Vertebrate Zoology – Evolution: How birds evolved from their dinosaur ancestors
Evolutionary studies, particularly the evolution of birds, have always evoked tremendous curiosity and fascination amongst the scientific community as well as the general public. The idea that birds actually evolved from dinosaurs goes back to the second half of the nineteenth century when two of the greatest scientists of that period, Richard Owen and T.H. Huxley, provided this hypothesis backed by documented anatomical evidence. The discovery of Archaeopteryx, an archetypal primitive bird, had provided basis for the presentation of this hypothesis. Subsequent discoveries of abundant and well-preserved fossils of dinosaurs as well as birds have offered extraordinary proof of the dinosaurian origin of birds, the beginnings of flight in birds and dinosaurs, and the origin of feathers before the beginning of flapping flight. (Zhou, 458); (Norell; Gaffney; Dingus, 11)
Discussion:
Brief history of dinosaurs
It is believed that the dinosaurs first evolved during Late Triassic and included both herbivores as well as predators including four-toed and bipedal meaning bird-like predatory dinosaurs. This period was marked by the appearance of dinosaurs like Coelophysis which were bird-footed, flesh-eating and gracile dinosaurs. The dinosaurs survived the end of the Triassic well into the Jurassic when many other kinds of reptiles other than dinosaurs also flourished. This was a period when birds became common in the air, water and on land. However, very little evidence of birds was found from most of the Jurassic period and fossils like Archaeopteryx were found only from the end of the Jurassic period which was only 150 Myr back. (Paul, 5)
The bipedal predators or theropods were obligatory bipeds. Prior to the end of the Triassic period, the neotheropods had split up into two lines — the ceratosaurs and the tetanurans. These tetanurans prolified on all continents by mid-Jurassic and gradually split into two main groups consisting of the allosauroids and the coelurosaurs. It is these coelurosaurs which eventually gave rise to birds by the end of the Jurassic period. (Sereno, 2141) The coelurosaurs comprised of both large and small-bodied theropods which had characteristic features in the limb bones, vertebrae and skull. Coelurosaurs comprised of ornithomimids, tyrannosaurids, therizinosaurids, and maniraptors, the group that included birds. (Sereno, 451)
Source of the dinosaurian origin of bird’s theory
It was the discovery of an amazingly bird-like 150 Myr old fossil in the swamps of Germany in 1869 which started the whole debate about the dinosaurian origins of birds. The Archaeopteryx had long and strong legs with three toes having claws quite like birds. Its head, however, had reptilian jaws and its spine extended up to a reptile-like bony tail. The most amazing feature was the presence of feathers, a unique characteristic of birds, on both sides of its tail bones. (Davies, Evolution) Later findings of bird-like dinosaurs in the early 1990’s from Mongolia and Argentina gave credence to this theory. The fossilized remains of Mononykus from Mongolia and Alvarezsaurus from Argentina blurred the boundary between flightless birds and the running bipedal dinosaurs. The most recent discovery of a feathered dinosaur fossil in China has helped in a better understanding of the dinosaurian origins of birds. This particular species, Anchiornis huxleyi, a small 13″ theropod whose forelimbs and body were feathered, is believed to be even more primitive than the Archaeopteryx. In fact, Anchiornis is probably closer to the point at which divergence from dinosaurs took place and a new line of “dinosaurs with wings” first evolved. These fossils have provided invaluable information about the evolution of feathers, wings and flight amongst theropods which eventually paved the way for the appearance of the modern birds. (Long; Schouten, 25); (Platt, New Feathered Dinosaur Found; Adds to Bird-Dino Theory)
Alternative theories of bird origin
Some other researchers like Chatterjee, Martin and Feduccia had suggested that birds did not evolve from dinosaurs but from crocodilians or ornithosuchids. However, it has been studied that extinct or extant birds do not possess any unique crocodilian or ornithosuchid characters. On the other hand, there are more than 120 unique characters which are shared only amongst dinosaurs and birds. In fact, there are very few characters which can be used to distinguish theropods from the proto-birds. (Farlow; Brett-Surman; Walters, 215)
