Dental microwear analysis of notoungulates (Mammalia) from Salla (Late Oligocene, Bolivia) and discussion on their precocious hypsodonty

The extinct endemic South American notoungulates are remarkable for their early evolution of hypsodonty in the Late Eocene/Early Oligocene. Due to this 'precocious hypsodonty' they have been considered to be the earliest grazers, suggesting an early spread of grassland in South America (at least 10 Myr earlier than in North America). This hypothesis is based on the idea that the abrasion of their hypsodont cheek teeth might have been caused by silica phytoliths which are abundant in a rich-in-grass diet. The analysis of the dental microwear pattern (using an optical stereomicroscope connected to a tri-CCD camera) of hypsodont notoungulates from the Late Oligocene Salla Beds of Bolivia revealed the preponderance of thin parallel scratches on the fossil teeth of most of the taxa analyzed, like extant grazers. This microwear pattern above all indicates predominant sub-horizontal jaw movements in the chewing behaviour of notoungulates with "precocious" hypsodonty. This signals shearing movements as in a diet rich in tough plants. This peculiar chewing pattern and/or a larger ingested proportion of grass phytoliths or exogenous grit are further discussed as hypothetical causes of increasing wear on notoungulate teeth during middle Cenozoic times. We particularly argue that the putative precocious spread of grasslands in South America traditionally linked with notoungulate hypsodonty is still far from being supported. Many questions arise concerning the development of hypsodonty within notoungulates and this case perfectly illustrates that the relations between high-crowned mammals and the source of tooth wear in the fossil record may not be straightforward. (C) 2009 Elsevier B.V. All rights reserved.