Pony down: Did ancient warming shrink the earliest horses? Scientists are already noticing other animals getting smaller with contemporary climate change.
Image: /Danielle Byerley/Florida Museum of Natural History
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The first horses in North America would not have been able to hold their own in the Triple Crown. At just about 5.6 kilograms the Sifrhippus sandrae hoofed onto the scene some 56 million years ago about the size of a small dog.
But then a funny thing happened. In the next 130,000 years during the Paleocene–Eocene Thermal Maximum, these small equines got even smaller, reaching the tiny size of 3.9 kilograms—some 30 percent lighter than their initial heft. Just 45,000 years later, however, the genus had bulked up to seven kilograms. And the horses were not the only ones. Many other mammals in the area followed the same pattern.
These animals' sizes likely resulted from relatively rapid climate change, suggest the authors of a new study published online Thursday in Science.
The study "highlights the importance of temperature on evolution—particularly mammal evolution," says Felisa Smith, a professor of biology at the University of New Mexico in Albuquerque, who wrote an essay on the findings in the same issue of Science. And it adds a new high-resolution tracking of body size and temperature during a crucial—and long puzzling—time in geologic history.
View a slide show of the visual history of ancient miniature horses
Looking the small horse in the mouth
The researchers did not have complete skeletons to measure for all of the animals, so to track the size of the horses over time they looked at their teeth—in particular, their molars. "It turns out that teeth are much better than femurs," Smith says. A leg bone "does tell you something about size, but teeth are much better." And as far as teeth go, she says, "the best thing to know is the area of the first molar."
The teeth came from a fossil-rich area called Cabin Fork in Wyoming and are part of a substantial collection at the University of Florida built in part by study co-author Jonathan Bloch, an associate curator of vertebrate paleontology there. From the collection, the research team could estimate the size of about 44 diminutive adult horses.
Some 40 percent of other mammals in the area seem to have experienced similar shrinking and subsequent growth, notes co-author Ross Secord, a vertebrate paleontologist at the University of Nebraska–Lincoln. They stuck with the small horses, however, because they had much more solid records from which to accurately date the samples.
The researchers used oxygen isotopes left by freshwater in the fossils to track mean annual temperature from when the animals had been alive. In particular, they sampled the isotopes from teeth of a large, water-dwelling mammal Coryphodon. With these isotope readings, "you get a little, tiny window as to what the temperature was at that time," Smith says.
This close reading has excited Smith and others who have been tracking animal size over the ages. "Although we knew that temperature might set a maximum for body size," Smith says, the new findings actually present a mechanism—and do so in a very detailed manner, showing "how animals responded to a particular temperature at a particular place at a particular time."
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18 Comments
Add CommentSo where do we find the largest land animals today?
Reply | Report Abuse | Link to thisYou mean, the land animals we didn't hunt into extinction?
Reply | Report Abuse | Link to thisWell then the largest land animals ever to exist? Hint. It was not in the Arctic. The scientific conclusions of the article are ridiculous & yet another weak attempt to link everything to AGW & thereby shake lose research funding.
Reply | Report Abuse | Link to thisAll large animals found by humans become extinct, or very close to extinction. Humans are a viral infection which has to be actively treated by large doses of liberal angst and Green thought.
Reply | Report Abuse | Link to thisWe find the largest land animals in Africa, which fortunately didn't suffer as much from human technology and the inevitable destruction resulting from the rampant and uncontrolled development of tools, without regard for their environmental impact or danger to the users themselves - at least, until the arrival of the European exploiters.
Reply | Report Abuse | Link to thisBottom line--- we humans have already triggered the self perpetuating feedback loop that is driving the next global, mass extinction.
Reply | Report Abuse | Link to thisOur species is so adaptable that we will probably survive but in a very different world.
Following the demise of the dinosaurs both small and large mammals evolved and existed together, but there have been recurrent patterns of exploring the limits to size, followed by extinctions. Across many species this pattern seems to be at least partly independent of transient fluctuations in temperature.
