Sunday, May 9, 2010

Keeled breastbone and flight muscles

Basal Paraves used the same flight stroke and muscles as Pterosaurs which is sufficient for flapping flight. It does not require a deeply keeled sternum.
The following references correctly show that neither pterosaurs nor basal paraves used their m. supracoracoideus as a pulley. But that is not necessary for flapping flight.

A keel in bird anatomy is an extension of the sternum (breastbone) which runs axially along the midline of the sternum and extends outward, perpendicular to the plane of the ribs. The keel provides an anchor to which a bird's wing muscles attach, thereby providing adequate leverage for flight. Keels do not exist on all birds; in particular, some flightless birds lack a keel structure.


Pterosaur specimen with clearly visible breast-bone keel.
Pterosaurs flew using their forelimbs, which are modified by hypertrophy of the fourth finger into a long spar supporting a membrane of tissue which was the flight surface. The wings were probably flapped in a manner grossly similar to that seen in birds (a group which displays many different flapping strategies among and within different species and different situations). One of the chief arguments against active pterosaur flight has been their relatively shallow sternum keel, which is the anchor point for the pectoralis muscles, the main flapping muscle. However, pterosaurs display other skeletal features that may have made this less problematic than a direct comparison to birds may indicate.
At one stage, it was thought that the flight muscles of
pterosaurs were very birdlike, with the arm lifted by
a muscle, m. supracoracoideus, anchoring on the sternum
rather than the shoulders. In birds, this muscle
arcs over the glenoid to attach on the dorsal surface
of the humerus, elevating the wing with a pulley-like
system (e.g., Kripp 1943; Padian 1983a; Wellnhofer
1991a). Detailed reconstruction of the proximal arm
musculature of pterosaurs shows that this is not
the case, however, and that the [pterosaur] arm was more likely
lifted by large muscles anchored on the scapula and
back, and lowered by those attached to the sternum
and coracoid (fig. 5.8; Bennett 2003a). Unlike [modern] birds,
where two vastly expanded muscles are mainly used
to power flight, it appears that pterosaurs used several
muscle groups to form their flapping strokes.
The lack of a morphologically derived SC in Late
Jurassic and Early Cretaceous birds precluded a high velocity
recovery stroke which undoubtedly limited
powered flight in these forms. Subsequent evolution of the
derived SC capable of imparting a large rotational force to
the humerus about its longitudinal axis was an important
step in the evolution of the wing upstroke and in the ability
to supinate (circumflex) the manus in early upstroke, a
movement fundamental to reducing air resistance during
the recovery stroke.
The highly derived morphology of the SC,
a characteristic of modern birds capable of powered flight, was
not present in Archaeopteryx (Ostrom, 1976a,b; Wellnhofer,
1988, 1993), nor is there firm evidence for its presence in
recently described Mesozoic species (Chiappe, 1995; Sanz et
al. 1996).
The dorsal elevators, principally the deltoideus major, can effect the recovery stroke by themselves, as they did in Archaeopteryx. The German anatomist Maxheinz J. Sy proved this when he cut the tendons of the supracoracoideus in living crows and pigeons (1936). Sy found that pigeons were capable of normal, sustained flight; the only capacity they lost was the ability to take off from level ground.
Furthermore, the supracoracoideus muscle,
and hence an ossified sternum, is not necessary to effect the
recovery stroke of the wing. Thus the main evidence for
Archaeopteryx having been a terrestrial, cursorial predator is
invalidated. There is nothing in the structure of the pectoral girdle of Archaeopteryx that would preclude its having been a powered flier.
The presence of a sternal keel is only genuinely an issue if the flight muscles are arranged as in modern birds; such system is the bizarre “pulley system”, seen above. This muscle arrangement, which is characterised by the fact that the muscles responsible for pulling the wing down – thesupracoracoideus complexes – attach to the sternum, is only known from the clade of avialans known as Ornithothoraces, which includes modern birds and several extinct clades like the enantiornithe birds. The other flying tetrapods, the pterosaurs and the bats, don’t have this system; instead, the muscles that pull the wing up are attached to the vertebrae, like in most tetrapods. Instead of relying on the supracoracoideus complexes, they rely on the deltoideus complexes.
Thus, deinonychosaurs did not require large keels; many modern bats lack them too (pterosaurs do have sternal keels, although they’re more like deeper regular sternums than true keels), as they’re not required in this style of muscle arrangement
The basal deinonychosaur Anchiornis might offer a possible explanation. It had symmetrical feathers, but they were arranged in an unique away; in species with asymmetrical feathers, the most distally attached wing feathers are the longest ones. In Anchiornis, the longest are anchored near the wrist, making the center of the wing the broadest area. This is not an unusual profile among flightless maniraptors – oviraptors like Caudipteryx have this sort of arrangement as well. Anchiornis, however, differs in that the feathers at the front (as in, anchored more distally) of the longest feather decrease rapidly in size as they are closer to the end of the supporting digit; this results in a rounded, yet slightly pointy wing shape.
It is possible that this arrangement could had been an early adaptation to the demands of powered flight, before true asymmetrical feathers evolved. If so, it is possible that Anchiornis did engage in powered flight, or even a method of escape akin to rudimentary WAIR. So far, no tests have been conducted to examine the aerodynamic capacities of it’s wings.
In modern birds (Neornithes), the wing is composed of a layer of long, asymmetrical flight feathers overlain by short covert feathers [ 1–3 ]. It has generally been assumed that wing feathers in the Jurassic bird Archaeopteryx [ 4–9 ] and Cretaceous feathered dinosaurs [ 10, 11 ] had the same arrangement. Here, we redescribe the wings of the archaic bird Archaeopteryx lithographica [ 3–5 ] and the dinosaur Anchiornis huxleyi [ 12, 13 ] and show that their wings differ from those of Neornithes in being composed of multiple layers of feathers. In Archaeopteryx, primaries are overlapped by long dorsal and ventral coverts. Anchiornis has a similar configuration but is more primitive in having short, slender, symmetrical remiges. Archaeopteryx and Anchiornis therefore appear to represent early experiments in the evolution of the wing. This primitive configuration has important functional implications: although the slender feather shafts of Archaeopteryx [ 14 ] and Anchiornis [ 12 ] make individual feathers weak, layering of the wing feathers may have produced a strong airfoil. Furthermore, the layered arrangement may have prevented the feathers from forming a slotted tip or separating to reduce drag on the upstroke. The wings of early birds therefore may have lacked the range of functions seen in Neornithes, limiting their flight ability.

