Evidence from Clocks in the Rocks

Just as fossils provide important information about life-forms that inhabited our world in the distant past, the rocks that they are preserved in constitute our only evidence to interpret when these ancient animals lived. To recount this part of our investigation, we need to flash back from the lab to the field.

What originally attracted us to explore the site that contained the eggs and embryos in Patagonia was the stunning visual beauty of the area's rocky outcrops. Layer upon layer of crimson sandstone and mud-stone form a fantasyland of banded ridges and flats at Auca Mahuevo. This maze of ridges and ravines took nature millions of years to construct. First, the layers of sand and mud were laid down across the landscape when the dinosaurs lived. Then, these layers were buried under a thick blanket of subsequently deposited layers and remained under the surface of the earth for millions of years before the powerful tectonic and volcanic forces that created the mountains and valleys of the region lifted them back toward the surface. As they rose, rain and wind eroded the overlying layers of rock, leaving the ancient layers exposed on the surface once again. And once they became exposed, the rain and wind sculpted them into the breathtaking landscape spread out before us.

We knew from our experience in other expeditions that these kinds of exposures sometimes contained buried caches of fossil treasure. So as we drove down toward the badlands under the radiant morning sun that greeted the second day of our expedition, we could hardly wait to scramble out of our vehicles and begin prospecting. But once we had discovered the fossils, we also knew that these breathtaking outcrops also contained the key evidence for interpreting when the embryos died.

It is difficult for human beings to appreciate how long ago the animals preserved as fossils at Auca Mahuevo lived. The average life span of a person living in the United States is about seventy-five years. The United States itself is slightly more than two hundred years old. The earliest human civilizations based on agriculture from which we have discovered artifacts existed about ten thousand years ago. The Ice Ages ended about twelve thousand years ago, when such animals as saber-toothed cats, mammoths, mastodons, glyptodonts, and giant ground sloths went extinct. The earliest members of our human species lived around one hundred thousand years ago, and our earliest human relatives first walked the earth about 4.5 million years ago. All the dinosaurs, excluding birds, died out 65 million years ago. But the animals living at Auca Mahuevo lived millions of years before that. Based on previous studies of fossil animals that had been collected from the same layers of rock that were exposed in other parts of Patagonia, we knew that the rocks at Auca Mahuevo were deposited sometime between 70 million and 90 million years ago, a nearly incomprehensible span of time.

Geologists and paleontologists have developed a special time scale to serve as a geologic and evolutionary calendar. Based on major changes in the kinds of fossil organisms that lived at different times during the history of the earth, this time scale is divided into four major eras. From oldest to youngest, they are called the Precambrian (4.5 billion to 570 million years ago), the Paleozoic (570 million to 250 million years ago), the Mesozoic (250 million to 65 million years ago), and the era we now live in, the Cenozoic (65 million years ago to the present).

The earth first formed slightly over 4.5 billion years ago at the start of the Precambrian era, as the planets consolidated from rings of star dust orbiting our sun. The earliest fossils of ancient life that we have found were simple, single-celled organisms related to modern blue-green algae, which lived in Precambrian oceans about 3.8 billion years ago.

The first of our vertebrate relatives did not appear until almost 3.3

billion years later, early in the Paleozoic era, and even then, these fishlike creatures, with segmented support structures resembling a backbone, were not terribly imposing. They were only a couple of inches long, devoid of jaws, and probably fed inconspicuously on creatures that either lived in the mud at the bottom of the oceans or floated closer to the surface.

The first known animals and plants to move out of the oceans and live on land arose about 400 million years ago, during the Paleozoic era. Fossils of these animals and plants were preserved in rocks deposited in lush coal swamps, such as the rocks that now form the Appalachian Mountains in the eastern United States. Ancient relatives of horsetails, ferns, and tree ferns dominated the flora of these swamps, along with extinct groups of trees that have no close living relatives. Some of the early animals that colonized the land included different kinds of insects and other arthropods, who followed the plants in their invasion of the land. Dragonflies with wingspans of more than three feet patrolled the skies, and cockroaches over a foot long scavenged in the underbrush.

