Kathryn A. Hoppe

The isotopic ecology of late Pleistocene mammals in North America: Part 1. Florida

Mammoths and mastodons are common in Pleistocene deposits, yet these proboscideans and many other animals disappeared suddenly ≈10,000 years ago. In this study, we reconstruct the diets of proboscideans and associated mammals through isotopic analysis of carbonate in tooth enamel apatite in order to test nutritional hypotheses for late Pleistocene extinction. We analyzed specimens from six sites in Florida, ranging from full glacial (>21,000 BP) to late glacial (14,750 to 10,000 BP) age. The oxygen isotope composition of mammalian apatite covaries with meteoric water composition, which in turn varies with climate. Consequently, oxygen isotope analysis can be used to assess the potential for time-averaging or mixing of specimens from different geographic regions within fossil assemblages. The carbon isotope composition of an herbivore is controlled by the isotopic composition of the plants that it ingests. Carbon isotope analysis reveals that mastodons ate chiefly C3 plants, presumably trees, shrubs and herbs, whereas mammoths consumed chiefly C4 grass. Several nutritional hypotheses for late Pleistocene extinction entail the assumption that extinct taxa had specialized diets. The resource partitioning and focused feeding preferences of Floridas proboscideans corroborate this assumption, but they do not, in themselves, prove that nutritional stress was the cause of the late Pleistocene extinction.

Late Pleistocene mammoth herd structure, migration patterns, and Clovis hunting strategies inferred from isotopic analyses of multiple death assemblages

Abstract Many late Pleistocene fossil localities contain the remains of multiple mammoths. Some of these sites have been interpreted as representing the mass death of an entire herd, or family group, of mammoths. These assemblages have been cited as evidence of intense human predation and used to reconstruct mammoth population dynamics. However, these interpretations remain controversial because the taphonomic settings of many sites are still debated. To reconstruct the taphonomic setting of each site and the movement patterns of mammoths among sites, I used analyses of carbon, oxygen, and strontium isotope ratios in mammoth tooth enamel. The carbon isotopes of fossils vary with diet and local vegetation, oxygen isotopes vary with local climate, and strontium isotopes vary with local soil chemistry. If Pleistocene mammoths traveled together in small family groups, then mammoths from sites that represent family groups should have lower isotopic variability than mammoths from sites containing unrelated individuals. I tested this conjecture by comparing the isotopic variability among mammoths from two sites—one that represents the mass death of a single herd (Waco, Texas) and one representing a time-averaged accumulation (Friesenhahn Cave, Texas)—and then used these analyses to examine mammoths from three Clovis sites: Blackwater Draw, New Mexico; Dent, Colorado; and Miami, Texas. Low levels of carbon isotope variability were found to be the most diagnostic signal of herd/family group association. Although the variability of oxygen and strontium isotope ratios proved less useful for identifying family group assemblages, these signals did provide information about the movement patterns of individuals among different sites. High levels of variability in each of the isotope systems at Clovis sites suggest that all of the sites examined represent time-averaged accumulations of unrelated individuals, rather than the mass deaths of family groups. In addition, analyses of the mean isotope values of Clovis mammoths show that although most mammoths from Blackwater and Miami had similar values, the values of Dent mammoths were significantly different. This demonstrates that the Dent mammoths belonged to a separate population and suggests that Clovis mammoths did not routinely undertake long distance (≥600 km) migrations.

Isotopic analysis of tooth enamel carbonate from modern North American feral horses: implications for paleoenvironmental reconstructions

The accuracy of paleoenvironmental reconstructions based on isotope analyses of equid teeth is currently uncertain because the exact relationship between the isotope composition of modern feral equids and their environment has not been thoroughly studied. We analyzed the carbon and oxygen isotope values (δ13C and δ18O) of tooth enamel carbonate and the δ13C values of fecal samples from modern feral horses. We compared those values with the δ13C values of local vegetation and the δ18O values of local waters. Herds were studied in two contrasting localities: eastern Oregon, where grasslands consisted of 100% C3 species, and New Mexico, where >95% of the grasses were C4 species. Carbon isotope analyses of fecal material and tooth enamel suggest that horses consumed primarily grass, but some New Mexico horses also consumed significant amounts of shrubs and/or forbs. Microhistological analyses of fecal samples show that Oregon horses consumed 95% grass, and Oregon enamel δ13C values are consistent with a diet containing 100% C3 plants. Microhistological analyses of fecal samples from New Mexico indicate a diet averaging 75% grass, while enamel δ13C values suggest that diets averaged 85% C4 plants (range=72–97%). Thus, reconstructions of the C3/C4 ratio of grasses in ancient grasslands that are based on the δ13C values of fossil equid teeth may underestimate the abundance of C4 grasses. The mean δ18O values of tooth enamel paralleled the trends observed in the mean δ18O values of precipitation. However, the mean δ18O values of enamel carbonate from Oregon and New Mexico differed by only 3.3‰, which is less than the difference in the mean δ18O values of precipitation (6.5‰). In addition, the range of δ18O values within New Mexico enamel samples (6.5‰) was greater than the difference between mean enamel δ18O values at each site. Calculated values for the δ18O of water ingested by horses are 2–3‰ more positive than mean δ18O values for corresponding precipitation, suggesting that horses consumed waters that were enriched in 18O due to evaporation. While our results confirm that local climatic and hydrological conditions can influence the δ18O values of equid enamel, they also show that the δ18O values of equid teeth are not always a direct proxy for the isotope ratios of precipitation.

