Expedition to Patagonia 1999

Return to Auca Mahuevo with Lowell Dingus

Follow-On Discoveries from the 1997 Expedition...

Following our discovery of the first-known sauropod embryos and embryonic dinosaur skin in 1997, many questions regarding these animals and their environment were still unanswered. So, just as modern investigators must often return to the scene of an accident to gather more evidence after their initial investigation, so did we. Concerning the dinosaurs and their embryos, mysteries remained about how the sauropods 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? Also, did they return to the nesting site year-after-year or just use the site once? Finally, what other dinosaurs lived alongside them on the ancient floodplains of Patagonia?

To help solve these mysteries, Luis Chiappe and I, along with our colleague Rodolfo Coria (below, left), mounted a second expedition to Auca Mahuevo in March of 1999. 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. Jack and his team of collectors were responsible for completely reinvigorating the study of dinosaur eggs and embryos with their discovery of Egg Mountain in the late 1970s. 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 and knowledge for dinosaur eggs is boundless, and there is nothing she likes more than to spend her days patiently quarrying eggs and mapping where they are found in the ground.

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 15 years had shed considerable new light on the evolution of both primitive dinosaurs called ceratosaurs and on the early relatives of our own group of vertebrates, the mammals. Gerald's thesis has 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 through his work it is evident 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 eggshells. Gerald's tall sturdy stature, along with his gregarious personality and wild stories of past adventures, makes him a natural for field work. His keen sense of observation, as well as his geological background, proved invaluable to our efforts.

Most of Frankie and Gerald's work on our project involved studying large areas of the egg-producing layer to see where the individual eggs were preserved within the layer. If they were randomly placed, it would argue against the idea that the dinosaurs laid the eggs in distinct nests, but if the eggs were placed in discrete clusters, it would suggest that they had been laid in nests.

(Note: Read Frankie and Gerald's follow ups, reachable from the menu on the top left of this page.)

After flying to Buenos Aires in late February, we gathered equipment and supplies before leaving for Auca Mahuevo on the 27th. Our two-day trek to the site was largely uneventful, at least until we left the highway for the final 75 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 thunderhead 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 themselves transformed into deep ponds. This presented no problems for the larger, four-wheel-drive vehicles. However, our entourage included the small Fiat sedan owned Luis' brother, Ezequiel Chiappe, and it was not built for these kinds of road hazards. With only six or seven inches of clearance, it struggled to ford the deep ponds. By now, the rain was torrential, an in places the road was covered by almost two feet of water. As I was filling in for Ezequiel for the last stretch to camp, the wipers on the Fiat quit working, and I had to stick my head out of the window to see where I 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.

Our return to Patagonia was sponsored by the Natural History Museum of Los Angeles County, which played an essential role in helping us arrange for two new vehicles from Honda, which played critical roles in transporting our crew and the fossils. In the period since our last expedition, Luis had become an Associate Curator in that museum's Department of Vertebrate Paleontology. 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 ancient catastrophe at Auca Mahuevo. 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.

Initially, we considered turning around and trying to out-flank 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 truly trapped. We had no other options 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 reeked havoc on the sauropod's arid 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 drainage. We had seen when the semi moved through that the water was now only about a foot deep, but the Fiat was pretty well spent. We basically had to tow it through the receding current. But once we had forded the stream, which was still about a quarter mile in width, the road soon improved dramatically.

By 7:30 pm, we limped into the tiny, rustic village of Auca Mahuida, where we intended to camp this year (slide 2). 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 1940's when a fire killed 17 workers and the authorities decided to close the mine. The village was abandoned, creating a situation in which 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 30-minute trek from Dona Dora's. In addition, the people of the village allowed us to use an abandoned house for storing our equipment and food (slide 3), and the town was furnished with a working well. So, we would have access to all the fresh water we required without shipping it in from Neuquen or Plaza Huincuil.

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 out delight, Rodolfo's crew was preparing a celebratory asado to mark our reunion in the field. Having 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.

