) (7) environment of the Hell Creek formation (collagen t ½ ~ 2 ka) than it would have been in the mastodon lying within the Doeden Gravel Beds (present day mean temperature 7.5 °C; collagen t ½ 130 ka; Fig. 1).