Abstract
First ribs – the first or most superior ribs in the thorax – are rare in the hominin fossil record, and when found, have the potential to provide information regarding the upper thorax shape of extinct hominins. Here, we describe a partial first rib from Member 4 of the Sterkfontein Caves, South Africa. The rib shaft is broken away, so only the head and neck are preserved. The rib is small, falling closest to small-bodied Australopithecus first ribs (AL 288-1 and MH1). Given that it was recovered near the StW 318 femur excavation, which also represents a small individual, we suggest that the two may be associated. Three-dimensional geometric morphometric analyses were used to quantify the rib fragment morphology and compare it to extant hominoid and other fossil hominin ribs. While only the proximal end is preserved, our analyses show that South African Australopithecus share derived features of the proximal first rib more closely resembling A. afarensis and later hominins than great apes.
Highlights
While only the proximal end is preserved, our analyses show that South African Australopithecus share derived features of the proximal first rib more closely resembling A. afarensis and later hominins than great apes
Ribs are rare in the human fossil record because of their delicate structure; this scarcity is pronounced in the early hominin fossil record
In spite of the scarcity of these remains, it is important to note that, among ribs, the first rib is generally the best preserved in the fossil record as a result of its unique morphology relative to the rest of the ribs in the thorax.[6,7]
Summary
Ribs are rare in the human fossil record because of their delicate structure; this scarcity is pronounced in the early hominin fossil record. First ribs are known from Australopithecus afarensis (AL 288-1ax)[11], Homo erectus (KNMWT 15000 AG and AY&AZ)[12], A. sediba (MH1, UW88-148; MH2, UW88-198 and UW88-187)[8] and H. naledi (UW101-83)[13]. A. afarensis AL 288-1 and A. sediba (MH1 and MH2) were hypothesised as presenting a narrow upper thorax, as extant apes do, and this thoracic shape is probably related to suspensory locomotor behaviour.[8,11,14,15] This narrow upper thorax has been observed in the recently published small-bodied species H. naledi.[13] In contrast, other early hominin specimens, such as H. erectus (KNM-WT 15000) and the large-bodied A. afarensis (KSD-VP-1/1), have been proposed to have modern human-like, expanded upper thoraces.[12,16]
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