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It seems like there are mixed results because sometimes I read about a single missing link, like an archaeopteryx that somehow single-handedly explains all modern-day birds, but then I see conflicting articles about how different birds are descended from different dinosaurs like a t-rex or a velociraptor and so on. Which theory is correct? Did birds all descend from one common ancestor or multiple?
The answer is "one common ancestor", but I'll expand.
All organisms descend from one common ancestor so that question is not quite well-posed, but what you are actually asking I think is whether birds all descend from one common ancestor that was a bird, or whether their common ancestor wasn't a bird, which implies that different branches of birds became birds independently. In other words, are birds a "polyphyletic" group or a "monophyletic/paraphyletic" one (the difference between the latter is whether all of that common ancestor's descendants are birds, or whether it also had descendants that aren't birds).
The answer to that is that modern birds are monophyletic: they all descend from a common ancestor that, itself, was a bird. (and that common ancestor doesn't have any descendants that aren't birds)
But modern birds aren't the whole story - their group originates in the Cretaceous, and there are many groups of birds that are clearly recognizable as birds but also aren't modern birds - they might have teeth, they have subtle but unavoidable skeletal differences, etc. Like Enantiornithes and Confuciusornis. In other words, modern birds absolutely, unambiguously have a common ancestor that was itself a bird, which isn't hard because animals we would recognize as "birds" were already common by the time it appeared and the ancestor of our modern birds was just one of them. Whether those older, "extended" groups of birds themselves descend from a single species is harder to tell because fossil evidence is all we're going on, and it's even harder to tell when we go closer to the origin of birds and they no longer look unambiguously like birds. But at that point it's not so much a matter of "did this group originate from one ancestor or several" but "are these fossils that we've been putting in this group as closely related as we were assuming?" and "did this trait that these different fossils have evolve in their common ancestor, or did they evolve it independently, and what about those other fossils that seem related but don't have the trait: did they lose it from the common ancestor who had it or are they a sign that the common ancestor didn't have it to begin with?". Like, you might get different answers depending on how you define "bird". BUT paleontologists these days like monophyletic groups, so whichever groups they call "bird", those will have a common ancestor that was a "bird".
The Wikipedia page for Avialae expresses some of the issues with what "bird", or "Aves" means in the context of paleontology. To quote:
- Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
- Aves can mean those that fly (alternately Avialae)
- Aves can mean all reptiles closer to birds than to crocodiles (alternately Avemetatarsalia [=Panaves])
- Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "crown group"). (alternately Neornithes)
"Avialae" is one of those "extended groups of birds", in that paleontologists will often refer to any member of that group as "birds", but there's still a wide gap between "Avialae" and "Neornithes"; some stepping stones on that gap:
Neornithes (modern birds) are part of
Euornithes (birds have a certain articulation oriented like modern birds, as opposed to Enantiornithes, or "opposite-birds", who have it the other way, in addition to other differences like having teeth; note this page has a cladogram), which is one branch of:
Ornithothoraces (all descendants of the common ancestor of Euornithes and Enantiornithes), which is part of
Avebrevicauda ("short-tailed birds", to distinguish them from long-tailed Avialans like Archeopteryx) which is part of
Avialae, which as mentioned is basically birdinosaurs that could fly like Archeopteryx but it doesn't stop there because that's part of
Paraves, a group of dinosaurs that by and large had wings and feathers including the four-winged dinosaurs like Microraptor gui, which if we saw today we'd recognize as not like other birds but would we recognize as not a bird, really? That's part of
Pennaraptora, which is the first group to contain names that we definitely think of as not-birds; this group contains birds as well as Oviraptor and Deinonychus, but to quote the page:
The earliest known definitive member of this clade is Anchiornis, from the late Jurassic period of China, about 160 million years ago.
And Anchiornis, as its name suggests, is bird-like, feathered wings and all. This is where we get into "wait, Velociraptors were basically like turkeys?" reactions (Deinonychus is Jurassic Park's "Velociraptors").
(also these points are where the groupings become disputed, or fast-changing, or ambiguous between groups that are defined based on phylogeny, or similarity, or others and you can get different results depending on which group's Wikipedia page you look at; for example the Wikipedia page for Theropods puts Avialae directly under Maniraptora).
Pennaraptora is part of Maniraptora, which also contains your velociraptors, and Maniraptora is part of Coelurosauria which also contains your Tyrannosaurs. At this point we have clear non-bird dinosaurs, although it's likely they all had feathers.
I don't know which articles you read saying some birds would have descended from T-Rex and others from Velociraptor; that seems completely counter to what we know today of bird evolution, even if we extend "birds" to "Avialae" or even "Paraves". In fact the only way it would work is if we call T-rex or velociraptor themselves "birds". What you could have is conflicting articles about how close birds are to T-rex or Velociraptor; that part of the family tree is based entirely on fossils and so new fossils can change our understanding of how things are related… But the current phylogeny that makes, say, Avialae a group that's quite a few nodes away from velociraptors, and both of those a couple of nodes away from T-rex, seems quite robust. We have found many, many bird and proto-bird fossils in the last few decades that have clarified the picture on that scale (you can get an idea of how many by clicking the various links).
One thing those many, many bird and proto-bird fossils also made clear is that the traits of modern birds (feathers, wings, toothless beaks, etc) didn't evolve in a simple line from non-bird to bird. Many of those traits evolved convergently in several lineages, were lost in some, maybe regained in others, and feathers in particular turn out to be a widespread dinosaur feature that cannot be considered a uniquely bird trait anymore (unless we want to call T-rexes "birds"). Still, saying "beaks evolved several times" or "feathers evolved several times" doesn't mean that birds, let alone modern birds, evolved from several different ancestors. It can mean that the common ancestor of birds had lots of variously bird-like more-or-less distant cousins living around the same time.
Why are birds the only surviving dinosaurs?
The story of the disappearance of dinosaurs is a famous one. Less familiar is the tale of the dinosaurs that were left behind.
What is it about birds that allowed them to survive when all other dinosaurs died out? Watch our animation.
By the start of the Jurassic Period, 201 million years ago, dinosaurs had become the global superstars of the animal kingdom.
There were lots of them - and lots of different species - and they held the top carnivore and top herbivore spots in food chains.
How Dinosaurs Shrank and Became Birds
Modern birds descended from a group of two-legged dinosaurs known as theropods, whose members include the towering Tyrannosaurus rex and the smaller velociraptors. The theropods most closely related to avians generally weighed between 100 and 500 pounds &mdash giants compared to most modern birds &mdash and they had large snouts, big teeth, and not much between the ears. A velociraptor, for example, had a skull like a coyote&rsquos and a brain roughly the size of a pigeon&rsquos.
For decades, paleontologists&rsquo only fossil link between birds and dinosaurs was archaeopteryx, a hybrid creature with feathered wings but with the teeth and long bony tail of a dinosaur. These animals appeared to have acquired their birdlike features &mdash feathers, wings and flight &mdash in just 10 million years, a mere flash in evolutionary time. &ldquoArchaeopteryx seemed to emerge fully fledged with the characteristics of modern birds,&rdquo said Michael Benton, a paleontologist at the University of Bristol in England.
To explain this miraculous metamorphosis, scientists evoked a theory often referred to as &ldquohopeful monsters.&rdquo According to this idea, major evolutionary leaps require large-scale genetic changes that are qualitatively different from the routine modifications within a species. Only such substantial alterations on a short timescale, the story went, could account for the sudden transformation from a 300-pound theropod to the sparrow-size prehistoric bird Iberomesornis.
But it has become increasingly clear that the story of how dinosaurs begat birds is much more subtle. Discoveries have shown that bird-specific features like feathers began to emerge long before the evolution of birds, indicating that birds simply adapted a number of pre-existing features to a new use. And recent research suggests that a few simple change&mdashamong them the adoption of a more babylike skull shape into adulthood&mdashlikely played essential roles in the final push to bird-hood. Not only are birds much smaller than their dinosaur ancestors, they closely resemble dinosaur embryos. Adaptations such as these may have paved the way for modern birds&rsquo distinguishing features, namely their ability to fly and their remarkably agile beaks. The work demonstrates how huge evolutionary changes can result from a series of small evolutionary steps.
