The Avian Genesis: An Essay

 

“[Every] bird species past and present can be traced to a single ancestor that evolved perhaps 160 million years ago from dinosaurian stock among small, bipedal carnivores akin to Velociraptor.” (Scott D. Sampson, Dinosaur Odyssey, pp. 87)

As I write this at my dining room table, I can hear dinosaurs outside. They’re making quite a racket as they flit tree-to-tree this warm spring morning. The fact that birds are dinosaurs is a shocking statement, but it’s nothing less than the truth. The fact that birds evolved from dinosaurs is attested to in the fossil record and in their bones today. Birds of all shapes and sizes are nothing less than the sole surviving dinosaur lineage that survived the Cretaceous-Tertiary Extinction and evolved in a variety of ways to fit their new environments. During the Mesozoic, dinosaurs ruled the world – and they still do today. While there are 6000 species of reptiles and amphibians and 4000 species of mammals, there are 10,000 species of birds. (Of course, bird species are still overwhelmed by the jaw-dropping numbers of fish, insects, and microbes.)

Archaeopteryx of the Late Jurassic was capable of powered flight

For most of the 1900s, many scientists rejected the dinosaur-bird connection, pointing out that dinosaurs didn’t have clavicles (the ‘collarbone’ in humans). In birds, the clavicles fuse into a single element, the furcula or wishbone. The presence of a clavicle was considered a ‘litmus test’ for the bird-dinosaur relationship, and the lack of clavicles was interpreted as strong evidence against such a connection. However, over the past couple decades, clavicles have been found in a broad range of theropod dinosaurs (and even in some sauropods!), and those dinosaurs thought to be closest to bird origins had clavicles. While most paleontologists today concur that birds are a surviving dinosaur lineages, many ornithologists – those who study birds – are uncomfortable with the idea. They argue that birds started out as small, lizard-like creatures that lived in trees; as these arboreal reptiles adapted more airborne habits, they eventually evolved into birds. Many ornithologists contend that dinosaurs could not have been ancestral to birds because the dinosaurs that are supposedly ‘bird ancestors’ were ground-dwelling bipeds rather than small, arboreal quadrupeds. After all, Archaeopteryx of the Late Jurassic precedes the ‘bird-like’ dinosaurs of the Cretaceous around twenty million years. However, we must bear in mind that (a) the fossil record isn’t complete, (b) birds and bird-like dinosaurs very possibly evolved from common ancestors rather than birds evolving directly from bird-like dinosaurs, and (c) the emergence of birds may be somewhat earlier in time than the arrival of Archaeopteryx (and on this note we can finger the Middle Jurassic Serikornis of China or the Late Triassic Protoavis). Another argument posed against dinosaur-bird origins has to do with the hands. Reptiles started with a five-fingered hand, and the hands of primitive dinosaurs also have five digits; but in the theropods that supposedly gave rise to birds, they only have three digits. Birds, too, have three digits, so what’s the problem? The issue is that the bird-like theropods retained the thumb, index, and middle fingers (digits I, II, and III); but in modern birds, the hand has become part of the wing and is extremely reduced and modified, and the digits of their three-fingered hands are the index, middle, and ring fingers (digits II, III, and IV). However, it’s difficult, when looking at a blob of embryonic cartilage under a microscope, to assert that the developing digits are this one or that one, and it’s also possible that digits change their identity in the womb. Besides, we don’t have any equivalent dinosaur embryos to compare to those of modern birds, so this assertion ultimately boils down to guesswork. A third argument raised against the dinosaur-bird connection is that birds are said to have evolved not from the bird-hipped ornithischians but from the lizard-hipped saurischians; if a dinosaur lineage evolved into modern birds, wouldn’t we expect them to come from the bird-hipped dinosaurs? Two points must be made in reply: first, ornithischians didn’t have ‘bird’ hips; rather, their hips were like bird hips whereas saurischians hips were like lizard hips, though in reality both dinosaur lineages’ hip structures were different from their respective likenesses; and second, we know that one specialized group of smaller-bodied theropods, the maniraptors, independently evolved a backward-facing pubis, and it’s this group that seems to have given rise to birds. All this to say that there are unresolved issues with the dinosaur-bird relationship; however, the dearth of information in support of these relationships cannot be denied. In fact, the similarities far outweigh the differences (and we would expect evolution to have a few ‘tricks’ up her sleeve). Most paleontologists see the similarities as indicative of relationship, whereas many ornithologists see them as convergent evolution (wherein two organisms of different lineages evolve the same characteristics because those characteristics are beneficial or even required in their environment). Because the chances of such overwhelming convergence are less than the chances of a dinosaur-bird relationship, most scientists tend towards the dinosaur-bird connection. As an aside, we must never ignore bias: scientists who study dinosaurs are biased towards a dinosaur-bird relationship, whereas ornithologists – who are biased towards modern birds – tend to chafe at the idea that birds are just a subset of dinosaurs. Such an idea might ‘subordinate’ them to another field of scientific study. 