After a hard-earned night's sleep, we set out for the site on the morning of March 1st. Luis and I 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. At that point, the paleontological and geological teams split up and began their separate operations.

The first task for the fossil collectors was a back-breaking one. Luis and Rodolfo wanted to expose a large area of the egg-bearing layer near the quarry where we had found the embryos during the previous trip, but to do so would involve a considerable amount of excavation. The reason was that most of the layer of mudstone containing the eggs was buried under three to four feet of other rock layers on the nose of the small ridge where the embryos had been found. 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.

In addition to Frankie and Gerald, two students were instrumental in helping with work in the quarry. At the time, 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 post-doctoral fellow in Ornithology at the American Museum of Natural History. He hails from the Mauritius Islands in the Indian Ocean east of Madagascar, which were 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 involving humanity's role as an agent of extinction. Anwar served on our expedition as a keen-eyed collector, as well as an excellent cook.

So, with everyone pitching in, we hauled the picks and shovels up the ridge and set to work (slide 4). Our goal was to expose an area of the egg layer about 30 feet long by 15 feet across. This would require us to remove about 30 to 40 tons of overlying rock. It was a long hard day, but with about 10 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 in order to excavate the eggs without breaking them. But we could begin that job tomorrow.

A mile away on the other side of the field area, I was engaged in some digging of my own; however, fossil eggs were not the quarry. In 1997, I had collected several rock samples for magnetic analysis (slides 5 and 6). Once analyzed by Carl Swisher and Gary Scott at the Berkeley Geochronology Center, the samples showed that our egg-bearing layer was composed of mudstone that had been formed at a time when the Earth's magnetic poles were reversed. In other words, a compass needle that points north today would have pointed south at the time when the mudstone formed and the eggs were laid. Such reversals of the poles have occurred numerous times in the geologic history of the earth, although we are not sure exactly why. Nonetheless, geologists have developed a timescale, coordinated with the geologic timescale, that documents when the poles reversed in the past. Based on the initial magnetic analyses of the rocks at Auca Mahuevo, along with the kinds of fossils found in nearby layers, it appears that the egg-bearing layer at Auca Mahuevo was formed between 79 and 83 million years ago. But several hundred feet of sandstone and mudstone layers lay on top of the egg-bearing mudstone that contained the quarry, and I had not had time to measure or collect magnetic samples from these layers during our first trip. Our intent was to continue measuring the rock layers above the egg-bearing layer and collect more samples for magnetic analysis. Starting just above the layer that contained the quarry, we began working our way up the ridges.

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As I described the rock types and measured the thickness of the layers for the stratigraphic section, Julia Clarke and Alberto Garrido collected rock samples for paleomagnetic analysis. Julia was a graduate student studying at Yale University under the supervision of another of our close friends and colleagues, Jacques Gauthier. His research over the last 20 years has greatly improved our understanding of the evolution of dinosaurs and the origin of birds from small meat-eating dinosaurs. Julia is well versed in both geology and paleontology, so she served both as a fossil collector and field geologist. Alberto is an Argentine geology student from the University of Cordoba. He is a handsome, soft-spoken young man in his late 20s. In addition to his general geological training, Alberto is very familiar with the rocks of this region because he had grown up in the area and had volunteered as one of Rodolfo's field assistants since he was a teenager. His knowledge of the area, as well as his keen powers of observation, would help make this expedition another special trip of discovery.

Every 15 feet or so we would look for a suitable layer of siltstone or mudstone from which to collect a magnetic sample. Each sample required about 30 minutes to excavate, measure, and wrap in aluminum foil. By the end of the day, we had collected about eight more magnetic samples and measured about 200 feet of rock layers above the main egg layer. In the process, we had relocated a new egg layer about 75 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 the next day, but for now, it was time to rejoin the paleontological team and return to camp.

