This site presents the idea that birds developed from flying pterosaurs.
This is a credible alternative to the current, mainstream idea that birds developed from land-based dinosaurs.
Bird-like characteristics are found in basal Paraves. They are not found in dinosaurs.
That is because basal paraves are not descended from dinosaurs. They are descended from pterosaurs.
Let's look at body size and forelimb length. Notice that the changes appear for the first time at the origin of Paraves (not earlier).
HIGH RATES OF EVOLUTION PRECEDED THE ORIGIN OF BIRDS
Mark Puttick and colleagues investigated the rates of evolution of the two key characteristics that preceded flight: body size and forelimb length. In order to fly, hulking meat-eating dinosaurs had to shrink in size and grow much longer arms to support their feathered wings.
"We were really surprised to discover that the key size shifts happened at the same time,at the origin of Paraves," said Mr Puttick of Bristol's School of Earth Sciences. "This was at least 20 million years before the first bird, the famous Archaeopteryx, and it shows that flight in birds arose through several evolutionary steps."
Being small and light is important for a flyer, and it now seems a whole group of dozens of little dinosaurs were lightweight and had wings of one sort or another. Most were gliders or parachutists, spreading their feathered wings, but not flapping them.
The origin of birds (Aves) is one of the great evolutionary transitions. Fossils show that many unique morphological features of modern birds, such as feathers, reduction in body size, and the semilunate carpal, long preceded the origin of clade Aves, but some may be unique to Aves, such as relative elongation of the forelimb. We study the evolution of body size and forelimb length across the phylogeny of coelurosaurian theropods and Mesozoic Aves. Using recently developed phylogenetic comparative methods, we find an increase in rates of body size and body size dependent forelimb evolution leading to small body size relative to forelimb length in Paraves, the wider clade comprising Aves and Deinonychosauria. The high evolutionary rates arose primarily from a reduction in body size, as there were no increased rates of forelimb evolution. In line with a recent study, we find evidence that Aves appear to have a unique relationship between body size and forelimb dimensions. Traits associated with Aves evolved before their origin, at high rates, and support the notion that numerous lineages of paravians were experimenting with different modes of flight through the Late Jurassic and Early Cretaceous.
Note that in the following two references the researchers are working within the dino to bird paradigm. To make the evidence fit the theory they are also forced to claim improbable rates of evolution.
Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds
Recent discoveries have highlighted the dramatic evolutionary transformation of massive, ground-dwelling theropod dinosaurs into light, volant birds. Here, we apply Bayesian approaches (originally developed for inferring geographic spread and rates of molecular evolution in viruses) in a different context: to infer size changes and rates of anatomical innovation (across up to 1549 skeletal characters) in fossils. These approaches identify two drivers underlying the dinosaur-bird transition. The theropod lineage directly ancestral to birds undergoes sustained miniaturization across 50 million years and at least 12 consecutive branches (internodes) and evolves skeletal adaptations four times faster than other dinosaurs. The distinct, prolonged phase of miniaturization along the bird stem would have facilitated the evolution of many novelties associated with small body size, such as reorientation of body mass, increased aerial ability, and paedomorphic skulls with reduced snouts but enlarged eyes and brains.
The evolution of birds from theropod dinosaurs was one of the great evolutionary transitions in the history of life [ 1–22 ]. The macroevolutionary tempo and mode of this transition is poorly studied, which is surprising because it may offer key insight into major questions in evolutionary biology, particularly whether the origins of evolutionary novelties or new ecological opportunities are associated with unusually elevated “bursts” of evolution [ 23, 24 ]. We present a comprehensive phylogeny placing birds within the context of theropod evolution and quantify rates of morphological evolution and changes in overall morphological disparity across the dinosaur-bird transition. Birds evolved significantly faster than other theropods, but they are indistinguishable from their closest relatives in morphospace. Our results demonstrate that the rise of birds was a complex process: birds are a continuum of millions of years of theropod evolution, and there was no great jump between nonbirds and birds in morphospace, but once the avian body plan was gradually assembled, birds experienced an early burst of rapid anatomical evolution. This suggests that high rates of morphological evolution after the development of a novel body plan may be a common feature of macroevolution, as first hypothesized by G.G. Simpson more than 60 years ago [ 25 ].
The idea that birds are descended from pterosaurs has an interesting history. The idea was proposed by Harry Govier Seeley. But note the negative reaction he received. Not much has changed in that respect.
