Melanins are widespread pigments in vertebrates, with important roles in visual signaling, UV protection, and homeostasis. Fossil evidence of melanin and melanin-bearing organellesmelanosomesin ancient vertebrates may illuminate the evolution of melanin and its functions, but macroevolutionary trends are poorly resolved. Here, we integrate fossil data with current understanding of melanin function, biochemistry, and genetics. Mapping key genes onto phenotypic attributes of fossil vertebrates identifies potential genomic controls on melanin evolution. Taxonomic trends in the anatomical location, geometry, and chemistry of vertebrate melanosomes are linked to the evolution of endothermy. These shifts in melanin biology suggest fundamental links between melanization and vertebrate ecology. Tissue-specific and taxonomic trends in melanin chemistry support evidence for evolutionary tradeoffs between function and cytotoxicity. Melanin in Vertebrates Melanins (see Glossary) are dark to rufous pigments that are widespread in vertebrates and underpin critical functions in physiology and behavior [1]. Fossil evidence of melanin extending to over 300 million years ago has triggered a paradigm shift in paleobiology, prompting remarkable reconstructions of the coloration and behavior of extinct vertebrates [2-6]. New discoveries of internal melanins in vertebrate fossils have broadened our understanding of the functional diversity of ancient melanins [7-9] and invite a re-evaluation of the macroevolutionary history of melanin and its functions. Here, we synthesize trends in the fossil record of melanin and explore fossil evidence for the evolution of melanin function and the genetic basis of melanization. This highlights the value of the fossil record as a resource for tracking melanin evolution through deep time. Functions of Melanin in Ancient Vertebrates In extant vertebrates, melanin occurs as micron-sized organelles, melanosomes, in the integument, eyes and internal tissues and functions in photoprotection, visual signaling, thermoregulation, immunity, antioxidation, mechanical strengthening, and abrasion resistance [6,10,11] (Figure 1, Box 1). It is unclear which functions evolved first and which selection pressures dominate [11,12]. Fossils preserving evidence of melanin offer a unique temporal perspective. Melanin has been reported from fossil vertebrates from >25 localities from the Carboniferous to the Pliocene (Table S1 in the supplemental information online). The fossils include cyclostomes, fish, frogs, lizards, and other squamates, ichthyosaurs, plesiosaurs, turtles, pterosaurs, feathered and nonfeathered dinosaurs, birds, and mammals. This phylogenetically and temporally broad dataset yields evidence for ancient functions of melanin (Figure 2). Highlights In extant vertebrates melanin fulfils diverse roles including visual communication, photoprotection, antioxidation, and mechanical strengthening of tissues, but the evolution of these functions is debated. The discovery that melanosomes in fossil and modern ve...

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.

11The Rhaetian (latest Triassic) is best known for its basal bone bed, but there are numerous 12 other bone-rich horizons in the succession. Boreholes taken around the M4-M5 motorway 13 junction in SW England provide measured sections with multiple Rhaetian bone beds. The 14 microvertebrate samples in the various bone beds differ through time in their composition and 15 in mean specimen size. The onset of the Rhaetian transgression accumulated organic debris to 16 form a fossiliferous layer high in biodiversity at the base of the Westbury Formation. The 17 bone bed at the top of the Westbury Formation represents a community with lower 18 biodiversity. The bone beds differ in their faunas: chondrichthyan teeth are dominant in the 19 basal bone bed, but actinopterygians dominate the higher bone bed. These differences could 20 be taphonomic, but are more likely evidence for ecological-evolutionary changes. Further, a 21 change from larger to smaller specimen sizes up-sequence allows rejection of an earlier idea 22 that the successive bone beds represented multiple reworkings of older bone beds. 23 24

The evolution of modern sharks, skates and rays (Elasmobranchii) is largely enigmatic due to their possession of a labile cartilaginous skeleton; consequently, taxonomic assignment often depends on isolated teeth. Bullhead sharks (Heterodontiformes) are a group of basal neoselachians, thus their remains and relationships are integral to understanding elasmobranch evolution. Herein we fully describe †Paracestracion danielia bullhead shark from the Late Jurassic plattenkalks of Eichst€ att, Germany (150-154 Ma)for its inclusion in cladistic analysis (utilizing parsimonious principles) of morphological characters from complete †Paracestracion and Heterodontus fossil specimens as well as extant forms of the latter. The presence of two separate monophyletic clades within Heterodontiformes was confirmed, based on predominantly non-dental characters,