Just as modern investigators must often return to the scene of an accident to gather more evidence after their initial investigation, we also needed to reexamine the site of our discovery. There were still many mysteries about how the sauropods had laid their eggs: Did they lay them in discrete nests or scatter them randomly across the surface of the floodplain? If they were in nests, how many were laid at one time? Did all the eggs belong to one sauropod species, or did multiple species use the same site? Did they return to the nesting site year after year or use the site only once?
To help solve these mysteries, we needed some special expertise, so two specialists on dinosaur eggs came with us. One was Frankie Jackson from Montana State University and the Museum of the Rockies in Bozeman. Before traveling to Patagonia, she had spent more than ten years collecting and studying dinosaur eggs, embryos, and nests in Montana for Jack Horner, whose team of collectors was responsible for completely reinvigorating the study of dinosaur eggs and embryos in the late 1970s with their discovery of Egg Mountain. Frankie had served as Jack's chief collector and preparator throughout much of the collecting and research at the site. A tall, thin, self-effacing woman, Frankie speaks with a slow Southern drawl, which she honed to perfection in her native Alabama. Her passion for and knowledge of dinosaur eggs is boundless.
Our other egg specialist was Gerald Grellet-Tinner from the University of Texas at Austin. Gerald was a student of another of our close colleagues and friends, Timothy Rowe, a professor in the Department of Geosciences. Tim's research over the last fifteen years has shed considerable new light on the evolution of both primitive dinosaurs called ceratosaurs and the early relatives of our own group of vertebrates, the mammals. His pioneering studies of vertebrate fossils through the use of CAT scans have also made a significant contribution to the field of vertebrate paleontology. Gerald's thesis had taken the study of bird eggs in a totally new direction by applying the methods of cladistics to explore what the structure of bird eggshell can tell us about how birds evolved from one another. Initial results are promising and his work shows that the structure of eggshell contains important clues for understanding the genealogical relationships of egg-laying organisms. Gerald plans to extend his novel approach to the study of dinosaur eggshell while continuing his research at the Natural History Museum of Los Angeles County. His tall, sturdy stature, along with his gregarious personality and wild stories of past adventures, makes him a natural for fieldwork, and his keen sense of observation, as well as his geological background, proved invaluable to our efforts.
We were also joined by two intrepid correspondents, Malcolm Ritter of the Associated Press and Tom Hayden of Newsweek magazine, who helped bring our continuing efforts to the attention of the public.
Our return to Patagonia was sponsored by the Natural History Museum of Los Angeles County. Since our last expedition, Luis had become an associate curator in that museum's Department of Vertebrate Paleontology, and his museum played an essential role in arranging for two new vehicles from Honda through its Office of Corporate Sponsorship. The National Geographic Society, the InfoQuest Foundation, and the Ann and Gordon Getty Foundation once again generously provided essential funding for the trip. Finally, after more than a year of preparation, planning, and research, we were ready to return to the scene of the catastrophe. We planned to spend the entire month of March 1999 at the site, searching for clues that would help us solve more of the mysteries of the ancient calamity.
After flying to Buenos Aires in late February, we gathered equipment and supplies before leaving for Auca Mahuevo on the twenty-
seventh. Our two-day trek to the site was largely uneventful, at least until we left the highway for the final seventy miles of dirt roads that wound deep into the desert. As we turned north for this last leg of the trip, we noticed a dark thunderhcad looming just above the horizon. Within half an hour, the first sprinkles began to splatter on our windshields, but more ominously, the sky above us had transformed itself from a hot but tranquil blanket of blue into a roiling cauldron of dark, greenish gray tumult. Part of this storm had passed just ahead of us, and large puddles began appearing in the low spots of the dirt road. As we drove on, the puddles transformed into deep ponds, which presented no problems for the larger, four-wheel-drive vehicles, but our entourage included a small Fiat sedan owned by Luis's brother, Eze-quiel Chiappe. He had willingly volunteered to help us for a week, and his physical strength was a great asset during the early days of heavy lifting and excavation. But his car was not built for these kinds of road hazards; with only six or seven inches of clearance, it struggled to ford the deep ponds. The rain was torrential, and in places the road was covered by almost two feet of water. While Lowell was filling in for Ezequiel during the last stretch to camp, the wipers on the Fiat stopped working, and he had to stick his head out the window to see where he was going. Water splashed over the hood as we plunged into the pools on the road, and the car could barely cross to the next dry patch. Finally, we crossed over a small ridge that led down into a wide ravine; at the bottom, we could see a flash flood crossing the road. We were stuck in the middle of nowhere.
