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In one fossil specimen of the Parave Anchiornis huxleyi, the features are so well preserved that the melanosome (pigment cells) structure can be observed. By comparing the shape of the fossil melanosomes to melanosomes from extant birds, the color and pattern of the feathers on Anchiornis could be determined. Anchiornis was found to have black and white patterned feathers on the forelimbs and hindlimbs, with a reddish brown crest. This pattern is similar to the coloration of many extant bird species, which use plumage coloration for display and communication, including sexual selection and camouflage. It is likely that non-avian dinosaur species utilized plumage patterns for similar functions as modern birds before the origin of flight. https://en.wikipedia.org/wiki/Feather#Evolution FINAL DRAFT STARTS HERE What were the selective forces that evolved feathers in theropod dinosaurs and early birds? Considered the most complex integumentary (exterior) structure found in vertebrates, feathers are an evolutionary novelty that helps characterize birds, the Aves, from other animal groups. The important function of enabling flight in modern birds suggests that feathers evolved primarily for this function. However, this hypothesis is flawed. A lack of known ancestral feather morphologies has restricted the investigation of the origin of feathers, but recent findings have helped shed light on the issue (Prum, Brusch 2002). Feathers are highly complex and would have required a series of adaptations and improvements to become specialized enough to offer any aerodynamic advantage. There were likely other functions of early feathers that increased fitness, allowing them to be selected for and eventually become complex features capable of facilitating flight. The discovery of Archaeopteryx in 1861 was the first evidence of feathers in a species outside the Ave clade, and recent discovery of more fossil feathers from non-avian theropod dinosaurs like Sinosauropteryx and Protarchaeopteryx has shed light upon the function of pre-avian feathers (Prum, Brusch 2002). These theropods had feathers but did not fly, meaning there must have been a different selective advantage given to individuals. Ultimately this was the driving force behind the advancement of feathers. Color patterns derived from melanosome shape in fossils of non-avian theropods suggest feathers played a role in communication or intersexual selection. Supported by recent evidence in the fossil record about the complex and diverse history of feathers, it is likely flight is only an exaptation from many complex selective forces that were present in the Mesozoic era. Feathers are filamentous, branched keratin structures that involve a tubular follicle (Prum 1999). They are complex evolutionary novelties that have historically been considered to be related to scales, due to both forming from an initial thickening in the epidermis called a placode (Prum 1999). Since fossil evidence of the phylogeny of feathers is scarce little is known about their origin and evolution, however there are some examples of theropod fossils that exhibit structures thought to be homologes of feathers that give insight into their origin. In some theropod dinosaur fossils, structures interpreted as feather homologes range from simple and filamentous in Coelurosaurians to complex with distinct shafts and vanes present in the Maniraptor clade, suggesting feathers evolved throughout the theropod lineage (Chiappe and Dyke 2002). In light of this and other recent findings, Ornithology expert Richard Prum suggests the developmental process of modern feathers in birds provides a congruent short-term model of the evolutionary sequence of novelties that led to the feather seen today. In stage I, the earliest feathers were simply hollow tubular structures formed from the extension of the dermal layer in the skin. The most basic form, these structures were similar to the calamus, or rigid center, of modern feathers. The next stage of feathers grew an unorganized tuft of filamentous barbs, followed by a stage III characterized by a vein and many symmetrical fused barbs. In stage IV feathers developed tiny hooks and grooves on the barbules connected to the vein, further strengthening the connection of the parallel barbules. It isn’t until stage IV that feathers could possibly have any aerodynamic function (Prum, Brusch 2002). Finally at the final stage V subsequent novelties yielded feathers with asymmetrical vanes and an aftershaft, features important to flight (Prum and Brush 2002). This hypothesis for the evolution of the feather structure is supported by fossil evidence found in recent years. With the likely sequence of evolutionary novelties leading to modern feathers established, the selective pressures that may have been active can be investigated. Before the origin of flight feathers must have given some fitness advantage to individuals allowing for their selection and evolution. One specimen that provides insight on the fitness advantage of feathers is from the well-preserved fossil of a late Jurassic Parave of the Troodontidae family named Anchiornis huxleyi, a theropod closely related to the Avialae (bird) clade (Li et al 2010). In the fossil, the melanosome structures