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The Evolution of Feathers from Dinosaur Hair

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The Evolution of Feathers from Dinosaur Hair GAIA N'15, LlSBOAlLISBON, DEZEMBRO/DECEMBER 1998, pp. 399-403 (ISSN: 0871-5424) THE EVOLUTION OF FEATHERS FROM DINOSAUR HAIR Peter J. GRIFFITHS University of Wolverhampton, School of Health Sciences. Uchfield Street. Wolverhampton WV1 10J. UNITED KINGDOM ABSTRACT: The significance of finding feathered theropod dinosaurs is discussed in terms _ ofthe theories of the evolution offeathers, birds and endothermy. The plumulous proximal portion of the isolated Archaeopteryx feather suggests that endothermy had already evolved in the dinosaur· bird lineage by the Late Jurassic. It is suggested that adipose tis­ sue would have played a major role in providing insulation in endothermic ancestral thero­ pod dinosaurs. Small endothermic dinosaurs may have found an additional form of insulation an advantage. This may initially have evolved as"hair" in the dinosaurs, rather than the more morphologically complex branched feather. As avian epidermal appendages are composed ofthe unique <I> keratin family, it is unlikely that feathers were derived directly from archosaurian scales, but must have involved intermediate structures. It is suggested that feathers could consequently have been derived from dinosaur "hair". RESUME: La decouverte significative de dinosaures theropodes a plumes a entraine une dis­ cussion en fonction des theories sur I'evolution des plumes, des oiseaux et de I'endother­ mie. La partie proximale duveteuse de la plume isolee de I' Archaeopteryx suggere une evolution de I'endothermie dans la lignee dinosaure-oiseau a la fin du Jurassique. On pense que Ie tissu adipeux auraitjoue un role majeur en fournissant de la chaleur chez les dinosau­ res theropodes endothermes. Ainsi, les petits auraient pu en trouver un avantage supple­ mentaire. Chez les dinosaures, il y aurait d'abord pu avoir une evolution vers des "poils" plutot que des plumes ramifiees, morphologiquement tres compliquees. Etant donne que les tissus epidermaux des animaux a plumes sont composes de I'unique famille keratine <1>, iI est improbable que les plumes aient directement derive des ecailles archosauriennes, mais ont dO faire intervenir des structures intermediaires. Par consequent, les plumes auraient pu provenir des "poi Is" de dinosaures. INTRODUCTION supporting evidence forthe hypothesis thatfeathers evolved initially for the purpose of insulation or The recent discovery of several species of display, becoming secondarily adapted for flight. In feathered theropod dinosaur from the Liaoning addition, the discovery of feathered non-avian Province of China, locality of the early birds Sinornis dinosaurs gives support to the hypothesis that birds (SERENO & RAo, 1992) and Confuciusornis (HOU et evolved from the theropods, and that dinosaurs al. 1995), has fuelled the debate regarding the origin ancestral to birds were endothermic. and initial function offeathers, and also speculation concerning the possibility that dinosaurs were THE THEROPOD ANCESTRY OF BIRDS endothermic. Sinosauropteryx prima (J I & J I, 1996), a small compsognathid dinosaur is described as It is generally accepted by most palaeontologists having epidermal appendages resembling the that birds evolved from the theropod dinosaurs plumules of modern birds (CHEN, DONG & ZHEN, (OSTROM, 1973), and cladistic analyses suggests 1998). Protoarchaeopteryx robusta (JI & JI, 1997) that among the theropod dinosaurs, the dro­ and Caudipteryx zoui (JI et al., 1998) are both maeosaurs share the most characters with the earli­ described as having pennaceous feathers attached est known bird Archaeopteryx, and also with modern to the fore limbs and tail, while Caudipteryx is birds (GAUTH IER, 1986; HOLTZ, 1994). reported as also having plumulaceous feathers The hypothesis that birds evolved from the thero­ around the body (JI et al., 1998). The discovery of pod dinosaurs has recently received further support feathered non-avian theropod dinosaurs provides 399 artigos/papers P. J. GRIFFITHS with the discovery of a new species of fossil bird morphologically very similar to those of modern from the Upper Cretaceous of Madagascar, Rahona birds. ostromi (FORSTER et a/., 1998). Rahona has bony The earliest known fossil feather is that of Ar­ protrusions on the forearms which in modern birds chl;leopteryx lithographica (MEYER, 1861). A recent serve as the attachment points for flight feathers, analysis shows the feather to be morphologically suggesting not only that it was feathered but also identical to the tenth secondary flight feather of the that it was capable of flight. Most interestingly, Ra­ magpie which has a wing shape (and presumably hona has a large, retractable sickle-shaped claw on aerodynamic characteristics) similar