Initially, we considered turning around and trying to outflank the storm, but a semi truck soon pulled up behind us, and the driver told us that the road we had come on was now washed out behind us. For the moment we were trapped: we had no other option but to wait until the torrent ahead of us, which turned out to be over two feet deep, receded. It seemed rather ironic to us that, 80 million years after floods had wreaked havoc on the sauropods' nesting ground at Auca Mahuevo, our own expedition had, at least temporarily, fallen prey to the same natural hazard on the parched desert landscape of modern Patagonia.
Within a couple of hours, the thunderstorm passed to the southeast, and the stream ebbed somewhat. With the larger vehicles leading the way, we ventured into the current. We had seen when the semi moved through that the water was only about a foot deep, but to Eze-quiel's dismay, the Fiat was pretty well spent. We basically had to tow it through the receding water. But once we had forded the stream, which was still about a quarter mile in width, the road improved dramatically.
By 7:30 P.M. we limped into the tiny, rustic village of Auca Mahuida, where we intended to camp this year. Less than ten people live in this small outpost, nestled in a picturesque ravine of reddish sandstone layers. Like Dona Dora and her family, the inhabitants now eke out a living off the land by raising sheep, goats, horses, and cattle; however, the town initially developed around the activities at a nearby asphalt mine. Once a relatively prosperous community of several hundred miners, Auca Mahuida experienced a sudden eclipse in the late 1940s when a fire killed seventeen workers and the authorities decided to close the mine. The village was abandoned, and the modern community evokes a ghostly image of its former heyday. Most houses have crumbled, and the desert has reclaimed most of the surroundings. Nonetheless, the village is only a ten-minute drive from our site, so our daily commute would be much shorter than the thirty-minute trek from Dona Dora's. In addition, the people of the village allowed us to use a partially crumbled and abandoned house. A few rooms still had roofs intact to protect our equipment and food, but we soon realized that each time we entered our sanctuary would be an adventure in itself when Rodolfo was struck by a falling brick, which opened an inch-long cut across his forehead. In spite of this precarious shelter, the town was furnished with a working well, so we would have fresh water without shipping it in from Neuquen or Plaza Huincul.
Rodolfo and his crew greeted us as we drove into town. Within an hour, we had pitched our tents and begun to settle in. Much to our delight, Rodolfo's crew was preparing a celebratory asado to mark our reunion in the field. As we dried out by the fire and toasted our arrival, the tribulations of the afternoon deluge quickly gave way to enthusiastic discussions of past triumphs and impending adventures. We could hardly wait to get started.
We set out for the site on the morning of March 1 and spent most of the morning introducing our new crew members to the surroundings, including where we had found fossils the previous year and the basic geological features. The paleontological and geological teams then split up and began their separate operations.
The first task for our fossil collectors was backbreaking. Luis and Rodolfo wanted to expose a large area of the egg-bearing layer at the quarry where we had found the embryos on our previous trip, but this would involve considerable excavation. At the nose of the ridge where the embryos had been found, most of the mudstone layer containing the eggs was buried under three to four feet of other rock. This spot was not easily accessible, so we could not use a bulldozer to clear off the overburden. We all knew that a more primitive approach would be required.
With everyone pitching in, we hauled the picks and shovels up the ridge and set to work. Our goal was to expose an area of the egg layer about thirty feet long by fifteen feet across, which would require us to remove about thirty to forty tons of overlying rock. It was a long, hard day, but with about ten people helping, we were nearing the top of the egg layer by the end of the afternoon. Once we got close to the top of the egg-bearing layer, we would trade our picks and shovels for smaller rock hammers, chisels, and dental tools to excavate the eggs without breaking them. But we could begin that job the next day.