A Phantom Leap
In the 1990s, an influx of new dinosaur fossils from China revealed a feathery surprise. Though many of these fossils lacked wings, they had a panoply of plumage, from fuzzy bristles to fully articulated quills. The discovery of these new intermediary species, which filled in the spotty fossil record, triggered a change in how paleontologists conceived of the dinosaur-to-bird transition. Feathers, once thought unique to birds, must have evolved in dinosaurs long before birds developed.
Sophisticated new analyses of these fossils, which track structural changes and map how the specimens are related to each other, support the idea that avian features evolved over long stretches of time. In research published in Current Biology last fall, Stephen Brusatte, a paleontologist at the University of Edinburgh in Scotland, and collaborators examined fossils from coelurosaurs, the subgroup of theropods that produced archaeopteryx and modern birds. They tracked changes in a number of skeletal properties over time and found that there was no great jump that distinguished birds from other coelurosaurs.
&ldquoA bird didn&rsquot just evolve from a T. rex overnight, but rather the classic features of birds evolved one by one first bipedal locomotion, then feathers, then a wishbone, then more complex feathers that look like quill-pen feathers, then wings,&rdquo Brusatte said. &ldquoThe end result is a relatively seamless transition between dinosaurs and birds, so much so that you can&rsquot just draw an easy line between these two groups.&rdquo
Yet once those avian features were in place, birds took off. Brusatte&rsquos study of coelurosaurs found that once archaeopteryx and other ancient birds emerged, they began evolving much more rapidly than other dinosaurs. The hopeful monster theory had it almost exactly backwards: A burst of evolution didn&rsquot produce birds. Rather, birds produced a burst of evolution. &ldquoIt seems like birds had happened upon a very successful new body plan and new type of ecology&mdashflying at small size&mdashand this led to an evolutionary explosion,&rdquo Brusatte said.
The Importance of Being Small
Though most people might name feathers or wings as a key characteristic distinguishing birds from dinosaurs, the group&rsquos small stature is also extremely important. New research suggests that bird ancestors shrank fast, indicating that the diminutive size was an important and advantageous trait, quite possibly an essential component in bird evolution.
Like other bird features, diminishing body size likely began long before the birds themselves evolved. A study published in Science last year found that the miniaturization process began much earlier than scientists had expected. Some coelurosaurs started shrinking as far back as 200 million years ago&mdash50 million years before archaeopteryx emerged. At that time, most other dinosaur lineages were growing larger. &ldquoMiniaturization is unusual, especially among dinosaurs,&rdquo Benton said.
That shrinkage sped up once bird ancestors grew wings and began experimenting with gliding flight. Last year, Benton&rsquos team showed that this dinosaur lineage, known as paraves, was shrinking 160 times faster than other dinosaur lineages were growing. &ldquoOther dinosaurs were getting bigger and uglier while this line was quietly getting smaller and smaller,&rdquo Benton said. &ldquoWe believe that marked an event of intense selection going on at that point.&rdquo
The rapid miniaturization suggests that smaller birds must have had a strong advantage over larger ones. &ldquoMaybe this decrease was opening up new habitats, new ways of life, or even had something to do with changing physiology and growth,&rdquo Brusatte said. Benton speculates that the advantage of being pint-size might have emerged as bird ancestors moved to trees, a useful source of food and shelter.
But whatever the reasons may be, small stature was likely a useful precursor to flight. Though larger animals can glide, true flight powered by beating wings requires a certain ratio of wing size to weight. Birds needed to become smaller before they could ever take to the air for more than a short glide.
In 2008, Arkhat Abzhanov, a biologist at Harvard University, was elbow deep in alligator eggs. Since alligators descend from a common ancestor with dinosaurs, they can provide a useful evolutionary comparison to birds. (Despite their appearance, birds are more closely related to alligators than lizards are.) Abzhanov was studying alligators&rsquo vertebrae, but what struck him most was the birdlike shape of their heads alligator embryos looked quite similar to chickens. Fossilized skulls of baby dinosaurs show the same pattern&mdashthey resemble adult birds. With those two observations in mind, Abzhanov had an idea. Perhaps birds evolved from dinosaurs by arresting their pattern of development early on in life.
To test that theory, Abzhanov, along with Mark Norell, a paleontologist at the American Museum of Natural History in New York, Bhart-Anjan Bhullar, then a doctoral student in Abzhanov&rsquos lab, and other colleagues, collected data on fossils from around the globe, including ancient birds, such as archaeopteryx, and fossilized eggs of developing dinosaurs that died in the nest. They tracked how the skull shape changed as dinosaurs morphed into birds.
Over time, they discovered, the face collapsed and the eyes, brain and beak grew. &ldquoThe first birds were almost identical to the late embryo from velociraptors,&rdquo Abzhanov said. &ldquoModern birds became even more babylike and change even less from their embryonic form.&rdquo In short, birds resemble tiny, infantile dinosaurs that can reproduce.
This process, known as paedomorphosis, is an efficient evolutionary route. &ldquoRather than coming up with something new, it takes something you already have and extends it,&rdquo said Nipam Patel, a developmental biologist at the University of California, Berkeley.
&ldquoWe&rsquore seeing more and more that evolution operates much more elegantly than we previously appreciated,&rdquo said Bhullar, who will start his own lab at Yale University in the fall. &ldquoThe umpteen changes that go into the bird skull may all owe to paedomorphosis, to one set of molecular changes in the early embryo.&rdquo
Why would paedomorphosis be important for the evolution of birds? It might have helped drive miniaturization or vice versa. Changes in size are often linked to changes in development, so selection for small size may have arrested the development of the adult form. &ldquoA neat way to cut short a developmental sequence is to stop growing at smaller size,&rdquo Benton said. A babylike skull in adults might also help explain birds&rsquo increased brain size, since baby animals generally have larger heads relative to their bodies than adults do. &ldquoA great way to improve brain size is to retain child size into adulthood,&rdquo he said.
(Indeed, paedomorphosis might underlie a number of major transitions in evolution, perhaps even the development of mammals and humans. Our large skulls relative to those of chimpanzees could be a case of paedomorphosis.)
What&rsquos more, paedomorphosis helped to make the skull a blank slate on which selection could create new structures. By erasing the snout, it may have paved the way for another of birds&rsquo most important features: the beak.
Birth of the Beak
The problem with studying something that occurred deep in evolutionary time is that it&rsquos impossible to know exactly what happened. Scientists can never precisely decipher how birds evolved from dinosaurs or which set of features was essential for that transformation. But with the intersection of three fields&mdashevolution, genetics and developmental biology&mdashthey can now begin to explore how specific features might have come about.
One of Abzhanov&rsquos particular interests is the beak, a remarkable structure that birds use to find food, clean themselves, make nests, and care for their young. He theorizes that birds&rsquo widespread success stems not just from their ability to fly, but from their amazing diversity of beaks. &ldquoModern birds evolved a pair of fingers on the face,&rdquo he said.
Armed with their insight into bird evolution, Abzhanov, Bhullar and collaborators have been able to dig into the genetic mechanisms that helped form the beak. In new research, published last month in Evolution, the researchers show that just a few small genetic tweaks can morph a bird face into one that resembles a dinosaur.
In modern birds, two bones known as the premaxillary bones fuse to become the beak. That structure is quite distinct from that of dinosaurs, alligators, ancient birds and most other vertebrates, in which these two bones remain separate, shaping the snout. To figure out how that change might have arisen, the researchers mapped out the activity of two genes that are expressed in these bones in a spectrum of animals: alligators, chickens, mice, lizards, turtles and emus, a living species reminiscent of ancient birds.
They found that the reptiles and mammals had two patches of activity, one on either side of the developing nasal cavity. Birds, on the hand, had a much larger single patch spanning the front of the face. The researchers reasoned that the alligator pattern could serve as a proxy for that of dinosaurs, given that they have similar snouts and premaxillary bones. The researchers then undid a bird-specific pattern of gene expression in chicken embryos using chemicals to block the genes in the middle of the face. (For ethical reasons, they did not allow the chickens to hatch.)