the feathered maniraptorian Velociraptor

There are over a hundred shared characteristics between bird-like theropod dinosaurs and birds as we know them today. A few examples will suffice: these theropods’ powerful hind legs operated like the limbs of gigantic birds; they shared the presence of a clavicle or ‘wishbone’; they shared a tendency for certain bones to fuse in the legs; they shared a tendency for the skull to be remodeled in certain ways; there are similarities between the bones of the wrist and how the wrist worked; some bird-like dinosaurs folded their hands sideways in the same manner as a bird furls its wings; and birds likely received their warm-bloodedness from warm-blooded dinosaurs. These are just a few of the 100+ similarities between dinosaurs and birds, and two more must receive special recognition. First, many dinosaurs had hollow chambers in their vertebrae, and in life these bony caverns were filled with air sacs that connected to the lung, just as we see in many birds today. A single bone of an Apatosaurus spine is so full of holes and indentations that the actual bony tissue was reduced to thin, convoluted, and folded partitions; Allosaurus and other theropods had hollowed-out vertebrae, though to a lesser extent than found in the sauropods. Hollow backbones are found in today’s birds; in them, the hollows are filled by air sacs connected by tubes to the lungs. Avian lungs are superbly efficient, far better at extracting oxygen than mammal lungs, and this is due to the air sacs and the resulting pattern of air flow. Even modern birds with solid vertebrae have air sacs within their body cavities. This unique system of ‘bony air sacs’ comes directly from the saurischian dinosaur lineage (which included sauropodomorphs and theropods, the latter of which gave rise to birds). 

the Chinese 'dino-bird' Confuciusornis

Another similarity is the most obvious: feathers. Today, birds are the only organisms with feathers, and all modern birds have feathers. For decades it was postulated that many types of dinosaurs had feathers, but this wasn’t proven until remarkable ‘feathered dinosaurs’ began being discovered in China. The sedimentary layers in these prehistoric environments were of an exquisite type that allowed the imprints of features (such as feathers) that are usually destroyed prior to fossilization. Paleontologist Steve Brusatte notes, “These dinosaurs had the great misfortune to live in a dense forest surrounding a wonderland of ancient lakes, a landscape that was periodically obliterated by volcanoes. Some of these eruptions spewed out tsunamis of ash, which combined with water to flood the landscape in a viscous ooze that buried everything in sight. The dinosaurs were captured going about their everyday business, preserved Pompeii-style. That’s why the details of the feathers are so pristine.” These fossil beds had unearthed at least twenty different species of feathered, flying dino-birds. These specimens reveal the many similarities between birds and dinosaurs; we’re not just talking about feathers, but also wishbones, three-fingered hands that could fold against the body, and literally hundreds of other skeletal aspects. Brusatte adds, “There are no other groups of animals – living or extinct – that share these [characteristics] with birds or theropods; this must mean birds came from theropods. Any other conclusion requires a lot of special pleading.” One of the most remarkable of these feathered Chinese dinosaurs is Confuciusornis, which was the first bird to have a beak, and it lived 120 million years ago in Cretaceous China. This was a successful ‘dino-bird’, for over a thousand specimens have been discovered. China has given us numerous specimens of feathered ‘dino-birds,’ but it has also given us many non-bird dinosaurs that had feathers. Indeed, many theropod dinosaurs – such as Caudipteryx, Microraptor, and Beipiaosaurus – had feathery pelts. While some believe dinosaurs first evolved feathers, we know that pterosaurs had feathery pelts; this may indicate that feathers first evolved in the common ancestors of both pterosaurs and dinosaurs. Stepping backwards in time from the Cretaceous to the Late Jurassic, we find Archaeopteryx, known from ten specimens from Germany. Archaeopteryx gives us a snapshot of what a more primitive ‘dinosaur-bird’ looked like: this creature had several primitive dinosaurian feathers including teeth, three separate clawed fingers (rather than hand-bones morphed into wings), and a long bony tail; at the same time it possessed feathers, fused clavicles, and an elongated forelimb modified into a primitive wings. 