A mile away on the other side of the field area, Lowell and the geological team were doing some digging of their own, though fossil eggs were not their quarry. Several hundred feet of sandstone and mud-stone layers lay on top of the egg-bearing mudstone, but we had not had time to measure or collect magnetic samples from these layers during our first trip. Our intent now was to continue measuring the rock layers above the egg-bearing layer and collect more samples for magnetic analysis. Starting just above the same layer that contained the egg quarry, we began working our way up the ridges.
Lowell described the rock types and measured the thickness of the layers for the stratigraphic section. Julia Clarke and Alberto Garrido, an Argentine graduate student in geology from the University of Cordoba, collected rock samples for paleomagnetic analysis. Alberto is a handsome, soft-spoken young man in his late twenties who, in addition to his general geological training, is familiar with the rocks of this region because he grew up in the area and has volunteered as one of Rodolfo's field assistants since he was a teenager.
Every fifteen feet or so, we would look for a suitable layer of silt-stone or mudstone from which to collect a magnetic sample. Each sample required about thirty minutes to excavate, measure, and wrap in aluminum foil. By the end of the day, we had collected about eight magnetic samples and measured about two hundred feet of rock layers above the main egg layer. In the process, we had located a new egg layer about seventy-five feet above the mudstone layer where the first eggs and embryos had been found. Our project to measure these layers and collect more magnetic samples would continue into the next day, but for now, it was time to rejoin the paleontological team and return to camp.
Unfortunately, when we arrived, a surprise was awaiting us. Our provisions had been too much of a temptation for the dogs of the village, who had knocked open the door of the house where we had stored our food and helped themselves to most of the meat we had brought. Well-gnawed bones littered the yard outside the house. Obviously, even in this rather remote corner of the Patagonian desert, we were going to have to beef up the security system around our home to fend off these four-legged intruders.
On the morning of March 2, both teams picked up where they had left off the previous day. Under Luis and Rodolfo's direction, the fossil collectors began the more delicate operation of removing the thin layer of mudstone that still overlay the eggs in the quarry, and by the afternoon, the tops of eggs began appearing on the surface of the expanded area of excavation. It was reassuring to realize that all the hard work of the last two days had not been wasted.
Most of Frankie and Gerald's work on our project involved uncovering a large area of the egg-producing layer to see where the fossil eggs were preserved within the layer. If they were not randomly placed but formed in discrete clusters, it would suggest that the eggs had been laid in nests. In addition to Frankie and Gerald, two students were instrumental in helping with work in the quarry. Gareth Dyke was a graduate student at the University of Bristol in England. He volunteered in order to gain a kind of field experience unavailable to him at home, and his determination during the long hours of quarrying contributed greatly to the trip's success. Anwar Janoo was a postdoctoral fellow in the Department of Ornithology at the American Museum of Natural History. He hails from Mauritius
Island in the Indian Ocean east of Madagascar, which was once inhabited by the famous dodo bird —a large, flightless relative of pigeons that was exterminated during the early days of European exploration and colonization of the Indian Ocean. Anwar is one of the world's foremost experts on this fascinating bird, which represents one of the most compelling stories in modern science about humanity's role as an agent of extinction. Anwar served on our expedition as a keen-eyed collector, as well as an excellent cook.
An intriguing fact was that the eggs were appearing within an interval of the mudstone that was about two feet thick: some were near the top of that interval, whereas others were near the bottom. At this early stage of the excavation, we were not sure whether the eggs were distributed in just one thick layer or whether there were actually two layers of eggs closely packed on top of each other. Only several days of patient excavation and Frankie's detailed mapping of the eggs' distribution in the quarry could help us answer that question.
On the other side of the field area, Lowell, Julia, and Alberto continued measuring the rock layers and collecting samples for magnetic analysis. This work was rather tedious because, as mentioned earlier, all one can do in the field is collect the samples. Analyses to determine whether the rocks had formed when the earth's magnetic field was normal or reversed would have to wait until we returned to the United States and worked on them in the laboratory. But in the midst of this tedium, a punctuating moment of unexpected discovery emerged.
