While feathers have been known in fossil Birds from the Mesozoic since the nineteenth century, they remain extremely rare, with the vast majority of known specimens coming from a single source, the Early Cretaceous Jehol Biota of northwest China, and the remainder coming from a few other locations in Asia and Europe. To date no Bird with preserved feathers has been described from the supercontinent of Gondwana, which comprised the modern continents of South America, Africa, Antarctica, Madagascar, India, Australia and New Zealand, forming an amalgamated landmass covering much of the Southern Hemisphere during the late Mesozoic.
In a paper published in the journal Nature Communications on 26 May 2015, Ismar de Souza Carvalho of the Departamento de Geologia at the Universidade Federal do Rio de Janeiro, Fernando Novas of the Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’, Federico Agnolín and Marcelo Isasi of the Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’ and the Fundación de Historia Natural ‘Félix de Azara’ at the Universidad Maimónides, Francisco Freitas of Geopark Araripe and José Andrade of the Departamento Nacional da Produção Mineral describe a fossil Bird from the Early Cretaceous Crato Formation of Brazil which possesses the first known feathers in any Mesozoic Bird from the Southern Hemipshere.
The specimen is very small, about 6 cm in total length, and has large orbits and poor mineralization of the ends of the long bones, for which reason it is thought to be a juvenile. It is not assigned to a species or genus, but is recognized as an Enantiornithine, a member of a group of Mesozoic Birds related to but not ancestral to the modern Neornithine Birds, with a set of distinct features not found in any post-Mesozoic Birds, including most notably the retention of a bony snout with teeth rather than an keratinous beak.
Main slab and interpretative drawing of the Crato Bird specimen. (a) Main slab. (b) Interpretative drawing of the skeleton and feathers. (c) Reconstructed cross-section at the level of distal vanes of the feathers. (d) Reconstructed cross-section at mid-length of the rachis. (e) Reconstructed cross-section of the calamus. al, alula; cd, free caudal vertebrae; dv, dorsal vertebrae; fr, frontals; lc, left coracoid; lcp, left carpometacarpus; lf, left foot; lh, left humerus; lr, left radius; ls, left scapula; lu, left ulna; mx, maxilla; pub, pubes; py, pygostyle; ra, rachis; rc, right coracoid; rh, right humerus; rr, right radius; rt, right tibiotarsus; rtmt, right metatarsals; ru, right ulna; sp, colour spots; v, vanes. Dark grey represents the vanes, light grey represents the scapus. Scale bar is 10 mm. De Souza Carvalho et al. (2015).
One distinctive feature of the Jehol Bird is the presence of a pair of enlarged ‘ribbon feathers’ on the tail. All modern Birds, and some Enantiornithines, posses a tail comprising a fan of feathers, but other Enantiornithines, as well as Confuciusornithids (another group of Mesozoic Birds) posses tails comprising a single pair of elongate ‘ribbon feathers’. These are morphologically distinct from any feathers, with a wide flattened middle part with a line down it, interpreted variously as a central rachis (vein) surrounded by fused and undifferentiated vanes, or an flattened and expanded rachis with a central groove. To date no fossil exhibiting this has preserved the feathers in a way in which this could be resolved, but the ribbon-feathers of the Crato Bird are preserved in relief, allowing examination of their structure, which does indeed appear to comprise a flattened and laterally expanded rachis for much of its length, a structure quite unlike anything seen in modern Birds, and in some ways intermediate between a modern Bird’s feather and a greatly extended Reptilain scale.
Details of tail feather of the Crato Bird specimen. (a) Proximal end. (b) Distal end. cal, calamus; lg, longitudinal groove; ra, rachis; sp, colour spot; v, vane. Scale bar is 2.5 mm. De Souza Carvalho et al. (2015).
The Crato Formation outcrops on the northern flanks of the Chapada do Araripe, a plateaux on the border between Ceará, Pernambuco andPiauí States in northern Brazil. In is noted for its exceptionally well preserved fossils, which include Dinosaurs, Crocodiles, Fish, Pterosaurs, Crustaceans, Arachnids, Plants...
The Early Cretaceous Jehol Group of China has produced a remarkable number of well-preserved fossils of Mesozoic Birds, adding greatly to our understanding of the early history of this group...
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