Evolution of feathers and flight
It has been suggested that the evolution of feathers was not a prerequisite for flight and might have evolved in dinosaurs for the purpose of display, camouflage or insulation. In fact, feathers have also been found in Sinosauropteryx, a small coelurosaurian dinosaur, demonstrating that feathers were a common characteristic amongst a broader group of dinosaurs. Different theories have been proposed about the origin of flight in bird-like dinosaurs. The “arboreal theory” suggests that flight evolved among theropod dinosaurs that could climb large trees and glide downwards towards the ground. However, there are weak evidence to support this theory including the absence of large trees near the area where Archaeopteryx existed during that period, the lack of aerodynamic design in Archaeopteryx and the absence of arboreal specializations in theropods like Tyrannosaurus and Allosaurus. The “cursorial theory” suggests that flight began as a result of activities like leaping, running and maintaining brief, leg-powered glides in order to catch prey or escape from predators. However, biomechanical analysis suggests that arboreal, rather than cursorial flight is easier to evolve but evidence seems to prove otherwise. (Alexander; Vogel, 203); (Hill; Davis, The Origin and Early Evolution of Birds)
Another model has been proposed by some biologists from the Oxford University. Their theory, known as the “pouncing proavis theory” is based on traits like bird-like running, weight-reducing adaptations and the development of elongated, symmetrical feathers on the hands of theropod dinosaurs like Caudipteryx. According to this theory, a small, carnivorous theropod made some small adaptations to catch prey by leaping on them from elevated positions. The ability to steer its body during the leaping act might have benefited this leaping animal. Further development may have included extensions or enlargements of the forearms and hand possibly helped in this and may have depended on asymmetrical drag and later on asymmetrical lift. The location and function of the forearm or hand extensions may have been the same as that of ailerons on the wings of a plane. Further evolutionary enhancements in such a theropod would have led to aerodynamic effectiveness which resulted in better steering and provided enough lift to convert the leaps into swoops. The evolutionary selection process for longer swoops might have resulted in a gliding and/or flapping stage which then evolved into actual flight. (Alexander; Vogel, 207)
Anchiornis, the primitive feathered theropod which was found in China recently, had some characteristic features of wrist which suggested that it was highly mobile and indicative of the future wing-folding mechanisms found in later birds. Steering and flapping, which are integral parts of flight, are highly dependent on the wrist. The wrist also plays an important role in wings formation. (Platt, New Feathered Dinosaur Found; Adds to Bird-Dino Theory)
Dinosaur and bird physiology
All modern-day birds are endothermic and scientists have reasoned that their immediate coelurosaurian ancestors might have been endothermic as well. However, enough evidence has been gathered to prove that some of the dinosaurs were, in fact, ectothermic. Nevertheless, some dinosaurian features do suggest that some of the dinosaurs were endotherms. These features include an upright posture and locomotion as evidenced from trackways, body size and length of specific body parts, and soft-part anatomy like organs and feathers. An upright posture is normally associated with increased mobility and activity which in turn is related to food requirements and metabolism. Again, feathers might have served the process of insulation to the endothermic ancestors of birds. (Martin, 373)
Skeletal similarities between dinosaurs and birds
Early birds had a number of similarities with their immediate ancestors, the coelurosaurian dinosaurs (especially the Maniraptora including the Velociraptor) in terms of major skeletal structures. These similarities include: (i) the anterior pubis moved to a more posterior position and possessed a small distally positioned “boot” (ii) large orbits (iii) thin-walled and hollow bones (iv) elongated forelimbs and arms having clawed manus (v) posteriorly-stiffened and reduced tail (vi) four-toed foot but externally supported by 3 toes; opposable grasping hand with 3 fingers (vii) metatarsals were elongated (viii) eggshell microstructure was similar (ix) curved neck shaped like an “S” (x) furcula or wishbone formed by fused clavicles (xi) scapula strap-like (xii) presence of expanded pneumatic sinuses in the skull (xiii) sacrum incorporating five or more vertebrae (xiv) arms and pectoral girdle showing signs of wing power stroke (xv) the pneumatization of the ribs at the front and neck as well the vertebrae were accomplished by air sacs and diverticula which were linked to the lungs. Striking similarities have also been observed between the unusual skulls of oviraptorids and “highly derived birds” such as the cassowary. (Are Birds Really Dinosaurs? ); (Farlow; Brett-Surman; Walters, 218)
Molecular evidence of dinosaurian origins
In the realm of developmental biology, the digits borne on the wings of birds are embryologically considered to be 2, 3 and 4. In Paleontology, however, these wing digits have been considered as digits 1, 2 and 3 based on phylogenetic assessment of the fossil lineage suggesting that birds have evolved from theropod ancestors that had lost the fourth and fifth digits. Critics of this theory have suggested that birds have evolved from some other ancestors that had lost the first and fifth digits. Studies of developing limbs of chicken, including a polydactylous variety and mouse have confirmed that the wing digits are actually 1, 2 and 3 and support the hypothesis of theropod descent of birds. (Vargas; Fallon, 87)
Conclusion:
The origin of birds from dinosaurs is a fascinating study. The discovery of the first fossil protobird, the Archaeopteryx started most of the studies on the dinosaurian origins of birds. Most paleontologists now agree that birds have descended from a particular line of dinosaurs, the theropods, more specifically the coelurosaurs who had features that were remarkably similar to birds. The study of the skeletal remains of the Archaeopteryx, Anchiornis, Mononykus, Alvarezsaurus and many other fossils have confirmed the hypothesis that birds have evolved from feathered dinosaurs through small evolutionary steps which included small leaps into the air to catch prey followed by swoops which later evolved into steered swoops, glides and finally into full-fledged flight. This process was helped by numerous small skeletal and physiological adaptations that helped in flight as well as ensured survivality of that primitive dinosaur which later evolved into birds. The origins of flight, however, are still debatable with experts differing over cursorial, arboreal and “pouncing proavis theory.” There is also considerable debate over the issue of the origin and purpose of the evolution of feathers since feathers have been found in many non-avian dinosaurs as well. Despite all these debates, one cannot discount the fact that there are many characteristics of more than 120 that are shared by both dinosaurs and birds.
References
Alexander, David E; Vogel, Steven (FRW). Nature’s flyers.
JHU Press, 2004.
Davies, Gareth Huw. Evolution. Retrieved 28 April, 2009 from http://www.pbs.org/lifeofbirds/evolution/
Farlow, James O; Brett-Surman, M. K; Walters, Robert F. The Complete Dinosaur. Indiana University Press, 1999.
Hill, Jon; Davis, Katie. The Origin and Early Evolution of Birds. 2007. Retrieved 28 April,
2009 from http://www.geologyrocks.co.uk/tutorials/origin_and_early_evolution_birds
Long, John A; Schouten, Peter. Feathered Dinosaurs.
Oxford University Press U.S., 2008.
Martin, Anthony J. Introduction to the study of dinosaurs.
Wiley-Blackwell, 2001.
N.A. Are Birds Really Dinosaurs? Retrieved 28 April, 2009 from http://www.ucmp.berkeley.edu/diapsids/avians.html
Norell, Mark; Gaffney, Eugene S; Dingus, Lowell. Discovering dinosaurs. University of California Press, 2000.
Paul, Gregory S. Dinosaurs of the air.
JHU Press, 2002.
Platt, Kevin Holden. New Feathered Dinosaur Found; Adds to Bird-Dino Theory. National Geographic News, January 16, 2009. Retrieved 28 April, 2009 from http://news.nationalgeographic.com/news/2009/01/090116-feathered-dinosaur.html
Poling, Jeff. Feathers, scutes and the origin of birds. Retrieved 28 April, 2009 from http://www.dinosauria.com/jdp/archie/scutes.htm
Sereno, Paul C. The Evolution of Dinosaurs. Science, vol. 284, 25 June 1999,
pp: 2137-2147.
Sereno, Paul C. The Origin and Evolution of Dinosaurs. Annu. Rev. Earth Planet. Sci, 1997.
vol. 25, pp: 435 — 489.
Vargas, Alexander O; Fallon, John. F. Birds Have Dinosaur Wings: The Molecular Evidence.
Journal of Experimental Zoology (Mol Dev Evol), 2005, vol. 304B, pp: 86 — 90.
Zhou, Zhonghe. The origin and early evolution of birds: discoveries, disputes, and perspectives from fossil evidence. Naturwissenschaften, 2004, vol. 91, pp: 455 — 471.
