Discovery of a furcula in tyrannosaurid theropods and its functional and phylogenetic implications

PJM: Afdeling for Historisk Geologi og Palæontologi,Geologisk Institut, Øster Voldgade 10, 1350 København K, DENMARK
PJC: Royal Tyrrell Museum of Palaeontology, P.O.Box 7500, Drumheller, Alberta, T0J 0Y0, CANADA

Since the end of the last century, it was commonly believed that dinosaurs had lost their clavicles. Heilmann (1926) used this absence to argue that birds could not be descended from theropod dinosaurs, in spite of a long list of common features that seem to unite these groups. Since then, it has become apparent that clavicles may be present in at least some theropods (e.g. Camp, 1936; Barsbold, 1981; Russell and Dong, 1993) and more recent debate has centered on whether certain bilaterally symmetric elements found in some taxa are fused gastralia, or may actually represent furculae (Thulborn, 1984; Bryant and Russell 1993).


Figure 1. Photographs of one of the two Albertosaurus furculae in (A) anterior and (B) posterior views. Abbreviations: ar = apical ridge (possibly homologous to avian hypocleideum); ep = epicleideum. Photographs taken by Mr. Ole Bang Berthelesen.

Recent recoveries of furculae in two specimens of Albertosaurus from the Late Cretaceous of Alberta, Canada, reveal that these elements are distinguishable from fused anterior gastralia by their relatively smaller size, more slender proportions, sigmoid curvature, and spatulate epicleidea (fig.1). Both specimens bear a small ridge on the ventral, midline apex (fig.1). This corresponds topologically to the hypocleideum of ornithurine birds, and may indeed be homologous to that structure. The small span of the furcula in both tyrannosaurids and other non-avian theropods imposes restrictions on the width of the ribcage and orientation of the shoulder girdle. Correct articulation of the shoulder girdle elements requires that the coracoids meet one another at the midline of the anterior end of the ribcage. The individual mobility of each scapulocoracoid would have been limited by its articulation with the furcula anterodorsally and the sternum posteroventrally. In this configuration, the glenoid would face more ventrally than posteriorly and thus increase the anterior reach of the front limbs. The furcula apparently served as an anchor for an enlarged sternobrachialis head of the M. pectoralis, the principle muscle involved in humeral extension. Olson and Feduccia (1979) recognized this feature in Archaeopteryx, but its presence in a number of tetanurine taxa suggests that it represents an exaptation in the origin of avian flight.

Bryant and Russell (1993) reviewed the knowledge about presence of clavicles in dinosaurs, and proposed a test for determining whether the avian furcula is in fact homologous to the clavicles of tetrapods based on several levels of acceptance of negative evidence (clavicles absent). Their results showed that homology of the avian furcula with tetrapod clavicles was doubtful, for categories where absence of clavicles in fossil specimens was readily accepted as evidence for evolutionary loss. A rerun of their test incorporating recent hypothesies of theropod phylogeny (Holtz, 1994; Sereno et al. 1994) as well as new information on theropod clavicles, shows more consistently that these bones were primitively present in Theropoda.


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