Recent research:
The geometry of feather barbs (barb length and barb angle) determines feather vane asymmetry and vane rigidity, which are both critical to a feather's aerodynamic performance. Here, we describe the relationship between barb geometry and aerodynamic function across the evolutionary history of asymmetrical flight feathers, from Mesozoic taxa outside of modern avian diversity (Microraptor,Archaeopteryx, Sapeornis, Confuciusornis and the enantiornithine Eopengornis) to an extensive sample of modern birds. Contrary to previous assumptions, we find that barb angle is not related to vane-width asymmetry; instead barb angle varies with vane function, whereas barb length variation determines vane asymmetry. We demonstrate that barb geometry significantly differs among functionally distinct portions of flight feather vanes, and that cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles. This cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds, revealing a fundamental aerodynamic adaptation that has persisted from the Late Jurassic. However, in Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds. In both modern birds and enantiornithines, trailing vanes have larger barb angles than in comparatively stemward taxa like Archaeopteryx, which exhibit small trailing vane barb angles. This discovery reveals a previously unrecognized evolutionary transition in flight feather morphology, which has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor. Our findings suggest that the fully modern avian flight feather, and possibly a modern capacity for powered flight, evolved crownward of Confuciusornis, long after the origin of asymmetrical flight feathers, and much later than previously recognized.
Based on what is known about molecular mechanisms for morphogenesis and the possible selective advantages, the parallel shifts to midline ossification that took place in derived enantiornithines and living neognathous birds appear to have been related to the development of a large ventral keel, which is only present in ornithuromorphs and enantiornithines.
It has always been difficult to understand how birds evolved from dinosaurs because of the strange combination of features observed in taxa inferred to be situated near this great evolutionary transition. For example, the sternum, also called the 'breastbone', is a large bone to which the lower ends of the bird's ribs are attached. It is intrinsic to modern avian flight, providing the attachment surface for the two largest muscles in the body, the primary fight muscles the pectoralis and supracoracoideus. This bone is present in many dinosaurs inferred to be closely related to birds (e.g. Microraptor, Epidexipteryx) and most basal birds (e.g. Confuciusornis, enantiornithines) but strangely is absent in troodontid dinosaurs and the early birds Archaeopteryx and Sapeornis. 
In a paper published online before print September 08 in the Proceedings of the National Academy of Sciences, scientists from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (IVPP), and Shandong Tianyu Museum of Nature (STM) observed more than 200 specimens of Anchiornis, the earliest known feathered dinosaur, and nearly 100 specimens of Sapeornis, one of the basalmost birds, and recognized no sternal ossifications.
On the absence of sternal elements in Anchiornis (Paraves) and Sapeornis (Aves) and the complex early evolution of the avian sternum
Comment on the absence of ossified sternal elements in basal paravian dinosaurs
Within Scansoriopterygidae, sternal elements are reported but poorly preserved only in Epidexipteryx51.
A monophyletic Scansoriopterygidae was recovered by Godefroit et al. (2013); the authors found scansoriopterygids to be basalmost members of Paraves
We recognize six ossifications that together form the sternum, three of which were previously unknown. Here we show that although basal living birds apparently have retained the dinosaurian condition in which the sternum develops from a bilateral pair of ossifications (present in paravian dinosaurs and basal birds)
First, it is argued that most theropod dinosaurs lack ossified sternal elements; this may be true of basal members of the clade and indeed these elements are rare. However, recent work on derived maniraptoran theropods strongly suggests that these elements are present in most taxa but ossify late in skeletal ontogeny and even fuse in mature specimens ().
An acid- and transfer-prepared, juvenile Rhamphorhynchus muensteri, despite some fragmentation, is in an excellent state of three-dimensional preservation, exposing exquisite anatomical details hitherto unknown in other pterosaurs. Here we describe the axial pneumatizations of the cervical and anterior dorsal vertebrae and the sternum. The interior of the cervical centra is subdivided into a pair of large camerae, presumably by air sacs entering by large pleurocoels in the sides of the centra. This so-called ‘camerate’ type of pneumatization is hitherto unknown in pterosaurs. Another excavation enters from the ventral side into the base of the neural arch and stretches between the pre- and postzygapophyses. This type of cavity also penetrates from the ventral side into the base of the first few transverse processes of the dorsal vertebrae, although these lack central pleurocoels. The cristospine also has a complex pneumatic foramen.
Forfexopterus would have been large for an archaeopterodactyloid [pterodactyloid pterosaur]. The skull was low and long, measuring 510 millimetres (20 in) in length; it appears that no crest was present on either jaw. Forfexopterus is unique among archaeopterodactyloids in that the first phalanx bone of the wing finger was shorter than the second but longer than the third. In addition, it exhibits a unique combination of traits: it has in total approximately 120 long, slender teeth, which span from the tip of the jaw to 1/3 along the length of the skull (stopping before the nasoantorbital fenestra); the crest on the sternum, known as the "cristospine", is long; the location where the coracoid attaches to the sternum is further forward on the right side than the left; and the coracoid bears a flange.[1]
[Dawndraco] Sternum is large and shows an elongated and low cristospine.
Dawndraco is a genus of pteranodontid pterosaur from the Late Cretaceous of North America.