The result: The treated embryos developed a more dinosaurlike face. &ldquoThey basically grew a bird embryo back into something that looked more like the morphology of extinct dinosaurs,&rdquo said Timothy Rowe, a paleontologist at the University of Texas, Austin, who has previously collaborated with Abzhanov.
The findings highlight how simple molecular tweaks can trigger major structural changes. Birds &ldquouse existing tools in a new way to create a whole new face,&rdquo Abzhanov said. &ldquoThey didn&rsquot evolve a new gene or pathway, they just changed control of an existing gene.&rdquo
Like the studies of Brusatte and others, Abzhanov&rsquos work challenges the hopeful monster theory, and it does so on a genetic scale. The creation of the beak didn&rsquot require some special evolutionary jump or large-scale genetic changes. Rather, Abzhanov showed that the same forces that shape microevolution &mdash minor alterations within species &mdash also drive macroevolution, the evolution of whole new features and new groups of species.
Specifically, small changes in how genes are regulated likely drove both the initial creation of the beak, which evolved over millions of years, and the diverse shape of bird beaks, which can change over just a few generations. &ldquoWe show that simple regulatory changes can have a major impact,&rdquo Abzhanov said.
Bhullar and Abzhanov plan to dig deeper into the question of how the beak and bird skull evolved, using the same approach to manipulate different features of skull and brain development. &ldquoWe have just scratched surface of this work,&rdquo Bhullar said.
The "Birds Are Not Dinosaurs" Movement
Birds are dinosaurs. But some scientists argue that this just can&rsquot be so because. because. well, it just can&rsquot. What, specifically, are their arguments?
A huge quantity of evidence shows that birds are dinosaurs, and specifically a lineage of the coelurosaurian theropod group Maniraptora. Additional support for the coelurosaurian origin of birds arrives on a regular basis, since new Jurassic and Cretaceous fossil species that fit somewhere on the bird lineage are reported virtually every month. But&hellip I can&rsquot help but be interested in &lsquonon-standard&rsquo or &lsquoalternative&rsquo hypotheses on evolutionary history, and among the most interesting is the &lsquoBirds Are Not Dinosaurs&rsquo (or BAND) movement. It&rsquos made &ldquoamong the most interesting&rdquo by the fact that its proponents have seen themselves as crusaders, true sceptics and better scientists than those who support what is now the mainstream model they&rsquove &ndash I think unwittingly &ndash moulded themselves into a distinct social group, even going so far as wearing special badges at conferences. Back in 2012 I wrote a long chapter on the evolutionary history of birds (Naish 2012), and I thought it would be remiss to exclude a section on the BAND movement. That section is reproduced here&hellip
The theropod hypothesis of bird origins is not universally accepted. Some ornithologists and paleontologists argue that theropods cannot be ancestral to birds because they do not conform, in anatomy or lifestyle, to the true bird ancestor as imagined by these researchers. Supposedly, theropods are too large and too specialized for terrestrial cursoriality to give rise to birds, possess anatomical characters that bar them from avialan ancestry, and appear too late in the Mesozoic record to be ancestral to Archaeopteryx (e.g., Martin 1983 Feduccia 1996, 2002). These authors argue that a number of peculiar Triassic reptiles &ndash they include Megalancosaurus, Cosesaurus, and Longisquama and have been dubbed avimorph thecodonts &ndash might represent the real closest relatives of birds. None of these taxa is at all birdlike and all clearly belong elsewhere in reptile phylogeny.BANDit scientists have pointed to these animals - bizarre Longisquama (at left) and the climbing megalancosaurs - as 'better' bird ancestors than theropod dinosaurs. They're about as different from birds as you could imagine. Credit: Darren Naish
The objections proposed by these workers have never been a problem for the theropod hypothesis and are naive attempts to falsify a well-supported hypothesis. While the Jurassic record of small theropods is poor, numerous fossils (among the best are those of the Middle and Late Jurassic maniraptorans Anchiornis and Xiaotingia) show that deinonychosaurs and other maniraptorans were present before the Tithonian (that is, they are geologically older than Archaeopteryx). Furthermore, non-avialan maniraptorans were not all large (some deinonychosaurs and other maniraptorans were similar in size to, or smaller than, Archaeopteryx), nor do claims that they were fundamentally distinct from basal birds withstand scrutiny.Mesozoic maniraptoran dinosaurs that possess all kinds of bird-like and not-so-bird-like combinations of anatomical characters are published all the time. This reconstruction, by Emily Willoughby, depicts Serikornis from the Upper Jurassic of China. According to its describers, it is not a bird. Credit: Emily Willoughby Wikimedia (CC BY-SA 4.0)
It has been argued that the neornithine hand represents digits II&ndashIV and is therefore different from the maniraptoran hand, which is usually taken to represent digits I&ndashIII. It seems peculiar to argue that a single character, or even a complex of related characters, can trump a list of tens or hundreds of others. The strong character evidence nesting birds within coelurosaurs must mean either that the proposed II&ndashIV formula for the avialan hand is wrong, or that an unusual embryological event &ndash a so-called frameshift &ndash occurred in theropod evolution. If such a frameshift did occur, true digit I was lost and true digit II became digit I. However, evidence from Hox genes indicates that the condensation axis for the first embryonic hand digit in birds receives a Hox signal normally associated with digit I (Vargas and Fallon 2005). This shows that the avialan hand likely does represent digits I&ndashIII.
Overall, the &ldquobirds are not dinosaurs&rdquo movement can be dismissed as naive because it hinges on the idea that we should make predictions about the avian ancestor before looking at fossils or phylogeny. Evolutionary hypotheses should be formulated on phylogenies, not vice versa.Alan Feduccia and his colleagues would have it that some of these animals are large flightless birds (indeed, some may well be), whereas others are nothing whatsoever to do with the bird lineage and not at all close to it in evolutionary terms. The reality is that theropods show exactly what we would predict given that birds evolved from among them: some are only vaguely bird-like, others are somewhat bird-like, others are highly bird-like, and others are so close to birds that we argue over whether they're birds or not. Credit: Darren Naish
Because vaned feathers are known for deinonychosaurs and oviraptorosaurs, Feduccia and colleagues have more recently argued that feathered maniraptorans are secondarily flightless members of Avialae, thereby renouncing decades of argumentation in which they stated that deinonychosaurs have no close relationship with birds (Feduccia 2002 Martin 2004 Feduccia et al. 2007). This idea of secondary flightlessness is not wholly objectionable, and some supporters of the theropod hypothesis have argued similarly (Paul 2002), but it is not supported by large analyses that incorporate good sampling of characters and taxa. Furthermore, what makes the hypothesis of Feduccia et al. untenable is their corollary that this feathered maniraptoran clade has no close affinity with the rest of Dinosauria. It is difficult to take this seriously, given that non-avialan maniraptorans have obvious affinities with non-maniraptoran coelurosaurs, which in turn have affinities with non-coelurosaurian theropods, and so on.
I regret that I was not a fan of this study. Credit: Darren Naish
A cladistic analysis that found no strong support for the theropod affinities of birds (James and Pourtless 2009) is extremely flawed. This study excluded all characters where homology has been questioned and excluded nontheropodan dinosaurs. Furthermore, the phylogenies generated by James and Pourtless (2009) are unresolved polytomies, so it cannot be said that this study failed to find support for the theropod hypothesis in particular it actually failed to find support for any hypothesis! While some details of maniraptoran phylogenies may prove incorrect, the &ldquobirds are not theropods&rdquo movement is based on erroneous argumentation and fails to account for the data as well as the theropod hypothesis does.
For previous Tet Zoo articles relevant to the issues covered here, see&hellip
Feduccia, A. 2002. Birds are dinosaurs: simple answer to a complex problem. Auk 119, 1187-1201.
Feduccia, A., L. D. Martin, and S. Tarsitano. 2007. Archaeopteryx 2007: quo vadis? Auk 124, 373-380.
