The current scientific consensus holds that birds evolved from theropod dinosaurs. Using the strict phylogenetic definition of a clade as all descendants of a single last common ancestor, modern birds are dinosaurs and dinosaurs are, therefore, not extinct. Less scientifically, but more comprehensible than concepts like monophyly, the current state of knowledge could be summed up by a Clausewitzian Birds are a continuation of dinosaurs by other means.
This theory, first suggested by the discovery of the “missing link” Archaeopteryx in 1861 (technically 1855), has been most spectacularly illustrated by the recent discovery of feathered dinosaurs, most famously in Liaoning province, China. Yet the most compelling evidence still comes from a complete anatomical study of both groups. Birds share hundreds of anatomical features with other dinosaurs unique to the various dinosaurian clades they belong to, and also many behavioral similarities inferred from fossils.
History of the theories
Shortly after the 1859 publication of Charles Darwin's The Origin of Species, British biologist and evolution-defender Thomas Henry Huxley proposed that birds were descendants of dinosaurs. He cited skeletal similarities, particularly among some saurischian dinosaurs, fossils of the 'first bird' Archaeopteryx and modern birds. In 1868 he published On the Animals which are Most Nearly Intermediate between Birds and Reptiles, making the case. The leading dinosaur expert of the time, Richard Owen, disagreed, claiming Archaeopteryx as the first bird outside dinosaur lineage.
For the next century, claims that birds were dinosaur descendants faded, with more popular bird-ancestry hypotheses including 'crocodylomorph' and 'thecodont"'ancestors, rather than dinosaurs or other archosaurs. Especially, in the 1920's, G. Heilman wrote his influential book The Origin of Birds, in which the dinosaur-bird link is dismissed, based on the dinosaurs' supposed lack of a furcula (fused clavicles).
Then, in 1964, John Ostrom discovered a fossilized dinosaur he called Deinonychus antirrhopus, a theropod whose skeletal resemblance to birds seemed unmistakable. This triggered a new interest in dinosaur studies, later known as the 'Dinosaur Renaissance'.
Ostrom has since become a leading proponent of the theory that birds are direct descendants of dinosaurs. Further comparisons of bird and dinosaur skeletons, as well as cladistic analysis strengthened the case for the link, particularly for a branch of theropods called Maniraptora. Skeletal similarities include the neck, the pubis, the wrists (semi-lunate carpal), the 'arms' and pectoral girdle, the shoulder blade, the clavicle and the breast bone. In all, over a hundred distinct anatomical features are shared by birds and theropod dinosaurs. By the 1990s, most paleontologists considered birds to be surviving dinosaurs and referred to 'non-avian dinosaurs' (those that went extinct), to distinguish them from birds (Aves or avian dinosaurs).
A recent case in point:
"Archaeopteryx, therefore, is closely related to the theropods. This in turn means that theropod dinosaurs are the ancestors of the modern birds that followed Archaeopteryx. The find, according to G. Mayr, 'not only provides further evidence for the theropod ancestry of birds, but it also blurs the distinction between basal (the earliest) birds and basal deinonychosaurs, their fearsome-clawed ancestors. 'I do think that the question of a theropod ancestry of birds can now be considered settled once and forever,' Mayr said." According to paleontologist John Hutchinson,
"For those that have actually seen the relevant specimens and considered all of the relevant data (which is a basic procedure for any scientist), it is becoming increasingly difficult to draw the line between 'bird' and 'non-avian dinosaur'." 
Note that the dividing line between extant birds (Neornithes) and non-avian dinosaurs can be unequivocally drawn, and that the deinonychosaurs are more probably the sister taxon of all birds: they evolved along somewhat different lines, but from the same ancestors (which were apparently not really "fearsome-clawed" yet) - indeed, the basic structure found in Velociraptor's large-clawed toe was apparently also present in Archaeopteryx (Mayr et al., 2005) -, and as part of a general radiation of more or less bird-like maniraptoran dinosaurs which apparently originated in the early Jurassic or perhaps a bit earlier. These "paravian" (from para- and Aves) dinosaurs reached their first "bout" of high diversity in the Cretaceous, with the "raptor-clawed" deinonychosaurs and (probably) around half a dozen lineages of birds. Of this first radiation, only some Neornithes survived into the Neogene and subsequently underwent two more major radiations, the first filling many niches left vacant by the extinction of the dinosaurs (with the added benefit of flight) and the second producing a vast number of smaller, usually tree-dwelling species -- the perching birds.
