Threatened Window on the Ancient Past

Nature has preserved a priceless window on the past for us at Auca Mahuevo. As a result of our crew's discoveries and investigations, it has become possible to peer through that window and envision some of what life was like 80 million years ago on the plains of Patagonia. The vision that has emerged is breathtaking, and if it were possible for us to drop in for a visit, it would be difficult for us to recognize where we were.

Huge herds of lumbering sauropods more than forty feet long roamed the gently sloping floodplains in search of vegetation to eat, as South America drifted lazily to the west away from Africa, which lay just over the horizon to the east. At some time during the year, hundreds of females congregated in the flood basins and abandoned stream channels of the Auca Mahuevo nesting colony to dig their nests and lay their eggs. Using their enormous feet, each female scooped out a basin about three to four feet across in the dry mud of the flood basin or the sand of the abandoned streambeds. The nests were spaced five to ten feet apart, and each female laid between fifteen and forty eggs in her nest before she retreated to the periphery of the nesting colony to stand guard or wandered off across the plains in search of food. The parents probably did not stay near the nest to provide much care for their young, either while the eggs were incubating or after the embryos hatched.

Most of the year, the climate at Auca Mahuevo was dry, and the Patagonian plains baked under the relentless Cretaceous sun. During most nesting seasons, the sunlight warmed the eggs in the nests, pro-

New evidence is documenting that large carnivorous dinosaurs may have formed packs, perhaps for the purpose of hunting. The abelisaur Aucasaurus could have used this technique to kill the adult sauropods that nested at Auca Mahuevo.

viding the developing embryos with excellent conditions for incubation. Inside the eggs, the embryos grew to about a foot in length. Though their heads, even with their relatively large eyes, were only a couple of inches long, numerous pencil-shaped teeth had already erupted from their jaws before they hatched, and the unhatched babies exercised their jaw muscles, which they would soon need for feeding on vegetation. Their skin was reminiscent of their other reptilian relatives that lived alongside them; scales covered their delicate bodies and formed roselike and linear patterns.

However, the nesting season did not always proceed according to plan. Occasionally, storms large enough to generate floods in the shallow streams swept across the ancient plains and surrounding regions. The water in the channels overflowed the stream banks, carrying suspended particles of silt and clay that were deposited like a muddy blanket across the flood basin. On at least five occasions, this muddy blanket was thick enough to bury the incubating eggs in the nesting colony, killing the embryos inside and beginning the geologic processes that led to their fossilization.

But most years, when floods did not ravage the colony, the embryos' development inside the eggs proceeded uninterrupted, and many of the embryos hatched to begin life no longer than a baby crocodile. Over the next few decades, the survivors grew to be more than forty feet in length and to weigh several tons, making them some of the largest animals ever to walk on earth.

Floods were not the only hazard that threatened a sauropod's survival at Auca Mahuevo. At least two perilous types of predators lurked on the plains adjacent to the nesting colony. The one that we know more about was Aucasaurus, a twenty-foot-long relative of the fearsome Carnotaurus. This gracile carnivore moved swiftly on its two powerful hind legs. Although its arms were short and were probably not formidable weapons, its jaws were studded with dozens of serrated teeth for slashing flesh, and its hind feet were equipped with razor-sharp claws for taking down prey. As if these attributes were not menacing enough, its skull was adorned with small horns over the eyes. Clearly, solitary adult aucasaurs were capable of preying on hatchlings and juvenile sauropods that frequented the floodplain, and if the aucasaurs joined together to hunt in packs, even adult sauropods could have been in jeopardy.

But Aucasaurus was not the top predator on the ancient Patago-nian plains. Based on our current but incomplete knowledge, that distinction fell to an even larger theropod. To date, we have found only a few bits and pieces of its skeleton, but these fragments suggest that the animal was as large as the largest carnivorous dinosaurs yet discovered, such as Tyrannosaurus and Giganotosaurus. A solitary predator of this size would have constituted a threat even to an adult sauropod of the size that lived at Auca Mahuevo, and evidence exists to suggest that some of these superpredators congregated in packs, making them a danger for even the largest sauropods.

In all, this portrait of life in ancient Patagonia, painted in the picturesque rocks and fossils that form the modern desert landscape, has greatly augmented our scientific knowledge about dinosaurs and the environment they lived in. Through the window at Auca Mahuevo, we have caught our first glimpse of what sauropods looked like when they first hatched. We can now be certain that they did lay large eggs and that, at least in the case of these South American sauropods, they laid those eggs in well-developed nests contained within a massive nesting colony frequented by at least hundreds of mothers at one time. The rocky pages of evolutionary history at Auca Mahuevo document that these sauropods returned to the nesting colony numerous times to breed and thereby sustain their majestic evolutionary lineage.

These discoveries and the insights that they have provided represent a true paleontologic treasure. Yet the clues buried in the rocks required to reconstruct the ancient events at Auca Mahuevo were not easily found. Serendipity played its usual role, but beyond that, the discoveries represent determined and well-coordinated scientific sleuthing by all of the crew members who participated in our expeditions. As we have documented, such work does not simply involve the ecstasy of initial discovery. To see clearly through the window on the past that Auca Mahuevo provides, that initial thrill must subsequently be supplemented with laborious and meticulous scientific investigations, both in the field and back at the lab. That work requires the expertise of dozens of geologic and paleontologic specialists, as well as a lot of time and money, and we are extremely grateful to all of our colleagues who have lent their knowledge and to all of the organizations that have financially supported the research.