The earliest vertebrates to walk on land did not evolve until about 350 million years ago. Although often much larger, these amphibious creatures were built somewhat like salamanders and had to return to the water to lay their soft, membranous eggs. These first tetrapods — so named because they have four limbs — evolved from ancient relatives of the coelacanth, a lobe-finned fish that was long considered to be extinct until it was discovered living in the waters of the Indian Ocean in the 1930s. Early reptiles and relatives of mammals appeared on the scene about 300 million years ago, near the end of the Paleozoic era, but the origin of dinosaurs still lay millions of years in the future, after the start of the Mesozoic era.

The Mesozoic era, sometimes called the Age of Large Dinosaurs, is divided into three different periods, the earliest of which is the Triassic period, which lasted from 250 million years ago until about 206 million years ago. As we've already mentioned, the earliest known dinosaurs lived in the Triassic, about 230 million years ago, when most of the continental masses were fused into a huge single continent called Pangaea. The earliest known mammals originated near the end of the Triassic.

Next comes the Jurassic period, when the supercontinent of Pan-

Map of the world 80 million years ago, when sauropod dinosaurs laid their eggs at Auca Mahuevo. Areas in gray illustrate emerged continents.

gaea began to split apart, which lasted from 206 million years ago to 144 million years ago. This period saw the evolution of most of the largest dinosaurs that ever lived. We have already introduced some of them, including the sauropods, a group of enormous herbivores such as Apatosaurus (formerly called Brontosaurus), Diplodocus, and Bra-chiosaurus, which thrived in the tropical and subtropical climates that predominantly characterized this interval. But this period also witnessed the evolution of such terrifying carnivorous forms as the twenty-foot-long Allosaurus with its three-foot-long skull and four-inch-long, serrated teeth. In addition to these large dinosaurs, the earliest-known flying descendants of dinosaurs—birds—began to compete for dominance in the skies. Their primary rivals were pterodactyls and other flying reptiles, whose bodies were not covered with feathers and whose wings were constructed very differently from those of birds.

But the dinosaurs at Auca Mahuevo lived during the last period of the Mesozoic era, the Cretaceous period, which lasted from 144 million years ago until 65 million years ago. During the Cretaceous, rifting between the subcontinents of ancient Pangaea gave rise to the modern continents that we recognize today. This period also witnessed the origin of our modern biota, for many groups of living vertebrates arose then, as well as the flowering plants and their ubiquitous insect pollinators. In North America, this period saw the evolution of the duck-billed dinosaurs such as Anatotitan, the horned dinosaurs, such as Triceratops, and the fearsome carnivore Tyran-nosaurus rex. Tyrannosaurus was long recognized to be the "king" of dinosaurs, as denoted by its species name. At almost thirty-five feet long, its slender but powerful hind legs probably made it a relatively swift and agile predator for its impressive size, although some paleontologists have recently argued that it primarily filled the ecological role of scavenger. Regardless of the feeding niche it filled, Tyran-nosaurus's four-foot-long skull, studded with eight-inch-long teeth shaped like steak knives, made it the most imposing carnivore on the North American continent. In Argentina, as mentioned earlier, the Cretaceous saw the evolution of the largest dinosaur yet discovered, the herbivorous Argentinosaurus, and a ferocious carnivore that may well have been larger than 'Tyrannosaurus, Giganotosaurus. Even as we write this book, fossils are being excavated near Plaza Huincul in

Neuquen province from gigantic meat-eaters that are even larger than Giganotosaurus.

We knew that the animals at Auca Mahuevo lived between 70 million and 90 million years ago, near the end of the Cretaceous. But 20 million years is a long time, and we wanted to pin down the time of the embryos' death more precisely. This required that we collect small samples from many of the rock layers at the site. We knew that the layers had been laid down one on top of the other so that the lower ones were older than the higher ones. This fundamental geologic principle, called superposition, established several centuries ago by early-students of earth history, allows one to establish the relative age of fossils in a sequence of rock layers. It was, therefore, critical for us to record which layer or layers contained the fossils at Auca Mahuevo, because fossils from lower layers were older than fossils from higher layers. The question was, exactly how old were they?

