Environmental Change in the Great Plains: An Isotopic Record from Fossil Horses

Abstract

A B S T R A C TCarbon and oxygen isotope ratios of fossil horse tooth enamel from Nebraska and Texas show evidence for late Neogene environmental changes in the Great Plains. The earliest unambiguous C 4 dietary signal among Texas equids coincides with the age of the classic late Hemphillian-age Coffee Ranch fauna, which we suggest is ∼6.6 Ma based on volcanic ash correlations. C 4 vegetation was present in the diets of a small fraction of late Hemphillian equids in Nebraska and was thereafter ordinary in the diets of both Nebraska and Texas equids. There is no unequivocal evidence for abundant C 4 vegetation in the diets of pre-late Hemphillian equids, and we suggest that the ensuing dietary change reflects C 4 -biomass expansion in the latest Miocene. Carbon isotope ratios of post-Hemphillian horses in Nebraska can be divided into two statistically distinct populations on the basis of whether tortoise remains cooccur with horse remains, indicating that the two proxies (carbon isotopes and presence/absence of tortoises) record complementary environmental phenomena. The average d O record correlates with changes in ungulate diversity, the disappearance of crocodilians in Nebraska, and global change in the latest Miocene.

Nature's green revolution: the remarkable evolutionary rise of C4plants

“…This decrease corresponds to a fall in crossover temperature from greater than 40 to 17-21 8C (figure 2), and Quade & Cerling (1995) and (c) from Passey et al (2002). matches almost exactly with the dates of C 4 grass origins suggested by molecular clocks.…”

“…18 O values decline in the Middle to Late Miocene (13-5 Ma), and remain relatively stable from the Pliocene onwards (less than 5 Ma; figure 6f ), most likely reflecting a combination of physiological and behavioural changes in the animals and climatic change (Passey et al 2002). These patterns are also seen in palaeosol carbonates during the Late Miocene and Pliocene, once the effects of latitude have been removed by statistical procedures (Fox & Koch 2004).…”

“…13 C values can be interpreted as either a drought response in the C 3 food plants or a diet composed of up to 25% C 4 grasses (Passey et al 2002). The latter interpretation is consistent with d 13 C evidence from palaeosols (figure 6e), suggesting a 12-34% contribution by C 4 grasses to plant productivity (Fox & Koch 2003), and analyses based on the taxonomic identification of fossil phytoliths, indicating the dominance of C 3 grasses with a lesser such a long delay in the evolution of grazers like the modern bison, with their ability to subsist on a specialized C 4 diet, remains unclear (Passey et al 2002).…”

“…Some indication of a delay between the origin of C 4 -based diets and increasing C 4 plant productivity requires further investigation. It is also interesting to note that Late Miocene horses co-occurring with thermophile tortoises had a higher proportion of C 4 grasses in their diets (Passey et al 2002), perhaps suggesting heterogeneity in the Great Plains regional climate, and a greater abundance of C 4 grasses in warmer areas (Passey et al 2002). However, while these arguments are entirely reasonable, they cannot be the full explanation.…”

Carbon isotope fractionation between diet, breath CO2, and bioapatite in different mammals

“…The powders were rinsed in distilled water several times between and after these treatments, and were oven dried at 50e70 C. Rabbit and vole enamel was not treated because of sample size limitations. A comparison for modern tooth enamel of the treatment described above versus no treatment shows little isotopic difference, and no consistent isotopic offset between the two [28]. Koch et al [20] report a treated versus untreated offset of up to 0.3& for enamel treated using a slightly different method (2% NaOCl in place of 3% H 2 O 2 ).…”