the non-flying Caudipteryx with some chicks

Scientists note that many of the ‘signature components’ of modern birds first appear in their dinosaur ancestors; far from being unique to birds, these features developed in ground-loving dinosaurs for purposes wholly unrelated to flight. How, then, did flying dinosaurs come about? Scientists have devised a proposed ‘route of emergence’ for many features that serve as ‘spring-pads’ for the emergence of avian birds. The basic ‘bird blueprint’ – long, straight legs and feet with three skinny toes – first appeared in the Late Triassic some 230 million years ago as dinosaurs evolved into upright, bipedal, fast-running creatures. These hind-limb features are staples of both dinosaurs and birds. Some time later, some of these dinosaurs of the theropod lineage fused their left and right collarbones into a new structure, called a wishbone; this stabilized the shoulder girdle and enabled them to absorb the extreme shock forces of grabbing prey. This wishbone, designed to protect the hunter from the physical effects of hunting methods, would later serve as a spring to store energy for flying dinosaurs when in flight. Farther down the line, some theropods called maniraptorans evolved a curved neck (perhaps for scouting prey), and some of these species shrank in size. Smaller sizes gave them access to new ecological niches, such as trees and brush and even underground caves or borrows that were inaccessible to larger dinosaurs. Even later, some of these shrinking dinosaurs began to fold their arms against the body (probably to protect the delicate quill-pen feathers that were evolving around the same time). These were the paravians – a subgroup of the maniraptorans – and would be the immediate ancestors of birds. These paravians were smaller, nimbler, and smarter than their larger-sized cousins. The paravians include the dromaeosaurs (such as Deinonychus and Velociraptor) and the troodontids (such as Troodon). They had feathers, many of them had wings, and most likely looked and acted like birds. Somewhere among these paravians lies the blurred line between ‘bird’ and ‘non-bird’ (at least in modern parlance). After the Cretaceous-Tertiary Extinction Event, it was the ‘bird’ lineage of the paravians that survived, likely due to their ability to adapt due to evolutionary modifications they’d taken in the late Mesozoic. 

We have seen that feathers didn’t emerge with flying dinosaurs; we know that many non-flying dinosaurs had feathers, and some scientists speculate that even the dinosauromorphs – the archosaur ‘proto-dinosaurs – had feathers. We know that pterosaurs had feather-like coverings, so the first emergence of feathers may go as far back as the common ancestors to dinosaurs and pterosaurs. Of course, the earliest feathers looked very different from modern bird feathers. The earliest feathers were modified reptilian scales that looked more like fluff; they were made up of thousands of hair-like filaments that have been called ‘proto-feathers.’ There was no way dinosaurs with these proto-feathers could fly. What, then, was their purpose? They likely served to keep small dinosaurs warm (a type of thermoregulation); they may have been for display purposes (for mating, species recognition, intimidating rivals, or camouflaging into their environment). These proto-feathers became more like what we know today in the maniraptorans: the hairy stands grew longer and started to branch, first into a few simple tufts and then into a complex system of barbs projecting sideways from a shaft. This was the emergence of the quill (or, in science-speak, the pennaceous feather). As these quills lined up and layered across each other on the arms, complex feathers formed wings. Many of these maniraptors, especially the paravians, had wings of various shapes and sizes (Microraptor, a dromaeosaur, had wings both its arms and legs!). 

