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Eggs Hold Skulls of Titanosaur Embryos |
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Bijal P. Trivedi for National Geographic News |
| September 27, 2001 |
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Scientists have discovered six fossilized eggs containing embryos of titanosaurs. The skulls of the embryos have been "exquisitely preserved" and were petrified at a critical stage of cranial development. These embryonic snapshots give scientists biological information that has been lacking in efforts to fill in the evolutionary history of titanosaurs. They were a sub-group of the long-necked, plant-eating dinosaurs known as sauropods, of which the brachiosaurus, found in many museums, was also a member. "It's a very exciting discovery," said paleontologist Matthew Carrano of Stony Brook University in New York. "We don't have any sauropod embryos, and skulls of adult sauropods and titanosaurs are very uncommon, so this is a double bonus." Skulls of adult titanosaurs are rare and the anatomy of the head is poorly understood. The skulls of sauropods are small and delicate compared with the creatures' limbs and vertebrae. Few even partially intact specimens have been found. "We are always finding bodies without heads. For some reason the heads just seem to come off," said Carrano. Luis Chiappe of the Natural History Museum of Los Angeles County and his colleagues discovered the eggs in a massive dinosaur nesting site at Auca Mahuevot in Patagonia, Argentina. "This is the largest nesting ground ever discovered, with just miles and miles of eggs buried in the dirt and rock," said Chiappe. Defining Features It is ironic, scientists say, that the most well-preserved titanosaur skulls found so far came from embryos that are remarkably fragile. The study of the approximately one-inch embryonic skulls offers clues to a key feature of titanosaur development: the location of the nostrils. In most dinosaurs, the nostrils were toward the tip of the snout. But the nostrils of titanosaurs and other sauropods were located almost between their eyes—one of their most defining features. The newly discovered embryos, however, show that the nostrils of a juvenile titanosaur were toward the tip of a short puppy-like snout, said Chiappe. As the snout of a young titanosaur grew and elongated, the nostrils must have retracted to their later position near the eyes, said Chiappe. His team also discovered that the rotation, or relocation, of the "braincase"—the bony structure that holds the brain—is achieved before the growth of the snout. Previous work had suggested that the location of the braincase and nostrils had evolved together. Chiappe's findings indicate, however, that these two phases of anatomical development evolved independently of one another. Details such as the order of events during development are vital for determining the relationships between the approximately 50 species of titanosaurs that have been identified and their relationship to other groups of sauropods. Chiappe and his colleagues report on their findings in the September 28th issue of the journal Science. Other Clues From Nesting Site The bodies of all sauropods were similar, Carrano explained. These dinosaurs were huge, walked on four legs, and had very long heads and tails. The head is often the key to identifying one species from another. Skulls also reveal a lot about the diet and feeding habits of a particular dinosaur group, Carrano said. Titanosaurs were very common between 90 and 65 million years ago and lived on all the continents except Antarctica and Australasia. They were more robust than brachiosaurus but had shorter tails, a broader and more elongated head, pencil-like teeth, and scaly armored skin. Some of the larger titanosaurs reached lengths of up to 120 ft (36 meters) and weighed 80 to 100 tons, according to Chiappe. The fossils are not the only clues to the prehistoric world. Nests like those in which the embryo skulls were found also offer details about the age of the site, the environment, and climate during the time when these dinosaurs roamed. The number and distribution of eggs found in Auca Mahuevo suggest that hundreds of female titanosaurs gathered at this nesting site, dug holes in the dirt, laid eggs, and covered their clutch with vegetation, said Chiappe. The rock containing the six eggs is between 79 and 83 million years old. The eggs appear to have been deposited before a seasonal flood, which buried them gently as muddy water trickled into the plant-covered nests without removing any of the vegetation. The team of researchers said they were not able to determine which species of titanosaur the embryos represent. Many fossils belonging to new species have been found at the Auca Mahuevo site in recent years, said Carrano. "I wouldn't be surprised if these also turned out to be a completely new species," he said. "Either way, it is a fantastic find and is sure to yield important evolutionary insights." This story will be aired in the United States on September 27 on the television news show National Geographic Today. Recent National Geographic News stories on dinosaurs: Dinosaur Beak Probably Used to Strain Food, Not Kill Prey Study Paints New Picture of Dinosaur's Nose Skeleton of New Dinosaur "Titan" Found in Madagascar "Tidal Giant" Roamed Coastal Swamps of Ancient Africa "Feathered" Fossil Bolsters Changing Image of Dinosaurs Oddly Angled Teeth Make Masiakasaurus Stick Out Additional dinosaur resources from National Geographic: Paul Sereno: NG Explorer-in-Residence and dinosaur hunter Dinorama Wanted: Albertosaurus Dinosaur Eggs Destinations: Dinosaur National Monument Educational Video: Dinosaurs on Earth: Then and Now Children's Pop-up Book: Dinosaur Babies Related lesson plan: Use this National Geographic News article in your classroom with the Xpeditions lesson plan: Physical Characteristics of Places: The Fossil Record |
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