Dinosaurs—like their bird descendants and all known reptiles-hatched from eggs. Dinosaur eggs have been found on every continent but Antarctica and Australia. More than 220 egg sites have been discovered, three-quarters of which are in either North America or Asia. Most of the egg nests that have been found date from the Late Cretaceous Epoch and are from areas that were once relatively dry or semiarid for all or part of the year. Dinosaur egg nests discovered in the Gobi Desert of Mongolia were buried by sudden sandstorms that doomed many dinosaurs along with their unhatched young. Other nesting sites, such as those found in France, India, and northwestern North America, met similar fates in sandstorms, mud slides, and other rapidly occurring natural catastrophes.
Fossil dinosaur eggs come in various shapes and sizes, including round, oval, and elongated oval varieties. The smallest known dinosaur eggs are round and only about 3 inches (7.6 centimeters) in diameter. The largest, found in China, are elongate and about 18 inches (46 centimeters) long.
The largest known dinosaur eggs are not from the sauropods, as one might expect. They are from a theropod group called the therizinosaurs, whose eggs measured up to about 18.5 inches (47 centimeters) long. The producers of these eggs have been positively identified based on a remarkable embryonic skeleton found inside one such egg. The egg containing the embryo was discovered in 1996 in Xixia, China, as part of a nest containing 26 therizinosaur eggs.
The only way to clearly identify the kind of dinosaur that laid an egg is to find a fossilized embryo inside. This is an extremely rare occurrence in the fossil record. The next best guess is based on the kinds of dinosaur bone fragments found in the vicinity of fossil eggs and nests. Another helpful way to identify the producer of a fossil egg is to examine the microscopic structure of the eggshell to see how it compares to the shells of dinosaur eggs that do have embryos inside them.
For many years, sauropod dinosaurs have been associated with medium-sized eggs that were roundish in shape. Some of these were spherical and measured about 5 to 10 inches (12.8 to 25.6 centimeters) in diameter. Others were slightly elongated and measured about 9 inches long by 6 inches wide (23 by 15.3 centimeters). Because cases of mistaken identity have previously been made in the study of dinosaur eggs, many scientists have been reluctant to assume that round eggs were laid by sauropods until conclusive evidence was found.
The large, round eggs so often attributed to sauropod dinosaurs have been found in many places around the world, including Spain, France, India, and Argentina. Clutches of these eggs generally consist of about 12 closely packed eggs laid in a single layer, although cases with as few as 3 and as many as 20 eggs also have been found. The egg sites in these locations offer our best clues about the nesting habits of sauropods.
Sauropods laid their eggs in three patterns, a habit that was likely governed by the physical size of the mother:
• Six to twelve eggs in a clutch laid in a circular pit that may have been dug out using the large, inside thumb claws on the forelimbs of the adult sauropod. These nest patterns have been found in Spain and Argentina.
• Fifteen to twenty eggs laid in semicircular arcs instead of tight clutches. It appears that this type of pattern, observed in France, may have matched the turning motion of a squatting female as she laid her eggs. The turning motion may have helped her to avoid accidentally stepping on the eggs as they were being deposited.
• Eggs laid in linear pairs or in parallel rows. Found in France, this type of nest is most rare. It is thought to have been made by sauropods due to the shape and size of the eggs.
Sauropod nesting grounds sometimes contain nests that have been stacked on top of other nests. This evidence suggests a seasonal return of the dinosaurs to the same nesting grounds, where they created new nests on top of the remains of old ones.
The first indisputable evidence that sauropods laid round eggs came in late 1998 from the desolate badlands of Argentina known as Patagonia. A small expedition from the American Museum of Natural History was exploring a remote corner of northwestern Patagonia for evidence of fossil birds. The team was led by Luis Chiappe, an Argentine himself, and Lowell Dingus, both then of the American Museum of Natural History in New York. Joining them from Argentina was Rodolfo Coria (b. 1961) of the Carmen Funes Museum in Plaza Huincul. Coria is a skilled and conscientious paleontologist who has found himself in the middle of an extraordinary period of dinosaur discoveries in his native land. He led the teams that discovered and named Argentinosaurus and Giganoto--saurus, among the largest known sauropod and theropod dinosaurs, respectively. His base of operation is a humble little museum that has barely enough space to house these important discoveries. Scientists from around the world have begun to visit Coria's home turf, working alongside him and his team on many joint projects.