the gliding, sheen-coated Microraptor

Though these feathered wings are necessary for flight, they likely didn’t evolve for that purpose. Many modern flightless birds have wings, but these wings are far from vestigial; they’re used as display structures, for stabilization when climbing, as fins to help them swim, or as insulators to keep eggs warm. There are a variety of reasons wings may have evolved in dinosaurs; it’s likely that flight was a ‘happy little accident.’ The current consensus is that feathered wings evolved for display, and there’s strong evidence for it. In 2005 a study on fossilized feathers and remnant melanosomes (which give feathers color) revealed that the feathers of non-flying winged dinosaurs came in a rainbow of colors. Some were iridescent like those of shiny crows, and many were colorful like those of a peacock. Steve Brusatte writes,  “The totality of the evidence—that wings first evolved in dinosaurs too large and ungainly to fly, that these wings were originally colored, and that modern birds use their wings for display—has led to a radical new hypothesis. Wings originally evolved as display structures—as advertising billboards projecting from the arms, and in some cases, like Microraptor, the legs, and even the tail. Then these fashionably winged dinosaurs would have found themselves with big broad surfaces that by the unbreakable laws of physics could produce lift and drag and thrust. The earliest winged dinosaurs… probably would have considered the lift and drag produced by their billboards to be little more than an annoyance. In any case, whatever lift was generated wasn’t nearly enough to get such large animals into the air. But in more advanced paravians, which had the magic combination of bigger wings and smaller body size, the billboards would have been able to take on an aerodynamic function. These dinosaurs could now move around in the air, even if awkwardly at first. Flight had evolved—and it had happened totally by accident, the billboards now repurposed as airfoils.” (297-298, Brusatte) Flight, then, was an exaptation, a deviated adaptation of an original function; feathers originally evolved for other purposes (they were adaptations), but they happened to be critical at enabling dino-birds to take flight, and this was an exaptation of the adaptation. 

the flying 'dino-bird' Eoalulavis

We turn now to the critical question: “How did paravians become flyers?” We know that flight has evolved in the animal kingdom (excepting insects) at least three times: first in pterosaurs during the Triassic Period, then in birds sometime during the Mesozoic (likely in the Jurassic or Cretaceous Period), and finally in bats during the Cenozoic. All three of these animal groups evolved flight independently in episodes of convergent evolution. There are three main anatomical requirements for flight, and these are shared by pterosaurs, birds, and bats: (1) a lightweight body, generally accomplished via lightweight bones and sometimes fewer bones in the back; (2) airfoil to produce lift, in which the wing is an airfoil that, when combined with forward motion, produces lift, which enables the body to become airborne; and (3) energy to take off (most modern birds are strong enough to become airborne from a standing start, but heavy pterosaurs would’ve needed a good run to take off, and how the earliest dino-birds took flight is debated). Studying the fossil record, we can determine that flying dinosaurs didn’t evolve from just one branch of the paravian tree; rather, it looks like flight evolved multiple times. Brusatte notes, “Flight probably evolved multiple times in parallel, as different species of these dinosaurs-with their different airfoil and feather arrangements—found themselves generating lift from their wings as they leaped from the ground, scurried up trees, or jumped between branches.” (298, Brusatte) Some of these early birds were gliders; incapable of powered flight, they could nonetheless soar on air currents. The four-winged maniraptor Microraptor could glide, as proven by experiments in which scientists built an anatomically correct, life-size model and put it in a wind tunnel. Brusatte continues, “Most theropods were evolving at ho-hum rates, but then, once an air-worthy bird had emerged, the rates went into overdrive. The first birds were evolving much faster than their dinosaur ancestors and cousins, and they maintained accelerated rates for many millions of years… [Other] studies have shown that there was a sudden decrease in body size and a spike in rates of limb evolution right around this same point on the genealogy, as these first birds were quickly getting smaller and growing larger arms and bigger wings so that they could fly better.” (302, Brusatte) It’s interesting to note that Archaeopteryx, one of the oldest birds, was capable of powered flight; this means that the origin of ‘small, winged, flapping, bona fide birds’ likely stretched back to the Middle Jurassic, around 170 to 180 million years ago. This means there was around 100 million years in which flying dinosaurs coexisted with their ground-loving counterparts. By the end of the Cretaceous Period, there were at least fourteen branches of the modern bird family tree that had taken root; the Cretaceous-Tertiary Extinction wiped out the non-avian dinosaurs and many of these bird lineages so that only eight branches went on to flourish in the Tertiary Period.