Evolution of birds

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Archaeopteryx lithographica (Berlin specimen).jpg
Archaeopteryx , a basal member of Avialae
Ludiortyx hoffmanni.JPG
Ludiortyx , a fossil bird genus of the Cenozoic

The evolution of birds began in the Jurassic Period, with the earliest birds derived from a clade of 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).

Contents

Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, Passer domesticus), and either Archaeopteryx, [2] or some prehistoric species closer to 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 non-avian dinosaurs, Archaeopteryx, and modern birds is still under debate.

Origins

There is significant evidence that birds emerged within theropod dinosaurs, specifically, that birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. [4] As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes less so. This was noted in the 19th century, with 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]

The mounted skeleton of a Velociraptor, showing the very bird-like quality of the smaller theropod dinosaurs Velociraptor 28-12-2007 15-06-24.jpg
The mounted skeleton of a Velociraptor , showing the very bird-like quality of the smaller theropod dinosaurs

Discoveries in northeast China (Liaoning Province) demonstrate that many small theropod dinosaurs did indeed have feathers, among them the compsognathid Sinosauropteryx and the microraptorian dromaeosaurid Sinornithosaurus . This has contributed to this ambiguity of where to draw the line between birds and reptiles. [6] Cryptovolans , a dromaeosaurid found in 2002 (which may be a junior synonym of Microraptor ) was capable of powered flight, possessing a sternal keel and ribs with uncinate processes. Cryptovolans seems to make a better "bird" than Archaeopteryx which lacks some of these modern bird features. Because some basal members of Dromaeosauridae, including Microraptor , were capable of powered flight, some paleontologists have suggested that dromaeosaurids are actually derived from a flying ancestor, and that the larger members became secondarily flightless, mirroring the loss of flight in modern paleognaths like the ostrich. [7] The discoveries of further basal dromaeosaurids potentially capable of powered flight, such as Xiaotingia , has provided more evidence for the theory that flight was first developed in the bird line by early dromaeosaurids rather than later by Aves as was previously supposed. [8]

Although ornithischian (bird-hipped) dinosaurs share the same hip structure as birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs if the dinosaurian origin theory is correct. They thus arrived at their hip structure condition 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

The basal bird Archaeopteryx , from the Jurassic, is well known as one of the first "missing links" to be found in support of evolution in the late 19th century. Though it is not considered a direct ancestor of modern birds, it gives a fair representation of how flight evolved and how the very first bird might have looked. It may be predated by Protoavis texensis , though the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor. The skeleton of all early bird candidates is basically that of a small theropod dinosaur with long, clawed hands, though the exquisite preservation of the 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.

Reconstruction of Iberomesornis romerali, a toothed enantiornithine Iberomesornis romerali by durbed.jpg
Reconstruction of Iberomesornis romerali , a toothed enantiornithine

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 Cretaceous saw the rise of more modern birds with a more rigid ribcage with a carina and shoulders able to allow for a powerful upstroke, essential to sustained powered flight[ citation needed ]. Another improvement was the appearance of an alula, used to achieve better control of landing or flight at low speeds[ citation needed ]. They also had a more derived pygostyle, with a ploughshare-shaped end. An early example is Yanornis . Many were coastal birds, strikingly resembling modern shorebirds, like Ichthyornis , or ducks, like Gansus . Some evolved as swimming hunters, like 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. [12] Meanwhile, the earlier primitive birds, particularly the Enantiornithes, continued to thrive and diversify alongside the pterosaurs through this geologic period until they became extinct due to the K–T extinction event. All but a few groups of the toothless 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

Modern birds originated in the late Cretaceous. [13] They are split into the paleognaths and neognaths. The paleognaths include the tinamous (grouse-like birds, found only in Central and South America) and the ratites, which nowadays are found almost exclusively in the Southern Hemisphere. The ratites are large flightless birds, and include ostriches, rheas, cassowaries, kiwis and emus. The ratites are a 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.

Haast's eagle and moa in New Zealand; the eagle is a neognath, the moa are paleognaths. Giant Haasts eagle attacking New Zealand moa.jpg
Haast's eagle and moa in New Zealand; the eagle is a neognath, the moa are paleognaths.

The basal divergence within Neognathes is between 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. [13] This can be the result of climate cooling fragmenting tropical biomes and producing widespread allopatric speciation plus an effect of some lineages diversifying in the expanding arid and cool 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 Bird Diversity 2013.png
The diversity of modern birds

The phylogenetic classification of birds is a contentious issue. Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). A preponderance of evidence suggests that most modern bird orders constitute good 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]

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 hybridization. This may arise from human alteration of habitats enabling related allopatric species to overlap. 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

Related Research Articles

<i>Archaeopteryx</i> Extinct genus of bird-like dinosaurs

Archaeopteryx, sometimes referred to by its German name, "Urvogel" is a genus of bird-like dinosaurs. The name derives from the ancient Greek ἀρχαῖος (archaīos), meaning "ancient", and πτέρυξ (ptéryx), meaning "feather" or "wing". Between the late 19th century and the early 21st century, Archaeopteryx was generally accepted by palaeontologists and popular reference books as the oldest known bird. Older potential avialans have since been identified, including Anchiornis, Xiaotingia, and Aurornis.