Figuring out the exact age of the fossils from Auca Mahuevo was difficult and involved detailed scientific analysis. First we compared fossil animals at Auca Mahuevo with fossil animals from other localities where the age was known. If the kinds of animals were very similar, then the fossils from Auca Mahuevo were assumed to be about the same age. Using just the dinosaurs, however, it was hard to tell, because no other rock layers at other sites contained many of the same kinds of dinosaurs. But some fossils in rock layers higher in the sequence near our site were of animals that had lived in the ocean, such as clams, snails, and microscopic plankton. These suggested that the dinosaurs at Auca Mahuevo were at least 70 million years old. In addition, rocks that underlie the layers at Auca Mahuevo contain dinosaurs that were estimated to be older than 90 million years. How was the age of these animals established?

Some rocks containing similar fossils in other parts of the world also contain ancient layers of weathered volcanic ash, and the volcanic ash contains small crystals of minerals that were formed during the volcanic eruption. Some of these crystals contain atoms, including uranium and potassium, that break apart into other atoms at a constant rate in a process called radioactive decay. The atoms that break apart are called parent atoms, and the atoms that result are called daughter atoms. Using sophisticated scientific instruments, geologists can measure how long it takes for half of the parent atoms to break up into their daughter atoms; this amount of time is called the half-life. Geologists can also measure how many parent and daughter atoms are present in the small crystals that were formed when the volcano erupted and can use these data to calculate the age of the layer of volcanic ash and estimate how old fossils in nearby rock layers are. Sometimes rock layers that can be dated by this method of radioactive decay do not actually contain fossils. Nonetheless, if the fossils from other rock layers in the stratigraphic section are above the dated laver, we know that they are younger, and if the fossils are below the dated layer, we know that they are older.

These methods of radiometric age estimation are based on the same process of radioactive decay utilized in the carbon-14 technique for dating ancient human remains and artifacts. The carbon-14 technique is not applicable for dating rocks and fossils older than about

Diagram showing basic concept of radiometric dating.

o=parent atoms in crystal lattice x=daughter atoms in crystal lattice

After two half-lives, half of remaining parent atoms have decayed to daughter atoms.

o=parent atoms in crystal lattice x=daughter atoms in crystal lattice fifty thousand years, however, because then the half-life of carbon-14 (C-14) is only about 5,700 years. In objects older than about fifty thousand years, there is usually not enough C-14 remaining to obtain accurate measurements for calculating the age. The half-lives of the uranium (U-238) and potassium (K-40) are much longer. Half of the U-238 atoms decay to lead (Pb-206) atoms in about 4.5 billion years, and half of the K-40 atoms decay to argon (Ar-40) atoms in about 1.3 billion years, which makes these techniques suitable for estimating the age of much older objects but less useful for dating more recent objects.

We hoped that the greenish sands at Auca Mahuevo might contain minerals that were erupted out of volcanoes when the greenish sands were formed, so we collected many samples for analysis back in the laboratory. Unfortunately, none of the rock samples that we brought back from these greenish layers surrounding the fossil eggs contained mineral crystals that had been erupted out of volcanoes. Instead, they represented tiny pieces from a large pool of molten rock that had cooled and crystallized deep underground, so we could not be sure that these crystals formed at the same time as the rock layers that contained our fossils. Hence, we could not estimate the age of the rock layers and eggs through radioactive decay. It was frustrating to fail in this initial attempt, but that is the nature of scientific research. We knew that we would have to expand our search for layers of ancient volcanic ash into other adjacent regions.

In addition to collecting rock samples for possible dating through radioactivity, we had collected rock samples for dating through magnetic analysis.

The earth is like a giant bar magnet with a north pole and a south pole. Throughout the known history of the earth, our planet's magnetic poles have commonly reversed directions, such that the magnetic end of a compass needle that points north today would have pointed south then. Recent geologic research has suggested that such reversals of the magnetic poles can happen over a period of a few hundred years. This may seem like a long time to humans, but it is only a brief instant in comparison to the vast expanse of geologic time. In the last 65 million years, for example, the magnetic poles have switched positions about thirty times, and the last time the poles switched was about 750,000 years ago. But how do geologists know this?