James, F. C., and J. A. Pourtless. 2009. Cladistics and the origins of birds: a review and two new analyses. Ornithological Monographs 66, 1-78.
Martin, L. D. 1983. The origin of birds and of avian flight. In R. F. Johnston (ed.), Current Ornithology, 1, 105-129. New York: Plenum Press.
Martin, L. D. 2004. A basal archosaurian origin for birds. Acta Zoologica Sinica 50, 978-990.
Vargas, A. O., and J. F. Fallon. 2005. Birds have dinosaur wings: the molecular evidence. Journal of Experimental Zoology (Molecular and Developmental Evolution) 304B, 86-90.
The views expressed are those of the author(s) and are not necessarily those of Scientific American.
Huxley, Archaeopteryx and early research Edit
Scientific investigation into the origin of birds began shortly after the 1859 publication of Charles Darwin's On the Origin of Species.  In 1860, a fossilized feather was discovered in Germany's Late Jurassic Solnhofen limestone. Christian Erich Hermann von Meyer described this feather as Archaeopteryx lithographica the next year.  Richard Owen described a nearly complete skeleton in 1863, recognizing it as a bird despite many features reminiscent of reptiles, including clawed forelimbs and a long, bony tail. 
Biologist Thomas Henry Huxley, known as "Darwin's Bulldog" for his tenacious support of the new theory of evolution by means of natural selection, almost immediately seized upon Archaeopteryx as a transitional fossil between birds and reptiles. Starting in 1868, and following earlier suggestions by Karl Gegenbaur,  and Edward Drinker Cope,  Huxley made detailed comparisons of Archaeopteryx with various prehistoric reptiles and found that it was most similar to dinosaurs like Hypsilophodon and Compsognathus.   The discovery in the late 1870s of the iconic "Berlin specimen" of Archaeopteryx, complete with a set of reptilian teeth, provided further evidence. Like Cope, Huxley proposed an evolutionary relationship between birds and dinosaurs. Although Huxley was opposed by the very influential Owen, his conclusions were accepted by many biologists, including Baron Franz Nopcsa,  while others, notably Harry Seeley,  argued that the similarities were due to convergent evolution.
Heilmann and the thecodont hypothesis Edit
A turning point came in the early twentieth century with the writings of Gerhard Heilmann of Denmark. An artist by trade, Heilmann had a scholarly interest in birds and from 1913 to 1916, expanding on earlier work by Othenio Abel,  published the results of his research in several parts, dealing with the anatomy, embryology, behavior, paleontology, and evolution of birds.  His work, originally written in Danish as Vor Nuvaerende Viden om Fuglenes Afstamning, was compiled, translated into English, and published in 1926 as The Origin of Birds.
Like Huxley, Heilmann compared Archaeopteryx and other birds to an exhaustive list of prehistoric reptiles, and also came to the conclusion that theropod dinosaurs like Compsognathus were the most similar. However, Heilmann noted that birds had clavicles (collar bones) fused to form a bone called the furcula ("wishbone"), and while clavicles were known in more primitive reptiles, they had not yet been recognized in dinosaurs. Since he was a firm believer in Dollo's law, which states that evolution is not reversible, Heilmann could not accept that clavicles were lost in dinosaurs and re-evolved in birds. He was therefore forced to rule out dinosaurs as bird ancestors and ascribe all of their similarities to convergent evolution. Heilmann stated that bird ancestors would instead be found among the more primitive "thecodont" grade of reptiles.  Heilmann's extremely thorough approach ensured that his book became a classic in the field, and its conclusions on bird origins, as with most other topics, were accepted by nearly all evolutionary biologists for the next four decades. 
Clavicles are relatively delicate bones and therefore in danger of being destroyed or at least damaged beyond recognition. Nevertheless, some fossil theropod clavicles had actually been excavated before Heilmann wrote his book but these had been misidentified.  The absence of clavicles in dinosaurs became the orthodox view despite the discovery of clavicles in the primitive theropod Segisaurus in 1936.  The next report of clavicles in a dinosaur was in a Russian article in 1983. 
Contrary to what Heilmann believed, paleontologists now accept that clavicles and in most cases furculae are a standard feature not just of theropods but of saurischian dinosaurs. Up to late 2007 ossified furculae (i.e. made of bone rather than cartilage) have been found in all types of theropods except the most basal ones, Eoraptor and Herrerasaurus.  The original report of a furcula in the primitive theropod Segisaurus (1936) was confirmed by a re-examination in 2005.  Joined, furcula-like clavicles have also been found in Massospondylus, an Early Jurassic sauropodomorph. 
Ostrom, Deinonychus and the dinosaur renaissance Edit
The tide began to turn against the 'thecodont' hypothesis after the 1964 discovery of a new theropod dinosaur in Montana. In 1969, this dinosaur was described and named Deinonychus by John Ostrom of Yale University.  The next year, Ostrom redescribed a specimen of Pterodactylus in the Dutch Teyler Museum as another skeleton of Archaeopteryx.  The specimen consisted mainly of a single wing and its description made Ostrom aware of the similarities between the wrists of Archaeopteryx and Deinonychus. 
In 1972, British paleontologist Alick Walker hypothesized that birds arose not from 'thecodonts' but from crocodile ancestors like Sphenosuchus.  Ostrom's work with both theropods and early birds led him to respond with a series of publications in the mid-1970s in which he laid out the many similarities between birds and theropod dinosaurs, resurrecting the ideas first put forth by Huxley over a century before.    Ostrom's recognition of the dinosaurian ancestry of birds, along with other new ideas about dinosaur metabolism,  activity levels, and parental care,  began what is known as the dinosaur renaissance, which began in the 1970s and continues to this day.
Ostrom's revelations also coincided with the increasing adoption of phylogenetic systematics (cladistics), which began in the 1960s with the work of Willi Hennig.  Cladistics is an exact method of arranging species based strictly on their evolutionary relationships, which are calculated by determining the evolutionary tree implying the least number of changes in their anatomical characteristics. In the 1980s, cladistic methodology was applied to dinosaur phylogeny for the first time by Jacques Gauthier and others, showing unequivocally that birds were a derived group of theropod dinosaurs.  Early analyses suggested that dromaeosaurid theropods like Deinonychus were particularly closely related to birds, a result that has been corroborated many times since.  
Feathered dinosaurs in China Edit
The early 1990s saw the discovery of spectacularly preserved bird fossils in several Early Cretaceous geological formations in the northeastern Chinese province of Liaoning.   In 1996, Chinese paleontologists described Sinosauropteryx as a new genus of bird from the Yixian Formation,  but this animal was quickly recognized as a more basal theropod dinosaur closely related to Compsognathus. Surprisingly, its body was covered by long filamentous structures. These were dubbed 'protofeathers' and considered homologous with the more advanced feathers of birds,  although some scientists disagree with this assessment.  Chinese and North American scientists described Caudipteryx and Protarchaeopteryx soon after. Based on skeletal features, these animals were non-avian dinosaurs, but their remains bore fully formed feathers closely resembling those of birds.  "Archaeoraptor", described without peer review in a 1999 issue of National Geographic,  turned out to be a smuggled forgery,  but legitimate remains continue to pour out of the Yixian, both legally and illegally. Feathers or "protofeathers" have been found on a wide variety of theropods in the Yixian,   and the discoveries of extremely bird-like non-avian dinosaurs,  as well as non-avian dinosaur-like primitive birds,  have almost entirely closed the morphological gap between non-avian theropods and birds.
Digit homology Edit
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 (finger bones) in the hand. This is an important and fiercely debated area of research because its results may challenge the consensus that birds are (descendants of) dinosaurs.
Embryologists and some paleontologists who oppose the bird-dinosaur link have long numbered the digits of birds II-III-IV on the basis of multiple studies of the development in the egg.  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 have identified the primary axis in birds as digit IV, and the surviving digits as II-III-IV. The fossils of advanced theropod (Tetanurae) hands appear to have the digits I-II-III (some genera within Avetheropoda also have a reduced digit IV  ). If this is true, then the II-III-IV development of digits in birds is an indication against theropod (dinosaur) ancestry. However, with no ontogenical (developmental) basis to definitively state which digits are which on a theropod hand (because no non-avian theropods can be observed growing and developing today), the labelling of the theropod hand is not absolutely conclusive.
