Discovering New Egg Layers

Our primary goal in undertaking the second expedition was to gain a better understanding of the reproductive behavior of sauropods. Given the evidence that was preserved in the rocks at the site, the answer to one aspect of this behavior had become clear. Many modern animals return to the same area to lay their eggs during different breeding seasons. Did the ancient giant dinosaurs of Patagonia do this also?

The evidence to answer this question was in the stratigraphic section that showed all of the different rock layers exposed at the site. To make the drawing, Lowell began at the lowest rock and worked his way up, layer by layer, to the highest. He had taken great care, when compiling the section, to record the color, thickness, rock type, and fossil content of each layer of rock that he found, not the most exciting job in the world even for a geologist. In all, the layers of rock that crop out at Auca Mahuevo are almost five hundred feet thick, and there are more than thirty-five distinct, major rock layers. The project took more than a week, but in the end, this hard and sometimes tedious work paid off. With all this geologic data recorded in his notes, we could now determine how many different layers of rock contained sauropod eggs, which would provide the clues we needed to decide if the sauropods had used this nesting site more than once.

As we mentioned earlier, this sequence of rock layers is important for telling time back when sauropods roamed the ancient Patagonian landscape. In a sense, each layer represents a page in the book of geologic history for Auca Mahuevo. By applying the geologic principle of superposition, we can leaf through that book page by page, from older, lower layers into younger, higher layers. Eggs and embryos found in the lowest rock layers were laid at some time before eggs

Auca Mahuevo
Stratigraphic section of Auca Mahuevo showing the four egg layers and the occurrences of theropod and sauropod bones.

found in higher rock layers, which is why it was so important for us to determine how many different rock layers contained eggs. Each egg-bearing layer represents a different nesting event because each rock layer represents a different page or moment in the history of the site. So, if we found sauropod eggs in more than one of the rock layers at Auca Mahuevo, we could be certain that the sauropods had laid their eggs more than one time at the site.

During our first trip to Auca Mahuevo in 1997, we had discovered that the eggs on the flats and the eggs in the embryo quarry both came from a twenty-four-foot-thick layer of mudstone that began about fifty feet above the lowest layer of rock exposed at the site. At first, we weren't sure whether the eggs from the flats and the eggs from the embryo quarry about a mile away actually came from the same layer. However, after hiking on the mudstone layer that contained the embryos all the way back to the flats, we were sure that all the eggs came from the same mudstone layer, leading us to conclude that there was one enormous nesting site covering a couple of square miles. That conclusion still stands, and our subsequent investigations showed the area to be even larger. The evidence we discovered during later trips to the site, however, documented that the sauropods had laid their eggs at Auca Mahuevo several different times.

We got our first indication of this in May 1998 before our second expedition. Lowell, Luis, Rodolfo, Suzi Zetkus, and Pablo Puerto had briefly returned to the site because National Geographic was planning to do an article on our discoveries. Brooks Walker, a photographer working for the magazine, accompanied us throughout our weeklong stay. Most of our time was spent helping Brooks take photos around the flats and at the embryo quarry for the article, rather than conducting scientific work. But one day, late in the week, Brooks climbed up to the top of a small ridge overlooking the site to get some panoramic shots. The ridge was across the road to Auca Mahuida from where we had found the eggs on the flats and the embryo quarry. Consequently, we had not spent much time looking for fossils there yet. As Brooks was taking pictures, he noticed some eggs weathering out of the ground near the base of a small peak on top of the ridge. After finishing his photos, he came back down and told us about his discovery.

Of course, we were thrilled to learn that more eggs were present in a new area of the site. We suspected at first that the eggs were com ing out of the same layer of mudstone that had contained the eggs on the flats and at the embryo quarry, but we had to check out Brooks's new site to be sure. As we walked east about a half mile from the road to the ridge, it soon became clear that the eggs Brooks had found were at a much higher elevation than the eggs on the flats. We knew that more hiking would be required to determine where the new layer fit in the sequence of layers, especially in relation to the layer that contained the eggs on the flats, but we would have to wait until our next expedition to conduct that detailed exercise.