In the words of Rinchen Barsbold,
"[I]n the dinosaur, more precisely the theropod, lineages "ornithization" took place fairly early. During this process, which was in its initial stages during the Late Triassic development of theropods, lineages were formed that led to birds. It is easily assumed that Archaeopteryx is merely one of those lineages (even if a dead end), but regardless of the extent to which "real" birds deviate from it, the similarity to its theropod ancestor is visible." (Barsbold, 1983)
Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The initial specimen was found in the Solnhofen limestone in southern Germany, which is a lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils. The Archaeopteryx is a transitional fossil, with features clearly intermediate between those of modern reptiles and birds. Brought to light just two years after Darwin's seminal The Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, specimens are commonly mistaken for Compsognathus.Additional resource:
Ceratopsian crest as acoustic amplifier can be found in published work:
Anton, J.A. Dinosaurs Incognito. 2009. VDM Verlag. Germany. pp. 192.
Archaeopteryx is undeniably a bird, but also exhibits a number of reptilian features, the most obvious ones being the long tail, teeth, dinosaurian three-fingered hands with clawed fingers, and dinosaurian legs (Dinosauria On-Line, 1995, 1996).
It seems more likely than not the bird lineage originated in today's Eurasia. The fossil record is too scant yet to settle this question, but it appears from the close similarity between Archaeopterx and the earliest unequivocal deinonychosaurs that the lineages had not separated a long time before the Late Jurassic. The distribution of fossils suggests that the bird-deinonychosaur split occurred around the time of the separation of Gondwana from Laurasia; by the Early Cretaceous, avian flight was apparently advanced enough to enable the different bird lineages to perform successful trans-oceanic voyages.==Features linking birds and dinosaurs==
The first good specimen of a 'feathered dinosaur' was the 1861 discovery of Archaeopteryx, in Germany, in the Solnhofen limestone, which is a lagerstätte; one of the rare and remarkable geological formations known for their superbly detailed fossils. Coming just two years after Darwin's seminal The Origin of Species, the evidence of a transitional fossil between reptiles and birds spurred the debate between evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, the specimens are commonly mistaken for Compsognathus.
Since the 1990s, a number of feathered dinosaurs have been found, providing clear evidence of the close relationship between dinosaurs and birds. Most of these specimens were local to Liaoning province in northeastern China, which was part of an island continent in the Cretaceous Period. However, the feathers were only preserved by the lagerstätte of the Yixian Formation; it is therefore possible that dinosaurs elsewhere in the world may also have been feathered, even though the feathers have not been preserved.
The feathered dinosaurs discovered so far include Beipiaosaurus, Caudipteryx, Dilong, Microraptor, Protarchaeopteryx, Shuvuuia, Sinornithosaurus, Sinosauropteryx and Jinfengopteryx, along with dinosaur-like birds, like Confuciusornis. All of them have been found in the same area and formation, in northern China. The Dromaeosauridae family, in particular, seems to have been heavily feathered and at least one dromaeosaurid, Cryptovolans, may have been capable of flight.
Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the association of multiple skeletal features also shared by the two groups is the more important link for paleontologists. Furthermore, it is increasingly clear that the relationship between birds, dinosaurs and the evolution of flight is more complex than has been previously realized. For example, while it was once believed that birds simply evolved from dinosaurs and went their separate way, some scientists now believe that some dinosaurs, such as the dromaeosaurs, may have actually evolved from birds, losing the power of flight while keeping the feathers in a manner similar to the Ostrich and other ratites.
