Artist’s reconstruction of Sifrhippus sandrae (right) touching noses with a modern Morgan horse (left) that stands about 5 feet high at the shoulders and weighs approximately 1,000 lbs. Sifrhippus was the size of a small house cat (about 8.5 lbs) at the beginning of the Eocene (approximately 55.8 million years ago) and is the earliest known horse. Image: Danielle Byerley, Florida Museum of Natural History.
Sifrhippus, the earliest known horse, first appeared in the forests of
North America more than 50 million years ago, it would not have been
mistaken for a Clydesdale. It weighed in at around 12 lbs—and it was
destined to get much smaller over the ensuing millennia.
lived during the Paleocene-Eocene Thermal Maximum, a 175,000-year
interval of time some 56 million years ago in which average global
temperatures rose by about 10 F, caused by the release of vast amounts
of carbon into the atmosphere and oceans.
a third of mammal species responded with significant reduction in size
during the PETM, some by as much as one-half. Sifrhippus shrank by about
30% to the size of a small house cat (about 8.5 lbs) in the PETM’s
first 130,000 years and then rebounded to about 15 lbs in the final
45,000 years of the PETM.
have assumed that rising temperatures or high concentrations of carbon
dioxide primarily caused the phenomenon in mammals during this period,
and new research led by Ross Secord of the University of
Nebraska-Lincoln and Jonathan Bloch of the Florida Museum of Natural
History at the University of Florida in Gainesville offers new evidence
of the cause-and-effect relationship between temperature and body size.
Their findings also offer clues to what might happen to animals in the
near future from global warming.
In a paper published in the Feb. 24 issue of the journal Science,
Secord, Bloch and colleagues used measurements and geochemical
composition of fossil mammal teeth to document a progressive decrease in
Sifrhippus‘ body size that correlates very closely to temperature
change over a 130,000-year span.
associate curator of vertebrate paleontology at the Florida Museum of
Natural History, said multiple trails led to the discovery.
was the fossils themselves, recovered from the Cabin Fork area of the
southern Bighorn Basin near Worland, Wyo. Stephen Chester, then an
undergraduate student at Florida, now an anthropology Ph.D. candidate at
Yale and a co-author on the paper, had the task of measuring the
horses’ teeth. What he found when he plotted them through time caught
Bloch and Secord by surprise.
pointed out that the first horses in the section were much larger than
those later on,” Bloch recalled. “I thought something had to be wrong,
but he was right—and the pattern became more robust as we collected more
postdoctoral researcher in Bloch’s lab for the first year of the
project, Secord performed the geochemical analysis of the oxygen
isotopes in the teeth. What he found provided an even bigger surprise.
was absolutely startling when Ross pulled up the first oxygen isotope
data,” Bloch said. “We looked at the curve and we realized that it was
exactly the same pattern that we were seeing with the horse body size.
the first time, going back into deep time—going back 10s of millions of
years—we were able to show that indeed temperature was causing
essentially a one-to-one shift in body size within this lineage of
horse. Because it’s over a long enough time, you can argue very strongly
that what you’re looking at is natural selection and evolution—that
it’s actually corresponding to the shift in temperature and driving the
evolution of these horses.”
Secord, who came to UNL in 2008 as an
assistant professor of Earth and atmospheric sciences and curator of
vertebrate paleontology at the University of Nebraska State Museum, said
the finding raises important questions about how plants and animals
will respond to rapid change in the not-too-distant future.
has implications, potentially, for what we might expect to see over the
next century or two, at least with some of the climate models that are
predicting that we will see warming of as much as 4 C (7 F) over the
next 100 years,” he said.
predictions are based largely on the 40% increase of atmospheric carbon
dioxide levels (from 280 to 392 parts per million) since the start of
the Industrial Revolution in the mid-19th century.
Ornithologists, Secord said, have already started to notice that there may be a decrease in body size among birds.
of the issues here is that warming (during the PETM) happened much
slower, over 10,000 to 20,000 years to get 10 degrees hotter, whereas
now we’re expecting it to happen over a century or two,” Secord said.
“So there’s a big difference in scale and one of the questions is, ‘Are
we going to see the same kind of response?’ Are animals going to be able
to keep up and readjust their body sizes over the next couple of
temperatures are not the only change animals will have to adapt to,
Secord said. Greenhouse experiments show that increased atmospheric
carbon dioxide lowers the nutritional content of plants, which he said
could have been a secondary driver of dwarfism during the PETM.
co-authors on the paper are Doug M. Boyer of Brooklyn College, Aaron R.
Wood of the Florida Museum of Natural History, Scott L. Wing of the
Smithsonian National Museum of Natural History, Mary J. Kraus of the
University of Colorado-Boulder, Francesca A. McInerny of Northwestern
University, and John Krigbaum of the University of Florida.
The research was funded by grants from the National Science Foundation, with additional support from UNL.