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. In other words, paleontologists assert that archosaurs of different lineages tend to lose the same digits when digit loss occurs, from the outside to the inside. The three digits of dromaeosaurs, and Archaeopteryx have the same phalangeal formula of I-II-III as digits I-II-III of basal archosaurs. Therefore, the lost digits would be V and IV. If this is true, then modern birds would also possess digits I-II-III.  Also, one 1999 publication proposed a frame-shift in the digits of the theropod line leading to birds (thus making digit I into digit II, II to III, and so forth).   However, such frame shifts are rare in amniotes and—to be consistent with the theropod origin of birds—would have had to occur solely in the bird-theropod lineage forelimbs and not the hindlimbs (a condition unknown in any animal).  This is called Lateral Digit Reduction (LDR) versus Bilateral Digit Reduction (BDR) (see also Limusaurus). 
A small minority, known by the acronym BAND (Birds Are Not Dinosaurs)  including ornithologists Alan Feduccia and Larry Martin, continues to assert that birds are more closely related to earlier reptiles, such as Longisquama or Euparkeria, than to dinosaurs.   Embryological studies of bird developmental biology have raised questions about digit homology in bird and dinosaur forelimbs.  However, due to the cogent evidence provided by comparative anatomy and phylogenetics, as well as the dramatic feathered dinosaur fossils from China, the idea that birds are derived dinosaurs, first championed by Huxley and later by Nopcsa and Ostrom, enjoys near-unanimous support among today's paleontologists. 
Thermogenic muscle hypothesis Edit
A 2011 publication suggested that selection for the expansion of skeletal muscle, rather than the evolution of flight, was the driving force for the emergence of this clade.   Muscles became larger in prospectively endothermic saurians, according to this hypothesis, as a response to the loss of the vertebrate mitochondrial uncoupling protein, UCP1,  which is thermogenic. In mammals, UCP1 functions within brown adipose tissue to protect newborns against hypothermia. In modern birds, skeletal muscle serves a similar function and is presumed to have done so in their ancestors. In this view, bipedality and other avian skeletal alterations were side effects of muscle hyperplasia, with further evolutionary modifications of the forelimbs, including adaptations for flight or swimming, and vestigiality, being secondary consequences of two-leggedness.
Archaeopteryx has historically been considered the first bird, or Urvogel. Although newer fossil discoveries filled the gap between theropods and Archaeopteryx, as well as the gap between Archaeopteryx and modern birds, phylogenetic taxonomists, in keeping with tradition, almost always use Archaeopteryx as a specifier to help define Aves.   Aves has more rarely been defined as a crown group consisting only of modern birds.  Nearly all palaeontologists regard birds as coelurosaurian theropod dinosaurs.  Within Coelurosauria, multiple cladistic analyses have found support for a clade named Maniraptora, consisting of therizinosauroids, oviraptorosaurs, troodontids, dromaeosaurids, and birds.    Of these, dromaeosaurids and troodontids are usually united in the clade Deinonychosauria, which is a sister group to birds (together forming the node-clade Eumaniraptora) within the stem-clade Paraves.  
Other studies have proposed alternative phylogenies, in which certain groups of dinosaurs usually considered non-avian may have evolved from avian ancestors. For example, a 2002 analysis found that oviraptorosaurs were basal avians.  Alvarezsaurids, known from Asia and the Americas, have been variously classified as basal maniraptorans,     paravians,  the sister taxon of ornithomimosaurs,  as well as specialized early birds.   The genus Rahonavis, originally described as an early bird,  has been identified as a non-avian dromaeosaurid in several studies.   Dromaeosaurids and troodontids themselves have also been suggested to lie within Aves rather than just outside it.  
Many anatomical  features are shared by birds and theropod dinosaurs.
Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The first 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. Archaeopteryx is a transitional fossil, with features clearly intermediate between those of non-avian theropod dinosaurs and birds. Discovered just two years after Darwin's seminal 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, at least one specimen was mistaken for Compsognathus. 
Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. The first of these were initially described as simple filamentous protofeathers, which were reported in dinosaur lineages as primitive as compsognathids and tyrannosauroids.  However, feathers indistinguishable from those of modern birds were soon after found in non-avialan dinosaurs as well. 
A small minority of researchers have claimed that the simple filamentous "protofeather" structures are simply the result of the decomposition of collagen fiber under the dinosaurs' skin or in fins along their backs, and that species with unquestionable feathers, such as oviraptorosaurs and dromaeosaurs are not dinosaurs, but true birds unrelated to dinosaurs.  However, a majority of studies have concluded that feathered dinosaurs are in fact dinosaurs, and that the simpler filaments of unquestionable theropods represent simple feathers. Some researchers have demonstrated the presence of color-bearing melanin in the structures—which would be expected in feathers but not collagen fibers.  Others have demonstrated, using studies of modern bird decomposition, that even advanced feathers appear filamentous when subjected to the crushing forces experienced during fossilization, and that the supposed "protofeathers" may have been more complex than previously thought.  Detailed examination of the "protofeathers" of Sinosauropteryx prima showed that individual feathers consisted of a central quill (rachis) with thinner barbs branching off from it, similar to but more primitive in structure than modern bird feathers. 
Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent the more important link for paleontologists. Furthermore, it is increasingly clear that the relationship between birds and dinosaurs, and the evolution of flight, are more complex topics than previously realized. For example, while it was once believed that birds evolved from dinosaurs in one linear progression, some scientists, most notably Gregory S. Paul, conclude that dinosaurs such as the dromaeosaurs may have evolved from birds, losing the power of flight while keeping their feathers in a manner similar to the modern 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. Skeletal similarities include the neck, pubis, wrist (semi-lunate carpal), arm and pectoral girdle, shoulder blade, clavicle, and breast bone.
A study comparing embryonic, juvenile and adult archosaur skulls concluded that bird skulls are derived from those of theropod dinosaurs by progenesis, a type of paedomorphic heterochrony, which resulted in retention of juvenile characteristics of their ancestors. 
Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation led by Patrick M. O'Connor of Ohio University. In theropod dinosaurs (carnivores that walked on two legs and had birdlike feet) flexible soft tissue air sacs likely pumped air through the stiff lungs, 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.  
Computed tomography (CT) scans conducted in 2000 of the chest cavity of a specimen of the ornithopod Thescelosaurus found the apparent remnants of a complex four-chambered heart, much like those found in today's mammals and birds.  The idea is controversial within the scientific community, criticised for being bad anatomical science  or simply wishful thinking. 
A study published in 2011 applied multiple lines of inquiry to the question of the object's identity, including more advanced CT scanning, histology, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. From these methods, the authors found that: the object's internal structure does not include chambers but is made up of three unconnected areas of lower density material, and is not comparable to the structure of an ostrich's heart the "walls" are composed of sedimentary minerals not known to be produced in biological systems, such as goethite, feldspar minerals, quartz, and gypsum, as well as some plant fragments carbon, nitrogen, and phosphorus, chemical elements important to life, were lacking in their samples and cardiac cellular structures were absent. There was one possible patch with animal cellular structures. The authors found their data supported identification as a concretion of sand from the burial environment, not the heart, with the possibility that isolated areas of tissues were preserved. 
The question of how this find reflects metabolic rate and dinosaur internal anatomy is moot, though, regardless of the object's identity.  Both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so dinosaurs probably had them as well the structure is not necessarily tied to metabolic rate. 
Sleeping posture Edit
Fossils of the troodonts Mei and Sinornithoides demonstrate 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.
Reproductive biology Edit
When laying eggs, female birds grow a special type of bone in their limbs. This medullary bone forms as a calcium-rich layer inside the hard outer bone, and is used as a calcium source to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of a Tyrannosaurus rex specimen's hind limb suggested that T. rex used similar reproductive strategies, and revealed that the specimen is female.  Further research has found medullary bone in the theropod Allosaurus and ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue. 
