Pterosaur melanosomes support signalling functions for early feathers

Remarkably well-preserved soft tissues in Mesozoic fossils have yielded substantial insights into the evolution of feathers1. New evidence of branched feathers in pterosaurs suggests that feathers originated in the avemetatarsalian ancestor of pterosaurs and dinosaurs in the Early Triassic2, but the homology of these pterosaur structures with feathers is controversial3,4. Reports of pterosaur feathers with homogeneous ovoid melanosome geometries2,5 suggest that they exhibited limited variation in colour, supporting hypotheses that early feathers functioned primarily in thermoregulation6. Here we report the presence of diverse melanosome geometries in the skin and simple and branched feathers of a tapejarid pterosaur from the Early Cretaceous found in Brazil. The melanosomes form distinct populations in different feather types and the skin, a feature previously known only in theropod dinosaurs, including birds. These tissue-specific melanosome geometries in pterosaurs indicate that manipulation of feather colour—and thus functions of feathers in visual communication—has deep evolutionary origins. These features show that genetic regulation of melanosome chemistry and shape7–9 was active early in feather evolution.


Origin of the specimen.
The specimen is believed to have been originally poached from an undetermined outcrop of the Early Cretaceous Crato Formation (Araripe Basin, north-eastern Brazil) and then resided in private collections in Europe for an unknown period of time. The specimen was deposited by its private owner at the Royal Belgian Institute of Natural Sciences (RBINS) through the French fossil preparation company Eldonia in 2020. The Brazilian authorities were contacted immediately and the specimen is now part of the collections of the Museum of Earth Sciences, Rio de Janeiro (collection number: MCT.R.1884).
Negotiations between the RBINS, Eldonia and Brazilian authorities led to the official physical repatriation of the specimen to Brazil in early February 2022. The specimen was incompletely prepared when it arrived at Eldonia. Preliminary preparation under the supervision of FE allows us to certify that this specimen is not a composite and that restorations prior to its arrival at RBINS are limited to standard consolidation procedures.

Geological background.
The specimen is hosted within grey laminated limestones and was probably excavated from the Nova Olinda Member of the Crato Formation (Fm). This member is located at the base of the formation; all specimens of Tupandactylus imperator described thus far have been recovered from here 1-4 . The vertebrate assemblage in the Nova Olinda Member includes well preserved fish 5-8 , bird feathers 9-15 , lizards 16-19 , anurans 20-23 , turtles 24-28 and rare crocodilians 29-32 . The member also preserves abundant arthropods 33, 34 , arachnids 35 , crustaceans 36 and plants, including early angiosperms 37, 38 .
The deposits of the Crato Fm consist of a mixture of siliciclastic deposits and laminated limestones 39 . The Nova Olinda Member is a laminated unit that for most of its vertical thickness (ca. 13 m) comprises pale carbonate-rich and dark carbonate-poor laminae 3 . This composition indicates deposition in a stratified water body characterized by a persistent thermocline or chemocline. The preservation of fossil soft tissues probably reflects (at least in part) low oxygen conditions in bottom waters. The geochemistry, sedimentology and palaeontology of the sediments indicate deposition in a brackish, restricted environment (not open marine) close to shore, probably a lagoon 39-40 .

1.3.
Description of the specimen.
Systematic Palaeontology.  is not unexpected and is implied in sexual selection 46 . We therefore tentatively refer this specimen to Tupandactylus cf. imperator, pending further evidence on the intraspecific variability of the cranial crest in this taxon.

Soft tissue anatomy.
In addition to the monofilaments and branched feathers associated with the occipital process (and that are described in detail in the main text), the cranial crest exhibits two types of fibrous integumentary structures. First, numerous sub-parallel, light to dark brown fibres (100-150 µm wide and up to ca. 300 mm long (Fig. 1a, Extended data Figs. 1, 4)) are widespread across the cranial crest. Those elongate fibres are well defined in the posteroventral part of the crest. They are faint or not evident in the anterodorsal part of the crest, where only a thick, black layer, possibly representing decayed organic matter, is present.
Most of these fibres are mutually parallel, aligned with the sagittal axis ventrally and curved dorsally.
Ventrally, a series of ca. 100 striking dark brown structures, each 600-900 μm wide, are aligned close to, but isolated from, the occipital process by a 20 mm to 30 mm-wide zone that lacks soft tissues (save faint fibre impressions). Each dark brown structure gives rise to a series of these sub-parallel dark brown fibres (usually ca. five) (Extended Data Fig. 4a-c).