kirk johnson collected this beautiful fossil leaf in montana of an extinct member of the laurales. a group that contains avocado and cinnamon trees.
Knowing the plants tells us something about the climate in T. rex's time. We have modern relatives for many of these plants and so we know what climates suit them best.
According to these analogies, during most of T. rex's time, the climate was what you'd expect to find on average in North Carolina, but without so much change between seasons. Iight-sweaterweather. But from shifts in the types of plant fossils, paleobotanists Kirk Johnson and Leo Hickey figure the climate changed considerably during T. rex's time, the 2.5 million years that are recorded in the Hell Creek Formation.
Overall, the Hell Creek Formation preserves a time of warming with lots of new plants moving in. The weather was in the low fifties on average, as it is in the redwood forest, though not so wet through most of Hell Creek time. We certainly don't think the weather in T. rex's day was dry—we don't find any caliches, the littie balls of calcium carbonate that form in sediments from years of dry seasons. We do see those caliches in the pits we dig for duckbilled dinosaurs from more than 70 million years ago in western Montana. That's closer to the age of Scott Wing and Leo Hickey's plant fossils but near to the latitude of our T. rex find. So we conclude that the weather was getting moister by T. rex's time.
But in the middle of T. rex's time, according to Kirk, a relatively short drought happened, maybe lasting just tens of thousands of years. Perhaps the mean annual temperature dropped a few degrees as well. The proof for that is a band of rock with unusually small leaves in it, some of them relatives of modern roses. Without much water, leaves don't get big. And small-leafed plants need less water than big-leafed plants.
For the last nearly million years of dinosaur time, years marked by the uppermost stripes of the Hell Creek badlands, the plants were warm-weather types. Their fossils resemble those you'd see in older rocks from places nearly eight hundred miles farther south, such as New Mexico and southern Colorado. The average temperature rose by about a dozen degrees, making the Hell Creek into a lush subtropical land with warm weather year-round.
Still there had to be seasons in T. rex's time. Montana was a little farther north than it is today (about fifty degrees north latitude), so in summer the days were considerably longer and brighter than in winter. At times the floodplains would have been downright hot and soggy, especially in swampy areas near the inland sea. Sudden storms produced flash floods that killed dinosaurs, buried them, and eventually turned some to fossils.Upland and away from the water, temperatures
would have been more extreme. There fewer plants, and animals, including T. rex,, could make a living.
Somehow T. rex and and the rest of the dinosaurs stopped making a living, everywhere, about 65 million years ago. Maybe the weather was slowly getting more extreme as the age of dinosaurs neared its end. I'm not big on extinction theorizing (see Chapter 1), but I think gradual, significant degrading of the climate over a longer stretch of time, maybe millions of years, would have been enough to do in the dinosaurs.
That's what Leo Hickey thought, too, up until about ten years ago. But fossils he and Kirk Johnson have found more recendy made him change his mind. The evidence for a sudden change in the plant world at the Cretaceous boundary is a lot stronger than it is for dinosaurs. You can find certain pollen within one millimeter below the boundary (and none of that kind of pollen above the boundary), but there are no dinosaurs within several feet of that boundary. The weather did change significandy at the boundary, 65 million years ago. It got a lot warmer, up to a mean of sixty-one degrees Fahrenheit year-round in Hell Creek localities— about what it's like in Sao Paolo, Brazil, today. Then, after the dinosaur extinction, the weather got cooler again, dropping back to fifty-two degrees Fahrenheit within 2 million years. The way Leo sees it, the dinosaurs were running along happily right until the big chill. And the asteroid impact probably triggered the dip that dinosaurs couldn't handle.
This temperature drop was a pretty sudden shift, by geological standards. But it still might have taken many thousands of years to wipe out the dinosaurs, because as Leo points out, you can't measure the effects of climate change from fossils in intervals of much less than 250,000 years.
Not everyone agrees. Jack Wolfe, a paleobotanist with the United States Geological Survey in Denver, Colorado, thinks the weather turned truly awful practically overnight in T. rex's time, so awful that it killed off the dinosaurs.
Jack is one of many scientists who think one or more asteroids struck the earth at the end of dinosaur time, with catastrophic results. To me that's a hot theory in search of evidence. In 1991 Jack added one of the newest wrinkles to the theory, specifying the month when dinosaurs went extinct: June. Jack's findings were published in a science journal and many newspaper stories that summer.
Jack had looked at a lot of plant fossils from the end of dinosaur times in Wyoming. The plants' leaves had opened as if it were early summer. Pollen grains were found in clumps, as they are when plants are not mature. And leaf surfaces were wrinkled, Jack thought, from a flash freeze. But Leo and Kirk noted that the kind of pollen Jack found were dispersed in clumps of four when they matured. And they got a wrinkled texture on leaves by drying them. What's more, they found that the same leaf pattern showed up on leaves from throughout Hell Creek Formation time, which would have to mean multiple flash freezes by Jack's reckoning.
To Jack, the plant wilt was due to a frost—the aftermath of a huge asteroid striking the earth. The impact made a huge crater in the Gulf of Mexico, sending up a cloud of dust that blocked the sun. Just days after the big chill, the air started to warm from carbon dioxide released by the impact with limestone in the sea. The greenhouse effect of the CO2 increase raised the worldwide temperature ten degrees Fahrenheit, where it stayed for more than half a million years.
At least that's how Jack sees it. Jack's evidence for that change is fossils of leaves that grew three or four times larger than before the end of the dinosaurs, developing sharp tips like tropical leaves today.
To me, all that sounds like a lot to figure out from some fossil plants. And the people who best know the plants of T. rex's time, Kirk Johnson and Leo Hickey, just don't buy it. Kirk and Leo found that what Jack had called frost damage on leaves could just as well have come from rotting and drying out. Kirk and Leo also don't think it's possible to make temperature estimates as precise as
Jack does, and they don't think Jack did enough detailed analysis.
I'm no expert on plants. But I know enough about science to be careful of what I read in the newspapers, and sometimes the scientific journals.
I do know that T. rex was around for several million Junes in many environments. T. rex roamed only a small part of the dinosaur world, but from what we can tell, T. rex encountered several different communities of animals within its range.
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