Brooding and care of young Edit
Several Citipati specimens have been found resting over the eggs in its nest in a position most reminiscent of brooding. 
Numerous dinosaur species, for example 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.
Gizzard stones Edit
Both birds and dinosaurs use 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.  Gizzard stones are also found in some fish (mullets, mud shad, and the gillaroo, a type of trout) and in crocodiles.
Molecular evidence Edit
On several occasions, the extraction of DNA and proteins from Mesozoic dinosaurs fossils has been claimed, allowing for a comparison with birds. Several proteins have putatively been detected in dinosaur fossils,  including hemoglobin. 
In the March 2005 issue of Science, Dr. Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone of specimen MOR 1125 from the Hell Creek Formation in Montana. The seven collagen types obtained from the bone fragments, compared to collagen data from living birds (specifically, a chicken), suggest that older theropods and birds are closely related.  The soft tissue allowed a molecular comparison of cellular anatomy and protein sequencing of collagen tissue published in 2007, both of which indicated that T. rex and birds are more closely related to each other than either is to Alligator.   A second molecular study robustly supported the relationship of birds to dinosaurs, though it did not place birds within Theropoda, as expected. This study utilized eight additional collagen sequences extracted from a femur of the "mummified" Brachylophosaurus canadensis specimen MOR 2598, a hadrosaur.  However, these results have been very controversial. No other peptides of a Mesozoic age have been reported. In 2008, it was suggested that the presumed soft tissue was in fact a bacterial microfilm.  In response, it was argued that these very microfilms protected the soft tissue.  Another objection was that the results could have been caused by contamination.  In 2015, under more controlled conditions safeguarding against contamination, the peptides were still identified.  In 2017, a study found that a peptide was present in the bone of the modern ostrich that was identical to that found in the Tyrannosaurus and Brachylophosaurus specimens, highlighting the danger of a cross-contamination. 
The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but upon further inspection and peer review, neither of these reports could be confirmed. 
Debates about the origin of bird flight are almost as old as the idea that birds evolved from dinosaurs, which arose soon after the discovery of Archaeopteryx in 1862. Two theories have dominated most of the discussion since then: the cursorial ("from the ground up") theory proposes that birds evolved from small, fast predators that ran on the ground the arboreal ("from the trees down") theory proposes that powered flight evolved from unpowered gliding by arboreal (tree-climbing) animals. A more recent theory, "wing-assisted incline running" (WAIR), is a variant of the cursorial theory and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as trees, which would help small feathered dinosaurs escape from predators.
In March 2018, scientists reported that Archaeopteryx was likely capable of flight, but in a manner substantially different from that of modern birds.  
Cursorial ("from the ground up") theory Edit
The cursorial theory of the origin of flight was first proposed by Samuel Wendell Williston, and elaborated upon by Baron Nopcsa. This hypothesis proposes that some fast-running animals with long tails used their arms to keep their balance while running. Modern versions of this theory differ in many details from the Williston-Nopcsa version, mainly as a result of discoveries since Nopcsa's time.
Nopcsa theorized that increasing the surface area of the outstretched arms could have helped small cursorial predators keep their balance, and that the scales of the forearms elongated, evolving into feathers. The feathers could also have been used to trap insects or other prey. Progressively, the animals leapt for longer distances, helped by their evolving wings. Nopcsa also proposed three stages in the evolution of flight. First, animals developed passive flight, in which developing wing structures served as a sort of parachute. Second, they achieved active flight by flapping the wings. He used Archaeopteryx as an example of this second stage. Finally, birds gained the ability to soar. 
Current thought is that feathers did not evolve from scales, as feathers are made of different proteins.  More seriously, Nopcsa's theory assumes that feathers evolved as part of the evolution of flight, and recent discoveries prove that assumption is false.
Feathers are very common in coelurosaurian dinosaurs (including the early tyrannosauroid Dilong).  Modern birds are classified as coelurosaurs by nearly all palaeontologists,  though not by a few ornithologists.    The modern version of the "from the ground up" hypothesis argues that birds' ancestors were small, feathered, ground-running predatory dinosaurs (rather like roadrunners in their hunting style  ) that used their forelimbs for balance while pursuing prey, and that the forelimbs and feathers later evolved in ways that provided gliding and then powered flight. The most widely suggested original functions of feathers include thermal insulation and competitive displays, as in modern birds.  
All of the Archaeopteryx fossils come from marine sediments, and it has been suggested that wings may have helped the birds run over water in the manner of the Jesus Christ Lizard (common basilisk). 
Most recent refutations of the "from the ground up" hypothesis attempt to refute the modern version's assumption that birds are modified coelurosaurian dinosaurs. The strongest attacks are based on embryological analyses that conclude that birds' wings are formed from digits 2, 3, and 4, (corresponding to the index, middle, and ring fingers in humans. The first of a bird's three digits forms the alula, which they use to avoid stalling in low-speed flight—for example, when landing). The hands of coelurosaurs, however, are formed by digits 1, 2, and 3 (thumb and first two fingers in humans).  However, these embryological analyses were immediately challenged on the embryological grounds that the "hand" often develops differently in clades that have lost some digits in the course of their evolution, and that birds' "hands" do develop from digits 1, 2, and 3.    This debate is complex and not yet resolved - see "Digit homology".
Wing-assisted incline running Edit
The wing-assisted incline running (WAIR) hypothesis was prompted by observation of young chukar chicks, and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators.  This makes it a specialized type of cursorial ("from the ground up") theory. Note that in this scenario birds need downforce to give their feet increased grip.   But early birds, including Archaeopteryx, lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes. Since the downforce WAIR depends on is generated by upstrokes, it seems that early birds were incapable of WAIR.  Because WAIR is a behavioural trait without osteological specializations, the phylogenetic placement of the flight stroke before the divergence of the Neornithes, the group which contains all extant birds, makes it impossible to determine if WAIR is ancestral to the avian flight stroke or derived from it. 
Arboreal ("from the trees down") theory Edit
Most versions of the arboreal hypothesis state that the ancestors of birds were very small dinosaurs that lived in trees, springing from branch to branch. This small dinosaur already had feathers, which were co-opted by evolution to produce longer, stiffer forms that were useful in aerodynamics, eventually producing wings. Wings would have then evolved and become increasingly refined as devices to give the leaper more control, to parachute, to glide, and to fly in stepwise fashion. The arboreal hypothesis also notes that, for arboreal animals, aerodynamics are far more energy efficient, since such animals simply fall to achieve minimum gliding speeds.  
Several small dinosaurs from the Jurassic or Early Cretaceous, all with feathers, have been interpreted as possibly having arboreal and/or aerodynamic adaptations. These include Scansoriopteryx, Epidexipteryx, Microraptor, Pedopenna, and Anchiornis. Anchiornis is particularly important to this subject, as it lived at the beginning of the Late Jurassic, long before Archaeopteryx. 
Analysis of the proportions of the toe bones of the most primitive birds Archaeopteryx and Confuciusornis, compared to those of living species, suggest that the early species may have lived both on the ground and in trees. 
One study suggested that the earliest birds and their immediate ancestors did not climb trees. This study determined that the amount of toe claw curvature of early birds was more like that seen in modern ground-foraging birds than in perching birds. 
Diminished significance of Archaeopteryx Edit
Archaeopteryx was the first and for a long time the only known feathered Mesozoic animal. As a result, discussion of the evolution of birds and of bird flight centered on Archaeopteryx at least until the mid-1990s.
There has been debate about whether Archaeopteryx could really fly. It appears that Archaeopteryx had the brain structures and inner-ear balance sensors that birds use to control their flight.  Archaeopteryx also had a wing feather arrangement like that of modern birds and similarly asymmetrical flight feathers on its wings and tail. But Archaeopteryx lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes (see diagram above of supracoracoideus pulley) this may mean that it and other early birds were incapable of flapping flight and could only glide. 
