Resource partitioning among late Pleistocene herbivores of Natural Trap Cave, Wyoming

Abstract

Natural Trap Cave (NTC) preserves abundant late Pleistocene herbivore remains including equids (Equus and Haringtonhippus) and the sheep Ovis. The NTC equid sample contains a minimum of two taxa, while other large herbivore genera are represented by single species. In modern communities, sympatric herbivores partition resources by feeding on different plants or different plant parts. Previous dental isotopic studies indicated that most NTC herbivores consumed C3 plants, including grasses. To test the hypothesis that resource partitioning occurred among grass-feeders at NTC, we collected mesowear data from both equids as well as Ovis, using samples collected by teams from the University of Kansas between 1974 and 1985.Assessed on upper cheek teeth, mesowear derives from either attritional (tooth on tooth) or abrasional (tooth on abrasive food) wear. The paracone and metacone were examined on each specimen to determine cusp shape (sharp, rounded, or blunt) and occlusal relief (high or low). A diet of non-abrasive material yields high and sharp cusps and one of heavily abrasive materials results in low and blunt cusps, while rounded cusps indicate intermediate levels of abrasion. A large sample (N = 143) of equid teeth included two morphs. A smaller sample (N = 10) of Ovis teeth provided comparison with the equid teeth. Data collected consisted of individual variables of cusp heights and shape, as well as stages of mesowear combining both variables. Upper adult M1/2s and P4s were used.Results from the equid sample revealed that 8% exhibited high and sharp/rounded cusps, indicating little dietary abrasion; 58% had low and rounded cusps; and 34% had low and blunt cusps. The latter grouping is evidence for a highly abrasive diet. In the sample of Ovis, the results are strikingly different: 40% had high and sharp cusps and 60% had high and rounded cusps. Even though mountain sheep are often interpreted to be grass feeders, their mesowear suggests they did not eat an abrasive diet at NTC.The distribution of mesowear scores among the equid fossils conforms with an interpretation of two equid taxa at NTC, as well as supporting our hypothesis that these taxa partitioned grass resources. The very different nature of the mesowear signature seen in Ovis, compared to the equid fossils, also supports our hypothesis that these grass-feeders either consumed a more mixed or browse-dominated diet with different overall levels of abrasion, or else ate different parts of grass plants. As observed in the Serengeti grazing succession, we propose that equids may have moved through the area first, consuming taller, older, more abrasive parts of grass plants, followed by mountain sheep eating softer, younger plants.