Wednesday, June 9, 2010

Scapula structure

The Scapulocoracoid is the unit of the pectoral girdle that contains the coracoid and scapula. The coracoid itself is a beak-shaped bone that is commonly found in most vertebrates with a few exceptions. The scapula is commonly known as the shoulder blade. The humerus is linked to the body via the scapula, the clavicle is connected to the sternum via the scapula as well.
In birds (and generally theropods and related animals), the entire unit is rigid and called scapulocoracoid. This plays a major role in bird flight. In dinosaurs the main bones of the pectoral girdle were the scapula (shoulder blade) and the coracoid, both of which directly articulated with the clavicle.[2]
There were two major bones in the [pterosaur] chest that anchored most of the flight muscles. The first was the sternum, which is the breastbone in birds and the front plate of our chest. The second was the pair of scapulocoracoids, which were single bones made from the fusion of the shoulder blade (scapula) and coracoid on each side (we do not have a coracoid bone, but it was a broad plate that formed part of the shoulder in pterosaurs).

Some pterosaur bones are quite unusual. This scapulo-coracoid is photographed from both sides. The glenoid cavity of the shoulder joint can be seen, where the humerus articulates the wing to the body.

Strap-like scapula
The pectoral girdle of basal pterosaurs consisted of a long strap-like scapula.
Bats, birds and pterosaurs are united by the convergent ability to flap their forelimbs/wings. In this way they generate thrust for true flying. Gliders don’t flap and they don’t have elongated scapula anchors.

Scapula Orientation
Scapula orientation in theropod dinosaurs is quite interesting and it is worth looking, to begin with, at what orientation is displayed in primitive reptiles. The scapula is generally held at an angle of 90 degrees to the horizontal line held by the backbone in other words it was held in a perpendicular fashion. At the other extreme, extant birds rotated the scapula so that it lies parallel to backbone – a position also evolved by the pterosaurs.
Theropods, and non-avian dinosaurs in general (but not bird-like theropods), evolved a condition that can be described as something in between – an intermediate position if you will. The scapula is held in an oblique position laterally to the ribcage but actually determining the exact position is somewhat problematic. There are not that many fully articulated specimens that can be referred to and there is always the spectre of both taxanomic and taphonomic variation to throw yet another spanner into the works. 
In this way, the scapulae of unenlagiids lie close to the vertebral column, dorsal to the ribcage, with the flat costal surface of the scapular blade facing ventrally, a condition seen in microraptorans (i.e. Microraptor), basal avialans (e.g. Archaeopteryx, Rahonavis), and ornithothoracine birds (Senter 2006), in which the shoulder socket sits high on the back, and the margins of the glenoid are smooth, thus this surface becomes shalower and consequently more continuous with the rest of the lateral surface of scapula
(Burnham 2008). In sum, the lateral orientation of the scapular glenoid in unenlagiids
(and probably also in other basal averaptorans), together with the absence
of acute ridges delimitating the glenoid cavity, suggest that the humerus in these
taxa was able to be elevated close to the vertical plane, 
as proposed by Novas and Puerta (1997) (Figs. 5.1, 5.2).
It is important to mention that scansoriopterygids retained a caudoventrally oriented glenoid, a subrectangular coracoid with reduced biceps tubercle, and a distally fan-shaped scapular blade, all representing plesiomorphic character states in respect to paravians.



No comments:

Post a Comment