But the discovery since the early 1990s of many feathered dinosaurs means that Archaeopteryx is no longer the key figure in the evolution of bird flight. Other small feathered coelurosaurs from the Cretaceous and Late Jurassic show possible precursors of avian flight. These include Rahonavis, a ground-runner with a Velociraptor-like raised sickle claw on the second toe, that some paleontologists assume to have been better adapted for flight than Archaeopteryx,  Scansoriopteryx, an arboreal dinosaur that may support the "from the trees down" theory,  and Microraptor, an arboreal dinosaur possibly capable of powered flight but, if so, more like a biplane, as it had well-developed feathers on its legs.  As early as 1915, some scientists argued that the evolution of bird flight may have gone through a four-winged (or tetrapteryx) stage.   Hartman et al. (2019) found that, because of how basal flying paravians are phylogenetically distributed, flight most likely evolved five times among paravians instead of only once. Yi, Archaeopteryx, Rahonavis and Microraptor were thus considered examples of convergent evolution instead of precusors of bird flight. 
Secondary flightlessness in dinosaurs Edit
A hypothesis, credited to Gregory Paul and propounded in his books Predatory Dinosaurs of the World (1988) and Dinosaurs of the Air (2002), suggests that some groups of non-flying carnivorous dinosaurs - especially deinonychosaurs, but perhaps others such as oviraptorosaurs, therizinosaurs, alvarezsaurids and ornithomimosaurs - actually descend from birds. Paul also proposed that the bird ancestor of these groups was more advanced in its flight adaptations than Archaeopteryx. The hypothesis would mean that Archaeopteryx is less closely related to extant birds than these dinosaurs are. 
Paul's hypothesis received additional support when Mayr et al. (2005) analyzed a new, tenth specimen of Archaeopteryx, and concluded that Archaeopteryx was the sister clade to the Deinonychosauria, but that the more advanced bird Confuciusornis was within the Dromaeosauridae. This result supports Paul's hypothesis, suggesting that the Deinonychosauria and the Troodontidae are part of Aves, the bird lineage proper, and secondarily flightless.  This paper, however, excluded all other birds and thus did not sample their character distributions. The paper was criticized by Corfe and Butler (2006) who found the authors could not support their conclusions statistically. Mayr et al. agreed that the statistical support was weak, but added that it is also weak for the alternative scenarios. 
Current cladistic analyses do not support Paul's hypothesis about the position of Archaeopteryx. Instead, they indicate that Archaeopteryx is closer to birds, within the clade Avialae, than it is to deinonychosaurs or oviraptorosaurs. However, some fossils support the version of this theory that holds that some non-flying carnivorous dinosaurs may have had flying ancestors. In particular, Microraptor, Pedopenna, and Anchiornis all have winged feet, share many features, and lie close to the base of the clade Paraves. This suggests that the ancestral paravian was a four-winged glider, and that larger Deinonychosaurs secondarily lost the ability to glide, while the bird lineage increased in aerodynamic ability as it progressed.  Deinonychus may also display partial volancy, with the young being capable of flight or gliding and the adults being flightless.  In 2018, a study concluded that the last common ancestor of the Pennaraptora had joint surfaces on the fingers, and between the metatarsus and the wrist, that were optimised to stabilise the hand in flight. This was seen as an indication for secondary flightlessness in heavy basal members of that group. 
In Euornithes, the earliest unequivocal example of secondary flightlessness is Patagopteryx. 
Did birds descend from a single or multiple species of dinosaur? - Biology
The discovery that birds evolved from small carnivorous dinosaurs of the Late Jurassic was made possible by recently discovered fossils from China, South America, and other countries, as well as by looking at old museum specimens from new perspectives and with new methods. The hunt for the ancestors of living birds began with a specimen of Archaeopteryx, the first known bird, discovered in the early 1860s. Like birds, it had feathers along its arms and tail, but unlike living birds, it also had teeth and a long bony tail. Furthermore, many of the bones in Archaeopteryx's hands, shoulder girdles, pelvis, and feet were distinct, not fused and reduced as they are in living birds. Based on these characteristics, Archaeopteryx was recognized as an intermediate between birds and reptiles but which reptiles?
As birds evolved from these theropod dinosaurs, many of their features were modified. However, it's important to remember that the animals were not "trying" to be birds in any sense. In fact, the more closely we look, the more obvious it is that the suite of features that characterize birds evolved through a complex series of steps and served different functions along the way.
In theropods even more closely related to birds, like the oviraptorosaurs, we find several new types of feathers. One is branched and downy, as pictured below. Others have evolved a central stalk, with unstructured branches coming off it and its base. Still others (like the dromaeosaurids and Archaeopteryx) have a vane-like structure in which the barbs are well-organized and locked together by barbules. This is identical to the feather structure of living birds.
At right, asymmetrical flight feathers are present in a fossil of a dromaeosaurid that may have had the ability to glide.
Another line of evidence comes from changes in the digits of the dinosaurs leading to birds. The first theropod dinosaurs had hands with small fifth and fourth digits and a long second digit. As the evogram shows, in the theropod lineage that would eventually lead to birds, the fifth digit (e.g., as seen in Coelophysoids) and then the fourth (e.g., as seen in Allosaurids) were completely lost. The wrist bones underlying the first and second digits consolidated and took on a semicircular form that allowed the hand to rotate sideways against the forearm. This eventually allowed birds' wing joints to move in a way that creates thrust for flight.
Birds after Archaeopteryx continued evolving in some of the same directions as their theropod ancestors. Many of their bones were reduced and fused, which may have helped increase the efficiency of flight. Similarly, the bone walls became even thinner, and the feathers became longer and their vanes asymmetrical, probably also improving flight. The bony tail was reduced to a stump, and a spray of feathers at the tail eventually took on the function of improving stability and maneuverability. The wishbone, which was present in non-bird dinosaurs, became stronger and more elaborate, and the bones of the shoulder girdle evolved to connect to the breastbone, anchoring the flight apparatus of the forelimb. The breastbone itself became larger, and evolved a central keel along the midline of the breast which served to anchor the flight muscles. The arms evolved to be longer than the legs, as the main form of locomotion switched from running to flight, and teeth were lost repeatedly in various lineages of early birds. The ancestor of all living birds lived sometime in the Late Cretaceous, and in the 65 million years since the extinction of the rest of the dinosaurs, this ancestral lineage diversified into the major groups of birds alive today.
Origin and Evolution of Birds
Consider for a minute the diversity of birds. There are nearly 10,000 species! Is it possible to trace these birds back to one common ancestor? If so, who is it?
One of the major criticisms of Darwin’s Origin of Species was
the apparent lack of any evidence showing the evolution of birds. Then,
as luck might have it, only two years after he first published his book,
Archaeopteryx appeared in a site in Germany.
Today there are 8 preserved fossils of Archaeopteryx in various
museums of the world. What an amazing find for science because it stirred
scientists to try to figure out how birds were related to other creatures.
was amazing for a few reasons. First it superficially resembled both a
bird and a reptile. In fact, except for the feathers, the
bird-like feet, and the fact that it had a wishbone
(furcula) it didn’t really look like a bird. The jaws
had teeth in them, of which no bird today has teeth. It also
had the ankle bone fused to the shinbone. Clearly this
bird had features of dinosaurs AND birds. So where did birds evolve?
Three hypothesis on origin of birds finally arose:
- Therapod dinosaur hypothesis: The first was a hypothesis
that they came from the therapod dinosaurs. Therapods are meat eating
dinosaurs such as Allosaurus.
- Crocodiles – the second hypothesis was that they
came from crocodiles because they had an endolymphatic duct. Yet, as
more research was conducted, they discovered that there was a tremendous
amount of variation in this duct even among the lizards and other reptiles.
Not many people today give much attention to this hypothesis
- Neither crocodiles or dinosaurs:Neither on the dinosaur
line or the crocodile line. Reasoning because several dinosaurs were
very specialized already.
Today we can show that birds are related in many ways to Dinosaurs. By
using key characters we can use cladistics to understand better the relationships.
For instance we can look at features they share in common with animals
such as reptiles, and ancient dinosaurs in order to figure out where they
may have evolved. They can thus, be linked generally to Ornithodira
and more specifically to Manirapterans.