We had discovered the first eggs and embryonic skin on our second day in the field during the expedition in 1997. For some reason, our second day in the field at Auca Mahuevo on our 1999 trip also turned out to be magical. After a morning of measuring rock layers and collecting magnetic samples, the geological trio headed down a ridge to return to their vehicle to drive to the quarry where lunch was waiting. On the way down the ridge, Alberto walked past some light beige fragments of rock weathering out of the hillside. Bending down to pick up one of the larger fragments, he could immediately see that these weren't simply small rocks; they were fragments of fossil bone. After calling Lowell and Julia over to examine the fossils, the crew began to collect some of the larger chunks. Clearly, at least some of the frag-
merits were from small vertebrae a few inches long, though none of them knew what kind of animal the fragments came from. Nonetheless, our geologists were excited because Auca Mahuevo, although rich in dinosaur eggs and embryos, had not yet yielded any skeletons of adult dinosaurs. Perhaps that would now change.
After putting a number of the larger chunks in a plastic bag, the crew drove back to the quarry. A large tarp had been strung between two of the vehicles to provide a bit of shade from the withering noonday sun. As we all began to eat, the bag of fossil fragments circulated among the paleontologists to see if anyone could identify the fragments. Immediately, Luis and Rodolfo became intrigued, and after a few moments of close inspection, they agreed on several basic points. First, the fragments represented tail vertebrae of a dinosaur. Second, the tail vertebrae did not seem to belong to a sauropod, but instead came from either a meat-eating theropod or a plant-eating ornithischian. This point was particularly exciting because with all the eggs of sauropods that we had found, it seemed most likely that any skeletal remains discovered would also be from sauropods. Third, we should examine the site more closely to see if more of the skeleton lay buried beneath the surface.
The next day, Luis, Rodolfo, and the geological team examined the remaining vertebrae fragments on the surface. Then we began clearing away the loose dirt around some of the larger fragments with a paintbrush to see if more bones lay buried under the surface. Within a few hours, our efforts had paid off. One by one, four more tail vertebrae slowly appeared. A tone of elation and anticipation permeated our conversation, but there was also some trepidation as it became clear that we would face a massive excavation effort if the whole skeleton lay beneath the dirt.
We were certain that, as new bones were exposed, we were progressing from the back of the tail toward the front. We could all see that these new tail vertebrae were arranged in a line that pointed into the small ridge on which we were working, and that each new one that we uncovered was larger than the previous one, as we followed the tail into the hill. At this point, we were sure that whatever remained of the skeleton was leading into the small hill that it sat on. In terms of preservation, this was excellent news: perhaps only the end of the tail had weathered away before Alberto had found the skeleton. But to find out, we would have to do some digging to remove the rock above where we hoped that the skeleton was buried.
Rodolfo and his crew volunteered to excavate the new dinosaur skeleton, and on March 4, they began by shoveling a foot or two of mudstone off the area. Then, as they carefully picked through the remaining mudstone with pocketknives and other small tools, more tail vertebrae appeared. By the end of the day, all the rest of the tail vertebrae had been exposed, along with the hipbones and some of the large bones of one hind leg. To our relief and great joy, the bones were all well preserved, and they fit up against one another in the same positions they had occupied when the animal had died.
With the back half of the animal now exposed, Rodolfo and Luis could see the shapes of the bones, providing some clues for identifying it. The bones of the tail and hind leg were almost identical to those found in Carnotaurus, one of the meat-eating theropods called abelisaurs. We were all thrilled because remains of abelisaurs are extremely rare, and many parts of their skeleton had never been found before. Rodolfo was particularly ecstatic: as a specialist on large predatory dinosaurs, he had been hunting for more than a decade for a complete skeleton. It looked as if his quest might finally be fulfilled.
At the quarry, work was also progressing nicely under the supervision of Frankie and Gerald. By the end of the day, sixty eggs had been exposed and mapped. Frankie had divided the surface of the quarry into one-meter squares and recorded the position of each egg in three dimensions from one corner of the quarry. To our knowledge, no other study had been able to document the distribution of so many eggs, and we hoped that our good fortune might allow us to make some important advances in understanding how sauropods had laid their eggs. With work at both quarries yielding spectacular results, it was time to celebrate: the sizzling of that evening's asado was accompanied by the popping of champagne bottles.
