Our Nests of Eggs and Other Evidence

Some animals are gregarious, whereas others live more solitary lives. How did the giant sauropods behave, especially when they went to the nesting site at Auca Mahuevo to lay their eggs? It had previously been suggested that sauropods laid their eggs in enormous colonies. The studies of the Indian and Pyrenees egg sites, mentioned in the last chapter, documented that thousands of eggs had been laid in areas encompassing several square miles. To the paleontologists studying those sites, this suggested that the area represented a single enormous nesting ground. In addition to speculating that the sauropods of India and the Pyrenees may have returned to the site during several breeding seasons, the scientists concluded that the close spacing of the nests might be indicative of some territorial behavior among individual sauropods. This seemed reasonable to the researchers because the eggs and nests were fairly well preserved, which suggested that the mothers had not trampled the clutches of eggs while moving around the site. But this evidence for colonial nesting and territoriality did not satisfy many researchers in the field of dinosaur reproductive biology.

Notwithstanding such criticisms, evidence from fossilized footprints and bone beds (high concentrations of skeletons from a single dinosaur species in a single rock layer) has also been used to suggest that at least some sauropod dinosaurs were gregarious. The best evidence for this comes from what are known as trackways, sequences of fossilized footprints left by sauropods in soft mud and sand, which later hardened into rock after the layer of soft sediment was buried.

These dinosaur footprints preserved in over 100 feet of the Paluxy River bed in Texas show three sauropod trackways, each overlapped by theropod trackways (birdlike footprints). Discoveries like this one have led to the belief that sauropods traveled in herds and that large theropods may have stalked their prey in packs.

One of the early students of these dinosaur tracks was Roland T. Bird, a preparator and collector at the American Museum of Natural History. Additionally, Bird was one of the original "bikers." Throughout the 1930s and 1940s, Bird thundered across the American West on his Harley-Davidson motorcycle in search of dinosaur footprints.

Bird discovered numerous sites of dinosaur footprints throughout the Southwest, and in 1937 while riding across New Mexico and Arizona, he received a tip that gigantic fossil footprints had been found along the Paluxy River in central Texas. Upon arriving in the town of Glen Rose, he learned that the local inhabitants were well aware of the fossils. Eventually, Bird discovered the trackway of a large theropod dinosaur leading into the Paluxy River. With the help of a local work crew, he diverted the water and exposed the riverbed. Imbedded in the rock layer that formed the riverbed was an incredible sight. Fossil footprints documented that at least twelve sauropods, probably bra-chiosaurs, had walked in the same direction across a mudflat that bordered the ancient Gulf of Mexico. More incredibly, they had been followed at some later time by three large theropods. Analyses by Bird and later workers, including the foremost modern expert on dinosaur trackways, Martin Lockley from the University of Colorado near Denver, have shown that the theropod tracks overlap and impinge upon the sauropod tracks, documenting that the theropods passed after the sauropods. The sauropod tracks also show that the animals were walking side by side in the same direction at regularly spaced but substantial intervals apart from one another. Bird interpreted these trackways to mean that the theropods were stalking the sauropods and that perhaps one of the meat-eaters had even attacked one of the sauropods. In Lockley's analysis, however, no evidence in the theropod tracks suggests that the meat-eaters sped up to catch the sauropods. Nor does any evidence in the trackways suggest that the animals turned to fight one another. In fact, it is not possible to determine the time in between the passage of the two groups. It could have been moments, or even hours. The theropods may have been hunting the sauropods, but there is no evidence of an attack. Nonetheless, the evidence that the Paluxy sauropods were traveling in a herd seems convincing.

Bird discovered more compelling evidence of sauropod herding at the nearby Davenport Ranch in 1941. There, he discovered an outcrop

Female Dinosaur Furries Laying Eggs

Several skeletons of oviraptorids, parrot-headed theropods from the late Cretaceous, have been found lying on top of their egg clutches in the Gobi Desert. These discoveries document that the brooding behavior of birds was inherited from their dinosaurian ancestors.

of rock containing the trackways of twenty-three sauropods all moving in the same direction within a narrow corridor about fifteen yards wide. The overlapping of the trackways, based on the analysis of Lockley, documents that larger sauropods were leading the way and younger, smaller individuals followed in line. The herd was moving at a modest walking pace, veering from right to left. There is no evidence, though, to suggest that the larger adults had encircled the smaller juveniles to protect them, as some researchers had speculated earlier.

Even though sauropods had a complex social structure that, at least at times, involved herding, it is still not known whether they gave their eggs and hatchlings parental care, that is, whether the adults fed and protected their young. Parental care has, however, been documented for other types of dinosaurs, including some meat-eating theropods and plant-eating ornithischians.

