A report on the discovery appears in the June 8 issue of Nature.
There is little doubt that the animals they found were a dwarf species—not small or deformed individuals of a larger species.
To examine the skeleton, the team cut sample sections of the bone core so thinly that they were translucent. This allowed them to observe the bone structure under a microscope.
The researchers expected to see juvenile features—bone filled with the blood vessels that allow growth—because no one had ever seen an adult sauropod so small.
But they were surprised: They mainly saw the kind of fibrous bone structure typical of adult bones.
Some bones, though, had more blood irrigation than others—a sure sign they belonged to younger, still growing animals.
"Dinosaur bones have growth rings, similar to but a bit more complicated than those you find in trees," Mateus said. "We can actually see the yearly growth of the older animals, which you don't see in the juveniles."
The analysis of these growth rings, he says, revealed conclusively that these smallest of sauropods were dwarves.
Island Dwarfism
E. holgeri, according to Mateus, is conclusive proof that island dwarfism is an evolutionary fact.
The new study "proves for the first time that dwarfism happens," he said, "and we hope this can have implications for the study of all the other dinosaurs in the world."
There are implications for dinosaur behavior as well.
"We're seeing a herd of several animals of several different ages," Mateus said. "This means the babies are living with the adults. They live in packs."
One of the discoverers of the "hobbit" humans on the Indonesian island of Flores, Peter Brown studies early-human fossils at the University of New England in New South Wales, Australia. (See a map showing the site of the hobbit find.)
(See pictures of the tiny humans of Flores.)
Island dwarfism has been proposed as a reason for the small size of the hobbits—Homo floresiensis—and Brown finds the new sauropod report interesting.
"It's not surprising that the large sauropod dinosaurs dwarfed under island conditions with limited available calories," Brown said. "Their food requirements must have been enormous."
No doubt their food requirements were smaller than those of the mighty brachiosaurs, but even the smallest sauropod must have had a healthy appetite.
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