Abstract
South Africa and East Africa each have a rich palaeoanthropological heritage, but the taxonomy of fossil hominins from these regions is controversial. In this study, two morphometric methods related to the quantification of variability in morphology have been applied to pairwise comparisons of linear measurements of hominoid crania and mandibles. The log-transformed standard error of the m-coefficient (‘log sem’) is calculated from linear regressions. Like Procrustes Distances (PD), log sem statistics can serve to quantify variation in the shape of a cranium or mandible in the context of a constellation of landmarks. In this study, PD and log sem statistics are integrated and standardised using Z-scores, and applied probabilistically to Plio-Pleistocene hominins. As a test case, OH 7 and OH 24 as reference specimens of Homo habilis are compared to fossils representing other taxa. There is a wide spectrum of variation in Z-scores for specimens attributed to early Homo dated within the period between circa 1.8 Ma and 2 Ma. In terms of morphometric variation predating 1.8 Ma, Z-scores (Z<2) for Australopithecus afarensis, A. africanus and Homo habilis display a small range of variability. This study serves as a demonstration of a method whereby log sem and PD can be used together to facilitate an objective assessment of morphological variability, applicable in palaeontological contexts.Significance: Using a probabilistic approach, two morphometric methods are integrated to quantify morphological variability in Plio-Pleistocene African hominin mandibles and crania. Two Tanzanian specimens of Homo habilis (the OH 7 mandible of the holotype specimen, and the OH 24 skull) can be used as reference material for morphometric comparisons with other fossils (mandibles or crania) attributed to Australopithecus africanus, A. afarensis, H. erectus and H. rudolfensis. The results of these comparisons are expressed as standardised probabilistic Z-scores such that these statistics for skulls and mandibles can be expressed on a common scale.
Highlights
The taxonomy of fossil hominins appears increasingly more complex given recent discoveries and announcements of new species.[1]
In a morphometric approach based on pairwise comparisons, Thackeray and Odes[29] calculated ‘log sem’ statistics associated with regression analyses to compare OH 24 with other crania, based on measurements from original specimens published by Wood.[23]
The AL 400-1 mandible of A. afarensis has a Z-value of 0.71 when it is compared with OH 7 using the Procrustes Distances (PD) statistical method
Summary
The taxonomy of fossil hominins appears increasingly more complex given recent discoveries and announcements of new species.[1] This is true for early hominins, with new Australopithecus species recently named, and with respect to questions about the emergence of the genus Homo and of modern humans. Taxonomic attributions and affiliations are one of the major issues in palaeoanthropology, including, for example, the debate between Tobias and Robinson regarding specimens attributed to Australopithecus or early Homo.[2,3,4,5] By 1992, Tobias[6] triumphantly declared that Homo habilis was ‘widely accepted as a good taxon’. Just a few years later, Wood and Collard[7] suggested that H. habilis as well as H. rudolfensis should be placed within Australopithecus. The development of analytical and statistical tools is required to help clarify the complex picture such as this (as an example), notably for fossil specimens that cannot be sampled for DNA analyses
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