Working with another team of paleontologists from the American Museum of Natural History in the Gobi Desert of Mongolia, we were part of the crew that discovered the first fossil dinosaur skeleton — the parrot-beaked Oviraptor—actually sitting on its nest. Interestingly, when the first skeleton of Oviraptor was found in 1923, it was collected from on top of a clutch of eggs. When described in 1924, this was regarded as evidence for Oviraptor's predatory activities. Paleontologist Henry Fairfield Osborn assumed that the Oviraptor had died while seizing the eggs of a plant-eating Protoceratops—a primitive horned dinosaur common in the Gobi deposits. Osborn's assumption led to Oviraptor's condemning name, which means "egg seizer." But the eggs underneath that first Oviraptor skeleton did not contain any embryos, and their identity remained a mystery until seventy years later, when our crew discovered eggs of an identical shape and appearance that contained an embryo of an oviraptorid inside. Thus, this crucial piece of evidence showed that the skeleton collected in 1923 and those discovered decades later were actually brooding nests of eggs that contained their own kin. Subsequent discoveries by the American Museum team and by other paleontological expeditions of other Gobi skeletons in exactly the same pose provided solid evidence that certain types of dinosaurs, including Oviraptor, did care for their voung, and evidence from other nesting grounds in Montana has suggested the same. The meat-eating dinosaur Troodon, one of the closest relatives of birds, has also been found to nest on top of its brood.

For the sauropods, however, this is not so clear. Some researchers believe that, because the eggs are laid so close to one another, the huge size of the adults would prevent them from directly taking care of their own young. Furthermore, the rarity of hatchlings or juveniles in the nesting grounds containing sauropod eggs is considered to suggest extreme precociality, meaning that as soon as the sauropod embryos hatched, they left the nesting area and moved to feeding grounds. Although we have found many embryos at Auca Mahuevo, we have not found any well-preserved hatchlings or juveniles in the same rock layer with the eggs. Anwar Janoo found a few small bones that might be from a sauropod hatchling on top of a clutch near the egg quarry in 1999, but they were too poorly preserved for us to identify them with any certainty. If the paucity of hatchlings and juveniles is a real representation of what was happening at the nesting site, and not a misleading problem with the way fossils became preserved, our inability to find juveniles may support the conclusion that sauropods were very precocial. One possibility was that, shortly after hatching, the baby sauropods congregated in large numbers, forming herds equivalent to flocks of hatchlings of flamingos and some other kinds of birds. These juvenile flocks, usually called creches, are guarded by a group of adults. The same could have been true for the giant sauropods. This idea may explain why sauropod bone beds and herd trackways rarely contain fossils of hatchlings or small juveniles. In any case, we had plans to test this idea. And where better than in the richest sauropod nesting site yet discovered —Auca Mahuevo.

Our examination of the scientific literature revealed that most, if not all, of these ideas about the reproductive biology of sauropods had been reached with little supporting evidence. Speculation about colonial nesting and territoriality in sauropods had been based on a limited number of nests, and as we have discussed earlier, without even knowing the true identity of the eggs. We felt that Auca Mahuevo, with its extensive egg layers laid by one kind of sauropod, provided a unique opportunity for sampling and measuring the eggs at the site, which could in turn shed new light on these and other issues concerning sauropod nesting behavior. The crucial evidence to resolve these issues lay in how the eggs were distributed in the egg layers.

As mentioned earlier, we undertook two projects to determine the distribution of eggs at Auca Mahuevo. The first involved exca vating a large quarry near the embryo site and mapping the position of each egg that was found there. We anticipated that this would give us a good idea about whether individual eggs were arranged in clusters that might represent nests or whether the sauropods had laid the eggs more randomly across the surface of the floodplain without constructing or utilizing distinct nests. Some evidence from other sites suggests that sauropods did the latter. At a site in southern France, for example, large eggs long thought to belong to sauropods appear to be laid in large semicircles without any indication that they were clustered in nests. At main other sites, megaloolithid eggs —the kind of eggs typically attributed to sauropods —have been found in more scattered patterns rather than in discrete clusters.

Frankie Jackson was instrumental in our egg-mapping at Auca Mahuevo. With the experience she had mapping dinosaur eggs in Montana, we knew that she should be the one to record the position of each egg in three dimensions. Our team surveyed the area within the quarry and laid out a system of one-meter squares, much as is done in an archaeological excavation. The initial quarry excavation covered twenty-five square meters, slightly more than twenty-five square yards. As new eggs were exposed during our excavation, Frankie would record their horizontal distances along the sides of the gridded rectangle from one corner of the quarry. We also recorded the elevation of each egg in the quarry with respect to a fixed point. The positions of more than two hundred whole eggs were eventually mapped, far more than in any other previous study.

