Isotopic dietary reconstructions of Pliocene herbivores at Laetoli: Implications for early hominin paleoecology

Major morphological and behavioral innovations in early human evolution have traditionally been viewed as responses to conditions associated with increasing aridity and the development of extensive grassland-savanna biomes in Africa during the Plio-Pleistocene. Interpretations of paleoenvironments at the Pliocene locality of Laetoli in northern Tanzania have figured prominently in these discussions, primarily because early hominins recovered from Laetoli are generally inferred to be associated with grassland, savanna or open woodland habitats. As these reconstructions effectively extend the range of habitat preferences inferred for Pliocene hominins, and contrast with interpretations of predominantly woodland and forested ecosystems at other early hominin sites, it is worth reevaluating the paleoecology at Laetoli utilizing a new approach. Isotopic analyses were conducted on the teeth of twenty-one extinct mammalian herbivore species from the Laetolil Beds (∼ 4.3-3.5 Ma) and Upper Ndolanya Beds (∼ 2.7-2.6 Ma) to determine their diet, as well as to investigate aspects of plant physiognomy and climate. Enamel samples were obtained from multiple localities at different stratigraphic levels in order to develop a high-resolution spatio-temporal framework for identifying and characterizing dietary and ecological change and variability within the succession. In general, dietary signals at Laetoli suggest heterogeneous ecosystems with both C₃ and C₄ dietary plants available that could support grassland, woodland, and forested communities. All large-bodied herbivores analyzed yielded dietary signatures indicating mixed grazing/browsing strategies or exclusive reliance on C₃ browse, more consistent with wooded than grassland-savanna biomes. There are no clear isotopic patterns documenting shifting ecology within the Laetolil Beds or between the Laetolil and overlying Upper Ndolanya Beds, although limited data from the U. Ndolanya Beds constrains interpretations. Comparison of the results from Laetoli with isotopic enamel profiles of other African fossil and modern communities reveals significant differences in dietary patterns. Relative to extant taxa in related lineages, carbon isotopic ranges of a number of Laetoli fossil herbivores are anomalous, indicating significantly more generalized intermediate C₃/C₄ feeding behaviors, perhaps indicative of dietary niches and habitat types with no close modern analogs. Enamel oxygen isotope ranges of fossil taxa from Laetoli are consistently more ¹⁸O depleted than modern E. African herbivores, possibly indicating more humid conditions during that interval in the past. These data have important implications for reconstructing dietary trajectories of mammalian herbivore lineages, as well as the evolution of ecosystems in East Africa. Isotopic analyses of similar or related taxa at other hominin fossil sites yield signatures generally consistent with Laetoli, suggesting that mammalian communities in East Africa were sampling ecosystems with similar proportions of browse and grass. Collectively, the isotopic dietary signatures indicate heterogeneous habitats with significant wooded or forested components in the Laetoli area during deposition of the Laetolil and Upper Ndolanya Beds. Early hominin foraging activity in this interval may have included access to forest or woodland biomes within this ecosystem, complicating traditional interpretations linking early human evolutionary innovations with a shift to savanna habitats.