Evolution of birds

Source: Wikipedia, the free encyclopedia.
theropod dinosaurs named Paraves.[1] Birds are categorized as a biological class, Aves. For more than a century, the small theropod dinosaur Archaeopteryx lithographica from the Late Jurassic period was considered to have been the earliest bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked reptile clade, the Archosauria. Four distinct lineages of bird survived the Cretaceous–Paleogene extinction event 66 million years ago, giving rise to ostriches and relatives (Palaeognathae), waterfowl (Anseriformes), ground-living fowl (Galliformes), and "modern birds" (Neoaves
).

Neornithes (to avoid the problems caused by the unclear relationships of Archaeopteryx to other theropods).[3]
If the latter classification is used then the larger group is termed Avialae. Currently, the relationship between dinosaurs, Archaeopteryx, and modern birds is still under debate.

Origins

Main article: Origin of birds
See also: Feathered dinosaur, Paraves, Avemetatarsalia, and Dromaeosauridae § Alternative theories and flightlessness

There is

Thomas Huxley
writing:

We have had to stretch the definition of the class of birds so as to include birds with teeth and birds with paw-like fore limbs and long tails. There is no evidence that Compsognathus possessed feathers; but, if it did, it would be hard indeed to say whether it should be called a reptilian bird or an avian reptile.[5]

theropod
dinosaurs

Discoveries in northeast

Aves as was previously supposed.[8]

Although

independently. In fact, a bird-like hip structure also developed a third time among a peculiar group of theropods, the Therizinosauridae
.

An alternate theory to the dinosaurian origin of birds, espoused by a few scientists, notably Larry Martin and Alan Feduccia, states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama.[9] This theory is contested by most other paleontologists and experts in feather development and evolution.[10]

Mesozoic birds

See also: Avialae and Category:Mesozoic birds

The basal bird

Solnhofen Plattenkalk shows Archaeopteryx was covered in feathers and had wings.[5]
While Archaeopteryx and its relatives may not have been very good fliers, they would at least have been competent gliders, setting the stage for the evolution of life on the wing.

Iberomesornis romerali, a toothed enantiornithe

The evolutionary trend among birds has been the reduction of anatomical elements to save weight. The first element to disappear was the bony tail, being reduced to a pygostyle and the tail function taken over by feathers. Confuciusornis is an example of their trend. While keeping the clawed fingers, perhaps for climbing, it had a pygostyle tail, though longer than in modern birds. A large group of birds, the Enantiornithes, evolved into ecological niches similar to those of modern birds and flourished throughout the Mesozoic. Though their wings resembled those of many modern bird groups, they retained the clawed wings and a snout with teeth rather than a beak in most forms. The loss of a long tail was followed by a rapid evolution of their legs which evolved to become highly versatile and adaptable tools that opened up new ecological niches.[11]

The

Hesperornithiformes – a group of flightless divers resembling grebes and loons
. While modern in most respects, most of these birds retained typical reptilian-like teeth and sharp claws on the manus.

The modern toothless birds evolved from the toothed ancestors in the Cretaceous.

Neornithes were also cut short. The surviving lineages of birds were the comparatively primitive Palaeognathae (ostrich and its allies), the aquatic duck lineage, the terrestrial fowl, and the highly volant Neoaves
.

Radiation of modern birds

See also: Mesozoic–Cenozoic radiation

Modern birds originated in the late

paraphyletic (artificial) grouping because tinamous are part of their evolutionary clade and they have likely lost the ability to fly independently, becoming an example of convergent evolution.[14][15] However, the evidence about their evolution is still ambiguous, partly because there are no uncontroversial fossils
from the Mesozoic and partly because their phylogenetic relationships are still uncertain.

Galloanserae and Neoaves
.

The timing of divergence of these major groups are a matter of debate. It is agreed that modern birds originated in the Cretaceous and that the split between

Galloanserae and Neoaves occurred before the Cretaceous–Paleogene extinction event, but there are different opinions about whether the radiation of the remaining neognaths occurred before or after the extinction event.[16] This disagreement is in part caused by a divergence in the evidence, with molecular dating suggesting a Cretaceous radiation and the fossil record suggesting a Paleogene radiation. The latest attempts to reconcile the molecular and fossil evidence estimated the most recent common ancestor of modern birds at 95 million years ago and the split between Galloanseres and Neoaves at 85 million years ago.[13] Notably, these studies show that the rapid proliferation of lineages in Neoaves seems to coincide with the Cretaceous–Paleogene extinction event
, suggesting a role for ecological opportunity stimulating diversification in the aftermath of the mass extinction.

