The Arboreal and Cursorial Hypotheses of the Flight Origin - Research Paper Example

The Origin of Flight in Dinosaurs The origin of flight is a topic that has been widely debated since the Ancient Greeks, and is one that has not yet been fully understood. There are several anatomical developments in modern birds and paleontological discoveries, such as the furcula used for skeletal bracing (Hui, 2002) that can be used to discover how flight is possible and give clues as to how the skill evolved. Using two main theories, it is possible to speculate whether flight evolved from the tree down (from a jumping, gliding, arboreal ancestry) or from the ground up (from a bipedal, leaping, cursorial ancestry). In the course of this essay, these two hypotheses will be discussed and contextualized by an investigation into the evolution of feathers and the various theories on why flight developed. These factors will all be discussed with reference to Archaeopteryx lithographica, a species generally regarded as being the common ancestor to modern birds and the primary source of information about the development of flight mechanisms. Archaeopteryx A brief discussion of Archaeopteryx litographica (commonly known as Archaeopteryx) is necessary to understand how instrumental this species is to discovering the mechanisms of flight evolution. Various Archaeopteryx specimens have been found in the south of Germany, well-preserved due to the qualities of the limestone in which they were found. The Archaeopteryx is commonly described as being the size of a raven and having several features that make it identifiable in the context of modern-day birds – feathers and a wishbone, for example (Ostrom, 1975). Several of these features suggest that the Archaeopteryx was capable of flight, although, as with many things in evolutionary biology, this is something that can never be ascertained. Archaeopteryx also shared several features with dinosaurs, such as having chevrons of increased length in the tail and a specific shape of ankle bone (Ostrom, 1975). This bird-dinosaur morphology is the principle reason for thinking that the species is the missing link. The way that Archaeopteryx specimens are conserved in limestone has resulted in the preservation of feather imprints, allowing paleontologists to ascertain that this species may have used feathers in the development of flight mechanisms. This is particularly good evidence of some aerial motion because the feathers found show characteristics of flight feathers, meaning that feathers had previously began to evolve for a purpose other than flight (Paul, 2002). Finally, it is important to note that it has not been proven whether Archaeopteryx had the power of full flight or could simply glide (Padian & Chiappe, 1998). Arboreal Hypothesis The arboreal hypothesis (also known as the ‘tree down’ hypothesis) refers to the idea that dinosaurs first gained flight by jumping from trees and acquiring flight as an evolutionary mechanism to avoid fatal accidents from this method. This hypothesis seems ‘intuitive’ because ‘flight evolving from an arboreal gliding stage would seem to be relatively easy’ (Padian & Chiappe, 1998, p15) and because the force of gravity ‘helps rather than hinders’ (Lewin, 1983, p38). Some studies, such as that of Feduccia (1993) suggest that the shape of the manus (the ‘hand’ portion of the forelimb) and the pes (the ‘foot’ portion of the hindlimb) of the Archaeopteryx exhibit evidence of perching, tree-dwelling and trunk-climbing due to the curvature of these anatomical elements. However, since this paper was published, another specimen of Archaeopteryx has been discovered (known as the Thermopolis specimen) which has almost complete pes, and thus there is now mounting evidence that the hallux (first digit of the pes) did not display curvature necessary for perching (Mayr et al., 2007). If we consider the Archaeopteryx as arboreal, it is important to understand how and why flight would have developed in this way. The original theory as stated by Othniel C. Marsh in the late 19th century was that Archaeopteryx would use wings as a balancing mechanism during leaps between trees, utilizing a gliding model to conserve energy. A common refutation to this point is that Archaeopteryx would utilize energy to climb trees (Mayr et al., 2007) but the terrestrial running would have taken more and as such gliding would be an evolutionary advantage (Feduccia, 1993). This, if taken as proof of the ‘intermediate gliding stage’ (Lewin, 1983, p38) that is so necessary in supporting the arboreal hypothesis, would help solve this challenge to evolutionary biology. A major problem with using Archaeopteryx as proof of the arboreal hypothesis is that it possessed very long, sharp claws or talons (Ostrom, 