Some recent paper with free pdfs:
Preserved melanin pigments have been discovered in fossilised integumentary appendages of several amniote lineages (fishes, frogs, snakes, marine reptiles, nonâavialan dinosaurs, birds, and mammals) excavated from lagerstÃtten across the globe. Melanisation is a leading factor in organic integument preservation in these fossils. Melanin in extant vertebrates is typically stored in rodâ to sphereâshaped, lysosomeâderived, membraneâbound vesicles called melanosomes. Black, dark brown, and grey colours are produced by eumelanin, and reddishâbrown colours are produced by phaeomelanin. Specific morphotypes and nanostructural arrangements of melanosomes and their relation to the keratin matrix in integumentary appendages create the soâcalled '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 nonâmelanic 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 pigmentâbearing vesicles, pigment concentration, macroscopic colour patterns, and taphonomy; (ii) provides background context for the evolution of colourâproducing mechanisms; and (iii) encourages future efforts in palaeocolour reconstructions particularly of lessâstudied groups such as nonâdinosaur archosaurs and nonâarchosaur amniotes.
Fossils are a key source of data on the evolution of feather structure and function through deep time, but their ability to resolve macroevolutionary questions is compromised by an incomplete understanding of their taphonomy. Critically, the relative preservation potential of two key feather components, melanosomes and keratinous tissue, is not fully resolved. Recent studies suggesting that melanosomes are preferentially preserved conflict with observations that melanosomes preserve in fossil feathers as external moulds in an organic matrix. To date, there is no model to explain the latter mode of melanosome preservation. We addressed these issues by degrading feathers in systematic taphonomic experiments incorporating decay, maturation and oxidation in isolation and combination. Our results reveal that the production of mouldic melanosomes requires interactions with an oxidant and is most likely to occur prior to substantial maturation. This constrains the taphonomic conditions under which melanosomes are likely to be fossilized. Critically, our experiments also confirm that keratinous feather structures have a higher preservation potential than melanosomes under a range of diagenetic conditions, supporting hitherto controversial hypotheses that fossil feathers can retain degraded keratinous structures.
Christiana Katti Micaela Stacey-Solis, Nicole AnahÃ Coronel-Rojas and Wayne Iwan Lee Davies (2019)
The Diversity and Adaptive Evolution of Visual Photopigments in Reptiles
Frontiers in Ecology and Evolution 7:352
Reptiles are a highly diverse class that consists of snakes, geckos, iguanid lizards, and chameleons among others. Given their unique phylogenetic position in relation to both birds and mammals, reptiles are interesting animal models with which to decipher the evolution of vertebrate photopigments (opsin protein plus a light-sensitive retinal chromophore) and their contribution to vision. Reptiles possess different types of retinae that are defined primarily by variations in photoreceptor morphology, which range from pure-cone to rod-dominated retinae with many species possessing duplex (rods and cones) retinae. In most cases, the type of retina is thought to reflect both the lifestyle and the behavior of the animal, which can vary between diurnal, nocturnal, or crepuscular behavioral activities. Reptiles, and in particular geckos and snakes, have been used as prime examples for the "transmutation" hypothesis proposed by Walls in the 1930s-1940s, which postulates that some reptilian species have migrated from diurnality to nocturnality, before subsequently returning to diurnal activities once again. This theory further states that these behavioral changes are reflected in subsequent changes in photoreceptor morphology and function from cones to rods, with a return to cone-like photoreceptors once again. Modern sequencing techniques have further investigated the "transmutation" hypothesis by using molecular biology to study the phototransduction cascades of rod- and cone-like photoreceptors in the reptilian retina. This review will discuss what is currently known about the evolution of opsin-based photopigments in reptiles, relating habitat to photoreceptor morphology, as well as opsin and phototransduction cascade gene _expression_.