The pectoral girdle of StW 573 (‘Little Foot’) and its implications for shoulder evolution in the Hominina

The ca. 3.67 Ma adult skeleton known as ‘Little Foot’ (StW 573), recovered from Sterkfontein Member 2

breccia in the Silberberg Grotto, is remarkable for its morphology and completeness. Preservation of

clavicles and scapulae, including essentially complete right-side elements, offers opportunities to assess

morphological and functional aspects of a nearly complete Australopithecus pectoral girdle. Here we

describe the StW 573 pectoral girdle and offer quantitative comparisons to those of extant hominoids

and selected homininans. The StW 573 pectoral girdle combines features intermediate between those of

humans and other apes: a long and curved clavicle, suggesting a relatively dorsally positioned scapula;

an enlarged and uniquely proportioned supraspinous fossa; a relatively cranially oriented glenoid fossa;

and ape-like reinforcement of the axillary margin by a stout ventral bar. StW 573 scapulae are as follows:

smaller than those of some homininans (i.e., KSD-VP-1/1 and KNM-ER 47000A), larger than others (i.e.,

A.L. 288-1, Sts 7, and MH2), and most similar in size to another australopith from Sterkfontein, StW 431.

Moreover, StW 573 and StW 431 exhibit similar structural features along their axillary margins and

inferior angles. As the StW573 pectoral girdle (e.g., scapular configuration) has a greater affinity to that of

apesdGorilla in particulardrather than modern humans, we suggest that the StW 573 morphological

pattern appears to reflect adaptations to arboreal behaviors, especially those with the hand positioned

above the head, more than human-like manipulatory capabilities. When compared with less complete

pectoral girdles from middle/late Miocene apes and that of the penecontemporaneous KSD-VP-1/1

(Australopithecus afarensis), and mindful of consensus views on the adaptiveness of arboreal positional

behaviors soliciting abducted glenohumeral joints in early Pliocene taxa, we propose that the StW 573

pectoral girdle is a reasonable model for hypothesizing pectoral girdle configuration of the crown

hominin last common ancestor.