There, a surprise was awaiting us. It seemed that our provisions had represented too much of a temptation for the dogs of the village. They had knocked open the door of the house where we stored our equipment and food helped themselves to a lot of the meat we had bought. 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 we 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 (slide 7). By the afternoon, the tops of eggs began appearing on the surface of the expanded area of excavation. It was reassuring for us to realize that all the hard work of the last two days had not been wasted. The 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 process, 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.

Across on the other side of the field area, Julia, Alberto, and I continued measuring the rock layers and collecting samples for magnetic analysis. By this time, this work had gotten rather tedious because, as mentioned earlier, all one can do in the field is collect the magnetic samples. Analyses to determine whether the rocks had formed when the Earth's magnetic field was reversed or not would have to wait until we returned to the United States and worked on the samples in the magnetics laboratory. But in the midst of this tedium emerged a punctuating moment of unexpected discovery.

During the expedition in 1997, we discovered the first eggs and embryonic skin on our second day in the field. For some reason, our second day in the field at Auca Mahuevo on our 1999 trip would also turned out to be magical. After a morning of measuring rock layers and collecting magnetic samples, my geological trio headed down a ridge to return to the vehicle so that we could drive to the quarry where lunch was waiting. On the way down the ridge, Alberto Garrido happened to walk past some light beige fragments of rock weathering out of the hillside. Bending down to pick up one of the larger fragments, he could see immediately that these weren't simply small rocks. They were fragments of fossil bone. After calling Julia and me over to examine the fossils, we began to collect some of the larger chunks. It was clear that at least some of the fragments were from small vertebrae a few inches long. However, none of us knew what kind of animal they came from. Nonetheless, we 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, we drove back to the quarry. A large tarp had been strung between two of the vehicles in order to provide a bit of shade from the withering noon-day sun. As we began to eat, the bag of fossil fragments circulated among the paleontologists to see if they could identify the fragments. Immediately, Luis and Rodolfo (slide 8) became intrigued. 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 either from a meat-eating theropod or a plant-eating ornithischian. That was exciting because with all the eggs of sauropods that we had found, it seemed most likely that any skeletal remains discovered would also be those of sauropods. Third, we should take a closer look at the site tomorrow to see if more of the skeleton laid buried underneath the surface of the ground.

The next day, Luis, Rodolfo, and the geological team returned to the site (slide 9). After examining the remaining fragments on the surface, Rodolfo began clearing away the loose dirt around some of the larger fragments with a paint brush to see if more bones laid buried under the surface (slide 10). 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 it was also becoming 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 each one that we uncovered was larger than the previous one as we followed the tail into the hill. Consequently, as our field notes indicate, we could be 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 found the skeleton. But to find out, we would have to do some serious digging to remove the rock above where the skeleton might be buried.

Rodolfo and his crew volunteered to take on the task of excavating the new dinosaur skeleton. On March 4th, they began by shoveling a foot or two of mudstone off the area under which they hoped the back part of the skeleton lay (slide 11). Then, carefully picking through the remaining mudstone with pocket knives 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 hip bones and some of the large bones of one hind leg. To our relief and great joy, the bones were all well preserved. Furthermore, they fit up against one another in the same positions that they had occupied when the animal died.

