Research Review Paper: The Reason Dinosaur´s Feathers Evolved?

Growing up, I have always been interested in dinosaurs, loving movies such as Jurassic Park as a child, and bonding with my family by watching Animal Planet and NOVA specials on evolution and especially human and dinosaur evolution. At one point in my life, I wanted to become a paleontologist. I’m sure all of us know the fact that our modern birds evolved from dinosaurs and that one of the very first things that tipped paleontologists off to this fact was the discovery of bird-like dinosaurs and eventually feathered dinosaurs. The research question that I ask is a simple one, how and why did dinosaur feathers evolve? I will be using three very different research papers to learn more about this question in order to try to come up with some answers.

The first paper, “Feathers, dinosaurs, and behavioral cues: defining the visual display hypothesis for the adaptive function of feathers in non-avian therapods”, lists the three dominant hypotheses for the adaptive function of early feathers (Stage 1): flight, thermoregulation, and visual display. (Dimond et al. 2011), and further details the visual display hypothesis in hopes of generating questions for future research. Of these hypotheses, the least likely one was said to be the flight hypothesis, since Stage 1 feathers would’ve been incapable of aerodynamic lift, while the thermoregulation hypothesis was said to may be likely, however it depended on the concentration of Stage 1 feathers on the therapod (Dimond et al. 2011). A therapod is basically a bipedal carnivorous dinosaur, usually with small arms, for example, Tyrannosaurus Rex. The visual display hypothesis at the time of this article was said to have been little analyzed and to be an area of further research, particularly in whether or not Stage 1 feathers would’ve been capable of performing some sort of visual display in terms of pigmentation or pattern, as well as the ability of the receiving organism to even identify the display using its vision to discern fine details (Dimond et al. 2011). It was claimed that analyzing the adaptive functions of early feathers was difficult because at the time there were no fossil examples detailing the morphology of such Stage 1 feathers, and so that is the major reason why this paper isn’t very experimentally based (Dimond et al. 2011). This paper explained in easy-to-understand words the main questions that paleontologists and evolutionary biologists have regarding dinosaur feathers, and emphasized further research into the discovery of fossils of therapods with these Stage 1 feathers in order to have enough evidence to either support or deny the visual display hypothesis.

The second paper, “Bristles before down: A new perspective on the functional origin of feathers”, presents the possibility that the earliest feathers may have resembled bristle-like structures, that may have served tactile purposes sort of like the whiskers on a cat, using the evidence from a paper in 2011 by Cunningham in which high concentrations of mechanoreceptors were observed at the bases of bristles (Pearsons and Currie 2015). These bristles then later would develop into early feathers which continued becoming more complex. This paper claims that the thermoinsulation hypothesis for the origin of earliest feathers is inadequate because they could only serve for insulation in high enough concentrations, implying the development of early feathers in large concentrations (Pearsons and Currie 2015). The writers of this paper are open to the possibility that the development of large concentrations and development of these early bristle structures and feathers may have been encouraged in endothermic dinosaurs, since the phylogenetic distribution of dinosaurs with feathers seems to generally match up with the distribution of those that developed endothermy. However, thermoinsulation being the first functional origin of early feathers is very unlikely, again because of the concentration issue (Pearsons and Currie 2015). In this way, these early feathers may have served a similar purpose to that of hair on mammals. The researchers also aren’t fans of the sexual display hypothesis since they claim that nothing in particular to early feathers supports sexual display, and that sexual display is an explanation that could be applied to any visible structure on an animal (Pearsons and Currie 2015). This new, “tactile-bristle-hypothesis” presented in this paper was an idea for future paleontological research and consideration, to try to present an alternative explanation the origin of the earliest feathers. I particularly found this paper interesting for its mention of the endothermy-to-feather link, since it may explain why today’s birds are endothermic, since the dinosaurs with the most complex and developed feathers would have been the endothermic ones in order to better retain heat. This paper contributed more detailed ideas about feather morphology, which sets up some background for the next research paper I will be detailing.

