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. 2020 Jan 7;117(1):278-284.
doi: 10.1073/pnas.1911896116. Epub 2019 Dec 23.

Insights Into the Lower Torso in Late Miocene Hominoid Oreopithecus bambolii

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Free PMC article

Insights Into the Lower Torso in Late Miocene Hominoid Oreopithecus bambolii

Ashley S Hammond et al. Proc Natl Acad Sci U S A. .
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Abstract

Oreopithecus bambolii (8.3-6.7 million years old) is the latest known hominoid from Europe, dating to approximately the divergence time of the Pan-hominin lineages. Despite being the most complete nonhominin hominoid in the fossil record, the O. bambolii skeleton IGF 11778 has been, for decades, at the center of intense debate regarding the species' locomotor behavior, phylogenetic position, insular paleoenvironment, and utility as a model for early hominin anatomy. Here we investigate features of the IGF 11778 pelvis and lumbar region based on torso preparations and supplemented by other O. bambolii material. We correct several crucial interpretations relating to the IGF 11778 anterior inferior iliac spine and lumbar vertebrae structure and identifications. We find that features of the early hominin Ardipithecus ramidus torso that are argued to have permitted both lordosis and pelvic stabilization during upright walking are not present in O. bambolii However, O. bambolii also lacks the complete reorganization for torso stiffness seen in extant great apes (i.e., living members of the Hominidae), and is more similar to large hylobatids in certain aspects of torso form. We discuss the major implications of the O. bambolii lower torso anatomy and how O. bambolii informs scenarios of hominoid evolution.

Keywords: ape and human evolution; locomotion; lumbar vertebrae; pelvis.

Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
O. bambolii, IGF 11778. (A) Photograph of slab with 5-cm scale. Dashed lines in ilium indicate regions where wax is stabilizing the bone. (B) Composed line drawing. Dark gray indicates regions stabilized by wax. lf, left femur; lh, left humerus; lm, left manus; lt, left tibia; lr, left radius; lu, left ulna; p, pelvis; rb, ribs; rh, right humerus; rf, right femur; sk, skull; v, vertebrae. (C) IGF 11778 pelvis and lower lumbar vertebrae in dorsal view. 1= penultimate lumbar vertebra; 2 = ultimate lumbar vertebra; 3 = right iliac blade; 4 + 5 = left iliac blade pieces; 6 + 7 = left lower iliac body; 8 = left ilium with cranial portion of acetabular lunate surface; 9 = pubis with small portion of acetabular lunate surface; 10 + 11 = left pubis fragments; 12 = left pubic ramus; 13 = left ischial fragment with a portion of acetabular lunate surface; 14+15 = left ischium fragments. (D) Anterior view of ultimate and penultimate vertebrae. PrZ = prezygapophysis; TP = transverse process. Pelvic and vertebral elements are figured in additional high-resolution views in the SI Appendix.
Fig. 2.
Fig. 2.
Pelvic form in anthropoids. (A) Natural log-log scatterplot of lower iliac height and acetabulum diameter in extant (n = 350) and fossil anthropoids (n = 10). (B) Natural log-log scatterplot of bi-iliac breadth measured across the anterior superior iliac spines and acetabulum diameter in extant (n = 1,494) and fossil anthropoids (n = 4). IGF 11778 preserved measurement indicated with open circle, with arrow indicating the direction of actual value. (C) Sacroiliac (SI) joint orientation indicated by red arrows; lateral iliac blade orientation indicated by black arrows. (D) The cross-section of Bac 184 (approximating the section location in ref. 41) is consistent with a coronal orientation of the sacroiliac joint and iliac blade, and inconsistent with a parasagittal orientation. The curvature of the Bac 184 gluteal surface would preclude humanlike medial rotation of the anterior iliac blades into the sagittal plane, especially if combined with a narrow sacrum.
Fig. 3.
Fig. 3.
AIIS comparative anatomy. The IGF 11778 anterior inferior iliac spine (pink) is most comparable to Gorilla and Pan among extant and fossil hominoids. The hominins (ARA-VP-6/500, A.L. 288–1, and human) all display a projecting sigmoidal AIIS (black arrow) for the rectus femoris muscle and iliofemoral ligament attachment. The Ardipithecus (ARA-VP-6/500) figure from ref. was modified with permission from AAAS. Pelves not to scale.
Fig. 4.
Fig. 4.
The O. bambolii lower torso and evolutionary scenarios. (A) The lumbar spine was moderately flexible with 5 vertebrae, with at least 2 vertebral levels separating the last rib and the upper pelvis. (B) The transverse process (tp) of the last lumbar vertebra ran cranially along the iliac crest and was entrapped by the ilium. (C) The sacrum had 5 or 6 segments. (D) The sacroiliac joint (si) was narrow and paracoronally oriented, and the ilium was not dorsally retroflexed as evidenced by the shallow greater sciatic notch (sn). The lower ilium was moderate in length. The ASIS was not thick. (E) The AIIS was not protruding. (F) The ischium had a strongly projecting ischial spine (is). (G) The ischial tuberosity (it) may have may have been callosity-bearing. (H) The pubic symphysis was likely craniocaudally abbreviated. (I) The pubis was long, with a prominent adductor longus origin site. (J) The hip joint was not adapted for large cranially directed loads, as evidenced by the femoral head. (K) 3 evolutionary scenarios are compatible with these morphologies (refer to text for descriptions of the 3 scenarios). Evolutionary regimes are indicated by colors along the tree branches: purple, hylobatid-like (specifically Symphalangus-like); green, great ape; pink, hominin. Location of O. bambolii (“Oreo” label) indicated in each scenario by dashed line(s).

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