Comparisons of bird and dinosaur skeletons, as well as cladistic analysis, strengthens the case for the link, particularly for a branch of theropods called maniraptors. Specific similarities have already been listed.
Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation which was led by Patrick O'Connor of Ohio University. The lungs of theropod dinosaurs (carnivores that walked on two legs and had birdlike feet) likely pumped air into hollow sacs in their skeletons, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said. The study was funded in part by the National Science Foundation.
Modern computerized tomography (CT) scans of dinosaur chest cavities, performed five years ago, found the apparent remnants of complex, four-chambered hearts, more like mammals and birds.Additional resource:
Ceratopsian crest as acoustic amplifier can be found in published work:
Anton, J.A. Dinosaurs Incognito. 2009. VDM Verlag. Germany. pp. 192.
A recently discovered troodont fossil, Mei long, demonstrates that the dinosaurs slept like certain modern birds, with their heads tucked under their arms. This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.
Care of young
Several dinosaur species, e.g. Maiasaura, have been found in herds mixing very young and adult individual, suggesting rich interactions between them.
A dinosaur embryo was found without teeth, which suggests some parental care was required to feed the young dinosaur, possibly the adult dinosaur regurgitated food into the young dinosaur's mouth (see altricial). This behaviour is seen in numerous bird species; parent birds regurgitate food into the hatchling's mouth .
Another piece of evidence that birds and dinosaurs are closely related is the use of gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths. Because a particular stone could have been swallowed at one location before being carried to another during migration, paleontologists sometimes use the stones found in dinosaur stomachs to establish possible migration routes.
“Trees-down” or “ground-up”?
Within the cursorial hypothesis for the origin of flight, some fast running animals with long tails would have used their arms to keep balance while running. Increasing the surface of the 'wings' could have helped them. The feathers could also have been used as a trap to catch insects or other prey. Progressively, the animals would have sprung on longer distances, helped by their forecoming wings.
The arboreal hypothesis states that the ancestors of birds lived in trees. They would have sprung from branch to branch, progressively increasing the surface of their 'wings' to develop a good gliding ability. After gliding, they would have begun to flap to increase their flying efficiency.
There is little evidence for tree-climbing dinosaurs (only Epidendrosaurus and maybe Microraptor) compared to the numerous long-legged, ground-dwelling theropods, but forest sediments are only rarely preserved, which could account for this scarcity.
Remember that the initial ability to develop true avian (as opposed to gliding or four-winged) flight apparently evolved in a group of species. These then evolved into different lineages, each featuring a slightly different approach to the challenge, and probably were well advanced in this process already by the time of Archaeopteryx. Thus, the question might be rephrased more accurately: "did the ancestors of Neornithes develop flight from the ground up or from trees (slopes, cliffs...) down?"
Are some dinosaurs secondarily flightless?
An interesting theory, defended notably by Gregory Paul in his books Predatory Dinosaurs of the World (1988) and Dinosaurs of the Air (2002), states that some groups of carnivorous dinosaurs, especially deinonychosaurs but maybe others such as oviraptorosaurs, therizinosaurs, alvarezsaurids and ornithomimosaurs, are actually descended from forms that could fly. In this view, Archaeopteryx-like creatures are less closely related to extant birds than these dinosaurs are.
Though by most current cladistic analyses, Archaeopteryx is closer to birds than deinonychosaurs or oviraptorosaurs, such animals as Microraptor or Sinornithosaurus apparently lie close to the base of the deinonychosaurian clade and appear to have more flight adaptations than later deinonychosaurs. Archaeopteryx is still basal enough in its characteristics to suggest that early/mid-Cretaceous descendants of the earliest birds could theoretically have reverted to a more dinosaurian mode of life. Hesperornis, whose ancestors became secondarily flightless around the Jurassic/Cretaceous boundary, suggests that the avian beak was less likely to get lost again than avian flight ability, but that teeth might have re-evolved more easily than it seems at first glance.
