Recent studies have suggested the presence of keratin in fossils dating back to the Mesozoic. However, ultrastructural studies revealing exposed melanosomes in many fossil keratinous tissues suggest that keratin should rarely, if ever, be preserved. In this study, keratin's stability through diagenesis was tested using microbial decay and maturation experiments on various keratinous structures. The residues were analysed using pyrolysis‐gas chromatography‐mass spectrometry and compared to unpublished feather and hair fossils and published fresh and fossil melanin from squid ink. Results show that highly matured feathers (200–250°C/250 bars/24 h) become a volatile‐rich, thick fluid with semi‐distinct pyrolysis compounds from those observed in less degraded keratins (i.e. fresh, decayed, moderately matured, and decayed and moderately matured) suggesting hydrolysis of peptide bonds and potential degradation of free amino acids. Neither melanization nor keratin (secondary) structure (e.g. ⍺‐ vs β‐keratin) produced different pyrograms; melanin pyrolysates are largely a subset of those from proteins, and proteins have characteristic pyrolysates. Analyses of fossil fur and feather found a lack of amides, succinimide and piperazines (present even in highly matured keratin) and showed pyrolysis compounds more similar to fossil and fresh melanin than to non‐matured or matured keratin. Although the highly matured fluid was not water soluble at room temperature, it readily dissolved at elevated temperatures easily attained during diagenesis, meaning it could leach away from the fossil. Future interpretations of fossils must consider that calcium phosphate and pigments are the only components of keratinous structures known to survive fossilization in mature sediments.

Exceptionally preserved fossils are the product of complex interplays of biological and geological processes including burial, autolysis and microbial decay, authigenic mineralization, diagenesis, metamorphism, and finally weathering and exhumation. Determining which tissues are preserved and how biases affect their preservation pathways is important for interpreting fossils in phylogenetic, ecological, and evolutionary frameworks. Although laboratory decay experiments reveal important aspects of fossilization, applying the results directly to the interpretation of exceptionally preserved fossils may overlook the impact of other key processes that remove or preserve morphological information. Investigations of fossils preserving nonbiomineralized tissues suggest that certain structures that are decay resistant (e.g., the notochord) are rarely preserved (even where carbonaceous components survive), and decay-prone structures (e.g., nervous systems) can fossilize, albeit rarely. As we review here, decay resistance is an imperfect indicator of fossilization potential, and a suite of biological and geological processes account for the features preserved in exceptional fossils.

Fossils were thought to lack original organic molecules, but chemical analyses show that some can survive. Dinosaur bone has been proposed to preserve collagen, osteocytes, and blood vessels. However, proteins and labile lipids are diagenetically unstable, and bone is a porous open system, allowing microbial/molecular flux. These ‘soft tissues’ have been reinterpreted as biofilms. Organic preservation versus contamination of dinosaur bone was examined by freshly excavating, with aseptic protocols, fossils and sedimentary matrix, and chemically/biologically analyzing them. Fossil ‘soft tissues’ differed from collagen chemically and structurally; while degradation would be expected, the patterns observed did not support this. 16S rRNA amplicon sequencing revealed that dinosaur bone hosted an abundant microbial community different from lesser abundant communities of surrounding sediment. Subsurface dinosaur bone is a relatively fertile habitat, attracting microbes that likely utilize inorganic nutrients and complicate identification of original organic material. There exists potential post-burial taphonomic roles for subsurface microorganisms.

Abstract:We examined bristle-like appendages on the tail of the Early Cretaceous basal ceratopsian dinosaur Psittacosaurus with laser-stimulated fluorescence imaging. Our study reveals previously unknown details of these structures and confirms their identification as integumentary appendages. For the first time, we show that most bristles appear to be arranged in bundles and that they exhibit a pulp that widens towards the bristle base. We consider it likely that the psittacosaur bristles are structurally and developmentally homologous to similar filamentous appendages of other dinosaurs, namely the basal heterodontosaurid Tianyulong and the basal therizinosauroid theropod Beipiaosaurus, and attribute the greater robustness of the bristles of Psittacosaurus to a higher degree of cornification and calcification of its integument (both skin and bristles). Although the psittacosaur bristles are probably homologous to avian feathers in their origin from discrete cell populations, it is uncertain whether they developed from a follicle, one of the developmental hallmarks of true feathers. In particular, we note a striking resemblance between the psittacosaur bristles and the cornified spine on the head of the Horned Screamer, Anhima cornuta, an extant anseriform bird. Similar, albeit thinner keratinous filaments of extant birds are the "beard" of the Turkey, Meleagris gallopavo, and the crown of the Congo 2 Peafowl, Afropavo congensis. All of these structures of extant birds are distinct from true feathers, and because at least the turkey beard does not develop from follicles, detailed future studies of their development would be invaluable towards deepening our understanding of dinosaur filamentous integumentary structures.