One of our primary geological goals when we returned to Auca Mahuevo in March 1999 was to make sure that Brooks's eggs came from a new layer. To do that, Lowell, Julia, and Alberto traced the layer of mudstone that contained the eggs on the flats to the east along the base of the ridge where Brooks had found the new eggs. The egg-bearing layer on the flats ran right along the bottom of the ridge, whereas Brooks's egg site was situated near the top. More than seventy-five feet of mudstone and sandstone layers separated the two layers containing eggs. Consequently, we knew that the sauropods had laid the eggs preserved on the flats long before they had laid the eggs at Brooks's site.

At present, we cannot tell how long the eggs on the flats were laid before the eggs at Brooks's site. It's not likely that that much sediment could have been deposited within a few years or decades, but whether that seventy-five feet of rock took centuries, a few millennia, a few tens of thousands of years, or even a hundred thousand years to accumulate is unknown. Because we have not found any ancient layers of volcanic ash at the site to date through radioactive means, we just cannot be sure. We could not use the magnetic information contained in the rocks to make such estimates because all the rocks we have sampled thus far at Auca Mahuevo were deposited during an interval when the earth's magnetic field was reversed. Nonetheless, because of the law of superposition, we can be sure that the eggs contained in the mudstone layer at the flats and the embryo quarry represent a distinctly earlier page in the geologic history of Auca Mahuevo than the page represented by the layer containing Brooks's eggs-

It might seem a bit strange for a photographer to discover an important new locality laden with fossil dinosaur eggs. That was obviously not the primary reason why Brooks had come with us to Patagonia. However, he had followed us around the site for several days, and during that time he had developed a clear idea of what we were looking for. So when he saw the eggs on top of the ridge, he knew exactly what they were.

Professional photographers often accompany scientific expeditions, and Brooks was not the first photographer accompanying a fossil-collecting expedition to make an important discovery of dinosaur eggs and nests. Another such incident had happened about seventy years before in the Gobi Desert of Mongolia. During the 1920s and early 1930s the photographer J. B. Shackelford had documented the expeditions to Central Asia led by the famous explorer Roy Chapman Andrews and the paleontologist Walter Granger of the American Museum of Natural History. Like Brooks, Shackelford became finely tuned for fossil prospecting, and like Brooks, Shackelford's acute vision and attention to detail paid great dividends to Andrews and Granger. Andrews described Shackelford's moment of discovery in his account of the expedition:

My car was far in advance of the others and I asked Shackelford to stop the fleet while I ran over to the yurts for a conference with the inmates. During this time ... he wandered off a few hundred yards to inspect some peculiar blocks of earth which had attracted his attention north of the trail. From them he walked a little farther and soon found that he was standing on the edge of a vast basin, looking down on a chaos of ravines and gullies cut deep into red sandstone. He made his way down the steep slope with the thought that he would spend ten minutes searching for fossils and, if none were found, return to the trail. Almost as though led by an invisible hand he walked straight to a small pinnacle of rock on the top of which rested a white fossil bone. Below it the soft sandstone had weathered away, leaving it balanced and ready to be picked off.

Shackelford picked the "fruit" and returned to the cars, just as I arrived. Granger examined the specimen with keen interest. It was a skull, obviously reptilian, but unlike anything with which he was familiar. All of us were puzzled. Granger and Gregory named it Proto-ceratops andrewsi in 1923. Shackelford reported that he had seen other bones, and it was evident that we must investigate____

Granger brought in ... a part of an eggshell which we supposed was a fossil bird, but which subsequently was recognized as dinosaurian

We could hardly suspect that we should later consider this the most important [site] in Asia, if not in the entire world.

The skull that Shackelford had found turned out to belong to a primitive new member of the horned dinosaurs, and the eggshell that Granger had found following Shackelford's route led to the discovery of numerous nests of dinosaur eggs. These were the ones that Andrews and his team originally thought belonged to Protoceratops, but were later recognized as belonging to a dinosaur related to Oviraptor.

