The Purpose of Feathers in Non Avian Dinosaurs

There is a significant body of evidence within the scientific community over the past fifteen years to suggest-contrary to depictions in blockbuster movies and mass media-that dinosaurs, both avian and nonavian, had feathers of some sort. Recent spectacular discoveries have suggested that modern day birds are descendents of dinosaurs, suggestions of the origin of such feathers, and the discovery of various types of filamentous fibers and feathers that were prevalent in various species of both avian and nonavian dinosaurs throughout their history. Paradoxically, there is no abundance of research and hypotheses regarding the function these feathers and filamentous fibers had for nonavian theropod dinosaurs throughout history. This paper will be exploring-using existing research-that gap of knowledge and seeking to make an informed hypothesis regarding the purpose of feathers in theropod dinosaurs. More specifically, I will explore the role latitudes played on the presence of feathers in these creatures.

Before one can talk at length about feathers, we must define what feathers are and more specifically, what we define as feathers in relation to dinosaurs. In their paper titled, The Early Evolution of Feathers: Fossil Evidence From Cretaceous Amber of France, by Vincent Perrichot et al, they define feathers on dinosaurs as, “complex integumental structures of a three-level branched structure composed of a rachis (primary shaft), barbs (secondary branches) and barbules (tertiary branches)” (Vincent Perrichot et al. 1). Paleontologists report finding filamentous fibers as well as true feathers on various species of dinosaurs from various epochs. In a recent study done by Xing Xu et al, An Integrative Approach to Understanding Bird Origins, Xu cites, “…numerous specimens of most theropod groups and even three ornithischian groups preserving feathers have been recovered from the Jurassic and Cretaceous beds of northeastern China and from the Jurassic and Cretaceous beds of Germany, Russia, and Canada. (Xu et al 6). According to Prado et. al., in their study, New Occurrences of Fossilized Feathers: Systematics and Taphonomy of the Santana Formation of the Araripe Basin (Cretaceous), NE, Brazil, “Feathers are the most complex integuments of vertebrates, due to their variety of forms and roles. This structure is responsible for the thermoregulation, display, protection against radiation, toxicity, buoyancy and even to produce sound” (Prado et al 1). Experts have even found feathers in the form of bristle scales – an intermediary character between scales and feathers, placodes, follicles, scales attached to the epidermis of the fossil, and in many other forms on all types of dinosaurs.

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With an understanding of how scientists today define feathers and where they have been found on dinosaurs, we can begin to explore what the existing hypotheses are regarding their purpose when found on nonavian dinosaurs. Christopher Dimond et al, in a study exploring the visual cues behind adaptive feathers in nonavian dinosaurs states, “The three predominant hypotheses are 1) flight, 2) thermoregulation, and 3) visual display” (Dimond et al 1). Additionally, Dimond’s study cites other sources that hypothesize feathers could have been used for, “insulation, water repellency, courtship, camouflage and defense (Prum and Brush, 2003), parental care and brooding (Clarke and Middleton, 2006; Grellet-Tinner, 2006), shielding nests (Turner et al., 2007), flight or lift while running (Dial et al., 2006), and male-to-male competition (Cowen and Lipps, 1982)”. (Dimond et al 1). While all these hypotheses have been thoroughly researched and written about, I hypothesize that the major determining factor regarding the presence or absence of feathers in nonavian dinosaurs throughout history is the latitude in which the dinosaur lived. Nonavian theropod dinosaurs are known to have lived from the Late Triassic through the end of the Cretaceous. In A Review of Mesozoic Climates, Anthony Hallam definitively states, “there is overwhelming evidence, based on the distribution of distinctive sediments and fossils and oxygen isotope data, that the climate of the Mesozoic world was appreciably more equable than that of today” (Hallam 1).

With Hallam’s findings in mind, it can be assumed that even in the tropical “hot earth” world many scientists have described, nonavian dinosaurs might have needed feathers for colder nights and periods of darkness for insulation. Smaller species, that would require insulation for survival are an obvious hypothesis as to the types of fossils found, but in another study done by Xing Xu et al, they postulate that “Yutyrannus huali [a new gigantic basal tyrannosauroid] bears long filamentous feathers, thus providing direct evidence for the presence of extensively feathered gigantic dinosaurs and offering new insights into early feather evolution” (Xu et al. 1). Found in China, Y huali’s longer filamentous feathers could have been an adaptation to a much colder environment than the rest of the planet. Xu further explains, “Most gigantic Late Cretaceous tyrannosauroids, by contrast, lived in a warm climate that was conducive to the loss of an extensive insulative feathery covering, although populations inhabiting cold environments such as the land that is now Alaska would have been a notable exception” (Xu et al. 1). Xu’s findings concur our previous misconceptions that dinosaurs didn’t have feathers or filamentous structures. Why, for so long did we believe this to not be so?

Until the first dinosaur fossil with a feather like structure attached was discovered in Brazil almost thirty years ago, humans and popular culture portrayed dinosaurs as reptilian. Leathery skin, sometimes scaly, sometimes not, Dinosaur’s appearance has varied little over the course of human study until the very recent future. The reason for this disparity in scientific reporting was due to how difficult it is to find a fossil that preserves both the bone as well as the filamentous integument attached. The soil conditions upon death have to have been just right in order for preservation of this type to occur. Prado argues, “Because feathers are very delicate features, they rarely survive the physicochemical process that follow their burial. Thus, they are usually found as: (i) carbonized and iron traces, (ii) inclusions in ambers and coprolites, (iii) and as imprints” (Prado et al 1). Many of the most well preserved fossils with feather integuments still attached have been found in amber deposits recently, often perfectly preserving the skin attached to the feather. This can occur in a few circumstances. According to Prado’s research on feathers found at the Crato Fossil Bed in Brazil, “the Crato Member was laid down under clear and relatively shallow waters during an arid and dry climate, where the carbonate sediments were deposited in a low energetic input with formation of halite and anhydrite minerals” (Prado et al. 1). An alternative suggestion by David Martill and Gunter Bechly in their work, The Crato Fossil Beds: A Window into the Ancient World, argue “once an anoxic and hypersaline bottom water body was established, the organic matter produced in the surface waters was no longer broken down by heterotrophic bacteria accumulated in the sediment” (Martill 60). Due to the fragility of the structure of a feather or filamentous structure, conditions had to be just right to ensure preservation.