Seeley was also an authority of pterosaurs, and in 1901 published a popular book on the subject, Dragons of the Air. In it, he gave an overview of animal flight, reptiles, the discovery of pterosaurs and pterosaur skeletal structure. He initially believed that birds descended from pterosaurs, but under intense criticism from his peers, backed off this assertion and argued that they shared common ancestry. "It would therefore appear from the vital community of structures with Birds, that Pterodactyles and Birds are two parallel groups, which may be regarded as ancient divergent forks of the same branch of animal life," he wrote.
His popular book on Pterosaurs, Dragons of the
Air (1901) found that birds and pterosaurs are closely parallel. His belief
that they had a common origin has been proved, for both are archosaurs, just
not as close as he thought.
I suggest that pterosaurs and birds are as close as Seeley thought.
Ornithodesmus (meaning "bird link") is a genus of small, deinonychosauriandinosaur from the Isle of Wight in England, dating to about 125 million years ago. The name was originally assigned [by Harry Govier Seeley]to a bird-like sacrum (a series of vertebrae fused to the hip bones), initially believed to come from a pterosaur. More complete pterosaur remains were later assigned to Ornithodesmus, until recently a detailed analysis determined that the original specimen in fact came from a small theropod, specifically a dromaeosaur. All pterosaurian material previously assigned to this genus has been renamed Istiodactylus.
By the 1840s, however, there was little doubt that Cuvier had been correct, and some naturalists were very impressed by resemblances between the skeletons of the flying fiends [pterosaurs] and birds. As Richard Owen stated in an 1874 monograph of Mesozoic fossil reptiles:
Every bone in the Bird was antecedently present in the framework of the Pterodactyle; the resemblance of that portion directly subservient to flight is closer in the naked one to that in the feathered flier than it is to the forelimb of the terrestrial or aquatic reptile.
Just like Owen, Seeley saw no way to “evolve an ostrich out of an Iguanodon,” but Huxley turned the argument from convergence against his opponents. The traits supposedly shared between birds and pterosaurs had to do with flight, and given that both lineages had become adapted to flying, common traits in their skeletons were to be expected. The diagnostic traits in the hips, legs, and feet of dinosaurs, on the other hand,were found in all birds, not just ground-dwelling ones. This meant that these characters marked a true family relationship and not just a shared way of life.
The hyposphene-hypantrum articulation is an accessory joint found in the vertebrae of several fossil reptiles of the group Archosauromorpha. It consists of a process on the backside of the vertebrae, the hyposphene, that fits in a depression in the front side of the next vertebrae, the hypantrum. Hyposphene-hypantrum articulations occur in the dorsal vertebrae and sometimes also in the posteriormost cervical and anteriormost caudal vertebrae.[1]
Hyposphene-hypantrum articulations were present in the derived and birdlike dromaeosauridRahonavis, but are lost [not present] in modern day's birds, probably due to their highly modified vertebrae.[4]
Early Dinosauromorphs (early ancestors of dinosaurs) like Marasuchus, Lagosuchus and Euparkeria as well as ornithischian dinosaurs lack hyposphene-hypantrum articulations. Because these articulations are absent in both saurischian ancestors and all non-saurischian dinosaurs, they are considered a synapomorphy (a distinctive feature) of the Saurischia, as proposed by Gauthier (1986).[4] Hyposphene-hypantrum articulations are found in all the basal members of the Saurischia.[5] However, they became lost in several saurischian lineages. They were present in the derived and birdlike dromaeosauridRahonavis, but are lost in modern day's birds, probably due to their highly modified vertebrae.[4] Within the Sauropodomorpha, they were present in prosauropods and most sauropods, but became independently lost in two cretaceous sauropod lineages, the Titanosauria and the Rebbachisauridae.[1][3]
Chiappe (2001) united the Pygostylia in possessing four unambiguous synapomorphies. The trait that gives the group its name is the presence of apygostyle.Next is the absence of a hyposphene-hypantrum. Next is a retrovertedpubis separated from the main axis of the sacrum by an angle of 45 to 65 degrees. Last is a bulbous medialcondyle of the tibiotarsus.
In the tails of dromaeosauriddinosaurs and rhamphorhynchid pterosaurs, elongate osteological rods extend anteriorly from the chevrons and the prezygapophyses. Thesecaudal rods are positioned in parallel and are stacked dorsoventrally.
