Description of the human femoral diaphysis (THI94-UA28-7)

ThI94-UA28-7 (Fig 3) is the partial shaft of a left femur of adult size (length of the preserved part 218 mm). At the proximal end, it is irregularly broken at the distal portion of the lesser trochanter, and at the distal end, about 2 cm distal to the beginning of the divergence of the two components of the linea aspera. The bone is too incomplete to even roughly estimate its total length. The bone surface is well-preserved on most of the posterior and medial, and part of the lateral faces, but some cortical bone is missing on most of the anterior face. Neither the external aspect, nor that of the internal structure, which was examined at two different levels before the shaft segments were glued back together, show evidence of crushing. In medial view, the imperfectly preserved anterior border of the bone is virtually straight and the better-preserved posterior border confirms that the curvature of the shaft was certainly weak. In posterior view, the medial outline is slightly convex, and the lateral outline slightly concave. Variations in transverse diameter are slight, but the smallest section is probably located close to the mid-length of the complete bone; there is no evidence that it was distally located, as in many early Homo [90,91]. Not enough of the bone is preserved to estimate the degree of anteversion, but it was certainly low.

Except at the distal end, the cross section is distinctly compressed antero-posteriorly, especially proximally, but there is no medial-localized swelling below the missing neck. Unfortunately the lateral part of the bone is not preserved at this level, preventing estimates of the sub-trochanteric platymeric index. At about 55–60% of the biomechanical length (see Methods), at a level where the outline of the cross-section is virtually complete and the whole shaft is well preserved, the transverse diameter is much greater than the antero-posterior one, giving a very low pilastric index close to the minimum recorded values for modern humans, Middle Pleistocene hominins, as well as for early Homo (Table 1).

The lines and ridges that mark the attachments of the hip muscles on the posterior face of the proximal end can hardly be traced. There is neither a distinct gluteal ridge, nor any evidence of a hypotrochanteric fossa, but what is preserved of the gluteal zone is a rough area whose medial margin, which provided attachment for the m. adductor magnus, is quite clear and centrally located on the shaft. Distally, it proceeds in a straight line to the linea aspera; proximally, this margin is so clearly marked that it would probably have extended to the missing lesser trochanter, as there is no evidence of an inflexion towards the medial side. In modern humans this inflexion leads to the spiral line for attachment of the m. vastus medialis; here, although the surface of this area is well preserved, the spiral line is absent.

Distally, the linea aspera remains centrally located; it is simple, slightly protruding and, although it remains bordered laterally by a shallow depression, there is no indication of a pilaster. The pilaster is always present in modern humans, but rare in early Homo, although occasionally present (e.g. in BOU-VP-19/63: [91], and probably Dmanisi: [97]). The linea aspera splits distally into the supra-condylar ridges, but nothing can be said about the popliteal surface.

The internal structure of the cross-section of the shaft could be examined on natural breaks before the segments were glued together (Fig 3: 2). The most informative section is located at a level where only a thin layer of bone is missing on the anterior face, so that the complete section can be reconstructed with reasonable accuracy. It is of course impossible to precisely determine the position of this section relative to the complete bone, but it must be close to 60–65% of the biomechanical length and comparisons should therefore be made mostly with data at 65%, but also at 50%.

The total subperiosteal (TA) and cortical (CA) areas of the cross section of ThI94-UA28-7 are TA = 483 mm², and CA = 401 mm² respectively. The value of the anteroposterior and mediolateral second moments of area, which characterize the resistance to anteroposterior and mediolateral bending stresses, are Ix = 13380 mm4 and Iy = 24869 mm4 respectively. Due to the reconstruction of a thin layer of bone on the anterior surface, and to the fact that the photo is only a projection on a plane of surfaces that are at different levels, these values carry some imprecision, but they do provide a basis for comparison.

The cortex provides resistance to axial load, and it is commonly assumed that Middle Pleistocene Homo had a relatively greater cortical area than modern humans, but this is mostly true of the distal portion [98,75]. However Trinkaus and Ruff [75] noticed that it also depends on the deposition/resorption process during development, and that the ratio CA/TA did not change much during the Pleistocene. ThI94-UA28-7 is well above the mean of Upper Pleistocene Homo at 50% of the biomechanical length, but close to the mean values for all groups at 65%, and within the ranges of all groups at both levels.

Trinkaus and Ruff ([75] and references therein) observed that when the anteroposterior second moment of area (Ix) is plotted against the mediolateral one (Iy), there is a clear distinction between Lower/Middle Pleistocene Homo and later modern humans, with the potential resistance to medio-lateral stresses being greater in the former group. When added to their graphs, ThI94-UA28-7 plots very clearly with other Middle Pleistocene Homo, and far from later modern humans, both at the 50% and 65% distances (the latter is shown on Fig 4; ThI94-UA28-7 is still more distinct from modern humans at 50%). According to Trinkaus and Ruff [75], this difference has to do with pelvic and crural body proportions rather than with locomotor patterns.

On the whole, the GH diaphysis fits well within the group of known early Middle Pleistocene hominins. Among those that are geographically and chronologically close, it resembles the specimen from Aïn Maarouf [95] in the lack of pilaster, but otherwise differs by its lesser curvature, greater platymery, and relatively thicker cortical bone, but individual variability could account for all of these differences. The large femoral series from Atapuerca (Sima de los Huesos) [99,100] also display similar features with a lack of pilaster, medio-laterally expanded section, and thick cortical bone. Published anterior views ([99]: Fig 2) show that the minimum transverse diameter is located near mid-length, instead of more distally as in early African Homo, but more specimens need to be discovered at Casablanca before any meaningful comparisons can be made.