Exceptional fossils can preserve diageneticallyaltered biomolecules. Understanding the pathways that lead to such preservation is vital to utilizing fossil information in evolutionary and palaeoecological studies. Experimental taphonomy explores the stability of tissues during microbial/ autolytic decay or their molecular stability through maturation under high pressure and temperature. Maturation experiments often take place inside sealed containers, preventing the loss of labile, mobile or volatile molecules. However, wrapping tissues inside aluminium foil, for example, can create too open a system, leading to loss of both labile and recalcitrant materials. We present a novel experimental procedure for maturing tissues under elevated pressure/temperature inside compacted sediment. In this procedure, porous sediment allows maturation breakdown products to escape into the sediment and maturation chamber, while recalcitrant, immobile components are contained, more closely mimicking the natural conditions of fossilization. To test the efficacy of this procedure in simulating fossil diagenesis, we investigate the differential survival of melanosomes relative to proteinaceous tissues through maturation of fresh lizard body parts and feathers. Macro-and ultrastructures are then compared to fossils. Similar to many carbonaceous exceptional fossils, the resulting organic components are thin, dark films composed mainly of exposed melanosomes resting on the sediment in association with darkened bones. Keratinous, muscle, collagenous and adipose tissues appear to be lost. Such results are consistent with predictions derived from non-sediment-encased maturation experiments and our understanding of biomolecular stability. These experiments also suggest that organic preservation is largely driven by the original molecular composition of the tissue and the diagenetic stability of those molecules, rather than the tissue's decay resistance alone; this should be experimentally explored in the future.

Preservation of feather fibers from the Late Cretaceous dinosaur Shuvuuia deserti raises concern about immunohistochemical analyses on fossilsThe MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Saitta, Evan T., et al. "Preservation of feather fibers from the Late Cretaceous dinosaur Shuvuuia deserti raises concern about immunohistochemical analyses on fossils." Organic Geochemistry, 125 (November 2018): 142-151.

Conclusive evidence for sexual dimorphism in non-avian dinosaurs has been elusive. Here it is shown that dimorphism in the shape of the dermal plates of Stegosaurus mjosi (Upper Jurassic, western USA) does not result from non-sex-related individual, interspecific, or ontogenetic variation and is most likely a sexually dimorphic feature. One morph possessed wide, oval plates 45% larger in surface area than the tall, narrow plates of the other morph. Intermediate morphologies are lacking as principal component analysis supports marked size- and shape-based dimorphism. In contrast, many non-sex-related individual variations are expected to show intermediate morphologies. Taphonomy of a new quarry in Montana (JRDI 5ES Quarry) shows that at least five individuals were buried in a single horizon and were not brought together by water or scavenger transportation. This new site demonstrates co-existence, and possibly suggests sociality, between two morphs that only show dimorphism in their plates. Without evidence for niche partitioning, it is unlikely that the two morphs represent different species. Histology of the new specimens in combination with studies on previous specimens indicates that both morphs occur in fully-grown individuals. Therefore, the dimorphism is not a result of ontogenetic change. Furthermore, the two morphs of plates do not simply come from different positions on the back of a single individual. Plates from all positions on the body can be classified as one of the two morphs, and previously discovered, isolated specimens possess only one morph of plates. Based on the seemingly display-oriented morphology of plates, female mate choice was likely the driving evolutionary mechanism rather than male-male competition. Dinosaur ornamentation possibly served similar functions to the ornamentation of modern species. Comparisons to ornamentation involved in sexual selection of extant species, such as the horns of bovids, may be appropriate in predicting the function of some dinosaur ornamentation.

Preserved melanin pigments have been discovered in fossilised integumentary appendages of several amniote lineages (fishes, frogs, snakes, marine reptiles, non‐avialan dinosaurs, birds, and mammals) excavated from lagerstätten across the globe. Melanisation is a leading factor in organic integument preservation in these fossils. Melanin in extant vertebrates is typically stored in rod‐ to sphere‐shaped, lysosome‐derived, membrane‐bound vesicles called melanosomes. Black, dark brown, and grey colours are produced by eumelanin, and reddish‐brown colours are produced by phaeomelanin. Specific morphotypes and nanostructural arrangements of melanosomes and their relation to the keratin matrix in integumentary appendages create the so‐called 'structural colours'. Reconstruction of colour patterns in ancient animals has opened an exciting new avenue for studying their life, behaviour and ecology. Modern relationships between the shape, arrangement, and size of avian melanosomes, melanin chemistry, and feather colour have been applied to reconstruct the hues and colour patterns of isolated feathers and plumages of the dinosaurs Anchiornis, Sinosauropteryx, and Microraptor in seminal papers that initiated the field of palaeocolour reconstruction. Since then, further research has identified countershading camouflage patterns, and informed subsequent predictions on the ecology and behaviour of these extinct animals. However, palaeocolour reconstruction remains a nascent field, and current approaches have considerable potential for further refinement, standardisation, and expansion. This includes detailed study of non‐melanic pigments that might be preserved in fossilised integuments. A common issue among existing palaeocolour studies is the lack of contextualisation of different lines of evidence and the wide variety of techniques currently employed. To that end, this review focused on fossil amniotes: (i) produces an overarching framework that appropriately reconstructs palaeocolour by accounting for the chemical signatures of various pigments, morphology and local arrangement of pigment‐bearing vesicles, pigment concentration, macroscopic colour patterns, and taphonomy; (ii) provides background context for the evolution of colour‐producing mechanisms; and (iii) encourages future efforts in palaeocolour reconstructions particularly of less‐studied groups such as non‐dinosaur archosaurs and non‐archosaur amniotes.