Reply | Report Abuse | Link to thisFor example an early dog sized version similar to the rhino grew into the massive Brontotherium that was 8 feet tall at the shoulder and 16 feet long. It became extinct about 30 million years ago. Other members of the Titanotheres include the Arsinoitherium about 10 feet long and 6 feet tall at the shoulder. It also became extinct about 30 million years ago. The largest land mammal ever was the enormous rhino-like Indricotherium that was 18 feet tall at the shoulder and 33 feet long with a weight conservatively estimated at 20 tons. It had a long neck and could brouse from trees 28 feet tall. Its head was about 5 feet long. It became extinct about 18 million years ago. A little later the Chalicotherium had a horse-like head and feet with claws. By standing on its hind legs it could browse from trees 13 feet high. The waves of exploring the limits to size continued into the relatively recent past. The giant ground sloth could stand 18 feet tall and became extinct only 10,000 years ago. It weighed in at 8 tons. The Glyptodon, armoured similar to the armadillos became extinct about the same time. It was about 5 feet high and and 11 feet long. The pattern of exploring the limits to size also occurred with the marsupials in Australia. The giant wombat was 10 feet long, 6.5 feet high at the shoulder and weighed in at 3 tons. It became extinct about 46,000 years ago.
The invertebrate animals also explored the limits to size. There was an ancient Dragonfly with a 2.5 foot wingspread that lived 300 million years ago. A Nautilus 8.5 feet in diameter lived about 450 million years ago. The fossil record exhibits abundant evidence of exploring the limits to size from small to large. This learning from experience is reinvested in successive waves of more refined species whose descendents are with us today.
The fundamental pattern as to how this works is introduced at http://www.cosmic-mindreach.com/Darwin.html. Hierarchical patterns dominate the evolutionary process. Although adaptive pressures exist the hierarchical learning pattern is not consistent with random mutation and natural selection as the sole evolutionary determinant.
In addition to my previous post the pattern has displayed itself in human cultural evolution over the past few tens of thousands of years as generally described at http://www.cosmic-mindreach.com/Human_History.html
Reply | Report Abuse | Link to thisplease, don't write anymore unless you want to continue to seem an idiot
Reply | Report Abuse | Link to thisthe following is a joke - it is only a joke:
Reply | Report Abuse | Link to thisCould this have any bearing on the obesity epidemic?
the preceding has been a joke - it has only been a joke
- you may now go back to your previous, boring, activities -
The diminution in mammal size during this period probably had little to do with warmer temperatures. During the Mesozoic Era the largest terrestrial animals, the sauropods, thrived in an environment with warmer temperatures than we have today.
Reply | Report Abuse | Link to thisThe most likely cause for this phenomenon is a short term increase and then reduction in surface gravity as posited by the Gravity Theory of Mass Extinction.
I expect we will soon see serious consideration given to this theory by SA. Right up their alley.
Reply | Report Abuse | Link to thisI agree with you, pokerplyer! I love it when Liberals comment and ignorantly blame their own laziness for all the things they deem "bad" in life. Seriously, this article is about something that happened thousands of years BEFORE humans were on Earth, but still many Liberals are commenting on how it is the fault of humans. How can we be at fault if we did not exist yet?
Reply | Report Abuse | Link to thisI also love it when they try to take today's world issues and apply them to world issues that happened hundreds or thousands of years ago. As if humans have now or have always had the scientifically proven abilities to see or predict the future hundreds or thousands of years later.
For fun, if anyone wants to see true global warming in action...ask a Liberal to show you scientific facts that they use to justify their own statements. The Liberal you ask for proof will get then exhibit the three elements of global warming, they will get hot, stormy, and will end up with a meltdown in a childish rant on how unfair your being for wanting actual scientific proof; instead of just their "look around you" ignorant statements.
In the late Cretaceous period in southern Laramidia, the climate was warm, and there was ample water from the western mountains to foster a plethora of plant growth. The result was the evolution of very large dinosaurs, foraging and predating in close proximity. But in the Pleocene--Eocene Thermal Maximum you claim the climate produced small horses, and other mammals. Curious.
Reply | Report Abuse | Link to thisLook the fact of the matter is that the horses were not shrunk by the climate change that's a bunch of horse malarky. Just as the Ching Dog of the ncient China Civilization was BREAD DOWN, from 300 to 500 pounds, so was the Horse yet for different reasons. The key factor is "BREAD DOWN"!!!! Not climate change.
Reply | Report Abuse | Link to thisWhile Africa sports the largest living creatures, polar gigantism is in fact an actual phenomenon that does exist. The fauna of Antarctica have always been known to be significantly larger than their relatives at other latitudes, often several times the size. This has been observed in dinosaurs, birds, and invertebrates such as crabs.
Reply | Report Abuse | Link to thisAnd as for the Arctic-- that's pretty much frozen water there. It's difficult for creatures to stay there since it thaws out during the summer. However, consider the fauna of Canada-- polar bears and moose that are far larger than most bears or cervids.
Reply | Report Abuse | Link to thisOh No!!!!!! short horses!!!!!! what are we going to do?!?!?!
Reply | Report Abuse | Link to this