With the back half of the animal now exposed, Rodolfo and Luis could see that the shapes of the bones provided some clues for identifying the victim. So, we could be more certain about what kind of dinosaur it was. The shape of 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 genuinely 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 to uncover a complete skeleton. It looked like 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, 60 eggs had been exposed and mapped. Frankie took the lead in the mapping operation by dividing the surface of the quarry up into one-meter squares and recording the position of each egg in three dimensions from one corner of the quarry, using a system of grids much like is done in archaeological excavations (slide 12). The fact that this many sauropod eggs were found in such a small area was unprecedented. 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 terms of the how sauropods laid their eggs. After analyzing the data back in the laboratory, Frankie realized two things (see figure 2). First, the eggs were indeed clustered, suggesting that the sauropods did lay the eggs in distinct nests. Second, there appeared to be two separate layers of eggs in the mudstone of the quarry. 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. Their good fortune continued with the discovery of more bones from the skeleton. The arms were very 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 away before Alberto found the skeleton on the hillside, 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 still had not found a good skeleton of the adult sauropods that had laid the eggs, we now had a pretty good idea about who probably served as one of their primary predators. It was clear from the parts of the skeleton that had been exposed that this menacing meat-eater was about 20 feet long. Based on the long powerful hind legs and short arms, it clearly walked exclusively on its two back legs. When alive, it probably weighed between one and two tons. It would have been a terrifying adversary. Such an animal could have easily taken down a young sauropod by 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 5th. By now more than 80 eggs had been exposed in the embryo quarry, and we were hopeful that soon we would have several hundred eggs excavated and mapped. But as it turned out, 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 to be temporarily interrupted by another torrent from the approaching fall.

During that night, the rains arrived. However, it wasn't the kind of torrential downpour that we had experienced earlier on our way to the site. It was just a gentle, steady rain, but it lasted for two whole days. The effect was somewhat frustrating, however, because the ground was muddy enough 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 sticks tenaciously to the tools and fossils. Consequently, the eggs could have been easily damaged if we had tried to excavate them. In order to deny the rain an opportunity to dampen our spirits, we decided to head in to Plaza Huincuil to see some new exhibits at Rodolfo's museum. Since the last time we visited, the museum had finished a new, full-scale mount of the monstrous meat-eater Giganotosaurus. It was spectacular, and the 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.

However, the trip turned out to be both long and a bit harrowing. Night had fallen and the headlights on one of our older vehicles that I was driving began to fail. It became impossible for me to see more than a few car-lengths in front of us, and the headlights of on-coming cars were completely blinding. We stopped to get things 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. However, the problems soon reappeared. Rodolfo's preparator, Sergio, and I had to drive the last 75 miles to camp very slowly in order to see the dirt roads at all. Consequently, we didn't get back until about 2:00 AM. Ironically, it turned out to be fortunate that the rain continued throughout that night and all the next day. 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 8th, the rains finally ceased, and we were once again raring 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 30-foot by 15-foot surface that we had originally planned to map. In the abelisaur quarry, Rodolfo' crew's attempt to expose the skeleton continued.

Julia, Alberto, and I, having finally finished measuring the rocky layers and collecting magnetic samples at the site, turned our attention to figuring out how many egg layers actually existed. It was already clear that at least two separate layers were present (see figure, right). One contained at least most of the eggs on the flats and the eggs in the quarry. The other lay about 75 feet higher in the sequence. It had been discovered by Brooks Walker, a photographer who came with us to the site in May of 1998 to take photos for an article in National Geographic. But were there still other egg-layers, as yet unrecognized? A close examination of the eggs weathering out on the flats revealed that two separate layers of mudstone contained eggs. These layers were actually separated by about five or six feet of unfossiliferous sandstone and mudstone. In addition, another isolated cluster of eggs was discovered about 25 feet below the layer that contained the quarry. With Frankie's discovery that there were two egg layers in the main quarry, this meant that at least five separate layers contained eggs.

Our discovery that five rock layers contained clusters of sauropod eggs at Auca Mahuevo provided important evidence about the reproductive behavior of the huge sauropods. The ancient layers of rock at Auca Mahuevo were laid down one on top of the other, so each layer represents a different page in the geologic history of the animals and environment that existed at the site 80 million years ago. Since five different layers at five different levels in the sequence of rocks contain eggs, the giant sauropods clearly returned to the nesting site at least five different times to lay their eggs, a behavior called site fidelity. Since we don't know the precise age of each rock layer, we don't know whether they returned every year or not, but that is certainly possible given the probability that not all of the eggs laid by the sauropods at the site were preserved as fossils. In years when floods did not bury the eggs quickly, most of the embryos would have hatched and left the site. The eggshells would have been broken up, and the shell fragments would probably be dissolved by rain or destroyed by other natural processes, leaving no evidence of these breeding events in the fossil record. So, while we cannot be certain exactly how many times or how often the sauropods used the site, we do know that they used it at least five times.

