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Feathered Dinosaurs

Many fossils have now been found in China that indicates that at least the theropod dinosaurs had feathers, and there is other evidence that the similarities between dinosaurs and birds go much deeper than the feathers. When the arm bones of Velociraptor were restudied in 2007 it was noted that they had quill knobs, a clear indication of feathers as these knobs are the attachment sites in modern birds for feathers. It became apparent that the these dinosaurs were covered with feathers. Other characters now known to be shared by dinosaurs and birds are the possession of a wish bone, hollow bones, and they nested in a similar manner to birds.

In the 1970s a 70 cm bipedal beaked dinosaur was found in Mongolia by the Russian palaeontologist Sergei Kurzanov. He named it Avimimus portentosus (the amazing bird mimic) because he believed it was so bird-like, having similarities such as quill knobs on the bones. The significance of these quill knobs were overlooked until they were also found on other dinosaurs such as Velociraptor.

In the 1970s it was also claimed by some that the theropods were both warm-blooded and bird-like. The 1996 discovery of Sinosauropteryx prima in Liaoning, China, proved to be further evidence of how bird-like the theropods were, it was covered with downy feathers. The fossil was so well-preserved that small mammals were found in its gut and eggs in its oviduct. Other theropods found in the same deposit included Caudipteryx and Microraptor. More than 24 species of dinosaur are now known to have been covered with feathers, though palaeontologists usually refer to these types of feather as dinofuzz. The range of species now known to be feathered is so wide that it indicates it was a common feature in dinosaurs.

There is an accumulating body of evidence for the similarity between some dinosaurs and birds includes, growth rates, physiology, bone structure and feathers. A small dinosaur was found at the Liaoning site, Mei long (sleeping dragon), that had apparently died in the same position adopted by sleeping birds to reduce heat loss, the legs tucked under its body and its head tucked under its arm. It was covered by a a rain of volcanic ash.

About 228 million years ago in the Middle Triassic of Argentina lived one of the oldest known theropods, Herrerasaurus, in the bones of which are the first indications of bird-like characteristics such as feathers. The coelurosaurians (hollow lizards), referring to their hollow vertebrae, were an early theropod groups that displayed these bird-like characteristics that continued through into the birds of today.

The coelurosaurians include a wide range of carnivorous dinosaurs, varying in size from about 2 m (Coelurus) to about 14 m (Tyrannosaurus). The position of Coelurus in the evolution of Tyrannosaurids has been widely debated, mostly because it is known only from a partial skull and only some of the post cranial bones. It is widely believed to fit somewhere in the early stages of the evolution of feathered dinosaurs. Being of a generalised form, it had evolved some of the features that were to continue on into the birds, such as light, hollow bones and, together with the rest of the feathered dinosaurs, had a brain that was relatively larger than occurred in other predatory lines such as Allosaurus and a group that arose in Gondwana, the Abelisaurids.

The earliest known forms that may be part of the Tyrannosaurus lineage are Eotyrannus and the Chinese feathered form, Dilong. Among the most primitive of the group was Coelosurus fragilis. It lived about 150 million years ago in the Late Jurassic, making it a contemporary of Archaeopteryx, the first known bird. A suggested contender for the first form in the line leading to Tyrannosaurus was Tanycolagreus, a 3-4 m long predator that was lightly built, with long arms and fingers. There are features of its pelvic bones and femurs that are similar to those of the Tyrannosaurids.

A paper published in Nature, September 2009, describes a troodontid, a theropod dinosaur, Anchiornis huxleyi that lived between 161 and 151 million years ago, in the earliest part of the Late Jurassic in Tiaojishan Formation of western Liaoning, China. It had an extensive covering of feathers, with long pennaceous feathers attached to the pes. This specimen adds to other evidence, such as Microraptor gui, from the Early Cretaceous, that the earliest birds probably had 4 wings, as do both of these species.

Asilisaurus kongwe, that was possibly omnivorous, was an archosaur that lived in the Triassic, about  240 Ma, in the 10 million years before the dinosaurs arose and was an early member of the ornithodira clade that led to theropods and eventually birds.