On the following day, Rodolfo and his team continued to probe through the mudstone at the dinosaur quarry, and they found even more bones from the skeleton. The arms were small in relation to the rest of the body, although they were proportionally slightly larger than the arms in Carnotaurus. Nonetheless, the shapes of the arm bones were very similar. Some vertebrae from the neck were also uncovered, which suggested that the skull might be preserved somewhere under the surface. In all, it appeared that, except for the end of the tail, which had weathered awav before Alberto had found the skeleton, all of the bones might be present.
While reflecting on our new discovery, we realized that another piece of the ancient puzzle of Auca Mahuevo had fallen into place. Although we had still not found a good skeleton of the adult sauropods that had laid the eggs, we now had a pretty good idea about who had probably served as their primary predator. The parts of the skeleton that had been exposed showed that this menacing meat-eater was about twenty feet long. Based on the long, powerful hind legs and short arms, it clearly walked exclusively on its two back legs and probably weighed between one and two tons. Although Alberto's spectacular discovery lay many feet above the egg layer, there was little doubt that the species to which it belonged inhabited the area when the Auca Mahuevo sauropods gathered to nest. It would have been a terrifying adversary. Such an animal could easily have taken down a young sauropod bv itself, and if the predators pooled their efforts in packs, even adult sauropods could have been at risk.
Quarrying of the eggs also continued on March 5. By now more than eighty eggs had been exposed, and wc hoped that we would soon have several hundred eggs excavated and mapped. Unfortunately, the fickle Patagonian weather had other ideas about how we would spend the next two days. By the end of the afternoon, clouds were building on the horizon, and we covered both quarries with large tarps just in case the torrid summer heat was temporarily interrupted by another torrent from the approaching fall.
During that night, the rains arrived, though not the kind of torrential downpour that we had experienced earlier on our way to the site. It was just a gentle, steady rain that lasted for two whole days. Frustratingly, however, the ground was so muddy that we could not get to either of our quarries. Even if we had made it, it would have been impossible to work because the wet clay of the egg-bearing mudstone would have stuck tenaciously to the tools and fossils, risking damage to the eggs if we had tried to excavate them. To deny the rain an opportunity to dampen our spirits, we headed to Plaza Huincul to see some new exhibits at Rodolfo's museum. Since the last time we had visited, the museum had finished a new, full-scale mount of the monstrous meat-eater Giganotosaurus, which was spectacular. This new exhibit, along with a quick shower at a local motel, worked wonders to refresh our strength and enthusiasm, so we headed back to camp with renewed vigor.
The trip turned out to be both long and a bit harrowing. Night had fallen and the headlights on one of our older vehicles, which Lowell was driving, began to fail. It became impossible to see more than a few car lengths in front of us, and the headlights of oncoming cars were completely blinding. We stopped to get the vehicle checked out and bought a new battery at the last gas station before we left the paved road for the long stretch of dirt roads leading into the desert, but the problems soon reappeared. Lowell and Sergio Saldivia had to drive the last seventy miles to camp slowly in order to see the dirt roads at all, so we didn't get back until about 2:00 A.M. Ironically, it turned out to be fortunate that the rain continued throughout that night and all the next day, because we had a chance to sleep in, do some laundry, take a lazy afternoon nap, and catch up on writing our field notes. By about noon on March 8, the rains finally ceased, and we were once again eager to go.
The rains had caused some minor damage in the quarries, but fortunately, the fossils were not affected. The pits were quickly drained so that work could continue. At the egg quarry, Luis, Frankie, Gerald, and the rest of the team focused on removing some of the rain-softened overburden so that the excavation could be expanded to the full thirty-feet-by-fifteen-feet that we had originally planned to map. In the abelisaur quarry, Rodolfo's crew continued to expose more of the skeleton.
Lowell, Julia, and Alberto, having finally finished measuring the rocky layers and collecting magnetic samples at the site, turned their attention to figuring out how many egg layers actually existed. Clearly, at least two separate layers were present: one contained most of the eggs on the flats and in the quarry, whereas the other lay about seventy-five feet higher in the sequence. But were there still others, as yet unrecognized? A closer examination of the eggs weathering out on the flats revealed that two separate layers of mudstone, separated by about five or six feet of unfossiliferous sandstone and mudstone, actually contained eggs. In addition, another isolated cluster of eggs was discovered about twenty-five feet below the layer that contained the quarry. This meant that at least four separate layers contained eggs, which was an important clue for us in our attempts to understand the nesting behavior of the gigantic sauropods, as we will discuss later in more detail. Gerald collected samples of the eggshell from each layer. By studying the patterns of ornamentation and chemical composition of the shell in the lab, he hoped to determine whether more than one kind of sauropod had laid the eggs in the different layers.
