Earliest axial fossils from the genus Australopithecus
Introduction
Australopitheus anamensis fossils dated to between 3.9 and 4.2 Ma provide evidence that both craniodentally and postcranially A. anamensis was more primitive than its putative anagenetic descendant Australopithecus afarensis (Heinrich et al., 1993, Coffing et al., 1994, Leakey et al., 1998, Ward et al., 1999, Ward et al., 2013, Ward et al., 2017, Strait and Grine, 2004, Kimbel et al., 2006, White et al., 2006, Haile-Selassie, 2010, Lacruz et al., 2012). Postcranial evidence for A. anamensis indicates that it was a habitual biped that nonetheless retained some primitive features in its upper limbs that potentially signal a significant arboreal adaptation (Heinrich et al., 1993, Lague and Jungers, 1996; but see Ward et al., 2001). However, nothing is known about the axial skeleton of A. anamensis. This report on seven A. anamensis vertebrae represents the initial examination into the spinal biology and evolution of this earliest known Australopithecus species.
The discovery of four of these fossils was first announced by White et al. (2006) in a report on fieldwork conducted in December of 2000 at the Assa Issie 2nd locality in Ethiopia's Middle Awash study area. Also presented here are fossils subsequently identified from fieldwork conducted in 2002 at the same locality. Dated to 4.1–4.2 Ma, these are the oldest presently known axial remains for the genus Australopithecus. Because vertebrae function as the interface between all of the major body segments and contribute to spinal morphology, including curvature and muscle attachments, and play major roles in upright posture and bipedalism, these fossils can contribute toward our understanding of the behavior and evolution of A. anamensis.
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Materials and methods
Seven vertebrae of A. anamensis (see catalog numbers in Table 1) are housed in the National Museum of Ethiopia in Addis Ababa, Ethiopia. Data were collected from original fossils unless otherwise noted, and measurements are given in millimeters unless specified otherwise. A full list of linear and angular variables and their descriptions is given in Table 2. Comparative data from A. afarensis is represented by the A.L. 333-83 (C1), A.L. 333-101 (C2) and A.L. 333-106 (C6) from Hadar, Ethiopia
Description of fossils
ASI-VP-2/219: first cervical vertebra This is a C1 vertebra fragment preserving the anterolateral portion of the right superior articular facet fragment, medial roots of the accessory and transverse processes, partial foramen transversarium, and portion of the inferior articular process (Fig. 1). While the dorsal-most margins of the superior articular facet are absent, its lateral and anterior surface exhibits a steep curvature and displays transverse constriction on its anterolateral margin
C1 (atlas)
Bifurcation of the Assa Issie C1 superior articular facets appears to be synapomorphic in hominins, and its exclusive presence in humans among all extant taxa may be a signal of positional reorientation of the cranium above the vertebral column (Billmann et al., 2007), rather than along the more variable positional continuum of less specialized primate taxa. Likewise, like most modern humans, the Assa Issie C1 vertebrae and that of A. afarensis lack the presence of a ponticulus posticus common
Summary and conclusions
This first examination of the spinal column of A. anamensis is limited by its small sample size yet provides some evidence for a suite of evolutionarily derived vertebral features. The dimensions of the Assa Issie vertebral fossils are much larger than those of A.L. 288-1 and similar in size to the largest A. afarensis fossils from the A.L. 333 site at Hadar. In general, the Assa Issie vertebrae are as large as those of humans, Neandertals, and gorillas, especially at the more cranial cervical
Acknowledgements
The authors would like to thank Tim White for his invitation to study the Assa Issie fossils, and the Ethiopian Government (Authority for Research and Conservation of the Cultural Heritage, Ministry of Culture and Tourism) for granting us permission to study the fossil materials. We thank the staff of the National Museum of Ethiopia for their dedicated assistance and extend our deep gratitude to the entire crew of the Middle Awash project who put in many years of difficult fieldwork to make
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