The Plant Evidence

Finding fossil plant evidence isn't easy. Fossil bones are usually preserved in stream channels. Fossil plants turn up in stream channels and also in floodplains, ponds, lakes, and deltas. In ancient stream channels, at least, only the bigger, sturdier leaves, plants, and trees become fossilized. They're the only ones strong enough to w hold up to the force of the water that was moving them and the mud that covered them at some break in the flow where debris piled up.

But where you find fossil bones you don't usually find plants. The acid environments that preserve leaves break down bone. So when fossil leaf-hunters go to the Hell Creek Formation, they head for the brown bands in the cliffs, rock that's turned dark with the carbon from decayed plants. Often it's a layer of clay or shale right above a black coal stripe that doesn't preserve anything you can see with the naked eye. The brown and black layers, however, are just where dinosaur diggers don't go to look for bones. We find bones in the tan sandstone and gray mudstone. We find plants in the parts of the Hell Creek Formation closest to the ancient shoreline, in what are now the Dakotas. Those waterlogged sites protected plants from drying out and breaking down in air.

There are exceptions to every rule. If you look long enough and hard enough, you can find plant fossils with dinosaur bones. Leo Hickey, a Yale University paleobotanist, found plant fossils underneath a Tricer-atops near Jordan, and his former student Kirk Johnson (now of the Denver Museum of Natural History) finds plant fossils with fossil bones in North Dakota.

We're sifting through bags and bags of rock matrix from around our T. rex, hoping to find plants. We can't see much in the way of plants from our quarry so far, but that's how it usually is. If we find plant remains with our T. rex,, they're more likely to be pollen than leaves. (Fossil trees are very hard to come by—though a 165-million-year-old fossil forest was recentiy found in northwestern China.) And it isn't easy to know when you've got hold of pollen. Each pollen grain is as small as a dust particle—you need to look at them under a microscope at 100 x to 800x magnification to see them clearly.

Tiny as they are, pollen grains are amazingly durable. Each grain is coated with a waxy surface that will preserve it pretty much forever if it is in an acidic environment, not exposed to oxygen. We've got pollen

this pac-man design belongs to the evergreen pollen group, taxodiaceae pollenites hiatus, suggesting it is related to metasequoia and the bald cypress.

this pac-man design belongs to the evergreen pollen group, taxodiaceae pollenites hiatus, suggesting it is related to metasequoia and the bald cypress.

grains from long before the age of dinosaurs, going back more than 360 million years. To extract the pollen you can pour acid onto the rock in which pollen is found, then spin it in a centrifuge. Once the rock has dissolved, pollen sinks to the bottom. You spread that slurry on a slide, put it under the microscope, and look for pollen grains. Different kinds of plants produce different-looking pollen grains. Some, such as grains from pine trees, have bladders that look like water wings. Others have spikes like a medieval mace.

Fossil pollen experts (called palynologists) have found well over one hundred kinds of pollen grains in the Hell Creek Formation. Pollen is a pretty powerful tool for dating rocks and the fossils in them. Pollen is so common and so durable that you can find rnillions of pollen grains in a single ounce of rock. Still, there is only so much you can figure out about the environment from pollen. The fossil pollen looks something like that from plants today. We can tell some of it came from conifers, some from sycamores, and some from ferns, palms, and extinct broadleaf trees.

Fossil leaves tell us more. Despite the obstacles to leaf preservation, fossil plant experts have found many sites in the Hell Creek Formation that have plant fossils in them. Leo Hickey and Kirk Johnson have dug hundreds of holes in these badlands looking for leaves. They've come up with more than a hundred good plant fossil localities, the overwhelming majority of them in the Dakotas. In all, Kirk and Leo have examined nearly twenty thousand leaves from the Hell Creek Formation.

Kirk has studied leaves found at the sites of the two Black Hills T. rex discoveries in South Dakota. From the "Stan" site Kirk saw mostly flowering plants, some of them relatives of magnolias, sycamores, and the laurel family (which includes avocados). Less than 10 percent of the leaves were conifer needles. They included a relative of the sequoia and another more surprising conifer, the monkey-puzzle tree. That group of conifers is found now only in the Southern Hemisphere, but in dinosaur times they grew worldwide. A Jurassic tree of

this family is the biggest tree ever known—its partial trunk is a fossil 380 feet long. The tree itself was probably 460 feet high, nearly 100 feet taller than the tallest tree today.

Kirk also studied leaves found in the body cavity of "Sue." These were a different mix of plants, unlike any Kirk had seen in a hundred Hell Creek sites. Types of plant that were not peculiar, just the relative abundance of some kinds. For instance, he found fewer flowering plants and lots more sequoia-like conifers, ferns, and aquatic plants than he expected.

There are two good reasons why what we know of Stan's world looks so different from Sue's. We're not getting a complete picture of either, only a sample of what happened to be preserved. Sue was buried in a stream channel, and many plants from distant places may have washed in among the bones. Stan comes from a point bar, a higher elevation more likely to preserve what actually grew nearby. And Stan and Sue came from very different times. Stan comes from very near the end of Hell Creek Formation time, sediments close to the boundary at the end of the Cretaceous. Sue comes from near the beginning of Hell Creek time. That's a differ-

the three-lobed wheel is GUNNERA MtCRORETICULATA, one of the few fossil pollen grains from the hell creek formation that has a known relative today. its modern cousin is a large-leaved flowering plant of the moist tropics.

ence of millions of years, and the climate and vegetation changed a lot in that time. Until the last few million years of dinosaur time in the Hell Creek Formation, plant samples show a mix—half ferns, the rest pretty evenly divided between flowering plants and conifers. In the last 2 million years of the Hell Creek Formation, the plant samples show almost all flowering plants. As Kirk puts it, "that's a fundamental reorganization of vegetation, a bigger shift than at the boundary" of dinosaur time.

