Tooth root and craniomandibular morphological integration in the common chimpanzee (Pan troglodytes): alternative developmental models for the determinants of root length.

Background/aims: Root length is strongly related to tooth stability but demonstrates considerable intraspecific variation. Previous studies have demonstrated an intraspecific relationship between root length and facial length in diverse mammalian taxa. These findings are indicative of plasticity in root length but with no clear developmental mechanism. This study aims to further these findings by identifying patterns of covariance between postcanine tooth root length and the whole integrated craniofacial skeleton in order to allow more refined hypotheses of the underlying developmental mechanisms to be proposed.

Methods: 2D landmark coordinates were obtained from lateral radiographs of 27 adult Pan troglodytes skulls. The landmark configurations were divided into two blocks, one of craniofacial landmarks and another of landmarks related to mandibular tooth roots. Covariation of the two blocks was determined using partial least squares analysis.

Results: The correlation coefficient between the first pair of singular warps is 0.76, highly significant (p<0.02) and not sex related. Visualisation of this correlation shows a clear pattern of increasing root length variation along the tooth row with increasing facial height but not length.

Conclusions: The findings support previous conclusions that tooth roots demonstrate plasticity during their development. A correlation between root length variation along the tooth row and facial height rather than length can be interpreted in the context of previous findings of maxillary and mandibular rotation and compensatory remodeling during development. It is therefore proposed that the observed root length plasticity is due to variation in the eruptive distance associated with compensatory jaw rotation during development.