Tiny shrinking horses

English: This reproduction of a painting of an...
Reconstruction of Sifrhippus. Image via Wikipedia

The earliest known ancestors of modern horses occur in Palaeogene mammal-rich terrestrial sediments of the northwestern US, particularly those of the Wind and Bighorn Basins. The first with clear horse-like features was Sifrhippus (formely Eohippus, or Hyracotherium), but famously it had four hoofed toes and was about the size of a household cat. Subsequent development to a single load-bearing toe has long formed one of the classic cases for evolution. Sifrhippus lived at the end of the Palaeocene. From the large numbers of well-preserved skeletons, this was a herding animal. The large numbers of fossils have also made it a candidate for testing a hypothesis that individuals of a mammal and bird species become smaller as climate warms: Bergmann’s Rule. The background to this view is that in modern warm-blooded or endothermic animal species individuals tend to be smaller the closer they are to the Equator.

The end of the Palaeocene was marked by a now well-documented rise in global surface temperature that left a marked sign of increased 13C in sediments spanning the Palaeocene-Eocene boundary, which is widely believed to have resulted from massive exhalations of methane from the seafloor. Bergmann’s Rule arose because there appears to be a general decrease in size of most mammal fossils through the P-E Thermal Maximum.  Sifrhippus lived through the event and indeed did undergo 30% decrease in size at the start of the carbon-isotope shift marking the PETM. Moreover, after the isotopic excursion its fossils indicate a 70% increase in size (Secord, R. and 8 others 2012. Evolution of the earliest horses driven by climate change in the Paleocene-Eocene Thermal Maximum. Science, v. 335, p. 959-962).

The study was of Sifrhippus and other mammals over a period representing several thousand generations. It broke new ground in two ways: it used the size of the horses’ teeth to estimate body mass, and teeth of a variety of mammals afforded systematic measurements of both carbon and oxygen isotopes. The carbon isotopic analyses pin-pointed the span of the PETM locally, while oxygen isotopes charted local changes in average temperature. The results show remarkable coherence with Bergmann’s Rule, but reveal other interesting aspects of the PETM in North America. Oxygen-isotope in the teeth of different mammal species give some idea of their diet and habitat. Sifrhippus shows the highest enrichment of 18O in its teeth, which suggests that it ate leaves from which water evaporation selectively removed the lighter 16O, i.e. in open, dry areas. Another ubiquitous fossil, Coryphodon, consistently has lower 18O than other mammals, signifying that it was water-loviong and ate aquatic plants, i.e. not subject to evaporation. Matching O-isotopes for the two species across the PETM shows a greater shift in 18O for Sifrhippus than for Coryphodon, which suggests that hidden in the O-isotope record of temperature is information about rainfall variations during the PETM. To further support Bergmann’s Rule, changes in the size of Sifrhippus, do not correlate with the aridity index. So it seem that heat alone was responsible for dwarfing – the other possibility considered by the researchers was that decreased availability or quality of diet could have been responsible.

Reconstruction of Coryphodon. Image via Wikipedia