If you look at a cladogram
of Diapsids which includes snakes, lizards, crocodiles (archosaurs),
and dinosaurs and birds, you can get a better picture as to where birds
Looking in particular at the Ornithodira, Dinosaurs, Saurischian
dinosaurs, Therapods, Tetanurae, Coelesaurs, Manirapterans. (list
Summarization of the set of derived characters that link them
to the dinosaurs:
Once the idea that birds came from dinosaurs began, there was a scurry
to find fossil evidence that could link birds back to their dino-roots.
Several different dino-birds arose in the last century. One was Caudipteryx
In China a fossil
was found that was dinosaur-like but had feathers. It seems that the wings
would have been too small to allow it to fly, but, the fact that it had
wings made it big news! Thus, the idea was that the initial evolution
of feathers may not have been for powered flight. In fact, if you look
at the tail feathers, it looks as though they are symmetrical around the
shaft. This finding forced a reconfiguration of the systematics of the
Another fossil was found that, although it was not a fossil with wings,
it was a closely related dinosaur to birds that was very small and appeared
to be arboreal. This tiny fossil is only about 10 cm long and if it lived
in the trees could have glided from tree to tree.
For almost a century scientists have been debating this issue. The common
belief was that flight must have evolved from the trees down. This is
because every known modern semi-airborn animal (glider), seems to be arboreal.
Yet, another competing theory is that the wings are used to catch insects
and thus evolved from the ground up.
One set of reasoning for the ‘ground-up‘ hypothesis is that dinosaurs
could have been leaping to catch insects and wings allowed them to come
down in one piece. Part of the evidence is that the capturing of prey
was the same movement for flight.
Wing-Assisted Incline running. (copy
of the study).
In a 2003 article in science, Kenneth Dial proposed his theory of ‘wing-assisted
incline running’ as a way for wings to evolve. In the study he used chucker
partridges and had them run up grades from 0 to 90 degrees. From 0 to
45 degrees, they just used their legs, but greater than 45 they used their
wings too. When they flap their wings, they put traction on the surface
and thus, increase their ability to run up the incline
In 2008, Arkhat Abzhanov, a biologist at Harvard University, was elbow deep in alligator eggs. Since alligators descend from a common ancestor with dinosaurs, they can provide a useful evolutionary comparison to birds. (Despite their appearance, birds are more closely related to alligators than lizards are.) Abzhanov was studying alligators’ vertebrae, but what struck him most was the birdlike shape of their heads alligator embryos looked quite similar to chickens. Fossilized skulls of baby dinosaurs show the same pattern — they resemble adult birds. With those two observations in mind, Abzhanov had an idea. Perhaps birds evolved from dinosaurs by arresting their pattern of development early on in life.
To test that theory, Abzhanov, along with Mark Norell, a paleontologist at the American Museum of Natural History in New York, Bhart-Anjan Bhullar, then a doctoral student in Abzhanov’s lab, and other colleagues, collected data on fossils from around the globe, including ancient birds, such as archaeopteryx, and fossilized eggs of developing dinosaurs that died in the nest. They tracked how the skull shape changed as dinosaurs morphed into birds.
Over time, they discovered, the face collapsed and the eyes, brain and beak grew. “The first birds were almost identical to the late embryo from velociraptors,” Abzhanov said. “Modern birds became even more babylike and change even less from their embryonic form.” In short, birds resemble tiny, infantile dinosaurs that can reproduce.
This process, known as paedomorphosis, is an efficient evolutionary route. “Rather than coming up with something new, it takes something you already have and extends it,” said Nipam Patel, a developmental biologist at the University of California, Berkeley.
“We’re seeing more and more that evolution operates much more elegantly than we previously appreciated,” said Bhullar, who will start his own lab at Yale University in the fall. “The umpteen changes that go into the bird skull may all owe to paedomorphosis, to one set of molecular changes in the early embryo.”
Why would paedomorphosis be important for the evolution of birds? It might have helped drive miniaturization or vice versa. Changes in size are often linked to changes in development, so selection for small size may have arrested the development of the adult form. “A neat way to cut short a developmental sequence is to stop growing at smaller size,” Benton said. A babylike skull in adults might also help explain birds’ increased brain size, since baby animals generally have larger heads relative to their bodies than adults do. “A great way to improve brain size is to retain child size into adulthood,” he said.
(Indeed, paedomorphosis might underlie a number of major transitions in evolution, perhaps even the development of mammals and humans. Our large skulls relative to those of chimpanzees could be a case of paedomorphosis.)
What’s more, paedomorphosis helped to make the skull a blank slate on which selection could create new structures. By erasing the snout, it may have paved the way for another of birds’ most important features: the beak.
Are Birds Really Dinosaurs?
People often say that birds are related to dinosaurs, but that’s really not true – birds aren’t related to dinosaurs… they are dinosaurs!
About 65 million years ago, a huge extinction wiped out all dinosaur groups except for one groupd. That group of dinosaurs went on to become all the birds we see today. But let’s start from the beginning.
Dinosaurs were a diverse group of reptiles that first emerged during the Triassic period, 231.4 million years ago. They were the dominant life forms on land for 135 million years, until a great extinction wiped most of them out. The cause of that extinction is still a matter of debate, but the most likely option seems to be an asteroid impact (though volcano eruption is also quite possible).
Dinosaurs are generally split into two major groups – Saurischia and Ornithischia.
- The Saurischia include all the carnivorous dinosaurs (theropods) and the giant, herbivorous dinosaurs (sauropods). The theropods are the more “popular” carnivorous dinosaurs, such as the T-Rex or the velociraptor, while the sauropods are the largest animals to ever walk on land
- The Ornithischia are an order of beaked, herbivorous dinosaurs. The more notable dinosaurs of the group are the Stegosaurus and Iguanodon.
Interestingly enough, birds evolved from the Saurischian dinosaurs. Modern paleontology indicates that birds may have started to emerge during the Jurassic, some 150 million years ago.
… and birds
Birds are a member of Maniraptora, a group of theropods. It may seem strange that they actually emerged from dinosaurs, but today, most paleontologists agree that several dinosaurs were covered in feathers, which makes a bit more acceptable. Their bones were also very light, they laid eggs, and despite their overall look, they have many things in common even with modern birds.
Of course, the transition took place over millions and millions of years, with several key transitions. The most well-known transitional fossil is Archaeopterix, the so-called missing link between reptiles and birds.
Archaeopteryx lived in the Late Jurassic period around 150 million years ago, when Europe was an archipelago of islands in a shallow warm tropical sea. The species is one of the clearest examples of transitional species, showing characters from both dinosaurs and birds. Most of the fossils found include impressions of feathers, which only rarely become conserved in fossils. Because these feathers are of an advanced form (flight feathers), these fossils are evidence that the evolution of feathers began before the Late Jurassic.
However, it took quite a while before birds started diversifying. For tens of millions of years, birds still had clawed wings and teeth. Truly modern birds appeared emerged 100 million years ago, way before the massive extinction which wiped out the dinosaurs. While they were never really on top of the food chain, they are the only branch of dinosaurs to survive past the Cretaceous (if you haven’t already, get used to the idea: birds are dinosaurs).
Modern birds are characterized by a beak with no teeth and a high metabolic rate and rate of growth. Most can fly, with some exceptions. They also have several other adaptations specifically aimed at flying.
So, birds are reptiles?
Well… yes, in a way, birds are reptiles in a sense, but it’s more complicated than that.
Biologists use two types of classification systems, the Linnaean and the phylogenetic. Depending on the classification system you use, birds are or aren’t reptiles. In the Linnaean classification, a reptile is an animal that is cold-blooded and has scales – so in this sense, a bird is definitely not a reptile. But in modern biology, people tend to follow the phylogenetic classification – that is, animals are grouped by their ancestry, and in this way, birds kind of are reptiles. It’s counterintuitive, but the question is only particularly useful for classification purposes, and both these systems have different uses.
The question which emerges now is: if birds are indeed reptiles, what are their closest relatives? Strangely enough, birds are most closely related to crocodiles. But while crocodiles have remained mostly unchanged for tens of millions of years, birds have changed and adapted – which is why we see this stunning bird diversity today.