On March 9, some of the crew members took a long drive to Neuquen to buy supplies and take care of some bureaucratic tasks, but Rodolfo's crew worked on in the dinosaur quarry. More and more frus-tratingly, we had yet to discover the skull, or even any evidence that a well-preserved skull was present. The small bits and pieces of skull bones found up to this point suggested that the skull might have been badly damaged either before or during fossilization. But it was also possible that the skull had become detached from the neck and was buried somewhere close by. When Marilyn Fox arrived at the site, she assumed the job of digging carefully through the mudstone to find out, but despite several days of painstaking excavation, she could still not find a large piece of it.
On the tenth, a sense of tedium descended over our operation, mixed with a growing sense of anxiety. We were about halfway through the field season now, and our wealth of success began to seem like a mixed blessing. It seemed impossible to get both the eggs and the large new dinosaur skeleton collected and analyzed. The work at the egg quarry seemed interminable; there were eggs everywhere we dug. Our hopes for a fairly complete abelisaur skeleton had been realized, but to excavate such a large skeleton would require more than a week of intense digging and plastering by Rodolfo's crew. And once the plaster jackets had been constructed around the bones, the heavy jackets would have to be rolled over, plastered on the bottom, and lifted onto a vehicle that could transport them back to the museum in Plaza Huincul. Some sort of heavy equipment, such as a bulldozer or a crane, would be required for the lifting. But where would we find that in the middle of the desert? Rodolfo left camp for Plaza Huincul to see what kind of arrangements he could make.
To make matters worse, the recent rains had triggered a totally unexpected population explosion among the insects of the area. Clouds of mosquitoes began to hatch from the small ponds and pools that were left scattered across the desert. Our normally peaceful evenings of relaxed conversation quickly degenerated into long hours of swatting and slapping as we tried to fend off the buzzing, bloodsucking hordes. Our only advantage was that they slept during the day, which allowed our work to proceed without these annoying interruptions.
The next two days were spent wrapping up odds and ends, especially around the egg quarry. Frankie precisely documented the position of more than two hundred eggs using the system of grids laid out in the quarry. In addition, Luis, Gerald, and Gareth built plaster jackets around several clusters of eggs so that we could collect them for further study. One was huge, containing forty or fifty eggs and probably weighing almost a thousand pounds. Natalia Klaiselburd, who was released from her duties in the quarry to prospect, found three more fragments of eggs that contained patches of fossilized skin. Another embryo was found in an egg near the quarry. Throughout this period, the mosquitoes were still on the rampage.
With the eggs in the quarry now completely exposed, Lowell and his geological team spent most of the morning of the thirteenth examining the site. We focused our attention on a puzzling geological phenomenon: the mudstone in the egg layers was laced with smooth, shiny, grooved surfaces that looked similar to slickensides, the surfaces one sees in fault zones where blocks of rock slip past one another as movement occurs along the fault. However, the slickensides in the mudstone at Auca Mahuevo were not large and continuous, the way one would expect if they had been created by a large fault that extended for hundreds of yards or miles across the countryside. These were only a few inches to a foot long. Some of the magnetic samples from the mudstone that we had collected in 1997 were yielding screwy results, which we thought might have resulted from the movement of the mudstone blocks along the small slickensides. But we didn't have any idea what had caused the slickensides, since they weren't large enough to be major faults.
Fortunately, David Loope, a geology professor from the University of Nebraska, had arrived the previous evening, and we described what we had been seeing to him. Dave possesses a calm and considered demeanor, as well as an entertaining sense of humor that is as dry as the Patagonian desert, and he is a widely recognized expert on ancient sand dunes who had accompanied us on expeditions to the dinosaur-rich deposits of the Gobi Desert in Mongolia. There, his observations were critical in helping us understand how the beautifully preserved fossils had come to be killed and preserved by massive, water-soaked sand avalanches that had slid down the dune faces during occasionally heavy rainstorms. His extensive knowledge about how different kinds of rocks form would also prove invaluable to our studies of the rocks that entomb the eggs and embryos at Auca Mahuevo.
