First evidence of a bipartite medial cuneiform in the hominin fossil record: a case report from the Early Pleistocene site of Dmanisi

Abstract

A medial cuneiform exhibiting complete bipartition was discovered at the Early Pleistocene site of Dmanisi, Georgia. The specimen is the oldest known instance of this anatomical variant in the hominin fossil record. Here we compare developmental variation of the medial cuneiform in fossil hominins, extant humans and great apes, and discuss potential implications of bipartition for hominin foot phylogeny and function. Complete bipartition is rare among modern humans (< 1%); incomplete bipartition was found in 2 of 200 examined great ape specimens and also appears in the form of a divided distal articular surface in the Stw573c Australopithecus africanus specimen. Although various developmental pathways lead to medial cuneiform bipartition, it appears that the bipartite bone does not deviate significantly from normal overall morphology. Together, these data indicate that bipartition represents a phyletically old developmental variant of the medial cuneiform, which does not, however, affect the species-specific morphology and function of this bone.

Fig. 1

Degrees of bipartition in the medial cuneiform. (A,B) Complete bipartition; (C) Incomplete bipartition; (D) division of the distal articular surface. From Gruber (1877).

Fig. 2

Definition of measurements (modern human right medial cuneiform with division of the articular surface) (micro-computed tomography-based data of an archaeological specimen from Vaihingen an der Enz, Germany, specimen no. 190-10). SL, superior length; ML, middle length; IL, inferior length; PH, proximal height; PAH, proximal articular height; PAB, proximal articular breadth; DH, distal height; DAH, distal articular height; DAB, distal articular breadth.

Fig. 3

Right Pongo pygmaeus (AIM10141) medial cuneiform (incomplete bipartition): medial (A), distal (B) and lateral (C) views. Left Pan paniscus (RMCA15296) medial cuneiform (incomplete bipartition): medial (D), distal (E) and lateral (F) views.

Fig. 4

Dmanisi right medial cuneiform (D4111a/b) and first metatarsal (D3442). D4111a/b: (A–F) plantar part; (A′–F′) dorsal part; (A,A′) medial sides of plantar and dorsal parts, respectively; (B,B′) lateral surfaces; (E,E′) dorsal surfaces; (F,F′) plantar surface; (C,C′) proximal surface; (D,D′) distal surface. D3442/D4111a/b in anatomical association: medial (G) and lateral (H) views. (I) D3442 proximal articular surface.

Fig. 5

Medial cuneiform shape variation. Plot of ratio between distal and proximal articular surface areas [distal articular area (DAA) : proximal articular area (PAA)] vs. ratio between inferior and superior lengths (IL : SL). Black/white circles, Taforalt/Afalou; black/white squares, Laas/Afro-American; asterisks, Sigtuna bipartite medial cuneiforms; X, Dmanisi D4111.

Fig. 6

Homology of tarsal primordia in human. T, tibia; F, fibula; pi, pisiforme; i, intermedium; t, tibiale; f, fibulare; taph, tarsalia praehallucis; c1–3, centralia; ta I, tarsale distale I; ta II, tarsale distale II; ta III, tarsale distale III; ta IV, tarsale distale IV [redrawn after Čihák (1972)].

Fig. 8

Hypothetical pathways of medial cuneiform development. (A) Mesenchymal pre-chondrogenic condensation. (B) Interzone formation from aggregation of mesenchymal cells: b1, single mesenchymal primordium; b2, divided mesenchymal primordia (plantar and dorsal parts). (C) Separation of cartilaginous anlagen by cavitation and formation of synovial cavities: c1, single anlage; c2, two anlagen. (D) Ossification: d1, from one center; d2, from two centers. (E) Adult stage of bone development: e1, normal medial cuneiform; e2, bipartite medial cuneiform.

Fig. 7

(A) Primordia of the bipartite dorsal (cfId) and plantar (cfIp) medial, and intermedial (cfII) and lateral (cfIII) cuneiform in a transverse section of the foot of a 19-mm embryo [from Čihák (1972). With kind permission of Springer Science+Business Media]. (B) Two ossification centers of a medial cuneiform. Copyright © 2010 by the American Orthopaedic Foot and Ankle Society, Inc., originally published in Foot & Ankle International, O'Neal et al. 1995 and reproduced here with permission.