Discovering a Skeleton of the Sauropods Adversary

One might think that being fifty feet long and weighing several tons would make adult sauropods, such as those that lived around Auca Mahuevo, quite invincible. But a pilgrimage to the nesting site at Auca Mahuevo was clearly not without risks, even for giant sauropods and their offspring. Fearsome predators roamed the floodplain.

During the first expedition to Auca Mahuevo, no skeletons of adult dinosaurs were found at the nesting site despite several efforts to find some. That we had found only a few fragments seemed quite curious to us: With all of the eggs and embryos, why were there no fossils of adults?

That all changed with Alberto Garrido's discovery of the large carnivorous-dinosaur skeleton that we described while recounting events during the 1999 expedition. The portions of the skeleton that we had uncovered showed that the new dinosaur was either a smaller version of Carnotaurus or one of its close abelisaur relatives. We estimated that it was about 70 percent as large as the only known skeleton of Carnotaurus, yet based on the structure of the bones, we knew definitely that it was an adult. A number of questions kept flying through our minds as the long days of excavation passed. 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 one hundred miles away? Could it be a previously unknown species that was

Abelisaurs

The 25-foot-long Carno-taurus is a horned member of the abelisaur family, carnivorous dinosaurs that roamed South America during the late Cretaceous period.

smaller but similar to Camotaurus? To decide, we would have to transport the skeleton back to the lab and clean up the bones because the last and most crucial piece of evidence needed to establish its identity, the skull, was still missing. The few fragments of skull bones that we had uncovered were quite troubling because they suggested that the skull might have been destroyed. But perhaps the skull had just broken away from the rest of the body and was buried nearby in the hillside.

Because we had been unable to locate the skull, we had decided to take a large block of mudstone from the area where the skull could have been buried, hoping to find it inside when the block was prepared at the museum. Although our hopes were not too high, a couple of months after we returned home, Rodolfo called with great news. His chief preparator, Sergio Saldivia, had found most of the skull in the block. At least one side of it was fairly complete, which would allow us to make more concrete comparisons with other abelisaur skulls and determine its identity.

When the skeleton was fairly clean, 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 Camotaurus, Abelisaurus, and llokelesia, 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 the shapes of its bones showed that our predator from Auca Mahuevo was closely related to Camotaurus, it was distinctly different from that species. Compared to the skull of Camotaurus, our skull was proportionally longer but not as tall. In addition, Camotaurus has large, prominent horns on the skull above the eyes, whereas the Auca Mahuevo abelisaur had only small bumps. As mentioned earlier, the arms of the Auca Mahuevo abelisaur were proportionately longer than those of Camotaurus, although they were still quite short and reduced in relation to those of 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 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. Coining a new name is part of describing a new dinosaur for the scientific community. After some consideration, we named the new abelisaur Aucasaurus garridoi to commemorate two of 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 had also done much of the hard work 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

Aucasaurus

Although very similar in many aspects, the abelisaur Aucasaurus had a skull that was longer and lower than that of its close relative, the horned Carnotaurus. These two abelisaur theropods are the most completely known meat-eating dinosaurs discovered in Cretaceous rocks of the Southern Hemisphere.

The abelisaur Aucasaurus, whose skeleton was unearthed in our 1999 expedition, follows the track of a sauropod. Although closely related to Camotau-rus, the smaller Aucasaurus lacked the more prominent horns of its relative.

comparisons with other known dinosaur skeletons, and we were ready to write a scientific paper to announce our 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 was submitted to the primary scientific journal that publishes information about new research in vertebrate paleontology.

Beyond describing and naming the new dinosaur, we were still faced with one other mystery concerning this individual. How had it died? Once again, some clues were preserved in the rocks that entombed the specimen.

The rock that had produced the skeleton was unusual because no other layers at the site were quite like it. The skeleton was found in finely banded, purplish gray mudstone, and upon closer inspection, Lowell discovered that these laminations also contained small fossils of shelly invertebrates. Finely banded layers of mudstone are often deposited on the bottom of shallow lakes, ones that could easily have formed on the floodplain after storms. Similar layers can be observed forming on the bottom of many lakes today. We could once again use a geologic process that we see operating today to interpret how the banded mudstone formed 80 million years ago at Auca Mahuevo. Clearly, the carcass had been buried at the bottom of a shallow lake on the floodplain. Either the predator had died in the lake or its carcass had floated out into the lake before sinking to the bottom and being buried. Because part of its skull was broken apart, perhaps it had gotten in a fight and been killed when another dinosaur bit or struck its head. But at this point, we simply cannot be certain about the cause of death. Only further study of the bones might help answer that mystery.

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Responses

  • Lukasz
    Why are dinosaur eggs named different than the dinosaur?
    8 years ago

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