Elsevier

Journal of Human Evolution

Volume 132, July 2019, Pages 189-214
Journal of Human Evolution

Earliest axial fossils from the genus Australopithecus

https://doi.org/10.1016/j.jhevol.2019.05.004Get rights and content

Abstract

Australopitheus anamensis fossils demonstrate that craniodentally and postcranially the taxon was more primitive than its evolutionary successor Australopithecus afarensis. Postcranial evidence suggests habitual bipedality combined with primitive upper limbs and an inferred significant arboreal adaptation. Here we report on A. anamensis fossils from the Assa Issie locality in Ethiopia's Middle Awash area dated to ∼4.2 Ma, constituting the oldest known Australopithecus axial remains. Because the spine is the interface between major body segments, these fossils can be informative on the adaptation, behavior and our evolutionary understanding of A. anamensis. The atlas, or first cervical vertebra (C1), is similar in size to Homo sapiens, with synapomorphies in the articular facets and transverse processes. Absence of a retroglenoid tubercle suggests that, like humans, A. anamensis lacked the atlantoclavicularis muscle, resulting in reduced capacity for climbing relative to the great apes. The retroflexed C2 odontoid process and long C6 spinous process are reciprocates of facial prognathism, a long clivus and retroflexed foramen magnum, rather than indications of locomotor or postural behaviors. The T1 is derived in shape and size as in Homo with an enlarged vertebral body epiphyseal surfaces for mitigating the high-magnitude compressive loads of full-time bipedality. The full costal facet is unlike the extant great ape demifacet pattern and represents the oldest evidence for the derived univertebral pattern in hominins. These fossils augment other lines of evidence in A. anamensis indicating habitual bipedality despite some plesiomorphic vertebral traits related to craniofacial morphology independent of locomotor or postural behaviors (i.e., a long clivus and a retroflexed foramen magnum). Yet in contrast to craniodental lines of evidence, some aspects of vertebral morphology in A. anamensis appear more derived than its descendant A. afarensis.

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.

Section snippets

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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