August 1, 2008

Modern Species Diversity May Offer Window to Past


Scientists hope to gain insight into fossil record by studying current size ranges

By Sue Vorenberg

The New Mexican

Some would say it's impossible to draw a picture of something when you can only see

2 percent of it.

With that little information, you might just see a little color, or an edge, or a blue chunk of sky in the background.

But impossible or not, that's very much like the picture paleontologists have to work with when they try to rebuild the planet's distant biological past.

"The problem with any analysis is that the fossil record is only about 2 percent complete," said Spencer Lucas, paleontology curator and interim director of the New Mexico Museum of Natural History and Science in Albuquerque. "That makes it hard to speculate."

Still, that hasn't stopped paleontologists and scientists from trying to use what little information they have.

And a new paper published July 18 in the journal Science tries to squeeze that data even further.

Authors Aaron Clauset, a postdoctoratal student at the Santa Fe Institute, and Douglas Erwin, a paleobiologist at the National Museum of Natural History in Washington, D.C., have created a model of species body sizes and how they should compare over time.

The pair surveyed fossils of mammals that lived over the past 50 million years, calculating the size differences between various animals. Then they built a mathematical model out of it.

That more recent fossil record, they think, can help scientists better understand the size differences between animals in a group -- like mammals or lizards or dinosaurs -- from times much further back than 50 million years, such as the time of the dinosaurs, which ended 65 million years ago, Clauset said.

"This model may give us a way of estimating what we're missing in the fossil record," Clauset said.

That could be useful because fossils from the time of the dinosaurs, for instance, can be very deceptive.

Over time, generally only the largest animals survive as fossils, with remains of smaller ones eroding away quickly, Lucas said.

"So what we have in the record are mostly large things," Lucas said. "It's just a function that large animals are better preserved than smaller ones. And smaller ones are also harder to find."

It's not necessarily that every group grows to an equal range of sizes, but larger groups of creatures like mammals or insects seem to have similar size ranges relative to one another, Clauset said.

"One of the smallest mammals to exist is about two grams, which is about thumb-sized. That's the bumblebee bat," Clauset said. "A typical mammal is about 40 grams, or generally rat-sized. And the largest mammal (an African elephant) is maybe five million times bigger than the smallest, that bumblebee bat."

Overall in the size spectrum, there are a lot more small species than large ones. A few, like elephants or whales, will be huge, and a few, like the bumblebee bat, will be tiny. But most will be in that medium size range of rats or rabbits or dogs or cats.

If you look at insects, the smallest is much smaller than two grams. But the group overall would follow a similar size scale, with the largest reaching many times the size of the smallest, Clauset said.

Nobody knows why creatures are distributed in that size range, but it has been discussed often among paleontologists and biologists. The thought is perhaps that when a group like the dinosaurs go extinct, a new species will spread out to fill a variety of niches -- both in function and in size -- that were left vacant by the extinction.

"But they can't vary any way they want, they have to obey the laws of physics," Clauset said.

In the case of mammals, biology seems to stop things from being smaller than two grams. It might be because organs just don't work when they're smaller than that, or it may be because of other factors.

And the largest mammals, on the other hand, tend to go extinct at a slightly higher rate than the small things, but there's also an evolutionary push toward getting bigger because the largest animals, like elephants or woolly mammoths, are less likely to become prey.

To get a more accurate model, though, the scientists decided to remove the time period that includes modern humans, because we tend to increase extinction rates of large animals more than any other creature likely did in the past, Clauset said.

"This is a model of natural evolution where creatures don't get to tinker with their own DNA," Clauset said, adding that for that reason, you couldn't predict human size ranges in the future with it.

"If humans left, though, you could use it to estimate how long it would take for a group of species to return to its prehuman state," he said.

The scientists tested their model by running the history of mammalian evolution through it from the past 50 million years and found some very close matches, Clauset said.

"It's surprising when you see data fit a model as well as this does," he added.

Still, with the 2 percent fossil record that gets spottier as it gets older, Lucas said, he's not so sure that the two authors can really prove the model would work on creature ranges from the distant past.

There's just not enough data to confirm it, he said.

"It's interesting to have a model like this, but other than an intellectual exercise, I'm not sure what you can do with it," Lucas said.

Contact Sue Vorenberg at

[email protected]

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