Remember the quickly reduced neural spines, caudal ribs, and chevrons?Those all indicate that the caudal muscles of both dromaeosaurids and pterosaurs were substantially reduced.
The tails of pterosaurs and dromaeosaurids are so similar that, in the fossil-forging black-markets of China, the tail of one is often used to “complete” a partial skeleton of the other. Skeletal image of Rhamphorhynchus courtesy of Scott Hartman (www.skeletaldrawing.com).
The fourth trochanter is a shared characteristic common to archosaurs. It is a knob-like feature on the posterior-medial side of the middle of the femur shaft that serves as a muscle attachment, mainly for the Musculus caudofemoralis longus, the main retractor tail muscle that pulls the thighbone to the rear.
Also, a large process on the shaft of the [archosuar] femur, the fourth trochanter, served as the attachment point for major tail muscles, the caudofemoralis group of thigh retracting muscles.
It is now also possible to think a step further and consider the muscles of the tail. Let’s first try to do that in very general qualitative terms. Remember the quickly reduced neural spines, caudal ribs, and chevrons? Those all indicate that the caudal muscles of both dromaeosaurids and pterosaurs were substantially reduced.
To help consider the problem quantitatively, a technique I used was to create digital models of the tail skeleton of a Velociraptor and a Rhamphorhynchus (a pterosaur) and to sculpt the corresponding muscles over the skeletal models. The results of this modeling concur with the qualitative inference. In particular, raptors and pterosaurs were found to have very weak caudofemoral muscles (indeed, some pterosaurs may not have had caudofemoral muscles at all).
In the tails of dromaeosaurid dinosaurs and rhamphorhynchid pterosaurs, elongate osteological rods extend anteriorly from the chevrons and the prezygapophyses. These caudal rods are positioned in parallel and are stacked dorsoventrally. The fully articulated and three-dimensionally preserved caudal series of some dromaeosaurid specimens show that individually these caudal rods were flexible, not rigid as previously thought. However, examination of the arrangement of the caudal rodsin cross-section indicates that the combined effect of multiple caudal rods did provide substantial rigidity in the dorsoventral, but not in the lateral, plane. The results of digital muscle reconstructions confirm that dromaeosaurids and rhamphorhynchids also shared greatly reduced caudofemoral muscles in the anterior tail region. The striking similarities between the tails of dromaeosaurids and rhamphorhynchidssuggest that both evolved under similar behavioral and biomechanical pressures. Combined with recent discoveries of primitive deinonychosaurs that phylogenetically bracket the evolution of dromaeosaurid caudal rods between two arboreal gliding/flying forms, these results are evidence that the unique caudal morphologies of dromaeosaurids and rhamphorhynchids were both adaptations for an aerial lifestyle.
At the start, the tail skeleton of Deinonychus appears normal. Just past the hips, the neural spines, caudal ribs, and chevrons all have a typical shape and they all project to a respectable extent. But, as the tail progresses towards the tip (and it doesn’t take long) things start to get weird. The neural spines, caudal ribs, and chevrons all shrink in, with the former two disappearing entirely . . . and then come the caudal rods. Both the vertebrae and chevrons abruptly develop pairs of elongated rods of bone that project towards the hips. These rods are slender, but very long (the longest easily overlap seven other sequential vertebrae), and they split, each becoming two still thinner rods. Together, rods of the vertebrae form a quiver that virtually encapsulates the dorsal (upper) portion of the tail, and together the rods of the chevrons do the same to the ventral (lower) portion.
The tail of Deinonychus and its raptor relatives is bizarre, but it is not (as Professor Ostrom himself realized) unique. Among all known vertebrates, a similar tail anatomy has evolved in one other group. . . and now we come to why I have been allowed to spend so much time discussing dinosaurs on what is supposed to be a blog about pterosaurs.
While later and more advanced pterosaurs (like Pterodactylus) only had short, stubby tails, early pterosaurs had long ones. The caudal skeletons of these long-tailed pterosaurs (with the exceptions of the dimorphodontids and very primitive forms) are strikingly similar to that of Deinonychus. In the case of long-tailed pterosaurs, the function of the caudal rods has always seemed obvious. As flying animals, increased rigidity would have helped a tail to serve as a stabilizer or as a rudder.
