Uncovering Clues in the Rocks

To figure out what had killed all the unhatched sauropods in the eggs, we again had to appeal to the rocks that contained the fossils. In addition to providing the only direct evidence to estimate the time of the embryos' death, the picturesque layers of rocks at Auca Mahuevo preserved key clues for interpreting the cause of their death, which appeared to be directly related to the environment in which the sauropods had laid their eggs. The evidence for interpreting what environment the dinosaurs had lived in would be gleaned from observing the different kinds of rocks that formed the layers at Auca Mahuevo.

Throughout the world today, rocks are either being eroded or formed on the surface of the earth in an ongoing cycle that has continued since the early days of earth history when the first rocks were formed. In areas that are relatively high and steep, including the mountains and hills that dominate all the continents, rocks are weathered and eroded by rain, ice, and wind. Chemical reactions promoted by molecules dissolved in rainwater help break the bonds between mineral crystals forming rocks near the surface. Aided by the destructive activity of plant roots, as well as the expansion and contraction of ice as it forms and thaws, loosened debris is eroded by runoff from rains and the force of the winds. In colder regions, glaciers can also serve as powerful agents of erosion, scouring away great quantities of mountainous terrain. In areas that are relatively low and depressed, including river valleys and lakes, some of the material eroded away from higher regions is deposited in layers on land and in lakes by rivers and streams, as well as by winds and the melting of glaciers. Other material is washed or blown off the continents to be deposited in layers at the bottom of the oceans.

In all of these modern settings, different types of debris, called sediments, are laid down under different environmental conditions by erosion and deposition. We can observe how rain and ice erode mountains and carry the residue down from the heights in rivers and streams to be deposited across floodplains and within ocean basins. We can also document the kinds of sediments that are being deposited on the floodplains and in the oceans. By comparing sedimentary layers forming today to the kinds of rocks we find in ancient rock sequences, we can identify the kind of environment in which the ancient sediments were deposited. This is possible because sedimentary rocks are simply ancient sediments that, with time and the action of several geological processes, have become petrified. The sediments that we see being transported and deposited today will eventually become the sedimentary rocks of tomorrow.

Such observations led some early students of geology to a rather startling, yet intuitive, conclusion about how the earth has evolved since it formed. In essence, they reasoned that the present is the key to the past, because the processes of erosion and deposition that we can observe operating today have acted more or less similarly throughout the vast expanse of geologic history. This concept is a fundamental dictum of geology called the principle of uniformitarianism.

Illustrative examples are provided by the many volcanoes that have erupted in the past across the globe, generating both lava and ash. Once cooled, some lava forms a kind of volcanic rock called basalt that has a characteristic texture and composition of minerals. When ancient sequences of rocks contain this same type of hardened lava, we know that it was erupted out of an ancient volcano, whether or not the volcano still exists.

In 1980, for example, a tremendous eruption occurred in southern Washington State at Mount St. Helens. Millions of tons of volcanic ash were blasted several miles up into the atmosphere, where winds carried the ash to be deposited across the states of Washington, Idaho, Montana, and even into the Dakotas. The ash formed a layer of powdery dust as it settled out of the air on distant cities and landscapes. Again, its texture and mineral composition is quite distinc tive, and similar layers can be found in ancient sequences of rocks, testifying to their origin during volcanic eruptions. When petrified, these layers of ancient volcanic ash are often called tuffs. We had hoped to find these tuffs in the sequence at Auca Mahuevo in order to provide environmental clues and to establish the age of the rocks through radioactive decay analyses.

Similarly, rivers and streams can be seen to deposit layers of gravel, sand, silt, and mud as they make their journey from steep mountain canyons across gently sloping floodplains. The kind of sediment that a stream can carry depends on the velocity and turbulence of its currents. Swift, turbulent currents flowing down steep slopes can transport boulders, as well as smaller particles of sand, silt, and clay, but less turbulent currents cannot carry such large sedimentary debris. Consequently, coarse, heavy sediment is often deposited by swift rivers and streams near the mouth of a steep mountain canyon, where the slope of the stream flattens out. These boulders, pebbles, and coarse sand grains are dumped into the valleys adjacent to the mountains as alluvial fans, while much of the fine sand and mud is carried on downstream. Immense alluvial fans can be seen today at the mouths of canyons leading into Death Valley in California. As mentioned earlier, the gravel-rich layers behind Dona Dora's puesto represent the kind of coarse gravel that was deposited on alluvial fans adjacent to some ancient hills or mountains on the Cretaceous Patagonian landscape when titanosaurs roamed across southern South America.

On more gently sloping plains farther away from the highlands, often on the coastal plain, slower, less turbulent rivers and streams deposit lighter sediment in stream channels, flood basins, and lakes. The coarser sand often forms sandbars in the stream channels. Finer silt and clay is often deposited outside the banks of the channels during floods, when the rivers and streams overflow their banks and the slower currents carry the silt and mud far away from the main channels before it settles out. Such deposits of sandbars in channels and mud on the floodplain away from the channels have been observed throughout many modern river systems, and similar layers of sand and mud can be found in many ancient rock sequences. These are prime places to look for fossils of dinosaurs, as well as other animals and plants that inhabited the ancient floodplains, because their bones and leaves could quickly be buried by flood debris before they could decay or be scavenged. In fact, many of the well-known dinosaurs from North America and other parts of the world, including Tyrannosaurus and Triceratops, come from sedimentary rocks formed in these kinds of environments.

