The goal of this research by doctoral student Laura Stroik (Arizona State University), working under the supervision of Dr. Gary Schwartz, is to determine the model of dietary competitive interaction that characterized the origination and early diversification of Eocene euprimates (earliest "true" primates) in North America, ca. 55 million years ago. Three models of competitive interaction are examined: (1) absence of dietary competition, (2) presence of strong dietary competition, resulting in the competitive exclusion of non-euprimate species, and (3) presence of weak dietary competition, resulting in the coexistence of euprimate and non-euprimate species. Competition is defined by niche overlap, and each hypothesis is distinguishable by its unique predicted pattern of dietary niche overlap among species through time.
Dietary niches of fossil taxa are reconstructed using morphological measures of molar teeth that are found to discriminate dietary regimes within a comparative sample of extant mammals. The degree of separation between reconstructed niches of early euprimates and their competitors is evaluated statistically across a series of fossil samples at a single site, the Bighorn Basin in Wyoming. Competitive environments are examined just prior to the origin of euprimates and then during the initial phase of their radiation within North America. Observed patterns of dietary niche overlap over time, together with species abundance and climatic variables, are compared to the expected patterns of each competition model.
Establishing the dietary competitive environment in which euprimates arose enables a direct test of the standing hypotheses of euprimate origins by demonstrating whether diet was, in fact, a primary driver of early euprimate evolution. In addition, climatic warming processes comparable to those seen today were also present during the time when true primates originated; thus, the results of this project are directly applicable to studies of the effects of global climate change on modern animals, particularly those at risk of extinction. This research promotes the training of undergraduate students, and the results of this study (including methodological techniques, quantitative data, and computer algorithms) are accessible to other researchers and educators through an online database.
The earliest Eocene (approximately 56 million years ago) marked the appearance of the first North American euprimates, or "primates of modern aspect": adapids and omomyids. Despite the fact that leading hypotheses assert that traits involved in food acquisition underlie euprimate origination and early diversification, the precise role that dietary competition played in establishing euprimates as successful members of mammalian communities is unclear. This is because the degree of niche overlap between euprimates and all likely mammalian dietary competitors ("the euprimate competitive guild") is unknown. This research determined which of three major competition hypotheses – non-competition, strong competition, and weak competition – characterized the early Eocene euprimate competitive guild. Each of these hypotheses is defined by a unique pattern of dietary niche overlap over time between euprimates and their non-euprimate competitors, allowing an evaluation of the nature of dietary competitive interactions surrounding the earliest euprimates in North America. Dietary niches were reconstructed for groups of mammals within the fossil euprimate competitive guild using measures of molar shape, which were determined to discriminate diets in two living mammalian communities. The degree of dietary niche separation among fossil mammals was then evaluated across a series of samples from the Bighorn Basin site in Wyoming just prior to, during, and after euprimate origination. Overall, the results of this study indicate that there was minimal dietary niche overlap between the earliest euprimates and contemporary non-euprimates. This suggests that the lack of dietary competition with other mammals played a significant role in the success of euprimates upon their origination in North America. In addition, this demonstrates that the key innovations exhibited by the earliest euprimates may have afforded an advantage only in that they enabled euprimates to exploit an open, available niche within the mammalian community. An additional component of this project involved examining the effects of the abiotic environment (e.g., temperature, rainfall) on early euprimate evolution. This is particularly important today, as predicting the effects of climate change on the world's living organisms is currently one of the most significant challenges we face, and primates are among those most at risk for extinction. This study determined that changes in the sizes of the dietary niches of early euprimates and other mammals over time actually paralleled major climatic shifts. In addition, results indicated that the pattern of dietary niche overlap within early euprimates contrasts with the niche structure observed in living groups. This signifies that the pattern of dietary niche overlap in the Eocene euprimate competitive guild (euprimates and their competitors) may represent only the beginnings of a dynamic process that altered the structure of this community for millions of years. Furthermore, it is these same abiotic and biotic processes that still influence, and will continue to influence, the composition and structure of mammalian guilds and communities of both the present and future. Thus, the changes in the fossil euprimate community as it responded to periods of global climate change can be directly compared with current climatically driven shifts in species interactions. Therefore, an understanding of this past event has the rare potential to inform current conservation efforts using a natural example. At this point, the results of this project have been disseminated to the scientific community via four conference presentations and publication of the associated dissertation. Moreover, an additional manuscript has been accepted for publication by a scientific journal and further publication of additional study results will be pursued. Finally, eight undergraduate students were mentored and trained by the co-PI in molding, casting, specimen preparation, curation, data collection, and use of medical-imaging software. In addition, this mentorship allowed these students to develop and improve their experience in research design, implementation, and presentation. This project was also presented to an undergraduate association, and this interaction encouraged and promoted interest in careers and further education and training in science, particularly evolutionary biology and biological anthropology, of undergraduate students. Under the guidance of the PI, both of these opportunities have aided in the professional development of the co-PI by contributing to her experience as a mentor, teacher, and project-organizer.