<span class="mw-page-title-main">Dinosaur</span> Archosaurian reptiles that dominated the Mesozoic Era

Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is a subject of active research. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201.3 mya and their dominance continued throughout the Jurassic and Cretaceous periods. The fossil record shows that birds are feathered dinosaurs, having evolved from earlier theropods during the Late Jurassic epoch, and are the only dinosaur lineage known to have survived the Cretaceous–Paleogene extinction event approximately 66 mya. Dinosaurs can therefore be divided into avian dinosaurs—birds—and the extinct non-avian dinosaurs, which are all dinosaurs other than birds.

<span class="mw-page-title-main">Troodontidae</span> Extinct family of bird-like dinosaurs

Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil discoveries of complete and articulated specimens, have helped to increase understanding about this group. Anatomical studies, particularly studies of the most primitive troodontids, like Sinovenator, demonstrate striking anatomical similarities with Archaeopteryx and primitive dromaeosaurids, and demonstrate that they are relatives comprising a clade called Paraves.

<span class="mw-page-title-main">Dromaeosauridae</span> Family of theropod dinosaurs

Dromaeosauridae is a family of feathered coelurosaurian theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek δρομαῖος (dromaîos), meaning 'running at full speed', 'swift', and σαῦρος (saûros), meaning 'lizard'. In informal usage, they are often called raptors, a term popularized by the film Jurassic Park; several genera include the term "raptor" directly in their name, and popular culture has come to emphasize their bird-like appearance and speculated bird-like behavior.

<i>Microraptor</i> Extinct genus of dinosaurs

Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China. They date from the early Cretaceous Jiufotang Formation, 125 to 120 million years ago. Three species have been named, though further study has suggested that all of them represent variation in a single species, which is properly called M. zhaoianus. Cryptovolans, initially described as another four-winged dinosaur, is usually considered to be a synonym of Microraptor.

<span class="mw-page-title-main">Maniraptora</span> Clade of dinosaurs

Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the non-avian dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Dromaeosauridae, Troodontidae, Oviraptorosauria, and Therizinosauria. Ornitholestes and the Alvarezsauroidea are also often included. Together with the next closest sister group, the Ornithomimosauria, Maniraptora comprises the more inclusive clade Maniraptoriformes. Maniraptorans first appear in the fossil record during the Jurassic Period, and survive today as living birds.

<span class="mw-page-title-main">Coelurosauria</span> Clade of dinosaurs

Coelurosauria is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs.

<span class="mw-page-title-main">Feathered dinosaur</span> Dinosaur with feathers

A feathered dinosaur is any species of dinosaur possessing feathers. That includes all species of birds, and in recent decades evidence has accumulated that many non-avian dinosaur species also possessed feathers in some shape or form. The extent to which feathers or feather-like structures were present in dinosaurs as a whole is a subject of ongoing debate and research.

<i>Jinfengopteryx</i> Theropod dinosaur genus

Jinfengopteryx is a genus of maniraptoran dinosaur. It was found in the Qiaotou Member of the Huajiying Formation of Hebei Province, China, and is therefore of uncertain age. The Qiaotou Member may correlate with the more well-known Early Cretaceous Yixian Formation, and so probably dates to around 122 Ma ago.

<i>Rahonavis</i> Extinct genus of dinosaurs

Rahonavis is a genus of bird-like theropod from the Late Cretaceous of what is now northwestern Madagascar. It is known from a partial skeleton found by Catherine Forster and colleagues in Maevarano Formation rocks at a quarry near Berivotra, Mahajanga Province. Rahonavis was a small predator, at about 70 centimetres (2.3 ft) long and 0.45-2.27 kg, with the typical dromaesaurid-like raised sickle claw on the second toe. It was originally the first African coelurosaur until the discovery of Nqwebasaurus in 2000.

<span class="mw-page-title-main">Palaeognathae</span> Infraclass of birds

Palaeognathae is an infraclass of birds, called paleognaths or palaeognaths, within the class Aves of the clade Archosauria. It is one of the two extant infraclasses of birds, the other being Neognathae, both of which form Neornithes. Palaeognathae contains five extant orders consisting of four flightless lineages, termed ratites, and one flying lineage, the Neotropic tinamous. There are 47 species of tinamous, five of kiwis (Apteryx), three of cassowaries (Casuarius), one of emus (Dromaius), two of rheas (Rhea) and two of ostriches (Struthio). Recent research has indicated that paleognaths are monophyletic but the traditional taxonomic split between flightless and flighted forms is incorrect; tinamous are within the ratite radiation, meaning flightlessness arose independently multiple times via parallel evolution.