The final paper, “Additional information on the primitive contour and wing feathering of paravian dinosaurs”, had the goal of better understanding the feather morphology of various dinosaurs in order to gain insight into their roles and evolution (Saitta et al. 2017). Paravian dinosaurs are a group of therapods that are more closely related to birds than to oviraptosaurs. Several high resolution photographs of non-avian and avian dinosaur taxa were taken using a Nikon D800 camera with a 60 or 105 mm Miko Lens, the taxa included being Anchiornis, Sinosauropteryx, Confuciornis, Psittacosaurus, and Caudipteryx (Saitta et al. 2017). Anchiornis was a paravian taxon with contour feathers detached from the plumage, resulting in a shaggy appearance (Saitta et al. 2017). Contour feathers are what form the general shape of the therapod, hence the term “contour”. Because of this open-vaned structure, not only was Anchiornis flightless, but also shaggy looking, refer to Figure 1. These simplified, open-vaned feathers were also observed on Caudipteryx. Contour feathers were evidenced to have evolved before flight, from observations made of Sinosauropteryx, and these contour feathers then likely served functional roles as thermoregulation and display, thus it was found that understanding the evolution of contour feathers could yield insight into the function of the earliest feathers (Saitta et al. 2017). Psittocosaurus, however, had long bristle-like filaments that may have occurred in clusters, while Confusiornis had evolved relatively more derived structures in its wing feathers (Saitta et al. 2017). The tufts of Sinosauropteryx differed much to the Anchiornis contour feathers, and the “shaggy” morphology of Anchiornis shows how extinct morphologies make the linear evolution-developmental model of feather evolution difficult, since some of these feather structures seem derived while others appear to be ancestral, which was actually stated in the conclusion of the paper, “Ultimately, truly ‘modern’ contour feathers might be relatively more derived than originally thought.” (Saitta et al. 2017). However, although open-vaned feathers are incapable of flight, there is evidence that partially open-vaned ones may be capable of gliding, meaning aerodynamic functions may actually predate sociosexual display in some non-avian therapods. The paper explained this further with the modern-day example of the silky recessive allele in pigeons and doves, where a mutation causes brittle barbule formation, reducing aerodynamic efficiency to the point where real flight is lost but they can still glide to low perches. This only applies to heterozygotes, homozygotes are incapable of gliding (Saitta et al. 2017). This could mean before flying, paravian therapods with partially open-vaned feathers may have glided from tree to tree like flying squirrels, how cool is that? There was some possibility however that in the taxon Caudipteryx secondary flightlessness may have emerged (Saitta et al. 2017). Secondary flightlessness is when a species that could fly loses the ability to, a type of evolutionary reversal. This research paper went into even more depth about the nitty-gritty morphology of rachises and whatnot, but what I have stated here is what I believe matters the most to my initial question. It would seem that the development of feathers wasn’t as simple as one dinosaur had them and they got more and more complex and eventually flying happened, rather in some taxa evolved certain structures in their bristles and vanes, while others may have glided using partially open-vaned ones while others lost their ability to fly completely, meaning I can’t look at the evolution of dinosaur feathers as one straight line. The article called for future research into the distribution of newer contour feather morphologies and sorting out whether certain morphologies or more plesiomorphic or derived.

So to summarize my take-aways from each of these papers, there is still some level of debate about why exactly feathers evolved in dinosaurs, however we do have a pretty good idea of how exactly they evolved, possibly evolving from bristle-like formations that were encouraged to grow more complex and in greater concentrations in endothermic dinosaurs and finding some other uses as their morphologies began changing in many different taxa, some for gliding, others for water repellency, others for even flight. The future seems to be mainly concerned with grouping whether certain extinct feather morphologies were ancestral or derived, this being important because to use a certain taxon’s morphology as evidence for some trend we must know whether this is even a trend, or just a quirk about that particular taxon. Also, it appears as if the bristle hypothesis needs further detailing, since it does have some interesting implications for the importance of tactile sensation in the evolution of avian species. Finally, maybe it can be possible in the future to find molecular data on species so that we have another point of view when classifying phylogenies of dinosaurs, to see for sure if certain dinosaur feather morphologies characteristics are plesiomorphic or derived.