A species of dinosaurs known as the biggest land animals that ever lived, had some miniature cousins, a study has found. According to researchers, the little reptiles weighed about one fiftieth as much as their closest relatives, the well-known brachiosaurs-long-necked, massive plant-eaters.
Newly unveiled models of a group of Europasaurus on display at the Dinopark Münchehagen. (Courtesy Dinopark Münchehagen.)
It's not the first scientific sensation surrounding the discovery of a miniature version of a familiar animal. And the last time such a sensation arose, it didn't turn out very well.
Researchers in 2004 reported discovering fossils from a species of miniature humans on the Indonesian island of Flores. A growing number of scientists now dispute the finding, saying the bones probably came from an ordinary person with a disease.
The researchers in the dinosaur study say they're untroubled by that ongoing debate.
When the little reptilian fossils turned up at the edge of Germany's Harz Mountains in 1998, researchers assumed they were the remains of young dinosaurs. But palaeontologist Martin Sander of the University of Bonn, Germany, and colleagues argue in the new study that they were probably adults. They based this conclusion on the microscopic structure of the bones.
Their analysis appears in the June 8 issue of the research journal Nature.
Dinosaur bones have "growth marks," not unlike the annual rings on trees, Sanders explained. When the reptiles were young the growth marks are far apart, because the animal was growing fast; but the marks lie close together for adult dinosaurs, which had already reached full size.
"It is precisely these tightly compressed marks that we have discovered just beneath the surface of the fossil's bones," said Sander, a specialist in the micro-structure of dinosaur skeletons. "The dinosaurs must have been fully grown when they died."
These "dwarf" dinosaurs were slightly longer and heavier than a car, Sander said. "They stopped growing when they reached 6 metres [20 feet] in length and a ton in body mass," he estimated. Their brachiosaur cousins, by contrast, were up to 45 metres (148 feet) long and weighed 80 tons, as much as a small town of over 1,000 inhabitants.
The 150 million-year-old fossil bones were seen as a rarity even before the new research. During the dinosaurs' time, large parts of Germany were underwater, with only a few islands sticking out. Dinosaurs are land animals, so fossil dinosaurs are rare in Germany.
This island situation may well explain why the "pygmy dinosaurs" evolved, Sanders and colleagues say.
When the sea level rose, flooding more and more land, food might have become scarce. "The result was enormous pressure to evolve: smaller animals which needed less food had better chances of survival," said Nils Knötschke of the Dinopark in Münchehagen, Germany, a member of the research team.
"Shrinkage like this due to a reduction in the food available can take place extremely rapidly, sometimes within 10 or 20 generations," Sander added. In Britain, he noted, deer were introduced to the Shetlands which quickly evolved into a dwarf species of deer. And on Flores in Indonesia, there used to be a miniature elephant hardly bigger than a St. Bernard dog.
This all fits in with the discovery of "Flores Man," said the researchers, who voiced no concerns that skepticism over that case would spill over into their work.
Octávio Mateus of the Universidade Nova de Lisboa, a member of the research team, wrote in an email that his group has stronger arguments than those who proposed the "Flores Man" species, because of the "large sampling of bones with unique adult micro-structure features."
"Flores Man" was slightly over half the height of a regular adult male, whereas these dinosaurs would be less than one-seventh the length of their giant counterparts.
The researchers gave the diminutive dinosaurs the scientific name Europasaurus holgeri. The moniker honors Holger Luedtke, a hobbyist first found the fossils in 1998 in a quarry near Oker, on the northern edge of the Harz Mountains.
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