Lakes also form in depressions on the floodplain, and they tend to accumulate their own distinctive layers of sediment. Away from where streams feed into the lake, thin layers of muddy clay, which floated far out into the lake before settling down through the water, are formed on the bottom. These thin layers, called laminations, often form finely striped rocks once the sediment is compacted, and this kind of rock is found in many ancient sequences of rocks. This environment also provides excellent conditions for the preservation of not only fish and other animals that live in the lake, but also vegetation and carcasses of dead animals that float out into the lake. The fine mud that settles out of the water can bury these remains quickly and preserve skeletons in which almost every bone remains in a natural, lifelike position. Often, the water at the bottom of a lake is depleted in oxygen, which reduces the number of bacteria and scavengers that destroy sunken carcasses, and in these instances, soft tissues such as hair, muscles, and internal organs stand a better chance of being preserved. Ancient lake sediments have produced some of the most exquisitely preserved fossil specimens of dinosaurs, insects, plants, and stream channcl stream channcl

Floodplain Environment
Diagram showing features in floodplain environment.

other organisms that have ever been found. Famous fossil sites such as the Cretaceous Liaoning lake sediments in northeastern China and the Tertiary Green River lake beds in Wyoming represent good examples of these ancient, fossil-rich environments, and similar rocks would yield critical clues about what dinosaurs preyed on the sauropods at Auca Mahuevo.

Finally, after the river flows into the sea, any remaining sand and silt are deposited near the shoreline, but finer-grained silt and mud can be carried farther out into the ocean basin. Out in deep ocean waters, the shells of microscopic, single-celled plankton settle to the bottom after the organisms die to form layers of limy mud on the bottom that are eventually compacted into limestone.

Given this brief introduction, let's once again look at the rocks containing the sauropod eggs and embryos at Auca Mahuevo. The eggs and embryos were entombed in rusty brown layers of silt and mud, which were mixed in with coarser layers of greenish and reddish brown sandstone. The alternation of layers containing sandstone and mudstone closely resembled the kinds of sediments that are deposited across floodplains by streams and small rivers. Thus, evidence from the rocks indicated that between 79 million and 83 million years ago, the dinosaurs at Auca Mahuevo lived on a broad, gently sloping floodplain, crisscrossed by shallow streams and rivers. This floodplain formed as South America drifted away from Africa due to the enormous forces generated deep within the earth as the result of plate tectonics, the geological process that drives the continents across our planet's surface. Thin layers of sandstone were the geologic clues that documented the ancient presence of shallow stream channels and their sandbars. The sand was not too coarse, and few pebbles were present, which suggests that the streams were not as swift and turbulent as the ones that deposited the coarse gravel found in the ridges behind Doiia Dora's puesto. Clearly, there were no large hills or mountains nearby to provide steep stream gradients and large pebbles or boulders. In addition, the thickest layers of sandstone were about three to four feet thick, suggesting that the streams were not terribly deep. Over time, these streams migrated back and forth across the floodplain, cutting and then filling in the channels with sandbars, but the eggs were not found in these layers of sandstone.

Fossils of the eggs and embryos were found in the finer-grained lay ers of mud and silt. What might this clue tell us about the cause of death? These layers represented silt and clay particles that were carried over the banks of the streams when they flooded and inundated the adjacent lowlands. Similar events are common in our modern world; every year, heavy rains in many parts of the globe unleash torrential floods that inundate major river valleys, often destroying populous cities and leaving a blanket of mud wherever the flood passes. Objects as large as cars and houses can be transported or buried by the onslaught. Although the streams on the primeval Patagonian floodplain were not large enough to generate such enormous floods, they nonetheless overflowed their banks from time to time, carrying a blanket of mud and silt to be deposited as currents slowed, away from the main channels. Eighty million years ago, no cities stood in the way of these floods —only the vegetation that grew on the plain and occasionally a nesting ground of sauropods.

It appears that the dinosaurs intentionally looked for places away from the streams in safer areas of the floodplain to lay their eggs. Eggshell is actually pretty durable, and fragments can be carried over substantial distances without being completely destroyed. However, we didn't find any fragments of eggshell in the sandstone formed from the bars in the channels that were active at the time of the nesting, and the eggs did not appear to have been broken or even transported by the floodwaters. So, the currents depositing the mud could not have been too strong, and the eggs in the nesting ground could not have been very close to active channels. Why did the dinosaurs apparently avoid the areas near the active stream channels? Perhaps it was just chance, but it may also have been because they had some sense that the streams could destroy their nests. In any case, at times this strategy, if there was one, didn't work. For the floods, although not very powerful, nonetheless brought a wave of sudden death to the nesting ground.

Since the fossil eggs and embryos that we found in 1997 were exclusively buried in the layers of mudstone, not the sandstone, we concluded that the eggs and embryos had quickly been buried when the streams flooded. (Years later, we would find eggs that had been laid in sand rather than mud. But the sand formed the bed of a long-abandoned channel at the time that the dinosaurs laid their eggs, and the eggs had later been buried by mud during a flood.)

When the floods buried the eggs in their debris, it not only began the fossilization that preserved the eggs but also either drowned or suffocated the helpless embryos inside under a massive sheet of water or a sticky blanket of mud. Thus the floods not only killed the embryos, but by burying the eggs in a layer of protective mud, prevented them from being exposed to plunder by scavengers, which would have greatly decreased their chances of being preserved as fossils. This might sound pretty awful, but for us paleontologists living 80 million years later, it was a stroke of good luck.

At last, the probable cause of death had been established, and we were now ready to submit our scientific paper to a journal for publication. Because our discoveries represented so much new information and so many firsts for paleontology, we decided to try to get our paper published in one of the most prestigious scientific journals in the world, Nature. We sent them our paper in the middle of 1998, but it would be several more months before the paper was actually published. Although we thought that the media might be interested in our discoveries when the paper finally came out, we were not prepared for the overwhelming reaction.

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