Monday, January 27, 2014

Pycnofibres to Feathers

The evidence indicates that pterosaur pycnofibres were Stage II feathers.

Background - How a Feather Develops
Early stages in the development of a feather:

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.
Bird 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
 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” structure speculated 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 1971Bakhurina & 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 1991Padian & Rayner 1993Kellner 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.
VERY interesting material on page 49 and 51. 


On the other hand, dinosaurs only had bristles:
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.
  • A male turkey grows a cluster of long, hairlike feathers from the center of its chest. This cluster is known as the turkey's beard.
  • On adult males, these beards average about 9 inches long.
  • 10 to 20 percent of hens also grow beards.
  • The longest beard on record is more than 18 inches long.
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
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.
Pegomastax africanus
A bizarre dinosaur had vampire-like fangs, a parrot beak and porcupine bristles, researchers say.

More links:
The beard of the wild turkey
Sinosauropteryx coloured "feathers" debunked

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.

For reference:

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.

Colored porcupine bristles:

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