GOMPHOTHERE PALEOECOLOGY IN NORTH AMERICA AS INFERRED FROM STABLE ISOTOPE ANALYSIS

Stable carbon isotopes preserved in enamel can be used to reconstruct the diets of extinct organisms; over the past few decades, this method has been used extensively to understand the dietary preferences of extinct proboscideans, including mammoths and mastodons. While the dietary ecologies of these taxa are well-understood, gomphothere diets are less understood and more variable; populations of different ages and locations often display varying dietary preferences. Further, most paleoecological analyses on gomphotheres come from South America, where they survived later and were the only resident proboscideans. As a result, North American gomphothere dietary ecology remains poorly resolved.

To assess North America gomphothere dietary variability through time, we compiled published stable isotope data from the literature and supplemented these data with 125 new stable carbon and oxygen isotope records from Florida and Texas. These data include multiple genera, including Amebelodon, Gomphotherium, Gnathabelodon, Stegomastodon, Rhychotherium, and Cuvieronius, and span a period of approximately 16 million years. Results show disparate stable carbon signatures amongst the various gomphothere taxa. For example, Gomphotherium shows a shift from C₃ browsing to C₃/C₄ mixed throughout the Miocene. Rhynchotherium from Florida during the late Miocene is primarily a mixed feeder; in the Pliocene of Texas, Rhynchotherium diets vary markedly, with both C₃ browsing and C₄ grazing signatures apparent. Stegomastodon similarly shows both grazing and mixed feeding habits in the late Pliocene of Texas. Cuvieronius is here confirmed to have been a mixed feeder during the Pleistocene, with 89% of specimens having a δ¹³C_PBD of between -9.0‰ and -2.0‰.

Along with a depauperate fossil record and limited paleogeographic range during the Pleistocene, these results suggest that gomphotheres consumed a wide range of resources and were not specialized on either grasses or browse. Their potential "jack of all trades and master of none" feeding strategy may have proved challenging when competing with the primarily browsing mastodons and grazing/mixed feeding mammoths in North America, leading to the early extirpation of the gomphothere from North America.