It is now also possible to think a step further and consider the muscles of the tail. Let’s first try to do that in very general qualitative terms. Remember the quickly reduced neural spines, caudal ribs, and chevrons?Those all indicate that the caudal muscles of both dromaeosaurids and pterosaurs were substantially reduced.
To help consider the problem quantitatively, a technique I used was to create digital models of the tail skeleton of aVelociraptor and a Rhamphorhynchus (a pterosaur) and to sculpt the corresponding muscles over the skeletal models. The results of this modeling concur with the qualitative inference. In particular, raptors and pterosaurs were found to have very weak caudofemoral muscles (indeed, some pterosaurs may not have had caudofemoral muscles at all).
In Professor Ostrum’s description of Deinonychus, he expressed his interest in considering this striking example of convergent evolution in a later study. Regrettably, however, he never got around to it -- after all, he soon had a revolution on his hands.
However in 2010, Scott Persons, a graduate student from the University of Alberta proposed that Tyrannosaurus's speed may have been enhanced by strong tail muscles.[108]He found that theropods such as T rex had certain muscle arrangements that are different from modern day birds and mammals but with some similarities to modern reptiles.[109]
Archaeopteryx has a large number of bird-like characters, besides feathers, that are not found in any dinsoaur. These include a reversed hallux (first toe), crocodilie teeth (the same type found recently in a mutant chicken embryo), birdlike skull, movable quadrate, and the socket for the arm bone in the shoulder is pointed outward (in dinosaurs they are pointed downward because the legs of dinosaurs are tucked underneath the body) and retroverted pubis (not found in theropods but are found in ornithischian dinosaurs, which are not considered close relatives of birds). Archaeopteryx and all other birds also lack the caudofemoralis muscle, which is found in dinosaurs and crocodilians. The caudofemoralis muscle is attached to the femur or the thigh bone and to the tail, as the name suggests, so that the tail moves when the legs move. Instead, Archaeopteryx has suprapubic muscles, which connect the tail to the pubes but not to the leg bones, a quite different arrangement than the caudofemoralis muscle in theropod dinosaurs.
We do know that Archaeopteryx and Caudipteryx lack the M. caudofemoralis. That makes sense if Archaeopteryx is a flyer and Caudipteryx is a secondarily flightless bird.
Early birds like Archaeopteryx and Microraptor had long tails and hindlimb wings to help generate additional lift. A long tail with feathers can generate more lift if it is light weight than if it is heavy and muscular. Since a flyer does not need a muscular tail to balance it as it runs, losing the M. caudofemoralis is not maladaptive but adaptive in a primarily arboreal and volant animal. Even though Caudipteryx is most likely a flightless animal, it still retains the ancestral condition of the loss of the M. caudofemoralis. As Louis Dollo points out, evolution is irreversible. Caudipteryx cannot simply re-evolve the lost M. caudofemoralis. OTOH, if Caudipteryx was really a dinosaur that did not have a flying ancestor, then it makes no sense for it to lose such an adaptive feature as the M. caudofemoralis.
I will post more material on this topic in the next post.
Here is an elaboration of the basic ideas listed in the right sidebar:
The current mainstream thinking is that birds developed from coelurosaur dinosaurs. I propose that birds did not develop from any kind of dinosaur, but rather that birds developed from pterosaurs.
I propose that long-bony-tailed basal pterosaurs (eg. Rhamphorhynchidae) developed into long-bony-tailed feathered flying basal Paraves (eg. scansoriopteryx). Which then developed into short-tailed feathered birds (Pygostylia). Which then developed into modern birds (Neornithes).
Let's look at what would be involved in a transition from basal pterosaur wing to basal paraves wing. The analysis shows the following:
1. The wing membrane would need to retract toward the wing bones.
2. The pterosaur long wing finger would need to shorten.
3. The pterosaur protofeathers would need to develop into pennaceous feathers.
4. The pteroid bone would be lost.
Here is a good introduction to the basics of pterosaurs. Note the similarities to birds.
It mentions many interesting aspects, including the sternum, the pneumatic bones, the pteroid bone, the semi-circular canals, the flocculus etc. Also it includes the distinction between the primitive Rhamphorhynchoids and the advanced Pterodactyloids.
Figure 9. The first feathers were likely hollow cylinders (Stage I) with undifferentiated collars that developed from an evolutionary novel follicle collar (From: Prum and Brush 2003).