In contrast, another recent genomic study suggests that the Galloanserae and Neoaves diverged around the Early-Late Cretaceous boundary (100.5 million years ago), with the paleognaths and neognaths diverging even earlier (around 130 million years ago), and that most terrestrial neoavian orders gradually diverged from one another throughout the Late Cretaceous, roughly in sync with the concurrent radiation of flowering plants. This would suggest that a majority of all terrestrial avian orders coexisted with the non-avian dinosaurs and are K-Pg extinction survivors. In contrast, most major radiations of seabirds and shorebirds (as well as in paleognaths, despite their ancient origins) were found to have only occurred after the K-Pg extinction event, and primarily after the Paleocene–Eocene Thermal Maximum. This clashes with previous studies that found a very rapid radiation of avian orders only after the K-Pg extinction.[17][18] The results of this study have been disputed by other researchers, due to a lack of fossil evidence to support its conclusions.[19]

The birds that survived the end-of-Cretaceous extinction were likely ground-dwelling (not arboreal) and thus persisted despite the worldwide destruction of forests.[20][21]

An analysis of the variation of diversification rates through time further revealed a potential effect of climate on the evolution diversification rates in birds in which the generation of new lineages accelerates during periods of global cooling.

biomes.[13]

Bird skull evolution decelerated compared with the evolution of their dinosaur predecessors after the Cretaceous–Paleogene extinction event, rather than accelerating as often believed to have caused the cranial shape diversity of modern birds.[22][23]

Classification of modern species

The diversity of modern birds
See also: Sibley–Ahlquist taxonomy of birds and dinosaur classification

The

clades
. However, scientists are not in agreement as to the precise relationships between the main clades. Evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged.

Structural characteristics and fossil records have historically provided enough data for systematists to form hypotheses regarding the phylogenetic relationships between birds. Imprecisions within these methods is the main factor for why a lack of exact knowledge with regards to the orders and families of birds exists. Expansions in the study of computer-generated DNA sequencing and computer generated phylogenetics has provided a more accurate method for classifying bird species - although DNA data studying can only go so far, and questions are still unanswered.[24]

Current evolutionary trends in birds

See also: Bird conservation and Sexual selection in birds

Evolution generally occurs at a scale far too slow to be witnessed by humans. However, bird species are currently going

extinct at a far greater rate than any possible speciation or other generation of new species[citation needed
]. The disappearance of a population, subspecies, or species represents the permanent loss of a range of genes.

Another concern with evolutionary implications is a suspected increase in

Forest fragmentation can create extensive open areas, connecting previously isolated patches of open habitat. Populations that were isolated for sufficient time to diverge significantly, but not sufficient to be incapable of producing fertile offspring may now be interbreeding so broadly that the integrity of the original species may be compromised. For example, the many hybrid hummingbirds found in northwest South America may represent a threat to the conservation of the distinct species involved.[25]

Several species of birds have been bred in captivity to create variations on wild species. In some birds this is limited to color variations, while others are bred for larger egg or meat production, for flightlessness or other characteristics.

In December 2019 the results of a joint study by Chicago's Field Museum and the University of Michigan into changes in the morphology of birds were published in Ecology Letters. The study uses bodies of birds which died as a result of colliding with buildings in Chicago, Illinois, since 1978. The sample is made up of over 70,000 specimens from 52 species and spans the period from 1978 to 2016. The study shows that the length of birds' lower leg bones (an indicator of body sizes) shortened by an average of 2.4% and their wings lengthened by 1.3%. The findings of the study suggest the morphological changes are the result of climate change, demonstrating an example of evolutionary change following Bergmann's rule.[26][27][28]

See also

References

  1. ^ Wilford, John Noble (28 March 2016). "'Dinosaurs Among Us' Retraces an Evolutionary Path". The New York Times. Retrieved 28 March 2016.
  2. ISBN 978-0-12-226810-6
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  3. ^ Gauthier, J (1986). "Saurischian Monophyly and the origin of birds". In Padian K (ed.). The Origin of Birds and the Evolution of Flight. Mem. California Acad. Sci 8. pp. 1–55.
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  5. ^ a b Huxley, T.H. (1876): Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46-138 original text w/ figures
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  7. ^ Paul, Gregory S. (2002). Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: Johns Hopkins University Press. 472 pp.
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  11. ^ Shortening tails gave early birds a leg up
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  18. ^ Yirka, Bob; Phys.org. "New study suggests birds began diversifying long before dinosaurs went extinct". phys.org. Retrieved 2024-02-16.
  19. ISSN 0362-4331
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  22. ^ Wong, Kate. "How Birds Evolved Their Incredible Diversity". Scientific American. Retrieved 6 September 2020.
  23. PMID 32810126. Text and images are available under a Creative Commons Attribution 4.0 International License
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  24. ^ "Bird - Classification". Encyclopedia Britannica. Retrieved 2021-03-14.
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  26. ^ Vlamis, Kelsey (4 December 2019). "Birds 'shrinking' as the climate warms". BBC News. Retrieved 5 December 2019.
  27. ^ "North American Birds Are Shrinking, Likely a Result of the Warming Climate". Audubon. 4 December 2019. Retrieved 5 December 2019.
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Further reading
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