The present is the key to the past. Every geologist worth their sodium chloride knows this. The famous phrase, a distillation of the principle of uniformitarianism, can be seen emblazoned on mugs, bumper stickers, and more in college geology departments around the country. But while it’s true that phenomena currently in action can help us understand what transpired during prehistory, the flow of clues isn’t one way. The past may very well be the key to the future, especially when we want a preview of what can happen during periods of rapid climate change. Given that our species is now responsible for turning up the global thermostat, it'd be wise to look to the past for what we might expect of nature's future.

During the dawn of the Age of Mammals, around 56 million years ago, the global temperature rapidly stabbed upwards. In about 100,000 years temperatures rose over 9 degrees Fahrenheit . Paleontologists know this as the Palaeocene/Eocene Thermal Maximum (PETM for short), and there’s hardly a better place to see its effects than the deserts of Wyoming. The fossil localities here are famous for producing a detailed sequence of species from this time, recording how life responded to rapid climate change.

Insects, for example, fared quite well. So much so that plant fossils from the PETM show a spike in holes and divots created by hungry, hungry arthropods. And then there are the mammals. The mammals that lived in the ancient Wyoming basins during this heat wave were smaller than those that came before or after. The fauna seem to have shrunk in the heat, like biological Shrinkydinks.

But what does this change really mean? Up until now, paleontologists have considered two hypotheses. It could be that the larger mammal species evolved to become smaller over time in a straight-line fashion called anagenesis. Then again, the smaller species could be closely-related immigrants that had previously lived in warmer habitats and were able to thrive as the larger species went extinct. Either way, the hypothesis is that smaller-bodied mammals were probably better able to shed heat and cope with altered nutritional values of plants that go along with high CO2 levels.

But the new study by Brian Rankin and colleagues looks at another possibility – species selection. The logic is the same as that of natural selection, but bumped up one level. Just like individuals, the argument goes, some species will vary in ways that make them more successful in splitting off descendant species than others. In this case, large-bodied mammal species would have suffered in the hothouse world, while the species that were already small would have spun off an increased number of new lineages. While the large species went extinct, the small species would have proliferated.

Fortunately the Bighorn and Clarks Fork Basin faunas have been so extensively studied that paleontologists have been able to reconstruct body sizes, ancestor-descendant relationships (which is a rare feat), and identify immigrant species. This is what decades of fieldwork leads up to – a dataset of over 2,000 localities, 5,000 specimens, and 50 species carefully arranged according to the times and places those mammals lived. Plugging all that into a modified version of what naturalists call the Price equation, Rankin and colleagues were able to parse why the ancient beasts became downsized.

Climate Change
Climate change through the Cenozoic. Credit: Robert A. Rohde Wikipedia (CC BY-SA 3.0)

There wasn’t a single reason why mammal body size changed during this narrow span of Cenozoic time. The primary cause, Rankin and colleagues conclude, is the immigration of small species into the Bighorn and Clarks Fork Basins as the large native species disappeared. These were mammals like the archaic hoofed herbivore Ectocion parvus and the primate Teilhardina brandti. The paleontologists also found evidence for some native species, such as the carnivore Viverravus politus, becoming smaller with time.

Now here’s the strange part. The paleontologists found some influence of species selection in these basins, but it was in favor of larger mammals. This was a one-two punch of extinction and evolution. Some small animals, like the early primate Carpolestes simpsoni, went extinct at the same time that large ones, such as the superficially rodent-like Azygonyx, were leaving big descendant species.

Without species selection causing some large species to more successfully leave big-bodied descendants, the mammal fauna of the PETM would have been even smaller than what was left behind in the rock. And as the simmering heat decreased, species selection still favored larger species, with anagenetic change and immigration being less important. This might mean that species selection was a relative constant during the days of the Palaeocene and Eocene but became briefly suppressed when intense heat threw a brief advantage to smaller mammals.

Similar influences may soon come into play, if they haven’t already. During very short time frames, Rankin and coauthors write, the influence of climate change can be see within species. But in the long term – the extended effects that we can see coming down the line from anthropogenic climate change – species selection starts to come into play. Which species go extinct and which are able to keep spinning off descendants begins to shape the world in new ways. There’s no set path to evolution. No destiny. But by looking to the deep past, maybe we can begin to perceive the rough outlines of the new world we’re inadvertently creating.

For more, check out Shaena Montanari’s post on the same study.


Rankin, B., Fox, J., Barron-Ortiz, C., Chew, A., Holroyd, P., Ludtke, J., Yang, X., Theodor, J. 2015. The extended Price equation quantifies species selection on mammalian body size across the Palaeocene/Eocene Thermal Maximum. Proceedings of the Royal Society B. doi: 10.1098/rspb.2015.1097