Figure 10. The next step in feather evolution (Stage II) involved the differentiation of the follicle collar into barb ridges to generated unbranched barbs (From: Prum and Brush 2003).
The long, hollow filaments on theropods posed a puzzle. If they were early feathers, how had they evolved from flat scales? Fortunately, there are theropods with threadlike feathers alive today: baby birds.All the feathers on a developing chick begin as bristles rising up from its skin; only later do they split open into more complex shapes. In the bird embryo these bristles erupt from tiny patches of skin cells called placodes. A ring of fast-growing cells on the top of the placode builds a cylindrical wall that becomes a bristle.
Reptiles have placodes too. But in a reptile embryo each placode switches on genes that cause only the skin cells on the back edge of the placode to grow, eventually forming scales. In the late 1990s Richard Prum of Yale University and Alan Brush of the University of Connecticut developed the idea that the transition from scales to feathers might have depended on a simple switch in the wiring of the genetic commands inside placodes, causing their cells to grow vertically through the skin rather than horizontally. In other words, feathers were not merely a variation on a theme: They were using the same genetic instruments to play a whole new kind of music. Once the first filaments had evolved, only minor modifications would have been required to produce increasingly elaborate feathers.
The [rhamphorhynchoid] “hair-like” structures [pycnofibres]
are also unique in being preserved in fully three
dimensionally forms as compared to two
dimensional staining or impressions.The hairs [pycnofibres] are shown to be complex multi-strand structures instead of single strands or actual hairs.The complex nature of these filaments most closely resembles natal down feathers, but apparently without having barbules.As such, they may represent the earliest known form of feathers.This implies that such
integumentary structures may have originated
independently among pterosaurs from that of birds,
or that birds and pterosaurs may share a common
ancestor which had evolved this kind of insulation
before fight had been achieved in either group.
Feathers differ significantly from hair in that their multiple strands, the barbs,emanate from a single hollow structure, called the calamus. The integumentary structures seen in Pterorhynchus [a rhamphorhynchid] bear a striking similarity to that of a natal down feather with only the notable absence of having the additional barbules branching from the barbs. This absence is significant all the more because without the barbules, the barbs emanating from a calamus represents the hypothetical “Stage II” structurespeculated as being an incipient step in the evolution of feathers (Prum, 1999). Proto-feathers have been attributed to two pterosaurs which are of similar animals (Ji and Yuan, 2002; Wang, et al., 2002). Even more so, the morphology details seen in Pterorhynchus demonstrate that the integumentary structures of pterosaurs are not like hair, but are analogous to being proto-feathers. Specifically, they resemble natal down feathers where individual filaments are seen to spread from a single follicle.
Therefore, the individual filaments are not representative of hair,
but are analogous to being the barbs of a feather.
Barbules, if present, cannot be discerned which
suggests that they either did not exist, or that the
limits of preservation have obscured them.
Nonetheless, the morphology of having several barbs stemming from a short calamus indicates that the body covering of Pterorhynchus are feather homologues. Without barbules, these structures would represent the second stage of feather
development as speculated by Prum (1999). The
feather homologues of Pterorhynchus also
demonstrate that a primary function achieved by
these plumulaceous feathers was that of thermal
insulation, and that feathers with a true rachis and
barbs aligned into well developed vanes represent
a derived condition.
The wing
membranes are thought to have been stiffened by
internal fibers, called aktinofibrils (Martill and
Unwin, 1989; Wellnhofer, 1987, 1991). The distal
end of a wing membrane is preserved in Pterorhychus which shows clear aktinofibrils that
are aligned in parallel rows. However, at right angles
to the aktinofibrils are minuscule pinnate fibers
which though imperfectly preserved, resemble the
larger integumentary structures from the body.
These tiny tufts on the wings are set close together
in rows and the diamond or V-shaped pattern caused
from their general outlines are distinctly visible
throughout. These tufts extend across the entire
width of the membrane. They are also preserved
more as three dimensional structures, whereas the
aktinofibrils are preserved two dimensionally as
stains within the matrix. Several of the tufts show
distinct filaments that emanate from a round base,
like a calamus. Therefore, the evidence suggests
that the external surface of the pterosaur wing was
not naked, but covered by tiny pinnate fibers which
would have looked much like a fine layer of velvet.
