Fragmentary fossils help reveal Neandertal skull growth

Frank Williams and I have a paper coming out shortly, comparing skull growth in Neandertals and humans. We use the resampling-based “grdif” method (see here) to compare an ontogenetic series of 20 non-adult and 20 adult Neandertals with a giant ontogenetic sample of humans. While Neandertal skull growth has been looked at before, the fragmentary nature of the fossil sample has caused most earlier studies to focus either on single traits or relatively few, often reconstructed, non-adult Neandertals. The advantage of grdif is that it incorporates all fossils regardless of their preservation, and provides a statistical comparison of cross-sectional samples.

In general, and unsurprisingly, skull growth is quite similar between humans and Neandertals. They’re closely related groups, after all. Compare grdif statistics, which measure how much two samples differ in growth between age groups, for humans vs. Neandertals (left) and humans vs. Australopithecus robustus (right):

Growth difference.

Growth differences (grdif) between humans and Neandertal skulls (left), and human and A. robustus mandibles (right). If two groups undergo the same amount of growth between age groups or stages, grdif equals 0. Positive values mean the fossil group grows more, while negative values mean humans grow more. Left is a figure from the paper, right is from my dissertation.

The Neandertal-human comparison shows much less difference than the australopith-human comparison. In spite of this general similarity between Neandertal and human skull growth, there are some key differences. Many distinct Neandertal traits, such as the extremely broad nasal aperture, appear piecemeal over the course of growth, rather than all at once. Some recent studies using geometric morphometrics have pointed to different patterns of craniofacial growth in Neandertals, but these were limited in needing smaller samples of more complete fossils. While the grdif approach doesn’t have the power to examine complex shape the same way as GM, and doesn’t produce as pretty of pictures, grdif does help fill in the gaps by including even fragmentary fossils. This is important as it helps reveal when during growth anatomical differences between groups appear.

Our paper will be out (hopefully early) in 2016 in American Journal of Physical Anthropology. In the mean time, the basic strategy of grdif is explained in Cofran (2014), and the R code for using this method can be found on my Research page.

Cofran Z (2014). Mandibular development in Australopithecus robustus. American Journal of Physical Anthropology, 154 (3), 436-46 PMID: 24820665


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