The sternum of Campylognathoides was a rather large rectangular plate of bone with a short forward-facing crest called a cristospina.[2]
The cristospine is unique to pterosaurs and is analogous to the manubrium of the bat.
The [Bellubrunnus rhamphorhynchid pterosaursternum has a large cristospine that extends anteriorly and is nearly as long as the body of the sternum itself (Figure 10). The elongate cristospine is seen in at least one other small specimen of Rhamphorhynchus – CM 11433
The [Darwinopterus] sternum is almost complete, lacking the cristospine
Darwinopterus, [Wukongopteridae] like its closest relatives, is characterized by its unique combination of basal and derived pterosaurian features. While it had a long tail and other features characteristic of the 'rhamphorhynchoids', it also had distinct pterodactyloid features, such as long vertebrae in the neck and a single skull opening in front of the eyes, the nasoantorbital fenestra (in most 'rhamphorhynchoids', the antorbital fenestra and the nasal opening are separate).[6]

Andres and colleagues assigned both the wukongopterids and Pterorhynchus to the group Darwinoptera.[5]
Plieninger, 1901
Included groups
Darwinopterus (meaning "Darwin's wing") is a genus of pterosaur, discovered in China and named after biologist Charles Darwin. Between 30 and 40 fossil specimens have been identified,[1] all collected from the Tiaojishan Formation, which dates to the middle Jurassic period, 161-160.5 Ma ago.[2] The type species, D. modularis, was described in February 2010.[3] D. modularis was the first known pterosaur to display features of both long-tailed ('rhamphorhynchoid') and short-tailed (pterodactyloid) pterosaurs, and was described as a transitional fossil between the two groups.[4] Two additional species, D. linglongtaensis and D. robustodens, were described from the same fossil beds in December 2010 and June 2011, respectively.[5][6]
Darwinopterus, like its closest relatives, is characterized by its unique combination of basal and derived pterosaurian features. While it had a long tail and other features characteristic of the 'rhamphorhynchoids', it also had distinct pterodactyloid features, such as long vertebrae in the neck and a single skull opening in front of the eyes, the
nasoantorbital fenestra (in most 'rhamphorhynchoids', the antorbital fenestra and the nasal opening are separate).[5]
The scansoriopterygids would have lived alongside .. the rhamphorhynchoid pterosaurs Jeholopterus and Pterorhynchus [Darwinoptera] 

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