Ji and Yuan (2002) and Czerkas and Ji (2002) regarded
the fibrous integumentary structures of
pterosaurs as potentially homologous with avian
feathers, implying that feathers are basal to the
clade stemming from the last common ancestor
of pterosaurs and birds, but no evidence from the
fossil record indicates such a distribution.
Reptiles have placodes too. But in a reptile embryo each placode switches on genes that cause only the skin cells on the back edge of the placode to grow, eventually forming scales. In the late 1990s Richard Prum of Yale University and Alan Brush of the University of Connecticut developed the idea that the transition from scales to feathers might have depended on a simple switch in the wiring of the genetic commands inside placodes, causing their cells to grow vertically through the skin rather than horizontally. In other words, feathers were not merely a variation on a theme: They were using the same genetic instruments to play a whole new kind of music. Once the first filaments had evolved, only minor modifications would have been required to produce increasingly elaborate feathers.
Kellner et al
On the tenopatagium close to the body and on the tail, a third type of fibre [pycnofibre] with somewhat diffuse edges is observed (figures 3a and and44a). Type C fibres can be easily separated from other fibres by their dark-brown colour and their general lack of organization. They are distributed along the body, the tail and the tip of the actinopatagium close to the fourth wing finger phalanx (figures 1, ,22 and and44c). Sometimes clustering together, they are not found covering the external portion of the plagiopatagium and are apparently rare on the actinopatagium.
As Wang et al. (2002) pointed out, these fibres are best interpreted as structures covering the body, commonly referred to as ‘hair’ or hair-like structures (e.g. Sharov 1971; Bakhurina & Unwin 1995). This pterosaur hair, which is not homologous to the mammalian hair (a protein filament that originates deep in the dermis and grows through the epidermis), is here called pycnofibre (from the Greek word pyknos, meaning dense, bushy). The pycnofibres are further formed by smaller fibrils of unknown nature. They were possibly mostly composed of keratin-like scales, feathers and mammalian hair.
Two other Chinese specimens were reported with integumental covering, coming from the same stratum (the Daohugou Bed) as Jeholopterus. So far we have not had the opportunity to examine this material. The first one is a small unnamed anurognathid with extensive preservation of soft tissue, including fibres that have been interpreted as protofeathers (Ji & Yuan 2002). The published pictures show that the soft tissue interpreted as protofeathers is of the same nature as the pycnofibres of Jeholopterus. There is no indication of branching structures that are expected for feather precursors. Although from the phylogenetic position most authors tend to agree that pterosaurs are closely related to dinosaurs (e.g.Sereno 1991; Padian & Rayner 1993; Kellner 2004a), regarding those structures as protofeathers implies that dinosaurs and closely related taxa must originally have had similar integument covering that in more derived theropod taxa (including birds) eventually developed into feathers. There is presently no such evidence, despite much well-preserved dinosaur material (e.g. Zheng et al. 2009). If other phylogenetic positions regarding pterosaurs as more primitive within archosaurormorphs (e.g. Bennett 1996) or even closely related to protorosaurs (Peters 2000; but see Hone & Benton 2007) are accepted, the case regarding pycnofibres as protofeathers is even less appealing.
Note that there are some difficulties with the Kellner et al position.
First they think they can override the assessment given by the people who actually have the fossil by looking at a picture.
Second, lacking branching structure is not a problem, since there is no branching in Stage II feathers.
Third their assertion that "regarding those structures as protofeathers implies that dinosaurs and closely related taxa must originally have had similar integument covering that in more derived theropod taxa (including birds) eventually developed into feathers" is based on the dino to bird theory and is not relevant to the pterosaur to bird theory.
And interestingly they even find that:
"The published pictures show that the soft tissue interpreted as protofeathers is of the same nature as the pycnofibres of Jeholopterus."
So in essence they are saying that Jeholopterus (another pterosaur) could also have stage II feathers.
A specimen of the horned dinosaur Psittacosaurus from the early Cretaceous of China is described in which the integument is extraordinarily well-preserved. Most unusual is the presence of long bristle-like structures on the proximal part of tail. We interpret these structures as cylindrical and possibly tubular epidermal structures that were anchored deeply in the skin. They might have been used in display behavior and especially if one assumes that they were colored, they may have had a signal function. At present, there is no convincing evidence which shows these structures to be homologous to the structurally different integumentary filaments of theropod dinosaurs. Independent of their homology, however, the discovery of bristle-like structures in Psittacosaurus is of great evolutionary significance since it shows that the integumentary covering of at least some dinosaurs was much more complex than has ever been previously imagined.
