Hypsodonty and tooth facet development in relation to diet and habitat in herbivorous ungulates: implications for understanding tooth wear

Abstract

1. The evolution of high-crowned teeth or hypsodonty in herbivorous mammals is widely interpreted as a species-specific adaptation to increasingly wear-inducing diets and environments at evolutionary time scales, with internal abrasives (such as phytoliths in grasses) and/or external abrasives (such as dust or grit) as putative causative factors. The mesowear score (MS) instead describes tooth wear experienced by individual animals during their lifetime. 2. Under the assumption that the abrasiveness that causes the MS in individuals is the same abrasiveness to which species adapted by evolving hypsodonty, one would expect a close correlation between the MS and the hypsodonty index (HI). Alternatively, if these two measures reflect different aspects of wear, one would expect differences in the way that proxies of diet or environment/climate correlate with each parameter. 3. In order to test these hypotheses, we collated a dataset on the HI, MS, percentage of grass in the natural diet (%grass), habitat (open, intermediate, closed) and annual precipitation (PREC) in extant mammalian herbivores. The availability of a quantitative MS constrained the dataset to 75 species. Data were analysed with and without phylogenetic generalized least squares (PGLS). 4. Correlations with PREC were stronger for HI than for MS, whereas correlations with %grass were similar for HI and MS. Habitat had a significant influence on the relationship with %grass for HI but not for MS. Habitat also had a significant influence on the relationship between HI and MS. MS improved the predictive power of HI for %grass, but not for PREC. 5. These results suggest that while the MS indicates predominantly the wear effect of the diet (internal abrasives), HI represents an adaptation to a wear effect that comprises both diet and environment (external abrasives). The additional environmental wear effect must reduce tooth height without causing macroscopic changes in tooth facet development as described by the MS. 6. The most parsimonious explanation for the apparent discrepancy between HI and MS is that external abrasives of very fine particle size play a major role in naturally occurring tooth wear. The experimental testing of this hypothesis will enhance our understanding of the processes involved in tooth wear. ABSTRACT1. The evolution of high-crowned teeth or hypsodonty in herbivorous mammals is widely interpreted as a species-specific adaptation to increasingly wear-inducing diets and environments at evolutionary time scales, with internal abrasives (such as phytoliths in grasses) and/or external abrasives (such as dust or grit) as putative causative factors. The mesowear score (MS) instead describes tooth wear experienced by individual animals during their lifetime. 2. Under the assumption that the abrasiveness that causes the MS in individuals is the same abrasiveness to which species adapted by evolving hypsodonty, one would expect a close correlation between the MS and the hypsodonty index (HI). Alternatively, if these two measures...

Fossil pollen records reveal a late rise of open-habitat ecosystems in Patagonia

“…Although we are unable to challenge the adaptive value of hypsodonty from the fossil pollen analysis, we demonstrate quantitatively that grasslands per se were virtually absent prior to Quaternary in Patagonia: thus, whatever the results of the microwear analysis, these animals could not have been eating grass. Kaiser et al 30 showed that external abrasives of very fine particle size have a major role in naturally occurring tooth wear, which is consistent with the view of Pascual and Ortiz Jaureguizar 31 , who interpreted precocious hypsodonty in South American ungulates as an effective response to abrasive volcanic ash that may had dusted the leaves of plants.…”

Decoupling the spread of grasslands from the evolution of grazer-type herbivores in South America

“…First, a body of work has shown that hypsodonty in modern ungulates correlates not just with diet, but with habitat openness, precipitation and feeding height, regardless of grass abundance [27][28][29] ; thus, there is current utility of high-crowned teeth in environments where abundant abrasives, such as soil particles or volcanic ash, are ingested alongside plant matter. In particular, a recent study implies that fine-grained volcanic ash abrades teeth without resulting in the wear patterns associated with ingestion of coarser grit 27 . Second, although high levels of ingested grit are typically associated with open, arid ecosystems, large mesodont-hypsodont herbivores do exist in some forest or woodland habitats today ( Fig.…”

Patterns of diet and body mass of large ungulates from the Pleistocene of Western Europe, and their relation to vegetation

“…Morphological adaptations improving wear resistance of herbivorous mammal teeth, such as increased molar crown height (hypsodonty), reflect the conditions under which they evolved and have been shown to be adaptive both to environmental factors (such as precipitation and erosion rates, which regulate the accumulation of mineral particles on plant material), as well as to dietary factors (abrasive food items, mainly grass). The mesowear signal, on the other hand, has been demonstrated to show a purely dietary signal (nonabrasive vs. abrasive plant material, or browse vs. grass) (Louys et al, 2012;Kaiser et al, 2013;Kubo and Yamada, 2014). Mesowear analysis will thus provide a robust proxy for ungulate diet irrespective of abiotic environmental and climatic effects.…”