for National Geographic News
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.
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