Evidence that inoceramid bivalves were benthic and harbored chemosynthetic symbionts

Inoceramid bivalves have been interpreted as both benthic and pseudoplanktonic. The comparison of 18O/16O and 13C/12C signatures of inoceramid shells with surface-dwelling and bottom-dwelling organisms should provide a simple means of resolving the controversy; however, we have found that the stable- isotope pattern is ambiguous. The bivalves have oxygen values similar to their contemporary benthic foraminifera but have carbon values similar to their contemporary planktonic foraminifera. We attempt to resolve this paradox by interpreting those inoceramids analyzed as benthic organisms that harbored chemosynthetic symbionts. A similar pattern of heavy oxygen and heavy carbon values is found in shell carbonate of some modern bivalves with chemosynthetic symbionts living around cold seeps off the coast of Oregon.

A paleoecological paradox: the habitat and dietary preferences of the extinct tethythere Desmostylus, inferred from stable isotope analysis

Abstract The Desmostylia, an extinct order of mammals related to sirenians and proboscideans, are known from the late Oligocene to late Miocene of the North Pacific. Though often categorized as marine mammals on the basis of fossil occurrences in nearshore deposits, reconstructions of desmostylian habitat and dietary preferences have been somewhat speculative because morphological and sedimentological information is limited. We analyzed the carbon, oxygen, and strontium isotope compositions of enamel from Desmostylus and co-occurring terrestrial and marine taxa from middle Miocene sites in California to address the debate surrounding desmostylian ecology. The δ13C value of tooth enamel can be used as a proxy for diet. Desmostylus had much higher δ13C values than coeval terrestrial or marine mammals, suggesting a unique diet that most likely consisted of aquatic vegetation. Modern aquatic mammals tend to exhibit lower variability in δ18O values than terrestrial mammals. Both fossil marine mammals and Desmostylus exhibited low δ18O variability, suggesting that Desmostylus spent a large amount of time in water. Finally, the Sr isotope composition of marine organisms reflects that of the ocean and is relatively invariant when compared with values for animals from land. Sr isotope values for Desmostylus were similar to those for terrestrial, rather than marine, mammals, suggesting Desmostylus was spending time in estuarine or freshwater environments. Together, isotopic data suggest that Desmostylus was an aquatic herbivore that spent a considerable portion of its life foraging in estuarine and freshwater ecosystems.

Reconstructing grassland vegetation and paleotemperatures using carbon isotope ratios of bison tooth enamel

Carbon isotope ratios ( 13 C values) of herbivores reflect the 13 C values of dietary plants, and the 13 C values of grazers (animals that consume 90% grass) reflect the local abundance of C3 versus C4 grasses. Because grassland C3/C4 ratios correlate with climate, the 13 C of fossil grazers may serve as a proxy for reconstructing paleoclimates and paleovegetation patterns. However, the accuracy of environmental reconstructions based on herbivore 13 C values is often uncertain, because the relationship between the 13 C of many animals and the abundance of C4 and C3 grasses has not been precisely quantified. We analyzed the 13 C of tooth enamel carbonate from modern bison (Bison bison bison) from nine localities in the United States. The C4 grass biomass at these sites ranged from 1% to 95% of the total grass biomass. The mean 13 C of enamel for each population correlated well with the local abundance of C4 grasses and with variations in mean annual temperatures. The variability of enamel 13 C values did not differ among habitats and was not correlated with the abundance of C4 grasses. These results demon- strate that analyses of the 13 C values of fossil bison can be used as a quantitative proxy for reconstructing grassland C3/C4 ratios and paleotemperatures, and they will serve as a baseline for interpreting the 13 C of fossil bison and other large herbivores in North

Climate, CO2, and the history of North American grasses since the Last Glacial Maximum

Bisoscapes: Bison isotopes show that temperature and precipitation outweigh CO2 changes for North American grass landscapes. The spread of C4 grasses in the late Neogene is one of the most important ecological transitions of the Cenozoic, but the primary driver of this global expansion is widely debated. We use the stable carbon isotopic composition (δ13C) of bison and mammoth tissues as a proxy for the relative abundance of C3 and C4 vegetation in their grazing habitat to determine climatic and atmospheric CO2 controls on C4 grass distributions from the Last Glacial Maximum (LGM) to the present. We predict the spatial variability of grass δ13C in North America using a mean of three different methods of classification and regression tree (CART) machine learning techniques and nine climatic variables. We show that growing season precipitation and temperature are the strongest predictors of all single climate variables. We apply this CART analysis to high-resolution gridded climate data and Coupled Model Intercomparison Project (CMIP5) mean paleoclimate model outputs to produce predictive isotope landscape models (“isoscapes”) for the current, mid-Holocene, and LGM average δ13C of grass-dominated areas across North America. From the LGM to the present, C4 grass abundances substantially increased in the Great Plains despite concurrent increases in atmospheric CO2. These results suggest that changes in growing season precipitation rather than atmospheric CO2 were critically important in the Neogene expansion of C4 grasses.