from the feathers are well preserved enough to be seen with an ESM (electron scanning microscope). By analyzing their shape, the pigmentation can be estimated and the color distribution of the organism can be constructed. Anchiornis was found to have black and white patterned feathers on the forelimbs and hindlimbs, with a reddish brown crest (Li et al 2010). This pattern is similar to the coloration of many extant bird species, which use plumage coloration for display and communication. In other fossil specimens of the Upper Cretaceous non-avian theropod Ornithomimus edmontonicus both filamentous and pennaceous feathers are present (Zelenitsky et al 2012). Only filamentous feathers are observed in the juvenile (<1 year old) fossils, while evidence of more advanced shafted feathers complimenting filamentous feathers is present in a mature adult fossil specimen. The late appearance of shafted wing feathers suggests that they initially functioned as a secondary sexual characteristic; a feature developed at maturity to differentiate males and females but not directly involved in reproduction. Based on observations that many extant bird species also develop certain feathers at maturity as a secondary sex characteristic, it is likely that dinosaurs also used feathers for reproductive communication via courtship and display (Zelenitsky et al 2012). Evidence of color patterns and the late development of mature feathers both suggest that they likely played a role in sexual selection and communication before evoloving for flight. Based on recent fossil evidence and observations of extant bird species, it is likely that feathers originally evolved for use as communication and intersexual selection features. Later in the evolution of theropods, species evolved pennaceous feathers (modern contoured bird feathers) that complimented filamentous down-like feathers. The diversity of these pennaceous feathers suggests they likely played similar roles in theropods as they play today, affiliated with insulation, brooding, camouflage, and display (Foth et al 2014). These pennaceous feathers formed small ‘protowings’ on the distal forelimbs and hindlimbs and fan-like structures on the long and bony distal tail of many species belonging to the clade Paraves; the clade of dinosaurs most closely related to birds (Foth et al 2014). While these theropods lack important skeletal features that are associated to powered flight in birds such as the pectoral gurdle and channelized wrist, there is evidence that they could have used their protowings as aerodynamic features to aid in movement, such as gliding or jumping. Baby birds have underdeveloped wings very similar to the protowings of some Paraves like Anchiornis and Archaeopteryx, so observing extant baby birds is one way of observing how aerodynamically effective these dinosaur protowings were (Heers et al 2014). One study found that while not suitable for powered flight, even underdeveloped wings helped aid baby birds in aerodynamic maneuverability when jumping to heightened surfaces, running up slopes, and slowing aerial descent (Heers et al 2014). Since protowings are somewhat functional in extant birds at immature stages, theropods potentially used protowings for similar functions. The short-term development from protowing wielding baby to flight capable adult in extant birds is likely parallel to the long-term evolutionary transition from protowings to flight-wings. Once an established behavior, aerodynamic function increased fitness and positive selection promoted changes in feather structure, reductions in body mass, increases in wing size and increases in flapping velocities. These changes eventually resulted in the emergence of powered flight and the avialae lineage that thrives today. Feathers are complex evolutionary novelties that evolved in theropods and eventually were optimized for flight as an exaptation from other selective pressures. The current function of highly adapted feathers for flight is not the sole selective force that drove their evolution, or even one of the initial forces. Evidence from fossil specimens of non-avian theropod dinosaurs show that feathers had evolved before the emergence of birds and diversified into a complex feature likely under selection for many different selective forces including insulation, communication, and sexual selection, which allowed them to diversify immensely (Zhou 2014). Color patterns in early dinosaurs suggest display and communication were important functions of feathers, similar to how they are still used today by many bird species. Only after feathers were firmly established and adapted in the theropod clade could the function for aerodynamic movement have arisen. Once the Paraves developed protowings and aerodynamic movement gave a fitness advantage, multiple modifications gave rise to the Ave clade still extant today. Flight, a feature that defines an entire class of organisms, is only an exaptation, or a shift in the function of the trait, to what the feather was originally adapted and selected for. After undergoing complex selection likely facilitated by multiple selective pressures, feathers became advanced enough to perform aerodynamic functions, and the selection for powered flight further adapted feathers to the state they exist today.