By inserting the new data provided by the newly described tenth Archaeopteryx fossil into a major existing cladistic data matrix, Mayr et al. (2005) showed that Archaeopteryx was in a sister clade of a clade consisting of both two groups that are traditionally seen as non-avian theropods, namely the Deinonychosauria and the Troodontidae, as well as the more derived birds, represented in the analysis by Confuciusornis. As in Paul's hypothesis, in this scenario the Deinonychosauria and the Troodontidae are part of Aves, the bird lineage proper, and secondarily flightless. This is however a matter of taxonomical and phylogenetic definition; the only thing that appears clearly from Mayr et al.'s study is that of the two primitive birds compared — neither of which is necessarily very close to the ancestors of modern birds — Confuciusornis was closer to a distinct group of theropods, traditionally seen as non-avian, than to Archaeopteryx.
The paper launched a vigorous debateTemplate:Cn, in which the authors made clear that they considered their data still equivocal as to whether bird flight or major theropod diversification came first. Neither birds more modern than Confuciusornis, nor many interesting theropods were included, so the main point of the study is to harden the case that bird-like flight was present not only in the ancestors of modern birds. Whether it was developed independently several times as suggested by Barsbold or only once, with most if not all terrestrial theropods being secondarily flightless, is not resolved; although statistical evaluation of the data matrix tentatively suggested the latter, reliability is insufficient to draw a conclusion in this respect.
Birds Came First
An even more radical implementation of the possibility that some dinosaurs are secondarily flightless, is the Birds Came First-hypothesis ("BCF"), which holds that all dinosaurs are "postvolant". The hypothesis was developed by amateur paleontologist George Olshevsky, who shortly after reading Paul's Predatory Dinosaurs of the World in the early nineties had the insight that the arguments expounded there for the secondarily flightlessness of the Maniraptora, might well be adapted to argue for the same condition to be present in all Theropoda, indeed in all Dinosauria. "BCF" accepts the close relationship between dinosaurs and birds, but argues that, merely given this relationship, it is just as likely that dinosaurs descended from birds as the other way around. It should be emphasized that the term "birds" in this case refers to a morphological state, not to Aves as they have been cladistically defined. Olshevsky does not claim that modern birds split off from other dinosaurs very early; in fact precisely the opposite: he thinks that Aves is but the most derived expression of a vast diversification of flying dinosaurs all through the Jurassic.
BCF admits that most dinosaurs found are large and very derived in morphology compared to a hypothetical flying ancestor, and it also accepts the results of cladistic analysis connecting these large species into a cladogram, suggesting that the intermediate forms were also large. BCF could have avoided this problem by claiming that just a very basal form was volant and all subsequent forms large. Instead Olshevsky resorted to a far more radical position by emphasising the point that a cladogram doesn't logically imply the morphology of its intermediate steps. He claimed, basing himself partly on Cope's law, that there was a hidden stem lineage of small arboreal forms that during the Mesozoic was the real engine driving dinosaurian evolution, generating time and again larger ground-dwelling species. The smallness and arboreality of the forms was used to explain the fact that they rarely left a trace in the fossil record. Of course being small and arboreal doesn't imply the capacity to fly and Olshevsky allowed his hypothesis to diverge in three subhypotheses: the weakest was that the stem line consisted of small tree-living species; the stronger was that these could glide; the strongest that they possessed full powered flight.
BCF has not found acceptance among professional paleontologists. It was only published once, in a magazine, Mesozoic Meanderings, that Olshevsky himself produced. Paul perfunctorily dismissed the hypothesis in his Dinosaurs of the Air; in the peer-reviewed literature it is never even mentioned as such. However this does not mean the hypothesis has been completely ignored by professionals. Olshevsky is a well-known figure among dinosaur enthusiasts in the USA and has been for many years a very active participant in the various internet fora dedicated to its study. This had lead to much debate about BCF. The main objections from the professional side are that the hypothesis as a whole is too vague to be testable and that the empirical support for the most interesting subhypothesis - full flight capacity - is poor. Only for the group of the Tetanurae, which are already quite derived theropods, are there some slight indications, e.g. the presence af a furcula, and these can be explained as exaptations. Because of the many convergences needed, BCF is also not very parsimonious when analysed from a cladistic point of view, as it implies that flight was lost many times. The parsimony problem would only be remedied if many flying forms would be found basal to the various groups.
