Archaeopteryx + Link Between Dinosaurs and Birds

In 1861 in a stone quarry in Bavaria, Germany, a creature with bird-like characteristics was unearthed (shown below) which would shake the world’s understanding of dinosaurs and birds. The discovery of Archaeopteryx lithographica, meaning “ancient wing,” was a crow-size animal (size compared to human shown below) that lived 150 million years ago during the Jurassic Period and ignited discussion about evolution and served as a link between the relationship between dinosaurs and birds. It was more than just any old fossil, but served as an intermediate species between birds and dinosaurs, confirming evolutionary theory. Archaeopteryx had feathers and features that were originally thought to be exclusive to birds. It was about 0.5 meters long and is thought to be a progenitor to Eumaniraptora, a branch of theropod dinosaurs, whose descendants lead to birds. Archaeopteryx served as a missing link between explaining how birds evolved from dinosaurs which would be further supported by Charles Darwin’s new theory of evolution in his book, On the Origin of Species by Means of Natural Selection, that came out only 2 years before the discovery.

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Archaeopteryx_lithographica_(Berlin_specimen)

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History About the Debate of Whether Birds Came From Dinosaurs

The first person to voice the thought that birds came from dinosaurs was Thomas Huxley in the 1860s. However, in 1926 a paleontologist, Gerhard Heilman, rejected this and thought that birds originated separately from a predinosaur reptilian ancestor because bird-like dinosaurs didn’t have a furcula (“wishbone” which is shown below). However, in 1973, John Ostrom, a Yale paleontologist, supported Huxley’s idea that there was a link between dinosaurs and birds. He proposed that birds came from small bipedal dinosaurs called coelurosaurs. One specimen of Archaeopteryx that was discovered in 1951 was misidentified as a theropod dinosaur because of how similar Archaeopteryx was to a coelosaur. Thus, most paleontologists believe that a theropod origin of birds is the most plausible explanation for avian evolution. There is a minority of paleontologists who think otherwise due to differences in tooth and finger structures.

By I, Toony, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2321740

By I, Toony, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2321740

Dinosaur or Bird?

Previously, Archaeopteryx was called the most primitive bird, as it had feathers, hollow limb bones, and a bone shaped fircula, which were all features of birds. However, Archaeopteryx also showed a variety of reptilian features, like sharp teeth, clawed forelimbs, and a long bony tail. So was it more reptilian or more bird? Xing Xu, a paleontologist at the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing, analyzed a new species, Xiaotingia zhengi, and determined that it was closely related to Archaeopteryx due to several similar features. Xu found proposed that these species resembled dinosaurs, like Velociraptor and Microraptor. The Archaeopteryx displayed no unique features shared with birds that couldn’t be found with dinosaurs, making Archaeopteryx more of a feathered dinosaur, rather than a bird. Originally it had been labelled as a bird because it had feathers and a wishbone, but as more discoveries popped up, these avian characteristics were less unique. The idea that Archaeopteryx, once heralded as the first bird to exist, was not the first bird has been met with some discord. Some paleontologists believe that it is too early to say that it wasn’t the first bird, for there is the possibility that Archaeopteryx could have been able to fly and later its descendants regressed to be more dinosaur-like. Different methodologies in research have led to different conclusions about whether Archaeopteryx was a bird or not. More work needs to be done to work out the phylogenetic tree that links the dinosaurs and birds together.

File:Archaeopteryx lithographica by durbed.jpg

By Durbed [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

First in Flight?

Whether Archaeopteryx could fly or not is still under debate, but Archaeopteryx exhibited several features which point to it not being able to fly. The feathers on bones of its arms seem to be quite primitive, limiting its flight capability by decreasing the surface area of its “wing.” Its shoulder girdle is missing several key features for powerful flight. Finally, the hallux is not fully reversed so it would have difficulty grasping branches and the second toe had a hyperextensive claw similar to the claws found on dromaeosaurid theropod dinosaurs, not birds. All of this evidence makes it seem that Archaeopteryx was probably not able to undergo sustained powerful flight. Instead, the Archaeopteryx may have been able to glide or undergo limited flapping.

File:Archaeopteryx (Feather).jpg

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Origins of Flight for Birds Today

The ancestry of birds brings up the question of how did birds develop the ability to fly. There are other organisms that can fly, like bats and insects, but birds are the only ones that evolved to fly with feathers. Hence, it’s logical to presume that the origins of flight depended on feathers. However, feathers serve other functions, like display and thermal regulation. (Flapping requires warm bodies because the chemical reactions that are necessary for flapping proceed optimally at high temperatures.) Therefore, the combination of feathers and high body temperature allowed birds engage in flapping flight. Now, there are two major schools of thought, concerning how and when birds evolved to have feathers and high body temperatures: “Trees-Down” Theory and The “Ground-up” Theory.

“Trees-Down” Theory

During the 20th century, it was thought that flight came from primitive gliding activities, like flying squirrels (shown below), which go from tree to tree. According to the “Trees-Down” theory, feathers came about from early reptilian scales. These “protofeathers” were thought to arise from a mutation that elongated and enlarged scales. Those with longer scales were thought to have an advantage because longer scales would have increased the surface area, slowing down the rate of fall and allowing creatures to go from tree to tree. This limited risks of death from falling or getting eaten by predators on the ground. It was thought that over time, those with longer and more aerodynamic scales would be naturally selected. This would in turn lead to species that could undergo gliding flight, which may have evolved into intermediate forms, similar to Archaeopteryx, leading to weak flapping and then flapping flight. Regulation of body temperature was thought to not be necessary until birds were well differentiated from reptiles and when feathers could provide insulation.

Pratikppf at en.wikipedia [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

The “Ground-up” Theory

This theory proposes that the early birds lived on the ground and were bipedal predators that chased and captured prey with clawed forelimbs. This predatory behavior is supported by high body temperature, like flapping flight. Repetitive muscle contractions would be more easily sustainable by a warm-bodied predator. Feathers may have developed later to insulate and improve the retention of heat for the body. Feathers may have also aided in capturing prey or balance while running by increasing the surface area of their forelimbs. Later, feathers were thought to be enlarged which would make flight possible. John Ostrom proposed this idea because of his belief that Archaeopteryx is linked with theropod dinosaurs, which are active, terrestrial predators, and have high body temperatures.

By Durbed [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

By Durbed [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Verdict

Not surprisingly, paleontologists, ornithologists, and other scientists who believe that birds descended from dinosaurs support the “Ground-up” theory, while those who do not believe that birds are descendants of the dinosaurs support the “Trees-down” theory. Discoveries in China of Caudipteryx and Protoarchaeopteryx have found non-avian dinosaurs with feathers. These species were terrestrial predators, which mean they provide evidence for the ground-up theory and suggest that organisms did not have to come from trees to obtain feathers. Nevertheless, there is no overwhelming, complete acceptance of either theory.

Photo by Laitche [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0) or FAL], via Wikimedia Commons

Photo by Laitche [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0) or FAL], via Wikimedia Commons

Works Cited
Kaplan, Matt. “Archaeopteryx No Longer First Bird.” Nature (2011): n. pag. Web.
Sibley, David, Chris Elphick, and John B. Dunning. The Sibley Guide to Bird Life & Behavior. New York: Alfred A. Knopf, 2001. Print.
Song, Jianlan. “Archaeopteryx: Dinosaur or Bird?” Bulletin of the Chinese Academy of Sciences 25.4 (2011): 266-71. Chinese Academic Journals (Qingdao Server) – Chinese – Physics, Astronomy, Mathematics (Series A). Web. 7 Feb. 2016.
Thompson, Mark James. “Archaeopteryx.” Encyclopedia of Time: Science, Philosophy, Theology, & Culture. By H. James Birx. Los Angeles: SAGE, 2009. 46-48. Web. 7 Feb. 2016.
Xu, Xing, Hailu You, Kai Du, and Fenglu Han. “An Archaeopteryx-like Theropod from China and the Origin of Avialae.” Nature 475.7357 (2011): 465-70. Web.