Coelurosaurs were not the only dinosaurs to sport unique body coverings, though. In 2002, palaeontologist Gerald Mayr and colleagues reported on long, bristle-like structures growing out of the tail of Psittacosaurus. This dinosaur was not a theropod, but instead was one of the early ceratopsians ("horned dinosaurs") which were part of a separate radiation of dinosaurs known as ornithischians. Psittacosaurus was about as distantly related to feathered theropods as it was possible to be while still remaining a dinosaur, yet it had tail bristles which were similar in structure to the wispy fuzz of coelurosaurs such as Sinosauropteryx.
It was complemented last year by the announcement of Tianyulong, a different sort of ornithischian that also had a row of bristles going down its back. Since these dinosaurs had bristles structurally similar to the protofeathers of some theropods, it either indicates that such body coverings evolved at least twice within each part of the dinosaurian split, or, as strange as it might seem, such body coverings were a common trait among dinosaurs that was lost in some lineages and modified in others.
The discovery that structurally unique "filamentous integumentary appendages" are associated with several different non-avian dinosaurs continues to stimulate the development of models to explain the evolutionary origin of feathers. Taking the phylogenetic relationships of the non-avian dinosaurs into consideration, some models propose that the "filamentous integumentary appendages" represent intermediate stages in the sequential evolution of feathers. Here we present observations on a unique integumentary structure, the bristle of the wild turkey beard, and suggest that this non-feather appendage provides another explanation for some of the "filamentous integumentary appendages." Unlike feathers, beard bristles grow continuously from finger-like outgrows of the integument lacking follicles. We find that these beard bristles, which show simple branching, are hollow, distally, and express the feather-type beta keratins. The significance of these observations to explanations for the evolution of archosaurian integumentary appendages is discussed.
In addition, there is current evidence that favors the hypothesis that the initial function of feathers was related to insulation (Chen et al.1998), but no compelling evidence suggests that coelurosaurians were distinctly different from more primitive non-insulated theropods physiologically or ecologically. One possible piece of evidence suggesting a physiological change is miniaturization at the base of the Coelurosauria. It appears that miniaturization characterizes basal coelurosaurians. If this holds true, the development of substantial feathered coverings is likely to be solicited to insulate the small bodies of basal coelurosaurians.
Over the course of the last two decades, the understanding of the early evolution of feathers in nonavian dinosaurs has been revolutionized. It is now recognized that early feathers had a simple form comparable in general structure to the hairs of mammals. Insight into the prevalence of simple feathers throughout the dinosaur family tree has gradually arisen in tandem with the growing evidence for endothermic dinosaur metabolisms. This has led to the generally accepted opinion that the early feather coats of dinosaurs functioned as thermo insulation. However, thermo insulation is often erroneously stated to be a likely functional explanation for the origin of feathers. The problem with this explanation is that, like mammalian hair, simple feathers could serve as insulation only when present in sufficiently high concentrations. The theory therefore necessitates the origination of feathers en masse. We advocate for a novel origin theory of feathers as bristles. Bristles are facial feathers common among modern birds that function like mammalian tactile whiskers, and are frequently simple and hair-like in form. Bristles serve their role in low concentrations, and therefore offer a feasible first stage in feather evolution.
Sawyer et al. (2003b:27) observed that in turkeys (Meleagris), beard bristles, which are structurally similar
to the fibrous structures identified as feathers in
coelurosaurs, display “simple branching, are hollow,
distally, and express the feather-type β keratins,”
even though they are not feathers.
Sawyer et al. (2003b:30) argued that
the present study raises the possibility that [the]
“filamentous integumentary appendages” [of
coelurosaurs] may more closely resemble the bristles of the wild turkey beard, and may not
depict intermediate stages in the evolution of
feathers
Concavenator had structures resembling quill knobs on its forearm, a feature known only in birds and other feathered theropods, such as Velociraptor. Quill knobs are created by ligaments which attach to the feather follicle, and since scales do not form from follicles, the authors ruled out the possibility that they could indicate the presence of long display scales on the arm. Instead, the knobs probably anchored simple, hollow, quill-like structures. Such structures are known both in coelurosaurs such as Dilong and in some ornithischians like Tianyulong and Psittacosaurus. If the ornithischian quills are homologous with bird feathers, their presence in an allosauroid like Concavenator would be expected.[3] However, if ornithischian quills are not related to feathers, the presence of these structures in Concavenator would show that feathers had begun to appear in earlier, more primitive forms than coelurosaurs.
