The case of the shrinking horse: Global warming seen as the likely culprit

Comparing fossil size in the early horse Arenahippus: A larger non-ETM2 Arenahippus specimen is held in the left hand, a smaller mid-ETM2 Arenahippus specimen in the right hand. Photo: University of New Hampshire
Comparing fossil size in the early horse Arenahippus: A larger non-ETM2 Arenahippus specimen is held in the left hand, a smaller mid-ETM2 Arenahippus specimen in the right hand. Photo: University of New Hampshire

A species of horse that lived 50 million years ago shrank in size during a warm spell on Earth, adding to a growing body of evidence that this may be a mammalian response to rising global temperatures.

While size reductions in mammals have previously been linked to large global warming events, new research has found that this evolutionary process can happen in smaller events known as hyperthermals, indicating an important pattern that could help us understand the underlying effects of current human-caused climate change.

The researchers collected teeth and jaw fragments in the fossil-rich Bighorn Basin region of Wyoming.

Their focus was on several early mammals including Arenahippus, an early horse the size of a small dog, and Diacodexis, a rabbit-sized predecessor to hoofed mammals.

Using the size of the molar teeth as a proxy for body size, the researchers found a statistically significant decrease in the body size of these mammals during a second, smaller hyperthermal, called the ETM2.

Arenahippus decreased by about 14 percent in size, and the Diacodexis by about 15 percent, the study team reported in the journal Science Advances.

“We know that during the largest of these hyperthermals, known as the Paleocene-Eocene Thermal Maximum, or PETM, temperatures rose an estimated 9 to 14 degrees Fahrenheit and some mammals shrank by 30 percent over time, so we wanted to see if this pattern repeated during other warming events,” said Abigail D’Ambrosia, a doctoral student at the University of New Hampshire and lead author of the study.

An Arenahippus jaw fragment (with 2nd and 3rd molars), as discovered in the field. For scale: Tip of chisel is about 1cm wide. Photo:: University of New Hampshire
An Arenahippus jaw fragment (with 2nd and 3rd molars), as discovered in the field. For scale: Tip of chisel is about 1cm wide. Photo: University of New Hampshire

“The hope is that it would help us learn more about the possible effects of today’s global warming.

“We found evidence of mammalian dwarfism during this second hyperthermal. However, it was less extreme than during the PETM,” she said.

“During ETM2, temperatures only rose an estimated 5 degrees Fahrenheit and it was shorter, only lasting 80,000 to 100,000 years, about half as long as the larger PETM.

“Since the temperature change was smaller, this suggests there may be a relationship between the magnitude of a global warming event and the degree of associated mammal dwarfism.”

University of Michigan paleontologist Philip Gingerich and his students collected some of the fossils analyzed in the study. They worked with other team members to find fossils from the same sediment layers that yielded isotopic evidence of climate change.

The fossils used in the study are part of the University of Michigan Museum of Paleontology’s collections.

Gingerich, who co-authored the paper, said: “What is important about these findings is that they replicate, extend and hence strengthen observations first made in connection with the slightly older and larger PETM event of global greenhouse warming, showing that many mammals respond to global warming by evolving to be smaller.”

Researchers propose that the observed change in animal body size could have been an evolutionary response to create a more efficient way to reduce body heat. A smaller body size would allow the animals to cool faster. Nutrient availability and quality in plants may have also played a role.

Previous research shows that both the PETM and the ETM2 hyperthermals coincided with increased levels of carbon dioxide in the atmosphere. That could have limited nutrient quality in plants, which may have contributed to the smaller mammal body size.

Hydrological records during the PETM also suggest less rainfall and drought, which could have led to drier soils and even wildfires. This may have affected vegetation growth and, possibly, offspring size in mammals. After both hyperthermal events, body sizes of all mammals rebounded.

The carbon dioxide released during both hyperthermals has a similar footprint to today’s fossil fuels.

Researchers hope that developing a better understanding of the relationship between the change in mammalian body size during those events and today’s greenhouse gas-induced global warming may help to better predict possible future ecological changes in response to today’s climate changes.

In addition to D’Ambrosia and Gingerich, authors of the Science Advances paper are William Clyde of the University of New Hampshire, Henry Fricke of Colorado College, and Hemmo Abels of Delft University of Technology, in the Netherlands.

Preliminary findings of the study were presented at the Society of Vertebrate Paleontology’s 2013 annual meeting.

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