Tooth wear and diversity in early hominid molars: A case study

Abstract

Functional relationships between diet and tooth morphology form an integral part of paleontological research. The detailed description of occlusal relief and wear patterns of molars provides information about food ingestion and mastication. In early hominids overall molar morphology is fairly similar. Size measurements, such as buccolingual or mesiodistal diameter and 2-D cusp area of hominid molars show considerable overlap. The pioneering works of Butler, Mills, Hiiemae, Kay, Maier and others have shown that the wear pattern on the occlusal surface seems to reflect mastication behavior as an indication of diet. However, most of the interpretations are based on two-dimensional analyses. Occlusal relief measured in 3-D highlights functionally important features useful for quantifying the complex wear patterns on hominid teeth. However, until recently they could not be measured because techniques and methods were lacking. Nevertheless the results of 2-D analyses so far demonstrate that the occlusal surface of teeth records a significant part of the life history of an individual. The 3-D analysis of wear patterns on hominid teeth may provide additional information regarding the relationships between diet, chewing behavior and early hominid evolution. In this case study we employ a new 3-D approach to compare details on the occlusal surface of worn molars of Australopithecus afarensis, Australopithecus africanus and Paranthropus robustus in order to examine possible differences in tooth wear patterns. High resolution optical topometry enables us to measure parameters on 3-D computer models of teeth. Here, we compare various occlusal morphologies of worn lower second molars and attempt to interpret function, taking dental and masticatory principles into account. Our results indicate that diverse modes of occlusal wear in Australopithecus and Paranthropus are evident. A closer look at the occlusal relief and wear facet pattern shows that an assortment of mechanisms for crushing, shearing and grinding on a single tooth are common, since orientation and inclination of wear facets vary. The fact that A. afarensis molars show diverse functional areas with little variation among individuals suggests it had a dental toolkit to cope with a wide range of food qualities and may indicate a species-specific dietary spectrum. A. africanus and P. robustus molars, with their pronounced and relatively rapid flattening of crown relief and diverse individual wear patterns, point towards hard-object feeding and greater intraspecific variation in diet. P. robustus, however, with somewhat higher occlusal relief, can be interpreted as an omnivorous generalist with hard objects as fall-back foods.

Keywords