The origin of birds has been a contentious topic within evolutionary biology for many years, but more recently a scientific consensus has emerged which holds that birds evolved from theropod dinosaurs during the Mesozoic Era. A close relationship between birds and dinosaurs was first proposed in the nineteenth century after the discovery of the primitive bird Archaeopteryx in Germany, and has been all but confirmed by recent finds of feathered dinosaur fossils in the Liaoning Province of China. In the phylogenetic sense, birds are modern dinosaurs.
Birds share hundreds of skeletal features with dinosaurs, especially with derived maniraptoran theropods like the dromaeosaurids, which most analyses show to be their closest relatives. Although harder to identify in the fossil record, similarities in the digestive and cardiovascular systems, as well as behavioral similarities and the shared presence of feathers, also link birds with dinosaurs. The ground-breaking discovery of fossilized Tyrannosaurus rex soft tissue allowed comparison of cellular anatomy and protein sequencing of collagen tissue, both of which provided additional evidence corroborating the dinosaur-bird relationship.
Few scientists still debate the dinosaurian origin of birds, although some suggest descent from other types of archosaurian reptiles. Even among those who support dinosaurian ancestry, the exact phylogenetic position of early birds within theropods remains controversial. The origin of bird flight is a separate but related question for which there are also several proposed answers.
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,[1] making the case. The leading dinosaur expert of the time, Richard Owen, disagreed, claiming Archaeopteryx was a true bird, without connection to dinosaurs, or indeed any reptiles.[2][3]
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 1920s, 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." [1]
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'." [2]
Others point out that the dividing line between extant birds (Neornithes) and non-avian dinosaurs can be unequivocally drawn,[citation needed] 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 major lineages of "birds".[citation needed] Of this first radiation, only some Neornithes survived into the Neogene and subsequently underwent three more major radiations. Of these, the first two filled many niches left vacant by the extinction of the dinosaurs (with the added benefit of flight), in all likelihood initially producing the diversity of aquatic, oceanic and terrestrial, and later that of tree-dwelling Neoaves.[citation needed] Currently there is increasing evidence that the "higher waterbirds" (and Charadriiformes which are probably somewhat older) had already diverged into the several lineages by the Maastrichtian, but these were probably minor and rather unspecialized.[citation needed] The final radiation, around the end of the Paleogene, in time produced a vast number of usually small, tree-dwelling species -- the perching birds.[citation needed]
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)
Compare the observation of Bostwick, 20 years later:
"What is surprising is how these analyses change our perception of bird origins in particular; many features we ornithologists thought of as arising simultaneously in the immediate ancestor of Archaeopteryx and birds, such as feathers, the general "winged" form of the forelimb, and the bird-like pelvic girdle, are heterogeneously scattered back through the theropod clades. Taxa such as the relatively basal theropod clade Ornithomimidae, with its Gallimimus, Struthiomimus, and Pelecanimimus, reflect in form as in name the kinds of convergences observed." (Bostwick 2003)
Archaeopteryx
Archaeopteryx is still maybe the most compelling fossil evidence towards a dinosaur-bird link, especially when compared to some maniraptoran dinosaurs such as Deinonychus.
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, the Eichstätt (JM 2257) and Solnhofen (BSP 1999) specimens were for some time mistaken for Compsognathus.
Archaeopteryx is undeniably bird-like, 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 that 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 mid-Cretaceous, avian flight was apparently advanced enough to enable the different bird lineages to perform successful trans-oceanic voyages.
Features linking birds and dinosaurs
Over a hundred distinct anatomical[4] features are shared by birds and theropod dinosaurs. Some of the more interesting similarities are discussed here:
Feathers
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. The early bird was so dinosaur-like that, without a clear impression of feathers in the surrounding rock, a specimen was once mistaken for the small dinosaur 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.
Skeleton
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.
Lungs
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.[5]
Heart
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.[citation needed]
Sleeping posture
A recently discovered troodont fossil, Mei long, demonstrates that the dinosaurs slept like certain modern birds, with their heads tucked under their arms.[6] This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.
Brooding
Several Citipati specimens (formerly referred to as Oviraptor) have been found resting over the eggs in its nest in a position most reminiscent of brooding.[citation needed]
Care of young
Several dinosaur species, e.g. Maiasaura, have been found in herds mixing both very young and adult individuals, 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 [3].
Gizzard
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.