Claims have been made that the hands of theropod dinosaurs and birds are essentially different since allegedly, the three (nowadays fused) fingers on a bird's hand are II, III, IV, whereas it is clear from the fossils of theropod dinosaurs and their immediate ancestors that the three digits left on advanced theropod hands are I, II and III.
The identification of bird digits as II, III and IV relies primarily on embryological evidence: at some stage in the development of the hand, a mysterious “element X” is found which could be interpreted as finger I. This identification is still much disputed, other people claiming that bird digits are simply I, II and III.
Another suggestion is that a developmental frame shift occurred in gene expression somewhere in the dinosaur-bird transition, effectively making the digits II, III and IV look like I, II and III. However, this hypothesis currently lacks positive evidence.
At one time, it was believed that dinosaurs lacked furculae (fused left and right clavicles, or "wishbones"). This was considered an overwhelming argument to refute the dinosaur ancestry of birds by Heilmann (1926). However, it has been shown since then that numerous tetanuran theropod species indeed have a furcula, apparently a tetanuran invention. The presence of a furcula even in Allosaurus, a relatively basal tetanuran, has been confirmed, and in an early Jurassic therapod among others.
- Barsbold, Rinchen (1983): O ptich'ikh chertakh v stroyenii khishchnykh dinozavrov. ["Avian" features in the morphology of predatory dinosaurs]. Transactions of the Joint Soviet Mongolian Paleontological Expedition 24: 96-103. [Original article in Russian.] Translated by W. Robert Welsh, copy provided by Kenneth Carpenter and converted by Matthew Carrano. PDF fulltext
- Dingus, Lowell & Rowe, Timothy (1997): The Mistaken Extinction: Dinosaur Evolution and the Origin of Birds. W. H. Freeman and Company, New York. ISBN 0-7167-2944-X
- Dinosauria On-Line (1996): Dinosaurian Synapomorphies Found In Archaeopteryx. Retrieved 2006-SEP-30.
- Heilmann, G. (1926): The Origin of Birds. Witherby, London. ISBN 0-486-22784-7 (1972 Dover reprint)
- Mayr, Gerald; Pohl, B. & Peters, D. S. (2005): A Well-Preserved Archaeopteryx Specimen with Theropod Features. Science 310(5753): 1483-1486. Template:DOI
- Olson, Storrs L. (1985): The fossil record of birds. In: Farner, D.S.; King, J.R. & Parkes, Kenneth C. (eds.): Avian Biology 8: 79-238. Academic Press, New York.
- ↑ O'Connor, P.M. and Claessens, L.P.A.M. (2005). Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs. Nature 436:253.
- ↑ Xu, X. and Norell, M.A. (2004). A new troodontid dinosaur from China with avian-like sleeping posture. Nature 431:838-841.See commentary on the article.
- ↑ . Confuciusornis is currently considered closer to modern birds than Archaeopteryx; it is unlikely to have diverged from modern birds' ancestors very much after Archaeopteryx in any case , and its skull is very different from both. The flight apparatus is much more advanced than in Archaeopteryx, but to a sort of "intermediate" condition (whereas the Mayr et al. paper perhaps indicates it was not intermediate in phylogenetic position, as it came out as the sister clade of Microraptor).
- ↑ Included as a cladistic definer, e.g. (Columbia University) Master Cladograms or mentioned even in the broadest context, such as Paul C. Sereno, "The origin and evolution of dinosaurs" Annual Review of Earth and Planetary Sciences 25 pp 435-489.
- ↑ Matthew R. Carrano, John R. Hutchinson and Scott D. Sampson, 2005. "New information on Segisaurus halli, a small therapod dinosaur from the Early jutassic of Arizona" Journal of Vertebrate Paleontology 25.4, (December 2005) pp 835-849.
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