Forelimb anatomy in basal archosaurs: functional implications

Abstract

Basal archosaurs show profound apendicular transformations when compared to their closest archosauriform outgroup (Euparkeria or proterochampsids). Although, most studies on archosaurian apendicular anatomy were focused on hindlimb morphology. The objective of this contribution was to study morphological changes in the forelimb of basal archosaurs and to assess their phylogenetical and functional implications. The pectoral girdle shows an anteriorly expanded acromion process on the scapula, expanding the area of origin of part of the glenohumeral stabilizator musculature of the scapula (Mm. deltoideus clavicularis and coracobrachialis brevis dorsalis), and the presence of a postglenoid process on the coracoid, changing the orientation of the retractor musculature (M. costocoracoideus) to the sagittal plane. In the forelimb, an individualized and distally displaced medial tuberosity on the humerus, a condition present in the Crurotarsi but shared with the proterochampsids, and an ossified olecranon process on the ulna can be also observed. These characters can be interpreted as potential adaptations for erect forelimbs, as the olecranon is the insertion of the main forearm extensor (Mm. triceps brachii) and the scapulocoracoid-humerus articulation is further stabilized, allowing the limbs to move mainly on a parasagital plane. These are one of the first results that recover valuable postural information from the forelimb in this important group of diapsids. Therefore, most of these characters are relevant for the phylogeny of this group as most of them can be optimized as unambiguous synapomorphies of Archosauria.