The gomphotheres were a diverse and widespread group of proboscideans occupying Eurasia, North America, and South America throughout the Neogene. Their decline was temporally and spatially heterogeneous and the gomphotheres ultimately became extinct during the late Pleistocene; however, the genus Cuvieronius is rarely represented in late Pleistocene assemblages in North America. Two alternative hypotheses have been invoked to explain this phenomenon: (1) competitive exclusion by sympatric mammoths and mastodons, or (2) ecologic displacement due to an environmental transition from closed forests to open grasslands. To test whether competition for resources contributed to the demise of North American Cuvieronius, we present herein a large collection of stable isotope and dental microwear data from populations occupying their Pleistocene refugium in the Atlantic Coastal Plain. Results suggest that Cuvieronius consumed a wide range of resources with variable textural and photosynthetic properties and was not specialized on either grasses or browse. Further, we document evidence for the consumption of similar foods between contemporaneous gomphotheres, mammoths, and mastodons. The generalist feeding strategy of the gomphotheres likely facilitated their high Miocene abundance and diversity. However, this "jack of all trades and master of none" feeding strategy may have proved challenging following the arrival of mammoths and likely contributed to the extirpation of Cuvieronius in North America.

Supplementary Appendix 1 - Explanation of Paleoecological Proxies

An introduction and background to the paleoecological proxies used in this analysis, which includes (1) dental microwear, (2) stable carbon isotopes, and (3) stable oxygen isotopes. Application references included as well as more in-depth explanations to some of the nuances which may be observed in the data presented within the manuscript.

Supplementary Tables 1-2

Supplementary Tables 1 and 2, which include (1) the raw specimen information and data used in the analyses and (2) a summary of the p-values for statistical comparisons amongst proboscidean samples.

High-Resolution Color Bitmap Images of DMTA Sample Surfaces

Includes: (1) Cuvieronius hyodon (Sample # TMM 47200-172), (2) Mammut americanum (Sample # UF 80004), and (3) Mammuthus columbi (Sample # UF 86825).