Delineation of the vertebral evolutionary pattern of African apes and hominids throws considerable new light on the troublesome issue of both the locomotor pattern and phylogeny of perhaps the most enigmatic hominoid of the later Miocene, Oreopithecus
. Arguments as to its potential phylogenetic relationships and locomotor patterns have been many (reviewed in Harrison 1986
; Kohler & Moya-Sola 1997
; Rook et al. 1999
). However, all have been hampered by its extremely poor condition, largely the consequence of its extreme compression during fossilization. This has frequently led to excessively liberal interpretations of its badly compromised structure.
A case in point is the attribution of a lordotic spine to this taxon based on a sagittal section of specimen BA72, a crushed and compressed amalgam of three lumbar vertebrae (Kohler & Moya-Sola 1997
). It seems inconceivable to us that such sectioning can reliably indicate the presence/absence of wedging in centra after they have been compressed to less than one half of their dorsoventral diameter. A far more conservative approach is to rely on more straightforward morphological characters of greater inherent reliability, and which are more resistant to misinterpretation from crushing defects. Not all of these appear to have been considered.
One of the most important is the vertebral formula of Oreopithecus
. There is general agreement, based on the ‘1958 specimen’ (IGF 11 778), that Oreopithecus
had five lumbar vertebrae (Harrison 1986
; Kohler & Moya-Sola 1997
; Rook et al. 1999
). A largely overlooked vital statistic, however, is that it also had six
sacral vertebrae (Straus 1963
; method of Schultz 1961
; for details, see McCollum et al. 2009
). This can be safely concluded from specimen BA-50, which preserves five sacral foramina on the left side, and at least four on the right. Moreover, the masses of the right and left halves of the sixth sacral vertebra appear to be fully symmetrical (therefore the right side presumably had five full foramina as well). We have demonstrated elsewhere that the basal hominoid column almost certainly exhibited 13 thoracics (among living taxa, only Homo
have any significant incidences of fewer). Thus, the minimum
pre-coccygeal vertebral number in Oreopithecus
was 31, which, as noted above, is the likely pre-coccygeal vertebral number for basal hominoids and was probably modal for Early and Mid-Miocene apes as well. Except for P. paniscus
, a modal vertebral number as high as 31 is extremely rare in extant species, occurring in only 2.8 per cent of P. troglodytes
and 0.06 per cent of Homo
(McCollum et al. 2009
Much has been made of the putative ‘short, broad, ilium’ of Oreopithecus
and of its relatively broad retroauricular segment (Hürzeler 1958
). However, a substantial reduction in the size of the post-auricular region of the pelvis appears to have accompanied the spinal invagination underlying scapular relocation in all hominoids (see earlier). That reduction was in turn accompanied by a broadening of the pre-auricular portion of the pelvis and is therefore expected in any clade in which shoulder reorganization occurred (Lovejoy et al. 2009c
). This same developmental process is likely to have re-occurred a number of times in hominoid evolution, and is almost certainly universally responsible for the dorsal migration of the LTPs. Broadening of the ilium well beyond comparable dimensions in Proconsul
is therefore fully expected in virtually any
large-bodied Miocene hominoid that exhibits posterolateralization of the shoulder.
The fifth lumbar vertebra of the ‘1958 specimen’ lies (in situ
) directly within its bi-iliac space, sharing the same functional position as the trapped (immobilized) L7 of a typical Presbytis
and the L3 or L4 of Pan
(see Straus 1963; ). Therefore, Oreopithecus
exhibits a maximum
of only four potentially mobile lumbar vertebrae. This is fully consistent with its ‘classic’ adaptive regimen for suspension as also seen in Gorilla
, and with directly opposite polarity compared with their homologues in bipedal hominids in a host of major adaptive characters (). These included transformation of lumbars via their sacralization, direct reduction in lumbar number from the primitive condition and entrapment (immobilization) of at least one lumbar by contact with a posterodorsally extended iliac crest. Given its primitive vertebral number, and a series of others, such as its retention of an anterior keel on its lumbar vertebrae (Straus 1963), Oreopithecus
appears to have acquired extensive adaptations to suspension entirely independently of other Miocene clades (as did Nacholapithecus
; Nakatsukasa et al. 2007
). It is thereby unrelated to hominids, its similarities (which are few; ) being largely minor convergences. Any bipedality would have been largely driven by the same context that does so in hylobatids—excessively long forelimbs combined with highly abbreviated hindlimbs ().
Figure 5. (a) Sacra of a chimpanzee, (b) A.L. 288-1 and (c) a modern human. Note the extremely narrow sacrum of the chimpanzee compared with the two hominids. Note also the much broader alae in A.L. 288-1 compared with its centrum. Compare this with the similar (more ...)
Principal characters of Oreopithecus compared with those of other hominoids.
One additional supposedly hominid character in Oreopithecus
is worthy of brief note. The degree of protuberance of its AIIS is not
unusual for a non-hominid. What distinguishes the AIIS in hominids from those in apes is not
its protuberance (those of Gorilla
are often very prominent), but rather its emergence from a novel, separate physis, a hominid adaptation that is almost certainly associated with dramatic expansion of iliac isthmus breadth (Lovejoy et al. 2009b
). There is no evidence of a similar degree of broadening in Oreopithecus
(note its relative pelvic breadth in ) and certainly none suggesting its origin by means of a separate physis.