Molecular evidence
It has been found that modern-day birds are closely related to older dinosaurs at the molecular level. Scientists have analyzed protein from a 68 million-year-old Tyrannosaurus rex bone (a femur). The seven collagen types obtained from the bone fragments, compared to collagen data from living birds (specifically, a chicken), reveal that older theropods and birds are closely related.[4]
Debates
Origin of bird flight
Two main theories have been proposed for the origins of flight: arboreal ("trees down") and cursorial ("ground up")
The cursorial theory of the origin of flight was first proposed by Williston, and elaborated upon by Baron Francis Nopcsa. This hypothesis proposes that some fast-running animals with long tails used their arms to keep balance while running. Increasing the surface area of the outstretched arms could have helped them, and Nopsca theorized that the scales of the forearms had become elongated, evolving into feathers. 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. Nopsca also proposed the idea that three stages existed in the evolution of flight. First, passive flight was realized, in which the developed wing structures served as a sort of parachute. Second, active flight was possible, in which the animal achieved flight by flapping its wings. He used Archaeopteryx as an example of this second stage. Finally, birds gained the ability to soar.[7]
The arboreal hypothesis states that the ancestors of birds lived in trees. They would have sprung from branch to branch, favoring the evolution of lengthened metatarsals and a backwards-directed hallux in order to grasp branches. The front limbs and rear limbs would have become adapted for separate purposes; the front for climbing and the rear for leaping. It proposes that the forelimbs, used for climbing, remained long, rather than being reduced, as is common in the evolution of cursorial animals.[7] The surface of their 'wings' progressively increased 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. However, the fact that forest sediments are only rarely preserved could account for this scarcity.
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;[8] 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 debate,[citation needed] 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.
Digit homology
There is a debate between embryologists and paleontologists whether the hands of theropod dinosaurs and birds are essentially different, based on phalangeal counts, a count of the number of phalanges (fingers) in the hand.
Embryologists number the digits of birds II-III-IV on the basis of development in the egg.[9] This is based on the fact that in most amniotes, the first digit to form in a 5-fingered hand is digit IV, which develops a primary axis. Therefore, embryologists identify the primary axis in birds as digit IV, and the surviving digits as II-III-IV.
The fossils of theropod dinosaurs and their immediate ancestors imply that the three digits left on advanced theropod hands are I-II-III. If this is true, then the II-III-IV development of digits in birds is incompatible with theropod (dinosaur) ancestry. However, with no ontogenical basis to definitively state which digits are which on a theropod hand, the labelling of the theropod hand is inconclusive.
However, paleontologists have traditionally identified avian digits as I-II-III. They argue that the digits of birds number I-II-III, just as those of theropod dinosaurs do, by the conserved phalangeal formula. The phalangeal count for archosaurs is 2-3-4-5-3; many archosaur lineages have a reduced number of digits, but have the same phalangeal formula in the digits that remain. The three digits of dromaeosaurs, and Archaeopteryx have the same phalangeal formula of I-II-III as digits I-II-III of basal archosaurs. Because maniraptorans are descended from dromaeosaurs, the reduction of digits in maniraptorans is believed to have occurred from the inside to the outside. Therefore, the lost digits would be V and IV. If this is true, then modern birds would also possess digits I-II-III, with further reduction during evolution.[9]
Furculae
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,[10] apparently a tetanuran invention. The presence of a furcula even in Allosaurus, a relatively basal tetanuran, has been confirmed, and in an Early Jurassic theropod[11] among others.
References
- 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
- Bostwick, Kimberly S. (2003): Bird origins and evolution: data accumulates, scientists integrate, and yet the "debate" still rages. Cladistics 19: 369–371. doi:10.1016/S0748-3007(03)00069-0 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 (1995): Archaeopteryx's Relationship With Modern Birds. Retrieved 2006-SEP-30.
- 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. doi:10.1126/science.1120331
- 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.
Footnotes
- ^ Huxley, T. H., 1868: On the Animals Which Are Most Nearly Intermediate between Birds and Reptiles Annals and Magazine of Natural History
- ^ Owen, Richard, 1863. On the Archeopteryx of von Meyer, with a decription of the Fossil Remains of a Long-tailed species, from the Lithographic Stone of Solenhofen,Philosophical Transactions of the Royal Society of London, vol. 153
- ^ It should be noted that Owen was a creationist, and therefore did not believe birds evolved from anything.
- ^ Chatterjee, Immoor (09 September 2005). "The Dinosaurs of the Jurassic Park Movies". Geolor.com. Retrieved June 23.
{{cite web}}: Check date values in:|accessdate=and|date=(help); Unknown parameter|accessyear=ignored (|access-date=suggested) (help) - ^ 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.
- ^ a b Terres, John K. (1980). The Audubon Society Encyclopedia of North American Birds. New York, NY: Knopf. pp. 398–401. ISBN 0394466519.
- ^ . 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,[citation needed] 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).
- ^ a b Chatterjee, Sankar (17 April 1998). "Counting the Fingers of Birds and Dinosaurs". Science. doi:10.1126/science.280.5362.355a. Retrieved June 21.
{{cite web}}: Check date values in:|accessdate=(help); Unknown parameter|accessyear=ignored (|access-date=suggested) (help) - ^ 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 theropod dinosaur from the Early Jurassic of Arizona" Journal of Vertebrate Paleontology 25.4, (December 2005) pp 835-849.
External links
- DinoBuzz A popular-level discussion of the dinosaur-bird hypothesis