The most rewarding part of our work is that, although this book is nearing its conclusion, the story of Auca Mahuevo is just beginning to be revealed. Although three full-scale expeditions have been mounted and more than three years of research have been conducted, numerous questions still remain. Exactly which kind of sauropod laid the eggs? Were they laid by titanosaurs, as we suspect, or by another kind of sauropod? If titanosaurs did lay the eggs, which of the many known species was responsible? Also, exactly how did the eggs and embryos become fossilized? We are sure that floods buried the eggs and nests in mud, but what processes of mineralization operated quickly enough that the poorly formed embryonic bones and skin became fossilized before they could decay?

Other mysteries concern the predators that lived on the ancient floodplain. For example, what really killed the Aucasaurus individual that we discovered? What kind of dinosaur did the huge, isolated theropod bones that we found belong to? Does their presence indicate that Giganotosaurus survived for millions of years longer than previously thought and preyed on the sauropods of Auca Mahuevo, or was a previously unknown predator of gigantic proportions roaming the plains?

We would also like to learn more details about the environment that existed at Auca Mahuevo. What kind of plants were the giant sauropods eating? Our attempts to recover fossilized pollen have thus far proved unsuccessful, and although we have found fossil stems of horsetails and wooden branches, we have yet to find rock layers that preserve abundant fossil plants. Such discoveries could go a long way toward providing clues about what vegetation lived at the site and what the sauropods ate.

Several geologic jobs are also still unfinished. One important task is to map where the egg layers and other distinctive rock layers are exposed on the surface of the ground, which would require highresolution photographs to be taken from a low-flying airplane equipped with a specialized camera. We would then be able to map where the egg layers are found on the photographs. In addition, we have yet to find a layer of ancient volcanic ash near the site that can be used to obtain an age for the fossils through radioactive techniques. We think we know where one might be found about fifty miles west of Auca Mahuevo; however, we have not been able to get to that spot because of its lack of roads and the rough terrain.

The paleontological and geological research required to paint a complete portrait of the animals and environment that existed at Auca Mahuevo has just begun. Our view through the window to the past is still murky, and our efforts to see more clearly will no doubt require the participation of more geologic and paleontologic specialists. But over the coming years, as new expeditions and analyses are conducted, we anticipate that Auca Mahuevo will once again prove magical and reveal more precious secrets and treasures from the history of life for us to ponder and investigate.

For more discoveries to be made, however, the site must be protected from poaching and vandalism. In the past decade, with fossils fetching handsome prices at auctions and curiosity shops, paleontologists have seen their sites looted and excavated, resulting in a great loss of information. Fossils provide unique clues to reconstruct the history of ancient life, but as this book illustrates, many of those clues reside in the geological context of the fossil. Without this accompanying data —such as the exact rock layer in which it was found, the type of rock containing it, the precise location of the site, the associated fossil biota, and the environmental setting—a fossil loses much of its scientific value. This information must be preserved for posterity so that future generations of paleontologists can reinterpret the fossils in light of subsequent discoveries. Museums and their paleontologists not only have the training to collect both the fossils and their contextual information but also the infrastructure to protect that information for the future. These institutions make the fossils and their supporting data available to outside researchers and the public as well. Amateurs and commercial collectors, on the other hand, often recover minimal information about the geological and paleon-tological context of a fossil when they collect it, focusing only on the intrinsic value of the fossil. Furthermore, fossils recovered by amateurs and commercial collectors are often left outside the custodial care of an institution that can protect them for future generations, making them unavailable to researchers and the public.

As we mentioned earlier, we found disturbing evidence during our 2000 expedition that commercial or amateur fossil collectors had visited the site and destroyed some nests to collect eggs. If such activities become more widespread across the site, the important fossils and scientific evidence needed to answer the remaining mys teries about the site will be destroyed. As a result, knowledge about the magnificent animals that lived at Auca Mahuevo 80 million years ago could be forever lost, and our priceless view through Auca Mahuevo's window on the past could be forever clouded.

Fortunately, some initial steps have already been taken to protect this unique paleontological resource. Led by the efforts of our colleague Rodolfo, the government of the province of Neuquen in Patagonia is purchasing all the land containing the site and declaring it a fossil preserve. Eventually, plans call for a small field laboratory to be built at the site, which will be staffed by a government ranger to patrol the area and help carry on the research. Through efforts like this, paleontologists around the world have protected important fossil sites, unique repositories for our own natural heritage. Examples include Dinosaur Provincial Park in Alberta, Canada, which was designated a World Heritage Site by the United Nations, and Egg Mountain in Montana, which is owned by the Nature Conservancy. We hope that, through their being declared a fossil preserve, the rugged badlands of Auca Mahuevo will continue to yield new discoveries of fossils and related geologic insights for decades to come, and that these new fossil treasures will be available for all to see and ponder. Only in this way can Auca Mahuevo be preserved to inspire the next generation of paleontologists to continue our explorations and fill the gaps in our knowledge about past life on earth.

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  • aimee
    What are dinosaurs threatened?
    9 years ago

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