Dino Embryo Details Dazzle Scientists
A rare set of exquisitely preserved dinosaur embryos unearthed in the Patagonian badlands is revealing how dramatically the animals changed as they grew from mere 15-inch hatchlings into bus-sized monsters.
Dinosaur eggs have been found in more than 200 locations around the world, but embryos have been found at only eight. The fossils discovered in the southern region of Argentina include the first nearly complete fetal skulls of sauropods, a type of placid, long-necked, plant-eating dinosaur that includes apatosaurus (previously called brontosaurus) and titanosaurs, the largest animals ever to walk the Earth.
In a study published in today’s issue of Science, paleontologist Luis M. Chiappe of the Natural History Museum of Los Angeles County describes the inch-long fetal skulls and the ways in which the dinosaur heads changed as the animals grew from hatchlings to adults.
“It’s an amazing find,” said Jeffrey Wilson of the University of Michigan’s Museum of Paleontology in Ann Arbor. “We’ve known about sauropods and their ilk for 180 years. This is the first time we’ve seen an embryo.”
“These animals got to be the biggest thing that walked the Earth, and they start out life in an egg you can hold in your hand,” he added.
The 80 million-year-old fossil eggs and embryos described in the report are on display at the L.A. museum until Oct. 8. The shells and embryos are embedded in fossil rock and stand out when viewed under microscopes in the exhibit, called “Tiniest Giants.”
The find is even more startling considering the fragility of eggs and embryos. Eggshells, made of calcium carbonate, can dissolve easily. The bones of embryos are more fragile and softer than those of adults. The fossils were created when mudflows from flooded rivers covered the dinosaur nests, killing the babies in their eggs but also preserving them.
The skulls of adult titanosaurs have been notoriously difficult for paleontologists to find, suggesting that adults had relatively delicate heads compared to brethren like Tyrannosaurus rex.
“It is ironic that the two best skulls we have are from unhatched babies, which happen to be the rarest of all dinosaurs,” Chiappe said.
These embryos may have survived so well because they were relatively mature, with bones that were well developed and harder, he said. “These were probably very close to hatching.”
By comparing the embryonic skulls to those of adults, scientists have shown that the skulls changed extensively and fluidly as the dinosaurs grew, with the snouts lengthening and the nostril openings migrating from close to the snout up toward the tops of the animals’ heads.
“In 15 or 20 years, this foot-long embryo became an animal the size of a school bus or even two school buses. You can expect there are going to be a lot of changes as the animal develops,” said Chiappe, who chairs the department of vertebrate paleontology at the L.A. County museum.
The eggs, about the size of softballs, were laid in clutches of about 30 in shallow sandy nests. Geological information from the site indicates they were laid between 79 million and 83 million years ago, toward the end of the reign of the dinosaurs.
In 1997, during an expedition in Patagonia, Chiappe and his colleagues stumbled onto a former dinosaur nesting site they called Auca Mahuevo, full of thousands of fossilized dinosaur eggs. Chiappe, detailing the adventure in a new book with paleontologist Lowell Dingus called “Walking on Eggs,” describes it as “a scene of unparalleled paleontological carnage.”
Chiappe, an Argentine native, made his name in paleontology at New York City’s American Museum of Natural History by studying the links between dinosaurs and modern birds. He joined the L.A. museum in 1999.
The paleontologists were literally walking on eggshells as they stooped to examine the fossils in the ancient stream bed. The fossils, reported in 1998, were extraordinary: There were pieces of fragile dinosaur skin that had somehow survived long enough to become fossilized. The skin fragments had visible scales and delicate, flower-shaped patterns that popped out under a microscope.
The skin, and what it revealed about what dinosaurs truly looked like, was so thrilling that Chiappe still calls the find the most exciting of his career. But it was the embryos that were important scientifically. Today’s paper is the first major report on what the embryos say about dinosaur evolution and the first to pinpoint the type of dinosaur they were.
Some of the embryos still have a mouthful of sharp, pencil-shaped teeth, each smaller than a grain of rice. The specific shape of the teeth and some features of the crowns led Chiappe and his Argentine co-authors, Leonardo Salgado and Rodolfo A. Coria, to conclude that the embryos were titanosaurs. Some species of titanosaur could reach 100 tons and 120 feet in length. Based on adult fossils found in the area, Chiappe suspects these embryos were from a species that would have been about 35 feet long when full-grown.
Wilson, an expert on sauropods, said he hopes additional structures from the embryos, including limb bones, can be used to verify that identification. Identifying species of dinosaurs from embryos is very difficult because the babies look so different from adults, he said.
The nests also provide rare insight into the behavior of these dinosaurs. Because there are so many nests and they are so close together, Chiappe and his colleagues surmise that females nested together.
“We’re talking about thousands of females gathering to lay their eggs and hundreds of thousands of eggs,” said Chiappe, in an office where a high-power microscope stands at the ready to examine the fossils he keeps near his desk.
An analysis of fossil sediments around the nests suggests that the lumbering dinosaurs excavated small nests with their feet, Chiappe said. He is seeking fossilized plant evidence to prove a theory that the animals may have covered the nests with mounds of vegetation.
The nests are raising myriad questions about the behavior of dinosaur mothers. Chiappe, a rapid-fire speaker who leaps from one idea to the next, wants to answer them all.
What kind of contorted squats did the dinosaurs have to endure to stoop low enough to drop eggs without breaking them? Microscopic analysis shows the eggs are thinner than ostrich eggs and would surely break if dropped from more than 5 or 6 feet, Chiappe said.
Computer models of how the animals moved suggest the long-necked dinosaurs couldn’t turn their heads enough to look backward toward the nest. “It’s remarkable they managed to do that without even seeing what they were doing,” Chiappe said. “Their eyes were so far away from the nest.”