For some of the crew members, March 9th involved a long drive to Neuquen in order to buy supplies and take care of other bureaucratic tasks, but the work of Rodolfo's crew in the dinosaur quarry went on. It was becoming more and more frustrating that we had yet to discover the skull. In fact, we had still found no evidence that a well preserved skull was present. The only small bits and pieces of skull bones found to this point suggested that the skull might have been badly damaged either before or during fossilization. But perhaps it had just become detached from the neck to be buried somewhere close by. With the arrival of Marilyn Fox at the site, the job of digging carefully through the mudstone to find out fell to her. Marilyn is an expert fossil collector and preparator who works with Jacques Gauthier and Julia at Yale. Yet, despite several days of painstaking excavation, she still could not find a large portion of it.

By the 10th, a certain sense of tedium descended over our operations, 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 of sorts. The main problem involved how we could get both the eggs and the large new dinosaur skeleton collected and analyzed. In terms of the egg quarry, an entry in our field notes laments that the work seemed interminable. There were eggs everywhere. Concerning the abelisaur skeleton, our hopes for a fairly complete specimen 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. Then, 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 on to a vehicle that could transport them back to the museum in Plaza Huincul. Some heavy equipment, such as a bulldozer or a crane would be required for the lifting. But where would we find those in the middle of the desert? Roldolfo 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, blood-sucking hoards. Our only advantage was that they slept during the day, so that our work could proceed without these annoying interruptions.

The next two days were spent wrapping up odds and ends, especially around the egg quarry. Mapping eggs in the quarry was completed. Frankie precisely documented the position of more than 200 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 and contained 40 or 50 eggs. In all, this jacket probably weighed almost 1000 pounds. Three more fragments of eggs that contained patches of fossilized skin were found, along with another embryo in an egg near the quarry. Throughout this period, as our field notes document, the mosquitoes were still on the rampage.

With the eggs in the quarry now completely exposed, I spent most of the morning of the 13th 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 the surfaces one sees in fault zones where blocks of rock slip past one another as movement occurs along the fault (slide 13). These surfaces are called slickensides. However, the slickensides in the mudstone at Auca Mahuevo were not large and continuous, like one would expect if they had been created by a large fault that extended for hundreds of meters or kilometers across the countryside. They were only a few inches or 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. However, we didn't have any idea what caused the slickensides, since they weren't large enough to be major faults.

Fortunately, David Loope had arrived the previous evening, and I described what we had been seeing to him. Dave is a geology professor from the University of Nebraska who possesses a calm and considered demeanor, as well as an entertaining sense of humor that is as dry as the Patagonian desert. 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 slid down the dune faces during occasionally heavy rain storms. 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 that operates in some soils called vertisols. 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. The expansion causes blocks of the soil to move upward, creating the slippage surfaces or slickensides that we had observed. This "vertical" movement results from the fact that when expansion occurs, 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 vertisols probably would have formed within decades or a couple centuries after the mudstone was deposited during floods.

The vertisols appeared to have some other important implications. First, they could provide another line of evidence to suggest that the area occasionally received substantial amounts of rain, alternating with substantial periods of dryness. Since we suspected that floods killed the embryos, they would help establish that rains potentially generating floods did occur. Also, vertisols could have easily led to the development of a hummocky surface on the floodplain, with small bumps and depressions littering the landscape. Modern vertisols, such as those developed in southern Texas, tend to form this type of microtopography. 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 appear 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 get a better idea of 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 (slide 14). This was a difficult task because, although some of the clutches were easy to identify, others had almost completely weathered away. We decided to mark only clusters that contained remnants of eggshell arranged in a circle that were oriented vertically in the ground to make sure that a clutch was actually present. Using a tape measure, we divided the selected area into squares that were 30 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 (see Figure 1). 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. It was 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.