After gathering the data, Frankie analyzed the maps back in the United States using a variety of statistical methods. To help her in these analyses, she recruited Richard Aspinall, a computer scientist at Montana State University in Bozeman. Frankie and Richard produced a three-dimensional map of our quarry that could be rotated and viewed from the top, the sides, and any other angle. The analyses showed the eggs were definitely clustered, not randomly distributed, although some eggs lav scattered between the more well-defined clusters, suggesting that the clutches may have been laid in nests of irregular shape. At least nine clusters were documented, and seven of those contained between fifteen and thirty-four eggs. This suggested that individual sauropods had laid clutches of eggs during one sitting. Given the way streams flow, we could not envision how currents dur ing floods could have arranged the eggs in clusters like those we found in the quarry.

The number of eggs in a clutch also varies among modern egg-laying animals. The number usually correlates with the amount of food available during the nesting season, along with the abundance of parasites and predators. Ostriches usually lay between fifteen and thirty eggs; crocodiles are known to lay up to sixty eggs; and Komodo dragons tend to lay between twenty and thirty eggs. Thus, the large number of eggs in our clutches was not surprising. What was surprising, however, was that the number of eggs in a typical clutch was much larger than the number documented in suspected sauropod nests at other sites around the world, where there are usually less than ten eggs in a megaloolithid nest. Perhaps this significant difference reflects an abundance of food at Auca Mahuevo, a possibility that is also suggested by the immensity of the nesting colony.

The 3-D map also documented two separate egg layers in the quarry, separated by a few inches of mudstone, which suggested that

This three-dimensional map of eggs from layer 3 at Auca Mahuevo shows how these eggs are generally clustered in distinct clutches. The vertical scale of this diagram has been exaggerated to show better the existence of two levels of eggs, whose separation is depicted here by the intervening light gray plane.

This three-dimensional map of eggs from layer 3 at Auca Mahuevo shows how these eggs are generally clustered in distinct clutches. The vertical scale of this diagram has been exaggerated to show better the existence of two levels of eggs, whose separation is depicted here by the intervening light gray plane.

two separate episodes of nesting had been preserved. Few if any eggshell fragments were found between the clusters, as we would have expected if the eggs had sat out on the surface for more than one year and the eggs had been broken through natural deterioration. Clearly, the eggs had been buried by flood debris before such breakage could occur. This lack of eggshell fragments also suggested that each layer of eggs represented a single egg-laying episode rather than an accumulation of nests that had been laid over several breeding seasons. At least several mothers had laid their eggs at this spot during a relatively short period.

The mapping at the quarry helped to answer another question involving parental care. We noticed that the eggs were generally complete and, therefore, unhatched, and as already noted, we observed relatively few fragments of eggshell between the egg clutches. All this suggested that the females did not remain at the site after they laid the eggs, because if they had, we would expect to find many more broken eggs and eggshell fragments as a result of trampling. As previously mentioned, strong evidence suggests that dinosaurs more closely related to birds, such as Oviraptor and Troodon, brooded their eggs in much the same way that birds do today. Some ornithischian dinosaurs may also have exhibited sophisticated parental care. Jack Horner and his colleagues from the Museum of the Rockies in Boze-man have suggested that hatchlings of the duckbill dinosaur Maiasaura were fed in their nest by their parents. Although evidence for this is not as conclusive as it is for Oviraptor and Troodon, it is believed that all dinosaurs offered some parental care to their broods.

Extinct dinosaurs belong to a genealogical group in which crocodiles and birds are the only living members, which means that the common ancestor of crocodiles and birds was also the common ancestor of extinct dinosaurs. Despite their fearsome appearance and predatory habits, crocodiles are exceptionally tender parents. After carefully burying her eggs in a sandy mound of vegetation, the mother guards the nest from intruders for about three months while the eggs incubate before hatching. When she hears the chirps of the babies emerging from the eggs, she delicately uncovers them, gently scoops them up in her mouth, and carries them to the closest pond or body of water. Birds have evolved a number of more intricate behaviors in parental care. Almost all of them incubate and protect the eggs

When hatched, babv sauropods would have been barelv a tool long, smaller than the footprints of their parents. Several adults may have guarded the periphery of the nesting colony.

by sitting on the nest, and then they feed and defend the hatchlings. Because both crocodiles and birds care for their young, it is logical to infer that they inherited this behavior from their common ancestor. Furthermore, because this ancestor was also the ancestor of the extinct dinosaurs, most scientists agree that most dinosaurs must have provided their young with some kind of parental care. Based on our observations in the egg quarry at Auca Mahuevo, we ruled out the kind of parental protection in which parents directly care for their own nest, but less elaborate kinds of parental care may have taken place. Although it is impossible to prove, the sauropods from Auca Mahuevo could have communally guarded the whole nesting colony. Adults may have patrolled the periphery of the nesting area to ward off potential predators. We will never know for sure, but it seems unlikely that the eggs were left to the mercy of the fearsome predators that must have roamed the region at that time.