Dave immediately thought that the slickensides might be related to a geologic process called vertisols, which operates in some soils. He explained that, in clay-rich soils, alternating cycles of wetting and drying can lead to expansion and contraction of the material in the soil. The expansion occurs when it rains and the clay absorbs the moisture, which causes blocks of the soil to move upward, creating the slippage surfaces or slickensides that we had observed. This "vertical" movement occurs because the direction of least resistance is upward, since rock or dirt is present on the sides and below the soil, but only the atmosphere or a thin layer of soil overlies the expanding clay. The ver-tisols would probably have formed within decades or a couple of centuries after the floods deposited the mudstone.
The vertisols appeared to have some other important implications. First, they offered additional evidence that the area occasionally received substantial amounts of rain, alternating with substantial periods of dryness. Since we suspected that floods had killed the embryos, the vertisols would help establish that rains potentially generating floods did occur. Also, vertisols could easily have led to the development of a hummocky surface on the floodplain, with small bumps and depressions littering the landscape, and such depressions might have made good places for the sauropods to lay their eggs. Finally, the vertisols could explain why many of the eggs were somewhat squashed and some of the egg clusters appeared to be mixed up with one another. Movement along the slippage surfaces could have created these effects by crushing the eggs trapped in the moving blocks and displacing others.
Frankie's mapping in the quarry gave us a good idea about how individual eggs were distributed in clusters, but the quarry was too small to document how the clusters were distributed across the land surface. To determine how the clusters were distributed, Luis,
Frankie, and Gerald went back to the flats. There, we selected a 120-foot-by-90-foot rectangular area and marked all the enclosed egg clusters with colored balloons, which proved slightly difficult because, although some of the clutches were easy to identify, others had almost completely weathered away. To make sure that a clutch was actually present, we decided to mark only clusters that contained remnants of eggshells arranged in a circle and imbedded vertically in the ground. Using a tape measure, we divided the selected area into squares that were thirty feet on a side, thereby creating a gigantic grid. Then, we once again plotted the precise position of each of the egg clusters within the squares. When the grid was constructed and the clutches marked, the green and blue balloons created a rather surreal scene across the naked landscape of the Patagonian desert, as if we had created a monumental work of art worthy of Christo. We humorously reflected on how we should submit a grant to the National Endowment for the Arts to raise additional funds for the project.
By March 14, Rodolfo had returned. He and his crew now faced a pressing problem: we had to get the skeleton out of the ground before the field season ended, and we had only two weeks to do it. This work would involve several phases. First, we would have to remove the rocks overlying the skeleton and open up a large quarry to have sufficient space to work. Then, trenches would have to be excavated around the skeleton. Because the skeleton was too large to be lifted out of the ground as a whole, we had to decide where to cut between the bones so that it could be divided into smaller, more manageable blocks. But before these blocks could be cut and lifted, the skeleton would have to be covered with toilet tissue and plaster bandages to form a protective covering and reinforced with sturdy wooden struts to hold it together and support it during the trip back to the museum. Once this was completed, we would have to define the different blocks by cutting through the rocks and, unfortunately, through some of the bones. Then all of the blocks would have to be undercut so plaster bandages could be attached to the undersides. These bandages would keep dirt and fossils from falling out of the plaster jackets when they were flipped over in the quarry so that the bottoms of the blocks could be jacketed. Finally, we would have to build wooden pallets for the blocks to sit on and find a crane to take them from the quarry to the road, where they could be lifted onto a truck for the trip back to the museum.
All of that work kept Rodolfo, Alberto, Gerald, Anwar, and several other crew members busy for the rest of the field season. We decided to subdivide the skeleton into five blocks: a large one for the hips and tail, two smaller ones for the hind limbs and arms, another large one for the trunk and neck, and a last one for where we hoped the skull would be. Even after dividing the skeleton into five sections, some of the blocks weighed more than a ton with their petrified bones, wet plaster, surrounding rock, and wooden supports.