What emerged from our forensic investigations that we conducted between 1997 and 1999 was a somewhat fuzzy, almost impressionistic, portrait of the sauropods' life on the ancient floodplain during their breeding season. It is likely that huge herds of sauropod females lumbered to Auca Mahuevo during many, if not all the breeding seasons in which the nesting site was in use. It is not clear whether males accompanied them or not. However, it is clear that upon their arrival, each gravid female laid approximately 10 to 40 eggs in a nest on the gently sloping, hummocky ground away from the major stream channels. Most of the nests were spaced between five and fifteen feet apart from one another. Sometime after completing this task, the females probably left the site, leaving the eggs to incubate, although some adults may have remained in the area to guard the colony. The embryos grew inside the eggs to a length of about 12 inches before they were ready to hatch. At the end of their development inside the eggs, the embryos began to exercise their jaw muscles and grind their teeth, in preparation for eating vegetation when they hatched. Normally, many of the eggs hatched without incident, and the hatchlings, with their relatively large heads, strong jaw muscles, and small but fully developed teeth, immediately set about consuming any nourishing vegetation that they could find. Over the next 15-20 years, assuming they survived, the sauropods would have to increase their body size more than 30-fold to become some of the largest animals known to have walked the Earth. However, floods occasionally inundated the nesting site during the incubation period, burying the eggs under a layer of mud and killing the developing embryos inside.

By March 14th, 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 only had two weeks to do it. Consequently, we decided to start the process of collecting it. This work would involve several phases. First, we would have to remove the rocks overlying the skeleton and open up a large quarry in order to have sufficient space to work. Then, trenches would have to be excavated around the skeleton. But 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. Before these blocks could be cut and lifted, however, they would have to be covered with toilet tissue and plaster bandages to form the protective covering. Then, they would have to be reinforced with sturdy wooden struts in order to hold them together and support them 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, 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 on the bottom of the blocks would be responsible for keeping the 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 palates for the blocks to sit on and find a crane to take them from the quarry to the road, where they could be lifted on to a truck for the trip back to the museum.

All 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. There was 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, then a last one for where we hoped the skull would be based on the small skull fragments we had found. 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, the other members of the geological team and I embarked to a two-day trip to the west of Auca Mahuevo. We were in search of a volcanic ash described in a geologic paper from a site about 50 miles west of Auca Mahuevo. The scenery on the drive was spectacular. Huge volcanoes loomed over the horizon off to the west, but as we approached the site where the ash had been discovered, the roads became impassible. Most of the dirt roads leading toward the site were washed out, so we could not get to the precise locality described. However, after driving almost completely around the site on more prominent roads, we did find one track that got us close. At the end of the track were some exposures that looked like 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 AM. 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. When we finally managed to close them, most of us were pretty well 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 3:00 or 3:30, we slumped out of the car, laid out our air mattresses, and crawled back into our bags.

Arising about 7:30, we ate and assaulted the exposures. Near the top of a large ravine, I 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 in hopes that not all the crystals had weathered to clay. Luis also found a few fragments of fossil bone, but nothing to write home about. It was clear that 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. So 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 obviously 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, another area for mapping clusters of eggs on the flats had been measured out and mapped. In the first large grid, 74 clusters had been documented, whereas the second smaller grid contained about half that number.

Over the next three days, most of the team focused on plastering and flipping the plaster jackets containing the abelisaur skeleton. Meanwhile, my 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, I used a tape measure and the leveling bubble in my 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 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 flat-bedded truck for the trip back to Rodolfo's museum. The work went slowly. Some of the blocks were extremely heavy, weighing over 2000 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. It proved essential in our efforts. 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 come and transport the blocks to Plaza Huincul.

With the portions of the skeleton that we had uncovered, it was clear that the new dinosaur was either a smaller version of Carnotaurus or one of its very close abelisaur relatives. We estimated that it was about 70% as large as the only known skeleton of Carnotaurus. Yet, based on the structure of the bones, we definitely knew that it was an adult individual. A number of questions kept flying through our minds as the long days of excavation passed by. Could it simply represent a different sex than the other specimen of Carnotaurus? Could it be a specimen of Abelisaurus, another abelisaur known only from a skull that was collected from the same layers of rock about 100 miles away? Could it be a previously unknown species that was smaller but very similar to Carnotaurus? To decide, we would have to transport the skeleton back to the lab and clean up the bones. For, the last and most crucial piece of evidence needed to establish its identity was still missing: we had yet to locate the skull.

Because we had been unable to locate the skull, we decided to take a large block of mudstone from the area where it appeared the skull could have been buried, hoping that we would find the skull inside when the block was prepared back in the museum. However, our hopes of finding it inside were not too high, but a couple of months after we returned home, Rodolfo called us with great news. His chief preparator, Sergio Saldivia, had found most of the skull in the block, studded with imposing razor-sharp teeth for slashing flesh. At least one side of it was fairly complete, which would allow us to make more concrete comparisons with other abelisaur skulls and determine the identity of the new specimen.

When the skeleton was fairly clean (slide 15), Luis and Rodolfo started comparing it to other abelisaurs. Abelisaurs are primitive carnivorous dinosaurs known primarily from the late Cretaceous of the Southern Hemisphere. Most species, as well as the best preserved specimens, of this theropod family come from Patagonia, including Carnotaurus, Abelisaurus, and Ilokelesia, but abelisaurs are also known from the late Cretaceous of India and Madagascar. Fragmentary remains have been reported from Western Europe, but these are poorly preserved and inconclusively identified.

Although it was clear from the shapes of the bones that our predator from Auca Mahuevo was a close relative of Carnotaurus, it was distinctly different from that species. In contrast to Carnotaurus, the skull was proportionally longer but not as tall. In addition, Carnotaurus has large, prominent horns on the skull above the eyes, whereas the Auca Mahuevo abelisaur possessed only small bumps above the eyes. As mentioned earlier, the arms of the Auca Mahuevo abelisaur were proportionately longer than those of Carnotaurus, although they were still quite short and reduced in relation to most other meat-eating dinosaurs. The bones of this skeleton were superbly preserved, but our biggest surprise was finding small, fossilized casts and impressions of the predator's muscles preserved above the hips in the mudstone that surrounded the skeleton. The most important aspect of the new skeleton was its completeness. Only the end of the tail, along with part of the skull, was missing; the hands and feet were essentially complete, giving us our first look at what the entire arms and legs of abelisaurs looked like.

All of this put us in a very fortunate position. After analyzing the bony details preserved in the skeleton, we knew that Alberto had discovered a completely new species of dinosaur, which required a new scientific name (slide 17). It is part of the process of describing the new dinosaur for the scientific community. After some consideration, we decided to name the new abelisaur Aucasaurus garridoi to commemorate two its attributes. The name for the new genus, Aucasaurus, means that it is a new dinosaur from Auca Mahuevo, while the name for the new species, garridoi, celebrates that it was discovered by Alberto Garrido. Alberto had not only found the specimen, but he had also done much of the hard work that was required to collect it, so it seemed most fitting to recognize his extraordinary efforts.

About one year after the specimen was discovered, all of the jackets had been prepared. Rodolfo and Luis had made all the essential comparisons with other known dinosaur skeletons. At this point, we were ready to write a scientific paper to announce our new discovery. The article not only named the new abelisaur but also described several important anatomical features that had previously been unknown in abelisaurs. Aucasaurus is the most complete abelisaur skeleton ever collected and provides new insights into the evolution of this peculiar lineage of horned carnivores. Our paper has been submitted to the primary scientific journal that publishes information about new research in vertebrate paleontology.

Finally, on the afternoon of March 24th, 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 Huincuil on our way back to Buenos Aires. It had been another exceptional month of discoveries.

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