Public Release:  Global warming led to dwarfism in mammals -- twice

University of Michigan








Teeth and jaw fossils of early hoofed mammals and primates that spanned this later climatic event were collected in Wyoming's Bighorn Basin, and the size of molar teeth was used as a proxy for body size. The researchers found that body size decreased during ETM2, but not as much as the dwarfism seen in PETM fossils.

For example, the study revealed that a lineage of early horses the size of a small dog, called Hyracotherium, experienced a body-size decrease of about 19 percent during ETM2. The same horse lineage showed a body-size decrease of about 30 percent during the PETM. After both events, the animals rebounded to their pre-warming size.

"Interestingly, the extent of mammalian dwarfism may be related to the magnitude of the hyperthermal event," said team member Abigail D'Ambrosia of the University of New Hampshire.

An ancient ungulate called Diacodexis decreased about 20 percent in size during ETM2, and the primate Cantius decreased 8 percent.

The burning of fossil fuels and the resulting release of heat-trapping greenhouse gases--mainly carbon dioxide--is blamed for present-day climate warming. The ancient warming events may have been caused by the release of seabed methane clathrates, a kind of methane ice found in ocean sediments, though this topic remains an area of active research, Gingerich said. Methane is a more potent greenhouse gas than carbon dioxide, and atmospheric methane is eventually transformed into carbon dioxide and water.

The parallels between ancient hyperthermals and modern-day warming make studies of the fossil record particularly valuable, said team member Will Clyde of the University of New Hampshire.


In 2006, Gingerich proposed that mammalian dwarfing could be a response to the lower nutritional value of plants grown under elevated carbon dioxide levels. Under such conditions, plants grow quickly but are less nutritious than they would normally be.

Animals eating such plants might adapt by becoming smaller over time. Evidence from the ETM2 fossils is consistent with that hypothesis, and research on the topic is ongoing, Gingerich said.


The research was funded by the National Science Foundation (EAR0958821), Geological Society of America, Paleontological Society and Sigma Xi.

Philip Gingerich:
