True Grit

I read with considerable interest the Science News Focus article entitled“Timing Complicates History of Horses” (26 November 2004, p. 1467), by Erik Stokstad. Though he accurately presents Stromberg’s (1) compelling paleobotanical argument calling for reconsideration of the notion that equid tooth crown height parallels grassland expansion, I believe this article overlooks the chief explanation for the advent of hypsodonty. Grazing on abrasive sandy substrates such as gritty prairie soils more likely predicts the advent of hypsodont cheek teeth than does the change to a more siliceous diet. Modern horses from arid climates regularly consume significant amounts of grit (2) along with their diet which can lead to a condition known as sand colic, a leading cause of mortality in captive horses. A gritty mix of minerals harder than apatite seems far more likely an impetus for tall teeth than opal phytoliths alone. This hypothesis could be addressed by correlating the biostratigraphic position of all hypsodont taxa with the clastic composition (quartz and feldspar) of concurrently deposited paleosols. The Miocene paleopedological framework has been extensively detailed (3-5) such that this task should not be all that difficult to test. Furthermore, measures of cursoriality of hypsodont taxa could then be correlated with various substrates to get a better picture of the expanse of grassland biome during the Tertiary.

Paleontologists have long hypothesized that crown height might be a reliable proxy for paleodiets (6-7). Yet, notable exceptions have been recognized including browsing hypsodont llamas, camels and grazing bunodont kangaroos (8) which rightfully cast doubt on the reliability of lone fossil morphometric studies. Previous paleobotanical studies demonstrate that drought adapted C4 grasses with undeniable Kranz anatomy do not appear in the fossil record until around 5 million years ago (9). Although successfully shedding light on the origin of C4 metabolism around 5-7 million years ago (10-11), stable isotopic investigations of Great Plains fossils have generally failed to elucidate the precise cause of hypsodonty. Therefore, the Stromberg study (1) should come as little surprise since morphometric and stable isotopic analyses of both fossil horse enamel (10-11) and organic matter (12) have not been able to closely correlate the evolution of hypsodonty with the advent of the savanna grassland biome in North America. The presence of open grasslands on the Great Plains4 million years before hypsodonty evolved implies the earliest equid grazers such as Parahippus(13) still consumed a significant portion of browse in their diet or consumed the softer C3 sedges and marsh grasses off less abrasive substrates early on in their adaptive radiation towards a fully grazing lifestyle. My present research does not afford me the luxury to further investigate these paleontological hypotheses, but nor does it dissuade an interest in the fascinating history of the family Equidae, their remarkable hypsodont teeth and the profound paleoecological implications of this line of inquiry.

Shawn G. Clouthier

University of Michigan, Department of Internal Medicine, 1500 East Medical Center Drive, 6410 CGC, Ann Arbor, MI 48109–0942, USA.

References and Notes

  1. C.A.E. Stromberg, Palaeogeography Palaeoclimatology Palaeoecology 177, 59-75 (2002).
  2. Modern horses typically consume 1.5-2.5% of their body weight (~25 lbs forage per day) exclusively nipped from the substrate. If just a minor fraction of this graze contains roots and adherent soil horses may easily consume a pound or more of grit daily.
  3. G. J. Retallack, Palaeogeography Palaeoclimatology Palaeoecology 207, 203-237 (2004).
  4. G. J. Retallack, Palaeogeography Palaeoclimatology Palaeoecology 183, 329-354 (2002).
  5. G. J. Retallack, Palaios12(4), 380-390 (1997).
  6. G.G. Simpson, Horses: The Horse Family in the Modern World and through Sixty Million Years History, Oxford Press (1951).
  7. R.A. Stirton, Journal of Mammalogy 21(4), 445-448 (1940).
  8. R.S. Feranec, Paleobiology 29(2), 230-242 (2003).
  9. E.M.V. Nambudiri, W.D. Tidwell, B.N. Smith and N.P. Hebbert, Nature 276, 816-817 (1978).
  10. B.J. MacFadden,Fossil Horses: Systematics, Paleobiology and Evolution of the Family Equidae.CambridgeUniversity Press (1992).
  11. T.E., Cerling, J. M. Harris, B. J. MacFadden, Carbon isotopes, diets of North American equids, and the evolution of North American C4 grasslands. In Stable Isotopes and the Integration of Biological, Ecological, and Geochemicial Processes, Bios Scientific Publishers (1998).
  12. S.G. Clouthier,Carbon and Nitrogen Isotopic Evidence for Tertiary Grassland Distributions and the Evolution of Hypsodonty in North American Great Plains Horses (32 Ma to Recent)(1994).
  13. See the UFLMNH Fossil Horse Cybermuseum website ( for a description of the various members of the Equidae.