Fossils from the Liaoning deposits in China

The very well preserved fossils in the Liaoning deposits from northeast China are of Early Cretaceous age. The first to be recovered from these deposits were fossils of early birds such as Confuciusornis, the skeletons of which were surrounded by the impressions of feathers, beaks and claws. A complete skeleton of a small theropod was discovered in 1996, the anatomy and proportions of which were very similar to another theropod, Compsognathus. This fossil, Sinosauropteryx,  was described by Ji Qiang and Ji Shu'an. The author3 suggested the remarkable feature of this fossil is the fringe of filamentous structures along the spine and across the body, suggesting that the animal had some sort of covering on its skin similar to the pile on some carpets, and in the eye socket and the gut region, there was evidence of soft tissue. This made it clear that some sort of covering was present on the body of at least some small theropods.

The first fossil from the Liaoning deposits to show signs of having true bird-like feathers on its tail and along the sides of its body was Protoarchaeopteryx, and the anatomy of  this fossil was much closer to that of dromaeosaurians than Sinosauropteryx. Sinornithosaurus was another dinosaur found in the Liaoning deposits. It appears to have been covered in a 'pile' of short filaments, and its anatomy was very similar to that of Velociraptor. Other discoveries at this site include Caudipteryx, a large animals about the size of a turkey that had snort arms with a pronounced tuft of tail feathers, and along its arms shorted fringes of feathers. Other dinosaurs found at the site include smaller dromaeosaurians that were heavily-feathered, and Microraptor, that had 4 wings. This animal was small and its skeleton was similar to the classical dromaeosaurian form with a typically long, narrow tail, a bird-like pelvis, long arms suitable for grasping, and rows of sharp teeth in the jaws. Its body was covered by downy feathers and its tail had a fringe of primary feathers. The author3 found the preservation of flight feathers along the arms that formed wings similar to those of Archaeopteryx, and the most unexpected feature was the presence of feathers along the lower parts of the legs - hence 'four-wing'.

Birds, theropods, and dinosaur physiology

The author3 suggests that after studying the finds in the Liaoning deposits is is apparent that theropod dinosaurs had a variety of skin coverings, from a shaggy, filamentous type, through downy, feather-like coverings to contour and flight feathers that were fully formed. The discoveries of feathered dinosaurs at Liaoning have led to speculation about what sort of covering was present on not only other theropods, but also other dinosaur groups.

The author3 has suggested that based on the known distribution of body coverings, it seems reasonable to consider the probability of giant dinosaurs such as T. rex, a theropod related to Sinosauropteryx, having some sort of covering, possibly only as a juvenile. He also suggests that, especially since the discoveries at Liaoning, it has become obvious that feathered theropods and true birds coexisted during the Jurassic and Cretaceous. Archaeopteryx is from the Late Jurassic, about 155 Ma, was obviously covered in feathers and bird-like. It is now known that about 120 Ma in the younger Cretaceous there were many different 'dinobirds' such as Microraptor and related species that lived along side true birds.

According to the author3 the sheer diversity of the dinobirds is bewildering, obscuring to some extent, the origins of true birds that are so common at the present.

The author3 suggests that the now known fact that many theropod dinosaurs had 1 of several insulating coverings, such as feathers, is conclusive proof that, from a physiological perspective, at least the theropods were genuine endotherms. He gives 2 main reasons for this conclusion:

  • Many of these dinosaurs were, at 20-40 cm long, small bodied, and as small animals have a large surface to volume ratio, hence they lose body heat to the environment very rapidly. The presence of a covering of filaments, that has a similar function to that of the fur on mammals at the present, the presence of downy probably being necessary if they generated internal body heat.
  • Equally, an outer insulator layer to the skin would make basking difficult, and probably impossible, as their ability to gain heat from the sun would be inhibited by the insultory covering. Basking is used by ectotherms to gain body heat so an insulatory layer of fur or feathers would makr it impossible for such reptiles to gain heat.

 

Sources &Further reading

  1. Feathered Dinosaurs: the Origin of Birds, John Long and Peter Schuten, CSIRO Publishing
  2. Cosmos magazine, Feb/Mar 2010
  3. Norman, David, 2005, Dinosaurs: A Very Short Introduction, Oxford University Press

Links

  1. Peter Schouten
  2. Dinosaur-like creature spread in Triassic times
  3. Dinosaurs outgrow their baby feathers
  4. Closet veggie-lovers?: 'Predatory Dinos Ate Plants

 

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Last updated 27/01/2010

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading