Wednesday, March 10, 2010
reptiles to birds
Coyne states, "If evolution is true, then we should expect to see the reptile-bird transition in rocks between 70 and 200 million years old" (39). This has been one major challenge toward evolution: the transition from reptile to bird. Site and explain how examples including fossil transitions, behavior similarities, and the role of feathers/wings in the transitional stages proves this evolutionary transition.
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In 1860, fossils were discovered in Germany that finally showed the reptile-to-bird transition. "The Archaeopteryx has just the combination of traits one would expect to find in a transitional form. And its age, about 145 million years, places it where we would expect." (40) According to Jerry Coyne, the Archaeopteryx contains reptile features including a jaw and teeth, a bony tail, and seprerated fingers on its wings. At the same time, it contains birdlike features including feathers and a large toe. It is suggested that these assymetrical feathers may have played a role in flight, because the shape of the feathers are aerodynamically ideal for flying. The big toe feature may have been used for perching, as it is today in modern birds. Then again in the 1990s another discovery was made in China: the Sinornithosaurus millenii. This creature had a body covered in small feathers, but it also had claws, teeth and a bony tail. From this information, it is suggested that feathered dinosaurs may have branched off and given rise to birds. New research published in Science suggests that one major anatomical similarity between birds and repitles is their lungs. The lung design in birds allows for the air to move in only one direction. This adaptaion allows for them to constantly breathe in oxygenated air, thus leading to more efficient gas exchage. The research in Science involved an experiment with aligator lungs in which it was discovered that they too have air flow in one direction. This unique trait found in birds and reptiles is just one more piece of evidence that they came from a common ancestor.
ReplyDeleteFarmer,C.G. & Sanders,K. 2010. Unidirectional Airflow in the Lungs of Alligators. Science. vol.327:338-340
The first major transitional fossil found between birds and reptiles was the, like stated above, the Archaeopteryx. This was a significant discovery in Germany because this fossil showed that this animal had feathers, possibly for flight, while also possessing bony structures such as teeth and a jaw like the dinosaurs.
ReplyDeleteAside from fossil records, there hold some many similarities between birds and reptiles. They both lay shelled eggs and have scales (birds have scales on their legs). Furthermore, they possess nucleated red blood cells and skeletal similarities.
The theory that prevails today as for the evolutionary pathway is that birds have evolved from a group known as theropods from the late Triassic Period until the end of the Cretaceous Period, about 220 to 65 million years ago. These dinosaurs were able to run quickly, likely attributed to their hollowed out bones for less mass, while also possessing only three toes on each feet and teeth. The final thing linking birds and theropods together is, of course, the existence of feathers.
Some hypothesis about how reptiles evolved into birds is visible through the period of dinosaurs. Dinosaurs running from predators or running after prey likely developed powerful enough feet to eventually be able to propel themselves into the air and gain temporary flight. This could be tied to current day birds that flap their wings while utilizing their powerful legs to gain much higher levels of velocity. Another theory, the arboreal flight theory, observes that some reptiles glided from tree top to tree top, eventually gaining enough power for flight.
A most recent discovery, the Microraptor in 2003, favors the arboreal flight theory. This fossil showed that the dinosaur had four feathered wings, along with feathers on its hind legs. These feathers are hypothesized to have increased the time the Microraptor was able to stay in the air when traveling from tree tops.
Sources:
http://www.pbs.org/lifeofbirds/evolution/
http://nationalzoo.si.edu/Publications/ZooGoer/2005/3/birdevolution.cfm