Paleontology

Similarities between Birds and Dinosaurs a Story of Evolution



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The relationship between birds and dinosaurs was first mentioned (atleast according to popular myths within the paleontology circle) by Thomas Henry Huxley, also known as Darwin's Bulldog due to his relentless defence of Darwin's Natural Selection theory. The story goes something like this, one evening Huxley was eating a quail, also wondering about a fossil bone in his lab. He was sure that the bone was a tibia (lower leg) that belonged to a carnivorous dinosaur, however, he was baffled by an extra part on the bottom part of the bone. Huxley finished off his quail leg, and realized the uncanny similarity between the two bones: it was the ankle bone, and it looked exactly the same in both the quail bone (which was obviously more complete) and the dinosaur bone. Huxley reached the conclusion that the anklebone, also known as astragalus, was so similar between the two different animals, they had to be related.

In other words, birds evolved from theropod (carnivorous) dinosaurs.

And of course, there was a huge debate over this and many scientists did not take his new theory seriously. Furthermore, birds seemed to have evolved from the theropods in the Saurischia order, which were basically dinosaurs that were 'lizard-hipped', and not the Ornithischia order, which literally means bird-hipped. So the theory was not a favorite among many paleontologists of the time as really nothing more than the pelvic structure seemed any more similar between dinosaurs and birds.

The theory received a big body blow in 1916 when Danish doctor Gerhard Heilmann published 'The Origin of Birds' a paper on the evolution of birds. By then many more clues and proofs about the anatomical similarities between birds and theropod dinosaurs have been discovered, which were noted by Heilmann in his publication. He however, was critical of the lack of the collarbone in theropod dinosaurs which fused together to form the wishbone in birds. According to Heilmann such an important physical trait could not have been lost in theropod dinosaurs to have re-evolved in birds. Thus he concluded that, birds could not have evolved from dinosaurs. The idea of birds evolving from dinosaurs was dropped once again, this time for almost half a century.

During the late 1960s, a very talented paleontologist (perhaps one of the greatest) by the name of John H. Ostrom from Yale University discovered a new dinosaur which he named Deinonychus. This was one of the most significant fossil find in history. Ostrom concluded that dinosaurs were atleast partly warm-blooded, and they were active creatures with high metabolism rates, not the slow and sluggish creatures they were believed to have been until then.

This was one of the first of many theories put forward by Ostrom (and later his student Robert Bakker) that would completely revolutionize our understandings on dinosaurs. Around the mid 1970s, he started studying the first Archaeopteryx specimen (which was known as Pterodactylus crassipes until Ostrom published his theory) that had been sitting in the shelves of Teylers Museum in the Netherlands.

Ostrom recognized many features that were common when he started studying the Archaeopteryx specimen. He correctly identified Archaeopteryx as one of the first 'true' birds. In total he came up with 22 significant characteristics between Archaeopteryx and birds, such as a wishbone, flight features, wings, and a partially reversed first toe; some of the theropod traits that Archaeopteryx had were long ascending ankle bone, interdental plates, similar pubis and pelvic bone structures, and long chevrons in the tail. Ostrom concluded that Archaeopteryx was very similar to the family of theropods known as Dromaesauride.

Thus Archaeopteryx became known as a 'transitional species' between dinosaurs and birds.

It must be noted that although noted by Heilmann in his study in the early 1900s that dinosaurs lacked collarbones, it was later found that theropods not only had collarbones, they were actually fused into a wishbone! The reason behind such an obvious blunder in Heinmann's study could be attributed to the sparse records of fossils during his time, and misidentification of whatever wishbones there were.

Although Ostrom's theory seemed to have well established the similarity and the ultimate origins of birds from dinosaurs, there have been many scientists who have constantly opposed his theories. Two of the most prominent opposers to bird-from-dinosaur theories are Alan Feduccia from the University of North Carolina and Larry Martin from the University of Kansas. They argue that birds could not have evolved from dinosaurs as 'flight' is something that must have started from a tree-climbing reptile which would need to jump or glide, however all known ancestors of dinosaurs were ground-dwelling reptiles. Furthermore, they argue that theropod dinosaurs had ribcages compressed from side to side, while birds have theirs from back to front.

Most scientists argue however, that organisms change over the course of evolution, and it is not the differences that matter but similarities that they retain. Moreover, theropod dinosaurs could have very easily evolved flight trying to leap and catch insects and other small animals, and we know for a fact today that many smaller theropods infact, did hunt insects and smaller animals.

The debate seems to have come to rest atlast after the discovery of well preserved fossils of feathered dinosaurs in the 1990s and 2000s. The fossils had perfectly preserved fossilized feathers or very detailed impressions of feathers. Furthermore, behavorial studies of Oviraptorosaur have shown evidence about similarities between birds and dinosaurs too. The fossil of Oviraptorosaur was found sitting on its nest, its forearms folded like a bird insulating its egg and nest. Although the feathers were not found, scientists concluded the Oviraptor was insulating its nest and egg, much similar to what a bird does today.

The similarities between birds and dinosaurs are undebatable today. Even though there are opponents to the theory of the origin of birds, it is believed by an overwhelming majority within the scientific community that there are undoubted similarities between birds and theropod dinosaurs, and they are the direct modern day descendants of dinosaurs. Let's take a brief look at similarities between the birds and dinosaurs as we end this article.

Similarities:

"
Pubis (one of the three bones making up the vertebrate pelvis) shifted from an anterior to a more posterior orientation and bearing a small distal "boot".Elongated arms and forelimbs and clawed manus (hands).Large orbits (eye openings in the skull).Flexible wrist with a semi-lunate carpal (wrist bone).Hollow, thin-walled bones.3-fingered opposable grasping manus (hand), 4-toed pes (foot); but supported by 3 main toes.Reduced, posteriorly stiffened tail.Elongated metatarsals (bones of the feet between the ankle and toes).S-shaped curved neck.Erect, digitgrade (ankle held well off the ground) stance with feet postitioned directly below the body.Similar eggshell microstructure.Teeth with a constriction between the root and the crown.Functional basis for wing power stroke present in arms and pectoral girdle (during motion, the arms were swung down and forward, then up and backwards, describing a "figure-eight" when viewed laterally).Expanded pneumatic sinuses in the skull.Five or more vertebrae incorporated into the sacrum (hip).Straplike scapula (shoulder blade).Clavicles (collarbone) fused to form a furcula (wishbone).Hingelike ankle joint, with movement mostly restricted to the fore-aft plane.Secondary bony palate (nostrils open posteriorly in throat).Small, possibly feathered dinosaurs were recently found in China. It appears that many coelurosaurs were cloaked in an external fibrous covering that could be called 'protofeathers'. * " (1)* Even more recent studies have proven that certain theropods infact, had feathers and not just protofeathers.

Citations:

http://www.abc.net.au/science/slab/dinobird/story.htm

http://en.wikipedia.org/wiki/Feathered_dinosaurs

http://en.wikipedia.org/wiki/Thomas_Henry_Huxley/

http://en.wikipedia.org/wiki/Origin_of_birds

(1) http://www.ucmp.berkeley.edu/diapsids/avians.html

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