Having determined, with Brooks's help, that more than one layer of eggs was present at the site, we now needed to make sure that no others were present. We examined the layer containing the eggs on the flats in more detail. Was there really just one thick layer or were several layers closely packed together? In March 1999, Lowell and Julia Clarke spent two whole days walking around the flats and the ridges that rimmed them. This careful investigation revealed two separate layers of eggs in the mudstone, separated by four thin layers of sandstone, siltstone, and mudstone that together were about five feet thick. The lower layer did not contain nearly as many eggs as the upper layer, but it represented a different and slightly earlier page in the history of the site. Our survey of the flats also showed that both of these layers of eggs were well below the younger layer that Brooks had found. Perhaps these two layers of eggs were laid within years or decades of one another, but again we cannot be certain.

In addition, Lowell found another single cluster of eggs in a fourth layer about twenty-five feet below the lowest layer of eggs exposed on the flats. Not much of this layer is exposed because it lies right on the desert floor below the ridges and flats. Nonetheless, we know that the eggs were not washed down from the adjacent egg-bearing areas because they are complete and planted firmly in the ground, rather than sitting isolated on the surface.

Finally, Frankie's mapping of eggs at the quarry, along with statistical analyses of their distribution, which we will describe later, suggested that two different levels of eggs were probably present there. This means that at least four, if not five, different layers of sauropod eggs are present at Auca Mahuevo.

In order to identify the main four egg layers separated one from another by several feet of rock, we decided to number them in ascending order. We called the layer containing the isolated clutch Lowell had discovered twenty-five feet below the lowest layer on the flats egg layer 1. The lowermost egg layer at the flats was labeled egg layer 2. The egg layer containing our quarry and all the eggs we had found during our 1997 expedition was identified as layer 3. Finally, the uppermost egg layer that photographer Brooks Walker had first spotted was called egg layer 4.

That four distinct rock layers contained clusters of sauropod eggs at Auca Mahuevo provided important evidence about the reproductive behavior of these huge dinosaurs. Based on the principle of superposition, these giant sauropods clearly returned to the nesting site at least four 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, 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 four or five separate times.

Because the surface ornamentation of the eggs varies, both within each layer and between different layers, we wondered whether the same kind of sauropod had laid all the eggs. But after careful study in the lab, we concluded that the differences in the microstructure are not so great as to suggest that more than one species of sauropod nested at Auca Mahuevo. Female dinosaurs, like all other egg-laying reptiles, laid slightly different kinds of eggs during their lifetime. For example, eggs laid by modern reptiles, including birds, tend to be larger in older individuals, and variations may occur depending on the environmental conditions and food available during a particular nesting season. Recent studies of modern mallard ducks has documented that females lay larger eggs when they mate with their preferred male partner. One theory is that the female invests more resources in the eggs resulting from a mate that she regards to be more fit than the others. We don't know whether sauropods also behaved like this, although it is possible since mallards and sauropods are both dinosaurs. Nonetheless, such factors may account for the variations we observed at Auca Mahuevo.

Additionally, we wondered how variations in environmental factors might come into play. As mentioned earlier, dinosaur eggshell is made of calcite, which is highly soluble in water. After the eggs were laid, dissolution by rainwater or subsequent exposure to groundwater after burial could have altered the surface ornamentation of the eggs, eroding or accentuating ridges and valleys. The eggs from Auca Mahuevo have been at the mercy of such conditions for millions of years, so it seems reasonable to expect that some dissolution could have occurred. So, until we discover an egg with a different kind of embryo inside or one with distinct differences in its microstructure, we think it is more reasonable to suggest that all the eggs from all the rock layers at Auca Mahuevo came from the same species of sauropod.

This was not the first time that paleontologists had suggested that sauropods returned to a particular nesting site. In 1995, for example, paleontologists working in the southern Pyrenees Mountains of Spain reported that they had found a site that also contained several clusters of dinosaur eggs in one layer of red sandstone. The eggs were almost spherical and slightly larger than the Auca Mahuevo eggs, averaging about eight inches in diameter. Although no embryos were found inside the eggs, they were assumed to belong to sauropod dinosaurs.

