Evolution of the power stroke in early Equoidea (Perissodactyla, Mammalia)

Abstract

During the early evolution of Equoidea, two families co-existed, Equidae and Palaeotheriidae. Both groups show a similar ancestral molar morphology and evolved from bunodont to lophodont or selenolophodont (lophodont with crescent-shaped cutting edge in some form) respectively, with a clearly pronounced ectoloph. Fossils of the here studied brachydont equids and palaeotheriids are known from the early Eocene to the middle Miocene in North America and Eurasia. Due to the rich fossil record, dental evolution and related functional shifts can be investigated in detail in each family. In this study, we focused on changes in the different masticatory paths, identified different functions within the power stroke and evaluated the efficiency of the different modes in mastication to correlate tooth morphology to the potential palaeodiets. The analysis is based on three-dimensional (3D) polygonal surface scans, allowing the detailed investigation of morphological features. The results show that primitive equoids possess well-developed cutting and shearing structures, despite being generally referred to as simply bunodont. These structures enable the primitive forms to break down structural plant parts more efficiently than for example Phenacodus (‘condylarth’ outgroup), showing simple rounded cusps, and therefore representing a more primitive type of bunodont dentition. It is general consensus that the diet of the more derived early equoids shifts to a higher percentage of tough plant parts and they adopt different strategies to efficiently comminute those parts. Equids emphasise cutting and shearing both buccally and lingually, trending towards a one-phase power stroke, a pattern resembling that of modern hypsodont Equidae. Our results suggest that the derived brachydont equids specialised to consume leaves rather than grasses. In comparison, derived palaeotheriids mainly emphasise shearing and cutting function buccally and a distinct grinding function in a two-phase power stroke. The combination of different functions suggest a broader diet different from only leaves, likely including twigs or hard fruits.