Rudolfo Coria is a skilled and conscientious paleontologist who finds himself in the middle of an extraordinary period of dinosaur discoveries in his native land. He led the teams that found and named Argentinosaurus and Giganotosaurus, two of the largest known dinosaurs. His base of operation is a small museum with barely enough space to house these important discoveries. Scientists from around the world have begun to visit Coria on his home turf to work alongside him and his team on many joint projects.
During its second day in the field, the American Museum of Natural History expedition spotted a promising rock face in the distance that might contain fossils. Having driven as far as they could in their truck, the paleontologists stopped and began to walk in the direction of the rock face. Before too long, they noticed fossil fragments all around them on the ground. Step by step, the team members all began to find fossilized chunks of dinosaur eggs. "We realized that the entire place was virtually paved with these eggs and fragments of eggs," recalled Chiappe. "The concentration of eggs was so intense and rich that, in an area of roughly 100 yards by 200 yards, we counted about 195 clusters of eggs."
Each cluster of eggs contained a half-dozen or more eggs. Each egg was only about five or six inches (12 to 15 cm) in diameter and nearly round. The outside surface of the fossil egg fragments had a familiar pitted pattern that had been seen before in dinosaur eggs. The scientists soon realized that they were walking through a vast nesting site of some kind of dinosaur. Dinosaur eggs are one of the rarest fossil discoveries, yet here they were surrounded by thousands of them. It was the discovery of a lifetime.
The team immediately set to work collecting fossils. During the first short season, they recovered several excellent egg specimens and returned to the United States to examine them in a laboratory. In early 1998, Marilyn Fox, an expert at reconstructing fossil specimens, was carefully chipping rock out from inside one of the fossils when she discovered something extraordinary—tiny bones. The egg contained the fossilized remains of an unhatched dinosaur embryo. After weeks of slow and painstaking preparation with the most delicate of hand tools, the dinosaur embryo began to reveal its identity. It became clear that the tiny dinosaur embryo belonged to the clade that includes some of the largest dinosaurs—the titanosaurs. These members of the group were probably between 40 and 60 feet (12 and 18 meters) when fully grown.
To the delight of the scientists, many of the intact eggs and fragments contained fossilized pieces of embryonic titanosaurs. The team even recovered fossil skin casts—impressions of dinosaur skin—the first for any variety of embryonic dinosaur specimen. The skin pattern clearly showed the reptilian scales that made up the skin of the embryo. Had they lived, these embryonic titanosaurs that measured about 12 inches (30 cm) long inside the egg would one day have grown to be 60 feet (18 m) long.
The discoverers named the site Auca Mahuevo. The name combined a reference to an extinct volcano in the area named Auca Mahuida with the Spanish words más huevos, meaning "more eggs." The expedition team returned to the site for three more years.
The size and scale of the Patagonian egg site are so extensive that Coria believes the site will take many years to explore fully. He calls the site "unique," a once-in-a-lifetime opportunity to study the entire ecosystem of these dinosaurs. The area includes fossils not only of the titanosaurs and their eggs, but also of ancient plant life and other creatures, including other dinosaurs, that lived in the same area with the titanosaurs.
The titanosaur embryos from Patagonia join only a small group of embryonic remains known for any dinosaurs. There are just four other examples of embryonic dinosaur skeletons that have been positively associated with dinosaur eggs, and only one of these skeletons represents a species of sauropod.
Though quite rare, probable sauropod eggs have been found in many countries around the world. The first fossil egg fragments thought to be those of dinosaurs were of this kind. They were found in the Provence region of France and described by Jean-Jacque Pouech in 1859. Since then, eggs associated with sauropods, especially titanosaurs, have been found in India, Argentina, Spain, and Mongolia. Thus far, however, no sauropod eggs are known from the Jurassic Period.
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