Meanwhile, along with Luis, Lowell and the other members of the geological team embarked on a two-day trip to the west of Auca Mahuevo, in search of a volcanic ash described in a geologic paper we had read. The scenery on the drive was spectacular; huge volcanoes loomed on the horizon off to the west. But as we approached the site where the ash had been discovered, the dirt road was washed out, so we could not get to the precise locality described. After driving almost completely around the site on other roads, however, we did find one track that got us close. At the end of the track were some exposures that looked as if they contained the same sequence of rock layers exposed at Auca Mahuevo. Since the sun was setting, we decided to cook dinner and look at the exposures in the morning.
We bedded down soon after dinner, but we were awakened by vigorous thunder and lightning about 1:30 A.M. The storm was rapidly approaching, and within a few minutes the first sprinkles began to pelt our sleeping bags. We hadn't brought our tents because there wasn't room in the vehicle, so as the rain quickly increased, we crawled out of our bags, stuffed them in or under the vehicle, and scrambled inside. With five of us and some of our equipment, it was pretty cramped, and for a few minutes we couldn't get the power windows to close. By the time we finally managed to close them, most of us were soaked. Heavy rain and a bit of hail pummeled our vehicle, and it took about an hour and a half for the storm to pass. But by three or three-thirty, we slumped out of the car, laid out our air mattresses, and crawled back into our bags.
Arising early in the morning, we ate and then assaulted the exposures. Near the top of a large ravine, Lowell found a cream-colored layer of rock that was quite different from any of the others. It seemed to be a layer of altered volcanic ash, but there didn't appear to be any large crystals in it that might be used for dating through radioactive methods. Nonetheless we collected some plastic bags full of chunks, hoping that not all of the crystals had weathered to clay. Luis also found a few fragments of fossil bone, but nothing worth collecting. We would need to return some other time and try to get to the site described in the scientific paper, but that would probably require hiring either horses or a helicopter. At about noon, we headed back to camp. As we drove, we passed drifts of hailstones that had washed into nearby gullies. Although we had spent a miserable night exposed to the elements, we clearly had not borne the brunt of the storm.
We returned to find that Frankie, Gerald, and the rest of the crew had found three embryos in the eggs at the quarry. In addition, they had measured and mapped another area of eggs on the flats. In the first large grid, seventy-four clusters had been documented, whereas the second, smaller grid contained about half that number.
Over the next three days, most of the paleontological team focused on plastering and flipping the plaster jackets containing the abelisaur skeleton. Meanwhile, Lowell's geological team began looking closely at some of the exposures that contained eggs in the highest layer in the sequence. Brushing off the loose dirt, we could see that the eggs had been laid on a bumpy surface of the ancient floodplain. Assisted by Julia and Frankie, Lowell used a tape measure and the leveling bubble in his Brunton compass to map where the eggs sat on the surface. The surface of the ancient floodplain was definitely bumpy, with small mounds and depressions, just as David Loope had warned us to expect with vertisols. However, it did not appear that the eggs sat in these small depressions. We wondered why the depressions had not been used as nests, but we could not be sure. The best news was that, at last, the swarms of mosquitoes were now quickly dying off.
Most of our last five days in the field were devoted to moving the blocks containing the abelisaur skeleton out of the quarry and down to the nearest road, where they could be lifted onto a flatbed truck for the trip back to Rodolfo's museum. The work went slowly, as some of the blocks weighed over two thousand pounds. To flip these large rocks and move them around the quarry, we used our four-wheel-drive Passport that American Honda had given us to use during the expedition. After all the blocks were flipped and ready to move, our friends at a nearby oil station helped us find a crane, and Rodolfo arranged for a truck to transport the blocks to Plaza Huincul. Although the truck failed to come on the specified day, Rodolfo returned a couple of days later with the driver to pick up the plaster jackets that the crane had left by the side of the road. In addition to this work, we also entertained several individuals who had helped sponsor our expedition, as well as several members of the media both from local and international networks and publications.
Finally, on the afternoon of March 24, we packed up most of our gear and celebrated the success of our season with a final asado. The next morning, under once again rainy skies, we headed for Plaza Huincul on our way back to Buenos Aires. It had been another exceptional month of discoveries. But now, once again, we faced long months of preparing the fossils that we had collected and analyzing the data that we had gathered.
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