Some kinds of sedimentary and volcanic rocks contain minute particles made of iron-bearing minerals, such as magnetite. When these particles settle out through a water column or cool within a body of magma, they align themselves with the earth's existing magnetic field. So, in rocks, these microscopic, magnetic mineral grains reflect the direction of the magnetic field at the time that the rocks were formed, which we can then analyze in a magnetometer.

Magnetic polarity indicated by the rocks exposed at Auca Mahuevo.

Why the earth's magnetic field occasionally reverses direction is not well understood, but the sequence of these reversals has been recorded in the earth's rocks. By combining these sequences of reversals with ages from rock layers that can be dated through radioactive decay, geologists have reconstructed a calendar of when the magnetic poles were oriented as they are today and when they were reversed. Using samples of rock from Auca Mahuevo, from the layers containing the eggs and embryos, we might be able to estimate the age of the rock by comparing our data to this global magnetic calendar.

We had carefully collected rock samples from eight different layers associated with the fossil eggs. First, using a small pick and a rock hammer, we had dug into the hillside to expose a two-foot-square area of unweathered rock. (Weathered rock could easily yield unreliable magnetic information.) Next, we used a hand rasp to plane a horizontal surface between three and five inches across on a chunk of rock. Fortunately, the mudstone and siltstone layers in the sequence were fairly soft, so the rasp worked effectively. To check that the planed surface was level, we used a special geologic compass, called a Brunton compass, which has a leveling bubble inside. Once a truly level surface had been planed off, we used the Brunton compass to mark an arrow pointing toward the north magnetic pole, thus showing how the chunk of rock was oriented in the ground in relation to the earth's present magnetic field. Once the arrow was marked, we had to carefully dig the chunk out of the ground without breaking it. To keep it in one piece during the trip back to the lab, the chunk was wrapped in aluminum foil and secured with masking tape. Finally, the sample was labeled with a number, and the position of the sample was recorded on the drawing of the stratigraphic section, so that we would know where it had come from once the magnetic analysis was conducted. The magnetic analyses, which might determine whether the rocks had been formed during a "normal" or "reversed" period in the earth's magnetic history, had to wait until we got the samples back to the lab.

The rock samples that we collected for magnetic analysis were somewhat more helpful in establishing the time of the embryos' death than were the samples collected for radiometric analysis. Carl Swisher and Gary Scott of the Berkeley Geochronology Center determined that the rocks containing the sauropod embryos at Auca

Mahuevo were deposited during an interval of time when the earth's magnetic field was reversed. These results helped to narrow down the 70-to-90-million-year range based on marine fossils preserved in higher layers of rock and dinosaurs from lower layers of rock. The presence of rocks at Auca Mahuevo that had formed during an interval when the earth's magnetic poles were reversed means that the rocks had to have been formed less than 83 million years ago.

We knew this because the global magnetic calendar documents that the poles were oriented as they are today between about 100 million years ago and 83 million years ago. Because the Auca Mahuevo rocks were formed when the magnetic poles were reversed, they had to have been formed after the end of that long interval. In addition, recent studies of fossil pollen found in rock layers just above the egg-bearing layers at Auca Mahuevo suggest that the pollen is between 76 and 81 million years old. Although we have yet to sample for pollen in these same layers at Auca Mahuevo, the same sequence of rusty mudstone and sandstone layers that contains the eggs is present where the pollen was found, about one hundred miles to the south of our site. Since these pollen fossils were higher in the rock sequence than the egg-bearing layers, the pollen had to be younger than the eggs and embryos. The magnetic poles of the earth were not reversed between 76 and 79 million years ago, but they were reversed between 79 and 83 million years ago. We concluded, therefore, that the eggs and embryos from Auca Mahuevo were probably between 79 and 83 million years old.

The 4-million-year span in our age estimate might still seem like an eternity. After all, almost the entire history of our human lineage is contained in the last 4 million years of geologic time. Most fossil sites from these more recent periods of earth history can be dated more precisely because the analytical uncertainties in estimating the age are not as great. Sites containing remains of large extinct dinosaurs are at least 65 million years old, however, so, given the analytical uncertainties, we would have to be satisfied that the time of death had been refined a bit through our magnetic analyses. But another large mystery remained to be solved: What was the cause of death?

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