Evol Ecol Res 18: 201-223 (2017)     Full PDF if your library subscribes.

Hypsodonty, horses, and the spread of C₄ grasses during the middle Miocene in southern California

Robert S. Feranec¹ and Darrin C. Pagnac²

¹Research and Collections, New York State Museum, Albany, New York, USA and  ²Department of Geology and Geological Engineering, South Dakota School of Mines  and Technology, Rapid City, South Dakota, USA

Correspondence: R.S. Feranec, Research and Collections, New York State Museum, Albany, NY 12230, USA. email: feranec@gmail.com

ABSTRACT

Background: C₄ grasses were not abundant in North America during the middle Miocene (c. 15 Ma). They did not become abundant until around 7 Ma. One can analyse stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope values in the enamel of fossil horse teeth to determine the extent to which horses were eating C₄ grasses even during the period before those grasses became abundant.

Questions: In southern California, what proportion of a middle Miocene horse’s diet was made up of C₄ grasses? Was the amount enough to influence the size and shape of horse teeth?

 Organisms: Eighty-five specimens of five fossil horse species – Acritohippus stylodontus, Archaeohippus mourningi, Merychippus californicus, Scaphohippus intermontanus, and Scaphohippus sumani – from the middle Miocene (c. 16 Ma) of southern California (i.e. Barstow Formation, Cajon Valley Formation, and Temblor Formation).

 Methods: To determine if C₄ grasses were present in middle Miocene horse diets, we analysed stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope values from the enamel of the fossils. If the result did indicate C₄ foraging at a locality, we modelled the percentage of C₄ grasses in equid diets using Stable Isotope Analysis in R (SIAR) v.4.2.2.

 Results: Modelled percentage C₄ in equid diets was <20%. Each formation was statistically significantly different from the others in terms of δ¹³C values. Barstow specimens had the highest values, those from Cajon Valley the lowest, and those from Temblor were intermediate. Those results indicate that horses ate C₄ grasses within the Barstow and possibly the Temblor Formation but not the Cajon Valley Formation. Within the Barstow sample, Scaphohippus sumani had statistically significantly lower δ¹³C but statistically significantly higher δ¹⁸O values than Acritohippus stylodontus, suggesting a higher proportion of C₃ grasses in the diet of Scaphohippus sumani versus a higher proportion of C₄ grasses for Acritohippus stylodontus. The latter species also had higher tooth crowns, consistent with a diet richer in C₄ grasses. There were no statistically significant differences between species at Cajon Valley for either δ¹³C or δ¹⁸O. The δ¹³C values for Merychippus californicus suggest that the habitats of the Temblor Formation had a low percentage (<6%) of C₄ plants.

 Conclusions: C₄ grasses lived in the mid-Miocene landscape in southern California up to 8 million years before the rapid increase in C₄ ecosystems that occurred worldwide about 7 to 5 Ma. Horses foraged these grasses, so hypotheses related to horse morphological evolution must take C₄ plants into account.

Keywords: C₃ plants, C₄ plants, Equidae, grasslands, hypsodonty, Miocene, stable isotope.

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