We re only beginning to fully understand those changes in plant life. People have been looking for and finding T. rex-aged leaf fossils for decades, but a lot of the time they misidentified the leaves they found. That's why if you go into the Late Dinosaur Hall of some of the grand old natural history museums and look at the mural, you'll see oaks, elms, and willows in the background, and maybe even some grass in the foreground. We know now those plants didn't exist in T. rex's time. Most of what we find are extinct kinds of plants. They might belong to a living family of plants, but they were different from anything we know today. What we can say with certainty is that T. rex's world was full of many kinds of plants, far more kinds of trees than we'd find in western North America today.

We know more than two hundred kinds of plants from T. rex country, more than 90 percent of them flowering plants. Since only selected parts of certain habitats get preserved as fossils, those two hundred plants are only a fraction of what must have grown in Hell Creek Formation times. The actual diversity of plants was probably closer to what you'd find today in a rainforest as opposed to any temperate habitat.

Just what communities those plants lived in is hard to say from clumps of leaves washed together from various delta locales. We'd have a better answer if we could find a piece of dinosaur land preserved in place just as it was.

That's a lot to wish for, but if you're lucky, you might find such a place. In the summer of 1990, Scott Wing, a Smithsonian paleobotanist, got lucky. He was looking at a rock outcrop near the town of Worland, Wyoming. The rock is from the Meeteetse Formation, and it's 72 million years old, close to 7 million years older than the Hell Creek Formation, from which we dug up our T. rex.

Scott Wing came upon some volcanic ash (benton-ite), and just beneath it a patch of open vegetation exactly as it looked in dinosaur days. He'd dug up a sort of Pompeii for plants: fossils created when mounds of cool ash from a nearby erupting volcano were washed over the ground. The area Scott found preserved by the volcanic eruption stretches more than a mile by two miles. In it, Scott found more than 110 species of plants, sometimes preserved nearly whole.

From one single pit three feet by six feet and a foot and a half deep, Scott and Leo Hickey dug up more than fifty different kinds of plants. You don't find those numbers in many places in the world today. Many of the plants they found are delicate littie herbs that would never have been fossilized in the usual stream channel fossil sites. Others were big palm fronds.

This sort of concentration of plants is extremely rare from late in dinosaur time. But those were sites that missed out on low ground plants. Low-growing herbs made up less than 15 percent of previous samples of plants from that time. At Scott's site, however, these small broadleaf flowering plants made up more than 60 percent of the kinds of species, but just 11 percent in relative abundance of what's found (the remainder were ferns, cycads, and conifers, including a cone from a monkey-puzzle tree). As a result, our impression of what some landscapes looked like near to T. rex's time has now been greatly changed.

At another site sixty miles closer to the ancient volcano, Leo found standing tree stumps that were partly petrified, with the rest burned by hot ash. The environment Leo discovered seems to have been a forest of large conifers with ferns as a heavy ground cover.

What Scott Wing and Leo Hickey's discoveries provide us are our first snapshots of the vegetation on the

the most abundant of fossils is pollen. microscopic grains like these from the hell creek formation are found by the millions.

ground close to T. rex's time and neighborhood.

We know T. rex itself from places that in its time, 67 to 65 million years ago, were part of a huge web of floodplains. This delta occupied nearly one million square miles of the North American west. It lay east of the newly rising Rocky Mountains and their active volcanoes, catching the dirt and debris that washed down from the highlands. Large rivers emptied into the shallow and shririking north-south seaway that then severed North America, from the Arctic to the Gulf of Mexico. The flow of water also formed lakes and low ridges along stream channels in a flat and largely featureless coastal plain.

We get dinosaur fossils only from deserts or places like these floodplains, where sediment builds up over bones (see Chapter 1). So we don't know whether T. rex lived in the eroding highlands closer to the Rockies. The two T. rexes found in Canada were from slighdy more upland environments than the T. rexes from the floodplains of eastern Montana, Wyoming, Colorado, and South Dakota.

The fossil plant evidence we have suggests that the terrain looked nothing like the badlands of today. T. rex country "looked a lot more like New England in late spring today than like most other places," says Kirk Johnson. The land would have been green and lush, at least in the floodplains, with fern glades and a ground cover of small flowering herbs and creeping vines. Around the stream margins were little water lily-like plants. Small trees, palms, and ferns were prevalent.

T. rex lived 2 million to 7 million years after the time of the dinosaur environment Scott Wing found buried beneath a volcano's dust. But the environment in T. rex's day was probably similar in a lot of ways. Maybe it was even more lush.

Paleobotanist Kirk Johnson and artists are now busy creating the first diorama of a Cretaceous forest ever illustrated from fossils, for the Denver Museum of Natural History. They're drawing the plants in the Hell Creek Formation, using a camera lucida (a mirrored

DINO DAMAGE: THE EFFECT OF

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Responses

  • PHILLIPP
    What plant lived when a tyrannosaurus rex lived?
    8 years ago
  • bingo
    How tyrannosaurus rex become a fossil?
    8 years ago
  • Krista
    What are plant evidenve?
    8 years ago
  • bosco
    What plants existed when t rex lived?
    8 years ago
  • Veronica
    What was the plants for the tyrannosaurus rex?
    7 years ago
  • krystian black
    What evidence have people got that tyrannosaurus rex become extinct?
    7 years ago
  • kaitlin
    What plants were around when the tyrannosaurus lived?
    7 years ago

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