At this Spanish site, the organization of the sand grains within the layer that contained the eggs suggested that the sandstone had been deposited by waves along the shoreline of an ancient ocean. Although initial studies concluded that the dinosaurs had nested on this beach, subsequent examination of the rock layers containing the eggs demonstrated that the ocean had retreated from this ancient shoreline by the time the dinosaurs had laid the eggs. Fossils from the sandstone and other nearby rock layers indicated that these alleged sauropods had used the site sometime between about 71 million and 65 million years ago, making it slightly younger than our Patagonian site.

Across one area of about six thousand square yards, the collectors documented twenty-four egg clutches arranged in three clusters.

These clutches contained no more than seven eggs, significantly fewer than those at Auca Mahuevo. On average, the nests were about two to three yards apart from one another within the clusters, although the concentration of nests was not nearly as great as that discovered in Patagonia. Originally, researchers thought that the eggs were laid in depressions dug in the sand by the sauropods, but subsequent researchers have argued that one cannot be certain of this because there are no differences between the rock that entombs the eggs and the rock that underlies the eggs.

Other, smaller exposures of the sandstone layer containing eggs were found within a six-square-mile area surrounding the site. Based on the density of eggs at the best site, Spanish paleontologists estimated that up to three hundred thousand dinosaur eggs might be preserved in the red sandstone around the whole area. This led them to speculate that the sauropods nested gregariously, but this conclusion has been questioned by later studies of the eggs in this area. Other palcontologic teams working in this rugged region of Spain have identified several different layers of eggs that are similar in appearance and microstructure. These field studies indicate that, as at Auca Mahuevo, the alleged sauropods must have returned to the Spanish nesting ground multiple times to lay their eggs.

The other area of the world that has produced numerous dinosaur eggs in particular sites and many sites over a large region is in India. Large, round eggs, found in clusters that might represent nests, are preserved in rocks that formed near the very end of the Cretaceous period. These rocks are exposed in a region referred to as the Deccan Traps, a geologic province noted for its extensive lava flows that cover the southwestern corner of India. As in the case of the Spanish sites, the Indian eggs lack embryonic remains, but their large size has made paleontologists believe they were laid by sauropod dinosaurs. Some paleontologists have suggested that the large numbers of eggs discovered in this region are evidence that these putative sauropods returned to their nesting grounds multiple times. Although this is clear at both Auca Mahuevo and the Spanish site, the egg-bearing layers mixed in with the lava flows in the Deccan Traps are not as continuous. This has complicated attempts to match up the egg-bearing strata across the region and left some scientists skeptical about whether the Indian dinosaurs returned to their nesting grounds multiple times.

Based on the discoveries at Auca Mahuevo, however, we could now be certain that some sauropods returned to the same nesting site at least several times. But what was it like when they came to Auca Mahuevo to nest? Did the mothers come as individuals or in large herds? Did they dig nests or lay their eggs randomly across the flood-plain? Once the embryos had hatched, did the mothers take care of their babies? We tried to solve some of these important questions by mapping the eggs in the quarry and on the flats.

The heavy plaster jackets containing the Aucasaurus skeleton arc lifted by a crane for transportation to the Carmen Funes Museum in Plaza I luincul.

Building a large grid on the flats of egg-layer 3 for mapping the exposed egg-clutches.

Building a large grid on the flats of egg-layer 3 for mapping the exposed egg-clutches.

Sizi ZetkusEmbryonic Skin Sauropods Auca Mahuevo

The embryonic skin shows a diverse pattern of scales. These unique fossils provided the first glimpse of the skin of dinosaur embrvos.

Although somewhat crushed from rock compaction, the Auca Mahuevo eggs would have been round and 5 to 6 inches in diameter.

Although somewhat crushed from rock compaction, the Auca Mahuevo eggs would have been round and 5 to 6 inches in diameter.

This nearly complete embryonic skull is only one and a half inches long.
Titanosaur Egg

Luis Chiappe excavates the remains of a titanosaur skeleton found in egg-layer 4 during the 2000 expedition.

Embryonic Skin Sauropods Auca Mahuevo
The bared flats and hills where eggs were first discovered at Auca Mahuevo.

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  • Ciriaca
    What rock layer were sauropods in?
    9 years ago
  • heike fried
    Who discovered egg layers?
    3 years ago

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