One of the most striking observations of the skeletal anatomy of tyrannosaurids and abelisaurids is the proportionate size of the scapulocoracoid in relation to the rest of the forelimb. It looks almost absurd and, for me, the fact that there is virtually no contact between the scapula itself and any other articulating bone is always of interest. Indeed the scapula appears to be almost suspended in mid-body virtually independent of the skeletal structure except for its union with the coracoid which, in turn, is an essential component of the pectoral girdle.
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.
This variation shows itself in as much that more derived theropods, such as dromaeosaurids, display the scapula in a much more horizontal position than, by way of example, coelophysoids. This evolutionary trend of rotating the scapula is almost certainly due to different taxa utilising their forelimbs for different functions which in turn dictated how the attached levator, subscapularis and serratus muscles held the blade in place.
The scapula itself is usually, in non-avian theropods, described as strap-like and is generally, but not always so apparent in some taxa, anteroventrally expanded to form the contact with the coracoid whilst the posterodorsal terminus tends to be narrower. The scapula increases in thickness as it expands to form part of the glenoid cavity and also strengthens the union with the coracoid. The upright stance of theropods meant that the pectoral girdle itself was narrow and was positioned anteriorly of the ribs. The girdle was held fast by the furcula and this acted as reinforcement for the scapula blades.
Just why the scapula remained so large in theropods such as tyrannosaurids and abelisaurids is unclear. The bone itself can actually be described as mobile because of its virtual isolation from the rest of the appendicular skeleton although it is the anchor point for many muscle attachments including those that held it in place attaching from the upper extremity as well as those that provided the “motion” for the forelimbs.
It appears that the scapulas main function was protection for the thoracic cage as well as increasing stability and, of course, providing the attachment for the coracoid, clavicle and formation of the shoulder joint. Still have to say though – it does look weird.
|From Burch & Carrano 2012|
Sara H. Burch & Matthew T. Carrano (2012): An articulated pectoral girdle and forelimb of the abelisaurid theropod Majungasaurus crenatissimus from the Late Cretaceous of Madagascar, Journal of Vertebrate Paleontology, 32:1