1975). These would have been useful as a predator but would have hindered when trying to gain access to the top of the trees. However, it is the fact that Archaeopteryx and its closest ancestors have the morphology of a bipedal creature (Redfern, 2001) that is probably the biggest failure of the arboreal hypothesis. It would be very difficult for a species that is so well adapted for running on solid ground to climb trees particularly well, as the leg and pelvic structure used for bipedality would make it very difficult to grasp a trunk sufficiently. These two problems have, as yet, not been sufficiently accounted for in the literature and as such it is difficult to accept the arboreal hypothesis as being legitimate. Cursorial Hypothesis The cursorial hypothesis (also known as the ‘ground up’ hypothesis) is the alternate theory that suggests that dinosaurs gained access to flight by first evolving powerful arm movements to assist with ground running, and finally becoming so confident with leaps that flight became possible. The cursorial hypothesis had many doubters, particularly in the past, but some newer evidence has brought this terrestrial theory to the forefront of the debate. This hypothesis is as well-established in paleontology as the arboreal hypothesis, again being first suggested in the late 19th century, suggesting that it was indeed running bipedal creatures that evolved flight through a series of progressively larger jumps (Bishop, 2008). However, the cursorial hypothesis first began to hold an advantage over the arboreal in the late 1970s with the publication of a paper by Ostrom (1975). Ostrom took a different view of the cursorial hypothesis, believing that flight evolved from the ground up because early ancestors of birds had wings used to catch insects. These wings then evolved into a wing stroke. This means that one of the major issues of the arboreal hypothesis (why a wing stroke would have evolved from gliding) is not applicable to the cursorial hypothesis. However, it was sincerely debated as to how this would evolve as it is clearly less energetically favorable than gliding from trees. This question led to a team of researchers from Northern Arizona University, Flagstaff deciding to base their investigation into the origin of flight around the principles of aerodynamics (Lewin, 1983). They discovered that the cost of running and jumping for one second to be around two calories, but the advantages of such a long and wide area within which to catch insects would outweigh this in calorific intake (Caple, Balda & Willis, 1983). However, a running takeoff is not without its problems, even if it is energetically favorable – any jump from one foot or from both feet on a patch of uneven ground will cause problematic rolling (Lewin, 1983) which is severely problematic if not corrected immediately. The study by Caple et al. (1983) goes onto describe how this problem could be averted by having increased mass at the extremities of the creature. However, despite the amassing of evidence for the arboreal hypothesis, there are still some significant problems. It has been suggested that to achieve enough lift to make flight viable, the Archaeopteryx would have to run three times faster than modern birds (Lewin, 1983). However, this could be possible due to the light weight (Erickson et al., 2009) and distinctive hind legs (Paul, 2003) of the species. It is also notable that running lizards can reach such speeds (Padian & Chiappe, 1998). Other research into the physics of cursorial flight mechanisms has also suggested that the effect of drag would cause Archaeopteryx to have a very short and ineffective flight (Longrich, 2006). Again, however, this does not necessarily mean that the species was not instrumental in the early development of flight – short, ineffective flight is still flight. The Evolution of Feathers When considering flight as a mechanism, feathers on modern day birds are often thought of, and in some cases used to justify either hypothesis mentioned above. It is commonly accepted that feathers did not evolve for the purpose of assisting flight, but existed for a different function and then evolved into the form we currently see today, where different feather shapes can be associated with different flight mechanisms (Paul, 2002). It is hypothesized that feathers initially evolved from tufts found on some dinosaur species into anatomical thermoregulation, either as protection from or to retain heat. Another hypothesis is that these tufts evolved as a form of sexual display, which would also help to explain the diversity of feathers found on modern day birds (Regal, 1985). It is useful to the cursorial hypothesis that these feathers evolved prior to the evolution of flight, as any predatorial forelimb movement