A perspective on the evidence for keratin protein preservation in fossils: An issue of replication versus validation

Abstract

The preservation potential of biomolecules within vertebrate integument through deep time has recently been subject to much research and controversy. In particular, the preservation potential of proteins, such as collagen and keratin, is currently debated. Here, we examine claims from a recent study (Schweitzer et al., 2018, PLoS One), which concludes that feather keratin has a high preservation potential. We argue that this work provides insufficient evidence for protein preservation due to issues of methodology and data interpretation. Additionally, we contrast their approach and claims to those of other recently published studies in relation to the question of keratin protein preservation in fossils. We worry that most of the perceived evidence for Mesozoic polypeptide survival stems from repeated replication of methods prone to false detection, rather than triangulation by validating these claims with alternative methods that provide independent lines of evidence. When alternative explanations exist for the evidence cited as support for dinosaur proteins far exceeding their predicted preservation limits, it is most parsimonious to reject the more extreme taphonomic hypotheses. The evidence is instead more consistent with a mode of preservation in which keratinous structures do not fossilize organically as polypeptides, but rather as largely pigment and/or calcium phosphate remnants, which were originally held within the keratin matrix that is now lost. Unsupported taphonomic models (e.g., keratin polypeptide preservation) have the potential to influence our interpretation of fossil data, potentially resulting in erroneous paleobiological or evolutionary conclusions, as illustrated in another recent paper (Pan et al., 2019, PNAS) that we also discuss.

Micro-XRF Study Of The Troodontid Dinosaur Jianianhualong Tengi Reveals New Biological And Taphonomical Signals

“…Even though previous studies 36,44 cannot determine whether the calcium phosphate in the fossilized keratinous structure was endogenous or secondarily precipitated, our results support that the fossilized claw sheaths are at least partially associated to the secondary mineralization of phosphate, with a similar process to the fossilization of the bones. The variation of Ca and P among each part of the claw also supports that endogenous phosphates are likely present in the fossilized sheath claws.…”

“…Nevertheless, the presence of P, Ca, Sr, Th and Ce in the claw sheath and the skeleton suggests that the keratinous unguis and the bony skeleton might share a similar process of fossilization. Saitta and Vinther 44 proposed two major modes for the keratin preservation in fossils: (a) predominantly organic in anoxic, finegrained sediments via melanosomes or amorphous melanin related components and (b) predominantly as calcium phosphate in coarse-grained, relatively oxic sediments. Chemical mapping of DLXH 1218 indicates that a combination of both modes could happen.…”

Sediment-encased pressure-temperature maturation experiments chemically simulate natural diagenesis of melanin

“…We hypothesise that, like natural diagenesis, P/T-maturation favours hydrolytic loss of integumentary proteins -evidenced by signi cant volume loss leading to voids/impressions as noted in prior work 28,30 .…”

“…An alternative view 57 suggests that brown stains in fossils can be readily produced by remnant proteinaceous components oxidatively condensing with sugar/lipid moieties to yield N-linked melanoidin-like heterocyclic polymers (i.e., advanced glycation end products [ 60 (i.e., extensively waterlogged, reducing lacustrine or marine settings). In contrast, the use of a pressurised setup in this study inhibits water from boiling off, instead directing proteins along hydrolytic reaction pathways.…”

“…Closed system maturation involves sealing tissue samples inside tiny Au/Ag capsules (~2 mm diameter) that are welded shut and placing them inside water-pressurised autoclaves. In contrast, tissues of larger sizes have been wrapped in aluminium foil, a highly open system, and subjected to heat and pressure in Ar-gas-pressurised autoclaves, but samples have been known to completely leak out of foil, leading to irretrievable products 29,30 . These setups represent unnatural extremes in closed vs.…”