Due to the conditions in which these fossils must be preserved, there can be a case made for why they are only found so well preserved in the locations above. The hypothesis above that mostly all dinosaurs had some type of filamentous coating could still hold true as the locations in which they have been found are not ideal for feather preservation. It appears that in order for a feather integument to be preserved, the conditions in which the initial preservation occurred and where the fossil migrated to must be precise. Prado cites, “that lithology itself may not be a sure factor for skin preservation dinosaurs, it may be an important factor, and perhaps decisive, factor in feather preservation” (Prado et al 1). While there is a great amount of literature on lithology, what remains to be talked about is lithology with specific reference to geography. My hypothesis tends to focus on the importance of latitudes to explain why certain types of dinosaurs contained feathers, but another important factor to consider is the speed of burial near riverine environments. Hadrosaurids could provide a window into this phenomena. Mary Higby Schweizer, in her paper Soft Tissue Preservation in Terrestrial Mesozoic Vertebrates, argues, “it could be that hadrosaurids were more likely to preserve skin than other dinosaurs because, during life, hadrosaurids preferred an environment conducive to rapid burial and preservation of soft tissues” (Wegweiser et al. 2006). This is a fascinating argument, as it brings to light a host of various factors that could potentially cause the feather’s on other species of dinosaurs to not be preserved. Essentially, if a dinosaur’s natural environment was not conducive to a perfect fossilization of delicate integumentary structures, these records would be lost forever in nature.

Christopher Dimond et al, in their paper, Feathers, Dinosaurs, and Behavioral Cues: Defining the Visual Display Hypothesis for the Adaptive Function of Feathers in Non-Avian Theropods, claims that initial forms of feathers in theropod dinosaurs were mainly utilized to signal. Whether it was sexual signaling for mating, deterrence of predators, or camouflage while hunting, these feathers served the purpose of sending a message to a receiver. Throughout the paper, Dimond posits that feathers in theropods were utilized to “trigger a behavioral response in the receiver” (Dimond et al 2011). While they do not rule out feather’s for thermoregulation in theropods, their focus on visual display as an adaptive feature is marvel. While I agree with their position that signaling could have been an ancestral form of feathers in these types of dinosaurs, I believe that the main purpose for filamentous integuments on these types of dinosaurs was for thermoregulation in a climate that vastly varied in temperature. Even today, which by and large is considered to be colder, the variation in temperature on a given day-even in the tropics-is enough to make a hairless creature rapidly lose body heat. This loss of body heat in an environment less forgiving than a tropical one might have spurred an adaptation such as feathers over the course of history.

The study of dinosaurs is ever evolving as our technological ability to analyze every aspect of their world grows by the day. Specifically, when studying feathers and filamentous integuments, our technology has caught up recently enough to detect the most minute details that for years, we were unable to identify. This has provided the scientific community with a window into what I believe, is a common character in nearly all dinosaurs-similar to mammals today. Just as in mammalia, dinosaurs adapted over millions of years to be as fit as possible for their respective environments and for the changes occurring on Earth at the time. Our ability to detect these adaptations, while much improved, is mainly contingent upon the environment in which they are interred. With the advent of new technology, science will continue to explore the relationship between feathers in theropods and similar dinosaurs and the latitudes in which they cohabitated.

WORKS CITED

Dimond, Christopher C., Robert J. Cabin, and Janie S. Brooks. “Feathers, Dinosaurs, and Behavioral Cues: Defining the Visual Display Hypothesis for the Adaptive Function of Feathers in Non-Avian Theropods.” Bios 82.3 (2011): 58-63. Web.

Hallam, Anthony . “A Review of Mesozoic Climate.” Journal of the Geological Society of London 142.3 (1985): 433-55. Web.

Martill, David M. The Crato fossil beds of brazil: window into an ancient world. Cambridge: Cambridge Univ Press, 2011. Print.

Prado GMEM, Anelli LE, Petri S, Romero GR. (2016) New occurrences of fossilized feathers: systematics and taphonomy of the Santana Formation of the Araripe Basin (Cretaceous), NE, Brazil. PeerJ 4:e1916 https://doi.org/10.7717/peerj.1916

Perrichot, Vincent et al. “The Early Evolution of Feathers: Fossil Evidence from Cretaceous Amber of France.” Proceedings of the Royal Society B: Biological Sciences 275.1639 (2008): 1197-1202. PMC. Web. 22 Mar. 2017.

Schweitzer, Mary Higby. “Soft Tissue Preservation in Terrestrial Mesozoic Vertebrates.” Annual Review of Earth and Planetary Sciences 39.1 (2011): 187-216. Web.

Xu, X., Z. Zhou, R. Dudley, S. Mackem, C.-M. Chuong, G. M. Erickson, and D. J. Varricchio. “An integrative approach to understanding bird origins.” Science346.6215 (2014): 1253293. Web.