would only have to be slightly adjusted to accommodate aerial motion in the presence of flight feathers (Padian & Chiappe, 1998). It is worth mentioning here the two species of Caudipteryx, specimens of which have been discovered in a small area of China (suggesting that their range was not particularly large). These creatures are incredibly birdlike, and came around 25 million years after Archaopteryx, and like Archaeopteryx share many features with both birds and dinosaurs. The interesting thing about Caudipteryx is that the specimens found show that these creatures had feathers, but they were short and symmetrical and thus could not be classified as flight feathers. Additionally, the shape and size of the body of Caudipteryx would mean that flight would be unlikely. This suggests that, after the development of feathers, the lineage split and gave rise to a set of creatures which had evolved feathers and either lost or never developed full flight (Paul, 2002). It is thus important to note that the presence of feathers does not necessarily mean that the specimen has the power to fly. Again, it is useful to consider Archaeopteryx as the key species in developing the use of feathers for flight, as this is the earliest known specimen to have feathers that can be truly classified as flight feathers. Flight feathers are identifiable because they are asymmetrical to allow exploitation of wind currents and allow for heightened control during the time spent airborne. They also have a characteristic barb-barbule-barbicel arrangement which give the vane of the wing stability. With this stability comes a firmness of the wing that allows for support of the birds body weight during flight. Whether this supports the arboreal or the cursorial hypothesis is a matter of debate, but it can be considered useful to both theories that feathers evolved prior to flight. Conclusion After having discussed both the arboreal and cursorial hypotheses, it is easy to see the source of the confusion and mystery surrounding the origin of flight in dinosaurs and how this was transferred to Archaeopteryx and other ‘missing link’ species. Many things in evolutionary biology can never be confirmed, although the amount of well-preserved specimens of Archaeopteryx and the diversity of modern-day birds has definitely helped to spur on interesting debate on this topic. Although both theories have their weaknesses and strengths (both arguable using various sources), it is evident why the cursorial hypothesis has gained strength in the latter half of the 20th century and continues to be supported by modern evidence, such as the discovery of the Thermopolis Archaeopteryx in 2005. The work by Caple et al., (1983) lay the groundwork for a cursorial hypothesis based firmly in aerodynamic principles and this has allowed the theory to go from strength to strength. This, coupled with the major issue that concerns the arboreal hypothesis – the apparent bi-pedal nature of Archaeopteryx – means that this currently seems to be the most reasonable hypothesis. Works Cited Bishop, K.L., 2008. The Evolution of Flight in Bats: Narrowing the Field of Plausible Hypotheses. Quarterly Review of Biology, 83(2), p.153-169. Caple, G., Balda, R.P. & Willis, W.R., 1983. The Physics of Leaping Animals and the Evolution of Preflight. The American Naturalist, 121(4), p.455-476. Available at: [Accessed April 11, 2011]. Erickson, G.M. et al., 2009. Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in Archaeopteryx. PLoS ONE, 4(10), p.e7390. Available at: [Accessed April 11, 2011]. Feduccia, A., 1993. Evidence from Claw Geometry Indicating Arboreal Habits of Archaeopteryx. Science, 259(5096), p.790-793. Available at: [Accessed April 11, 2011]. Hui, C.A., 2002. Avian furcula morphology may indicate relationships of flight requirements among birds. Journal of Morphology, 251(3), p.284-293. Available at: [Accessed April 11, 2011]. Lewin, R., 1983. How Did Vertebrates Take to the Air? Science, 221(4605), p.38-39. Available at: [Accessed April 11, 2011]. Longrich, N., 2006. Structure and function of hindlimb feathers in Archaeopteryx lithographica. Paleobiology, 32(3), p.417-431. Available at: [Accessed April 11, 2011]. MAYR, G. et al., 2007. The tenth skeletal specimen of Archaeopteryx. Zoological Journal of the Linnean Society, 149(1), p.97-116. Available at: [Accessed April 11, 2011]. Ostrom, J.H., 1975. Archaeopteryx. Discovery, 11(1), p.15-23. Padian, K. & Chiappe, L.M., 1998. The Origin and Early Evolution of Birds. Biological Reviews, 73(1), p.1-42. Paul, G.S., 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds illustrated edition., The Johns Hopkins University Press. Redfern, R., 2001. Origins: the evolution of continents, oceans, and life, University of Oklahoma Press. Read More