BOTTOM LINE There is no evidence that the true dinosaur (eg. tyrannosaurs, compsognathus) filaments, were anything other than simply bristles. There is no evidence that those bristles have any relationship to feathers in the long-bony-tailed feathered flying creatures such as the basalmost paraves.
Believe it or not, we now know that some ornithischians had feathers as well. Two incredible fossil discoveries prove this. First, there is a well-preserved specimen of the small horned dinosaur Psittacosaurus (an early cousin of Triceratops) with a series of hair-like bristles running down the tail. Second, there is a beautiful specimen of the small, fast-running Tianyulong (a heterodontosaurid ornithischian) with similar bristles covering the neck, back and tail.
These bristles aren't exactly the same as the feathers of living birds. In fact, they look quite different. They are more like the quills of a porcupine: long, perhaps hollow, structures that stand up straight, forming something that would have looked like a Mohawk. They did not have vanes, or barbs, or barbules, or the other components of the characteristic ‘quill pen’ feathers of living birds. They were not used for flight, but were more likely used for display: to intimidate rivals, impress mates, or differentiate one species from another.
But although these bristles are different from the feathers of living birds, scientists are confident that they are essentially the same type of structure. In other words, they are comprised of the same material and are controlled by the same basic genes. In more technical terminology, they are 'homologous' to feathers.
The bristle-like structures of ornithischians are probably primitive versions of feathers. The earliest dinosaurs probably evolved simple feathers like this for display or to regulate body temperature, and later they were modified into more elaborate structures that were useful for an entirely new purpose: flight.
Figure 13. Hypothesized stages I–III of feather evolution. Stage I of this model assumes an unbranched, hollow filament, which developed from a cylindrical invagination of the epidermis around a papilla. In stage II, a tuft was formed by fusion of several filaments at their bases. Stage III represents the formation of a central rachis and development of serially fused barbs (III A) — to which, at a slightly later stage (III B), secondary barbs (barbules) were added. The two other stages, IV (bipinnate feathers with elaborate barbules and a closed vane) and V (the asymmetrical flight feathers of modern flying birds), are not shown (From: Sues 2001).
This developmental model provides functionally neutral criteria to evaluate the homology between
avian feathers and other fossil integumental structures.
The model predicts that feathers with single
unbranched keratin structures (stage I) or many
unbranched keratin filaments (stage II) preceded
the origin of the branched or pennaceous feather. From my direct observations of the two specimens of Sinosauropteryx (Chen et al., ’98), the integumentary structures appear to consist of unbranched filaments about 20 mm long. Reports of
the filamentous structures of Beipiaosaurus [a therizinosaur] indicate that they are 50–70 mm long and possibly
branched. It is uncertain whether the reported
branches in both species are bifurcations of single
structures or the merely the appearance of branching
created by closely adjacent, separate unbranched
filaments within the specimens. However,
the length and position of these structures in
Beipiaosaurus demonstrate convincingly that these
were not internal integumental structures.
All described feathers in nonavian theropods are composite structures formed by multiple filaments. They closely resemble relatively advanced stages predicted by developmental models of the origin of feathers, but not the earliest stage. Here, we report a feather type in two specimens of the basal therizinosaurBeipiaosaurus, in which each individual feather is represented by a single broad filament. This morphotype is congruent with the stage I morphology predicted by developmental models, and all major predicted morphotypes have now been documented in the fossil record. This congruence between the full range of paleontological and developmental data strongly supports the hypothesis that feathers evolved and initially diversified in nonavian theropods before the origin of birds and the evolution of flight.
Chen et al., (1998) described Sinosauropteryx as a compsognathid dinosaur. The structures are essentially filaments and have “no structures showing the fundamental morphological features of modern bird feathers, but they could be previously unidentified protofeather which are not as complex as either down feathers or even the hair-like feathers of secondarily flightless birds.” (Chen, et al., 1998). The structures are clearly epidermal in origin, unbranched, probably tubular, and may cover most of the body.
v3.jpg)