The quarry was also useful in documenting the spatial relationships between individual eggs, although it was not large enough to provide a lot of data on how the clusters were distributed across the nesting site. For that, we would need to document the position of clusters across a larger area. The flats where we had first discovered eggs, the better part of a mile away from the quarry, seemed to be the best place to do that because the eggs and clutches were weathering out on a large, relatively flat surface.

So Luis, Frankie, and Gerald Grellet-Tinner had returned to the flats and surveyed a larger grid in an area that seemed to contain a representative number of clusters. The rectangle that they laid out was about sixty-five yards long and thirty yards wide, with the whole area encompassing about two thousand square yards. The surface of the area was fairly flat. The difference between the highest and lowest point within the area was about two feet, which was not much more than the thickness of one clutch, so we assumed that only one layer of eggs was present. Within the mapped area, we found seventy-four randomly distributed egg clusters that we assumed represented distinct clutches. In a couple of spots near the middle of the area, clusters were packed together rather densely, only two to four feet apart from one another, but throughout most of the area, clusters were separated from each other by at least nine or ten feet. To double-check our observations, we constructed and mapped a second grid about three hundred yards from the first one. This area was less than half the size of the other, but our mapping showed the same high concentration of clusters: thirty-five clutches were found.

Given this evidence, what can we reasonably conclude about the nesting behavior of the Auca Mahuevo sauropods? First, the infre-quency of broken eggs and eggshell fragments between the clusters of eggs argues that these are clutches laid by a single individual during one nesting. If we accept that, then we can conclude that fifteen to thirty-four eggs were laid by one mother during a single sitting.

This map of egg clutches exposed cn the flat areas of Auca Mahuevo shows the high concentration of eggs present at this site. Maps like this one offer unprecedented evidence of the colonial nesting behavior of sauropod dinosaurs.

Second, although it is difficult to identify single nesting episodes involving more than one individual with complete certainty, the densely distributed egg clutches both in the quarry and on the flats suggest beyond anv reasonable doubt that these sauropods were gregarious nesters. The alternative, that the hundreds of egg clutches exposed on the flats at Auca Mahuevo were laid at widely disparate times by solitary mothers, seems quite unlikely. Several birds and crocodiles congregate into groups—often, enormous ones in the case of birds—during the nesting season. The causes for this are not well understood, but scientists believe that nesting colonies offer better protection for the eggs, thus increasing their chances for survival. Perhaps this same adaptation drove the Auca Mahuevo sauropods to nest in large numbers.

Third, as we noted earlier, the sauropods returned to Auca Mahuevo numerous times, undoubtedly over many different breeding seasons, because at least four different layers of rock contain the same kind of fossil eggs.

What emerges from our investigations between 1997 and 1999 is a somewhat fuzzy, almost impressionistic portrait of the sauropods' life on the ancient floodplain during their breeding season. It is likely that herds of gravid females lumbered to Auca Mahuevo during many, if not all, of the breeding seasons in which the nesting site was in use. It is not clear whether males accompanied them. Upon arrival, each female laid between fifteen and forty eggs in a nest on the gently sloping, hummocky ground away from the major stream channels. Most of the nests were spaced five to fifteen feet from one another. Sometime after laying the eggs, 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 twelve 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 to 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 fullv developed teeth, immediately set about consuming any nourishing vegetation that they could find. Over the next fifteen to twenty years, assuming they survived, the sauropods would increase in body size more than thirtyfold

Our discoveries documented that sauropods gathered in large numbers to lay their eggs in the soft substrate of a floodplain. Occasional heavy rains brought floods to the nesting colony, burying and preserving the eggs and embryos of these large dinosaurs.

to become some of the largest animals known to have walked the earth. However, floods occasionally inundated the nesting site during incubation, burying the eggs under a layer of mud and killing the developing embryos inside.

Except for the occasional carnage caused by the floods, this portrait of the nesting scene seems rather peaceful and pastoral. But based upon another discovery made during the 1999 expedition, that perception is misleading. A menacing terror lurked in the shadows of the floodplain.

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