<span class="mw-page-title-main">Neognathae</span> Infraclass of birds

Neognathae is an infraclass of birds, called neognaths, within the class Aves of the clade Archosauria. Neognathae includes the majority of living birds; the exceptions being the tinamous and the flightless ratites, which belong instead to the sister taxon Palaeognathae. There are nearly 10,000 living species of neognaths.

<span class="mw-page-title-main">Archaeopterygidae</span> Family of dinosaurs

Archaeopterygidae is a group of paravian dinosaurs, known from the latest Jurassic and earliest Cretaceous of Europe. In most current classifications, it contains only the genera Archaeopteryx and Wellnhoferia. As its name suggests, Protarchaeopteryx was also once referred to this group, but most paleontologists now consider it an oviraptorosaur. Other referred genera, like Jurapteryx, Wellnhoferia, and "Proornis", are probably synonymous with Archaeopteryx or do not belong into this group. Jinfengopteryx was originally described as an archaeopterygid, though it was later shown to be a troodontid. A few studies have recovered Anchiornis and Xiaotingia to also be members of the Archaeopterygidae, though most subsequent analyses have failed to arrive at the same result. Uncertainties still exist, however, and it may not be possible to confidently state whether archaeopterygids are more closely related to modern birds or to deinonychosaurs barring new and better specimens of relevant species. Teeth attributable to archaeopterygids are known from the earliest Cretaceous (Berriasian) Cherves-de-Cognac locality and the Angeac-Charente bonebed of France.

<span class="mw-page-title-main">Origin of birds</span> Evolution, adaptation, and origin of birds

The scientific question of within which larger group of animals birds evolved has traditionally been called the "origin of birds". The present scientific consensus is that birds are a group of maniraptoran theropod dinosaurs that originated during the Mesozoic era.

<span class="mw-page-title-main">Avialae</span> Clade including all birds and their ancestors

Avialae is a clade containing the only living dinosaurs, the birds, and their closest relatives. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally used.

<span class="mw-page-title-main">Alan Feduccia</span> American academic

John Alan Feduccia is a paleornithologist specializing in the origins and phylogeny of birds. He is S. K. Heninger Distinguished Professor Emeritus at the University of North Carolina. Feduccia's authored works include three major books, The Age of Birds, The Origin and Evolution of Birds, and Riddle of the Feathered Dragons.

<span class="mw-page-title-main">Paraves</span> Clade of all dinosaurs that are more closely related to birds than to oviraptorosaurs

Paraves are a widespread group of theropod dinosaurs that originated in the Middle Jurassic period. In addition to the extinct dromaeosaurids, troodontids, anchiornithids, and possibly the scansoriopterygids, the group also contains the avialans, which include diverse extinct taxa as well as the over 10,000 species of living birds.

<i>Xiaotingia</i> Extinct genus of dinosaurs

Xiaotingia is a genus of anchiornithid theropod dinosaur from Middle Jurassic or early Late Jurassic deposits of western Liaoning, China, containing a single species, Xiaotingia zhengi.

<i>Changyuraptor</i> Extinct genus of dinosaurs

Changyuraptor is a genus of "four-winged", predatory dinosaurs. It is known from a single fossil specimen representing the species Changyuraptor yangi, which was discovered from Early Cretaceous deposits in Liaoning Province, China. C. yangi belongs to the group of dromaeosaurid theropod dinosaurs called the Microraptoria.

<span class="mw-page-title-main">Biogeography of paravian dinosaurs</span> Geographic distribution of paravian dinosaurs

The biogeography of Paravian dinosaurs is the study of the global distribution of Paraves through geological history. Paraves is a clade that includes all of the Theropoda that are more closely related to birds than to oviraptorosaurs. These include Dromaeosauridae and Troodontidae and Avialae. The distribution of paraves is closely related to the evolution of the clade. Understanding the changes in their distributions may shed light on problems like how and why paraves evolve, eventually gaining the ability to fly.

References

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  24. ↑ "Bird - Classification". Encyclopedia Britannica. Retrieved 2021-03-14.
  25. ↑ Fjeldså, Jon; Niels Krabbe. (1990). Birds of the High Andes: A Manual to the Birds of the Temperate Zone of the Andes and Patagonia, South America. Apollo Books. ISBN   978-87-88757-16-3.
  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.
  28. ↑ Weeks, Brian C.; Willard, David E.; Zimova, Marketa; Ellis, Aspen A.; Witynski, Max L.; Hennen, Mary; Winger, Benjamin M. (2019). "Shared morphological consequences of global warming in North American migratory birds". Ecology Letters. 23 (2): 316–325. doi:10.1111/ele.13434. hdl: 2027.42/153188 . ISSN   1461-0248. PMID   31800170. S2CID   208620935.

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