# Bipedalism or bipedalisms: The os coxae of StW 573

## TL;DR

This study examines the pelvis of a 3.67 million-year-old Australopithecus prometheus fossil to explore the possibility of multiple species of Australopithecus with different forms of bipedal locomotion.

## Contribution

The study presents new evidence supporting the existence of two Australopithecus species at Sterkfontein with distinct pelvic morphologies and forms of bipedality.

## Key findings

- StW 573 and StW 431 show human-like iliac crest breadth and ischium length, suggesting a human-like bipedal pelvis.
- StW 573 and StW 431 are consistently classified with modern humans in discriminant function analysis.
- Other australopiths like Sts 14 and KNM-WT 15000 are more peripheral to modern humans in the analysis.

## Abstract

There has been a long debate about the possibility of multiple contemporaneous species of Australopithecus in both eastern and southern Africa, potentially exhibiting different forms of bipedal locomotion. Here, we describe the previously unreported morphology of the os coxae in the 3.67 Ma Australopithecus prometheus StW 573 from Sterkfontein Member 2, comparing it with variation in ossa coxae in living humans and apes as well as other Plio‐Pleistocene hominins. Statistical comparisons indicate that StW 573 and 431 resemble humans in their anteroposteriorly great iliac crest breadth compared with many other early australopiths, whereas Homo ergaster KNM WT 15000 surprisingly also has a relatively anterioposteriorly short iliac crest. StW 573 and StW 431 appear to resemble humans in having a long ischium compared with Sts 14 and KNM WT 15000. A Quadratic Discriminant Function Analysis of morphology compared with other Plio‐Pleistocene hominins and a dataset of modern humans and hominoids shows that, while Lovejoy's heuristic model of the Ardipithecus ramidus os coxae falls with Pongo or in an indeterminate group, StW 573 and StW 431 from Sterkfontein Member 4 are consistently classified together with modern humans. Although clearly exhibiting the classic “basin shaped” bipedal pelvis, Sts 14 (also from Sterkfontein), AL 288‐1 Australopithecus afarensis, MH2 Australopithecus sediba and KNM‐WT 15000 occupy a position more peripheral to modern humans, and in some analyses are assigned to an indeterminate outlying group. Our findings strongly support the existence of two species of Australopithecus at Sterkfontein and the variation we observe in os coxae morphology in early hominins is also likely to reflect multiple forms of bipedality.

There has been a long debate about the possibility of multiple contemporaneous species of Australopithecus in both eastern and southern Africa, potentially exhibiting different forms of bipedal locomotion. Here, we describe the previously unreported morphology of the os coxae in the 3.67 Ma Australopithecus prometheus StW 573 from Sterkfontein Member 2 (left), comparing it with variation in ossa coxae in living humans and apes as well as other Plio‐Pleistocene hominins. Statistical comparisons indicate that StW 573 and StW 431 (right) resemble humans in their anteroposteriorly great iliac crest breadth compared with many other early australopiths, with Homo ergaster KNM WT 15000 surprisingly having a relatively anterioposteriorly short iliac crest. StW 573 and StW 431 also appear to resemble humans in having a long ischium compared with Sts 14 and KNM WT 15000. A quadratic discriminant function analysis of morphology compared with other Plio‐Pleistocene hominins and a dataset of modern humans and hominoids shows that, while Lovejoy's heuristic model of the Ardipithecus ramidus os coxae falls with Pongo or in an indeterminate group, StW 573 and StW 431 from Sterkfontein Member 4 are consistently classified together with modern humans. Although clearly exhibiting the classic “basin shaped” bipedal pelvis, Sts 14 (also from Sterkfontein), AL 288‐1 Australopithecus afarensis, MH2 Australopithecus sediba and KNM‐WT 15000 Homo ergaster occupy a position more peripheral to modern humans, and in some analyses are assigned to an indeterminate outlying group. Our findings strongly support the existence of two species of Australopithecus at Sterkfontein and the variation we observe in os coxae morphology in early hominins is also likely to reflect multiple forms of bipedality.