Works Cited: Chiappe, Luis M., and Gareth J. Dyke. "The Mesozoic Radiation Of Birds." Annual Review of Ecology and Systematics 33.1 (2002): 91-124. Web.

Foth, Christian, Helmut Tischlinger, and Oliver W.M. Rauhut. "New Specimen of Archaeopteryx Provides Insights into the Evolution of Pennaceous Feathers."Nature 511 (2014): 79-82. Web.

Heers, Ashley M., Kenneth P. Dial, and Bret W. Tobalske. "From Baby Birds to Feathered Dinosaurs: Incipient Wings and the Evolution of Flight." Paleobiology 40.3 (2014): 459-76. Web.

Li, Q., K.-Q. Gao, J. Vinther, M. D. Shawkey, J. A. Clarke, L. D'alba, Q. Meng, D. E. G. Briggs, and R. O. Prum. "Plumage Color Patterns of an Extinct Dinosaur." Science 327.5971 (2010): 1369-372. Web.

Prum, Richard O., and Alan H. Brush. "The Evolutionary Origin And Diversification Of Feathers." The Quarterly Review of Biology 77.3 (2002): 261-95. Web.

Prum RO. 1999. Development and evolutionary origin of feathers. J. Exp. Zool. 285:291–306.

Zelenitsky, D. K., F. Therrien, G. M. Erickson, C. L. Debuhr, Y. Kobayashi, D. A. Eberth, and F. Hadfield. "Feathered Non-Avian Dinosaurs from North America Provide Insight into Wing Origins." Science 338.6106 (2012): 510-14. Web.

Zhou, Zhonghe. "Dinosaur Evolution: Feathers Up for Selection." Current Biology24.16 (2014): 751-53. Web.

What were the driving evolutionary functions of the earliest feathers in theropod dinosaurs and early birds?

Heers, Ashley M., Kenneth P. Dial, and Bret W. Tobalske. "From Baby Birds to Feathered Dinosaurs: Incipient Wings and the Evolution of Flight." Paleobiology 40.3 (2014): 459-76. Web. This article investigates the functional capabilities of early feathers based off fossil records. It considers different aerodynamic and non-aerodynamic functions and links the feathers from theropod dinosaurs to those in avian lineages. The research suggests that early aerodynamic feather function in maniraptors was improved upon to give birds flight.

Li, Q., K.-Q. Gao, J. Vinther, M. D. Shawkey, J. A. Clarke, L. D'alba, Q. Meng, D. E. G. Briggs, and R. O. Prum. "Plumage Color Patterns of an Extinct Dinosaur." Science 327.5971 (2010): 1369-372. Web. In this study, feather color patterns are mapped in a late jurassic theropod dinosaur. Melanosome shape and density indicate the pigmentation of feathers on the body, and indicate that these feathers likely played a role in sexual selection or communication rather than flight.

Manning, Phillip. L., Nicholas P. Edwards, Roy A. Wogelius, Uwe Bergmann, Holly E. Barden, Peter L. Larson, Daniela Schwarz-Wings, Victoria M. Egerton, Dimosthenis Sokaras, Roberto A. Mori, and William I. Sellers. "Synchrotron-based Chemical Imaging Reveals Plumage Patterns in a 150 Million Year Old Early Bird." Journal of Analytical Atomic Spectrometry 28.7 (2013): 1024. Web. This article looks at recent breakthroughs in paleobiology which allow the pigments of fossil feathers to be determined and therefore reach a more accurate determination of the appearance of fossil species. Different methods are used to analyze feather pigments, including x-ray absorption spectroscopy, are used to see chemical distribution within the fossils. The distributions are used to predict complete feather pigment pattern in three key specimens of Archaeopteryx, an important species in the transition from theropods to birds.

Prum, Richard O., and Alan H. Brush. "The Evolutionary Origin And Diversification Of Feathers." The Quarterly Review of Biology 77.3 (2002): 261-95. Web. The origin and developmental theory of feathers is investigated in this article and previous theories are critiqued and alternatives to the theropod origin of feathers are rejected. The structure of these early feathers is discussed and linked to later avian lineages.

Zelenitsky, D. K., F. Therrien, G. M. Erickson, C. L. Debuhr, Y. Kobayashi, D. A. Eberth, and F. Hadfield. "Feathered Non-Avian Dinosaurs from North America Provide Insight into Wing Origins." Science 338.6106 (2012): 510-14. Web. This article focuses on a non-maniraptor type of theropod dinosaur which is now known to have possessed feathers according to the fossil record. The study provides insight into the purpose of feathers in dinosaurs and the eventual evolution of the aviary wing.

Suggestions and sentence contribution https://en.wikipedia.org/wiki/Feathered_dinosaur I think there could be more information on the more advanced species of dinosaurs (in terms of feather development) and how flight actually evolved. Did these species use their proto-wings initially to fly or just glide? I would like to see more detail about how pigment in fossil feathers is analyzed and colors can be determined through chemical analysis of trace metals and the presence of melanosomes. Color in feathers is important evidence that supports the theory that feathers played a role in sexual selection. I'm confused on where the line between "bird" and "dinosaur" really is, and if its even a defined line at all. What makes a bird a bird and not a theropod? I think more information on this would be helpful.

Supporting the display hypothesis is the fact that fossil feathers have been observed in a ground-dwelling herbivorous dinosaur clade, making it unlikely that feathers functioned as predatory tools or as a means of flight. Zelenitsky, Darla K.; Therrien, Francois; Erickson, Gregory M.; DeBuhr, Christopher L.; Kobayashi, Yoshitsugu; Eberth, David A.; Hadfield, Frank (10/26/12). "Feathered Non-Avian Dinosaurs From North America Provide Insight into Wing Origins". Science 338 (510). doi:10.1126/science.1225376