to that of Ar­ the second toe ofthe hind foot, similar to the slashing chaeopteryx (GRIFFITHS, 1996). This is the only de­ claws of Velociraptor(OSBORN , 1924) and Deinony­ scribed individual fossil feather attnbutable to chus (OSTROM, 1969). Archaeopteryx, and so it is not known if the species It has been suggested that the second toe of possessed feathers specialised specifically for inSU­ Archaeopteryx may have been hyperextensible lation. (PAUL, 1988; SERENO, 1997). In addition, although However, an examination of the morphology of the pedal claws are considerably smaller than those the isolated Archaeopteryx feather reveals some in­ of the wing (GRIFFITHS, 1994), the claw of the teresting features. On distal portions of the feather, second toe is significantly larger than those of the individual barbs and even barbules can clearly be other toes, and has a different morphology. For observed forming the usual pennaceous structure example, in the Eichstatt specimen WELLNHOFFER which allows the barbs to zip together creating the (1974) reports that the claw of the second toe has a typical flight-feather vane capable of generating curvature with a greater angle from the vertical aerodynamic lift. In comparison, the morphology of chord across the articular facet to claw tip (160°) the feather is quite different proximally where indi­ compared to the other toes (132°_146°) , which vidual barbs cannot be distinguished. In this region, supports the suggestion that the second toe of the feather has a tufted appearance suggesting that Archaeopteryx may have been hyperextenslble. the hooklets attached to the barbules are fewer in While a slashing claw is found in Rahona , such a number or are absent, and that the barbs are much claw is not present in Iberomesornis (SANZ et a/., thinner than in the distal parts of the feather. Conse­ 1988)or Concornis(SANZ& BUSCALIONI, 1992)from quently the isolated Archaeopteryx feather appears the Lower Cretaceous of Spain, or Confuc/Usorms to have a plumulaceous region at the base which (HOU et a/., 1995) and Sinornis (SERENO & RAO, would efficiently trap air against the skin. This fea­ 1992) from China. This would suggest that the ture is identical to that found in the flight feathers of slashing claw of Rahona may be a retained modern birds which have few if any down feathers plesiomorphic character. on the wing. The plumulaceous base of the Archae­ opteryx feather indicates that it was capable of pro­ EVOLUTION OF ENDOTHERMY AND DOWN viding insulation and so supports the hypothesIs that FEATHERS Archaeopteryxwas endothermic (GRIFFITHS, 1996). The discovery of apparently flightless, feathered As the Archaeopteryx feather is already morpho­ non-avian theropod dinosaurs provides strong logically specialised and exhibits characteristics supporting evidence for the hypothesis that feathers enabling the dual functions both of flight and Insula­ evolved initially for the purpose of display or Insula­ tion, Archaeopteryx can give no indication of the ini­ tion, becoming secondarily adapted for flight during tial role of feathers, but does make clear that the subsequent evolution of birds (REGAL, 1975). An feathers capable of providing insulation as well as alternative hypothesis for the origin of feathers (FE­ flight were already present during the Late Jurassic. DUCCIA 1974) is that they evolved initially for flight, It is logical to assume that endothermy and feathers and subsequently become adapted for insulation evolved in taxa ancestral to Archaeopteryx. when the birds became endothermic, although this now seems less likely in view of the recent finds. THE ROLE OF ADIPOSE TISSUE FOR Modern birds are unquestionably endothermic INSULATION and have down feathers which are specialised for in­ In contrast to birds, mammals use two different sulation as well as flight feathers. At some point mechanisms for insulation, hair and also a layer of along the dinosaur - bird lineage, both endothermy adipose tissue under the dermis. Hair plays a similar and feathers for insulation evolved, although not role in insulation as down feathers, trapping a layer necessarily at the same time. The first discovered of air against the skin thus reducing heat loss by con­ fossil down feather has been classified as lIerdop­ vection and radiation. In adipose tissue, lipid drop­ teryx viai (LACASA, 1985) from the Lower Creta­ lets are stored in large specialised cells which are ceous of EI Montsec, Spain. This location has embedded in connective tissue, particularly in the yielded a variety of specialised feathers which are 400 THE EVOLUTION OF FEATHERS FROM DINOSAUR HAIR dermis. The dermal layer of adipose tissue prevents Cretaceous birds Iberomesomis, Concornis and the loss of heat from deep within the body, helping to also Sinornis were much smaller than Archaeop­ maintain core
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