Dino Dwarf: Island living may have led to ancient downsizing

Fossils unearthed in a German quarry hint that members of one species of dinosaur that lived in the region 152 million years ago evolved to be abnormally small—only 6 meters long and weighing a ton or so. That midget stature might have resulted from constraints of an island ecosystem.

The new species falls in the group of plant-eating dinosaurs called sauropods, which included the largest land creatures that ever lived. Brachiosaurus, a moderate-size sauropod, grew to a length of 23 m and tipped the scale at around 50 tons, says Martin Sander of the University of Bonn in Germany. Brachiosaurus‘ thighbone alone could measure 2.2 m, taller than basketball star Shaquille O’Neal.

Bones of the newly discovered German sauropod, which is closely related to Brachiosaurus, are considerably smaller. Paleontologists have recovered about 650 bones that represent 10 or so members of the yet-to-be-named species. Fossil thighbones measure up to 62 centimeters in length, a typical size for juvenile sauropods, says Sander. However, detailed analyses suggest that the largest of the new finds came from adults.

A sample drilled from the 62-cm thighbone shows that its lines of arrested growth—bone features analogous to the growth rings of a tree—become more closely spaced near the bone’s outer surface. That trend, plus features suggesting that the bones were remodeling themselves, indicates that the animal was fully-grown, Sander contends.

The German sauropod’s miniaturization resulted from an island-dwelling lifestyle, Sander and his colleagues proposed at a meeting of the Society of Vertebrate Paleontology in Denver last week. Scientists have observed a correlation between the area of a landmass and the body size of the largest animals that live there (SN: 12/01/01, p. 343: Available to subscribers at Move over, Leo. Give me more elbow room.). More recently, scientists in Indonesia discovered fossils of a new humanlike species and attributed its small size to island life (SN: 10/30/04, p. 275: Evolutionary Shrinkage: Stone Age Homo find offers small surprise).

At the time the German sauropod was alive, much of central Europe was a group of islands. The larger landmasses in that archipelago were about the size of Cuba and New Zealand, says Sander.

Not all paleontologists accept the dwarfism diagnosis. Kristi Curry Rogers of the Science Museum of Minnesota in St. Paul says that she’s open to the notion but cautions that bone remodeling sometimes occurs in juvenile dinosaurs. Also, the remodeling depicted in the slides that Sander presented appears to have erased some lines of arrested growth, making it difficult to discern the animal’s age or growth rate. Finally, she says, those images hint that the animal was still growing because they don’t seem to show a decreased number of blood vessels near the bone’s outer surface.

Analyses showing a detailed growth pattern of the new species and based on a technique similar to the one used recently for Tyrannosaurus rex (SN: 8/14/04, p. 99: Growth Spurt: Teenage tyrannosaurs packed on the pounds) could be convincing evidence, says Rogers. The discovery of other smaller-than-normal dinosaur species in the region would also support the dwarfism scenario, she says.