How to Shrink a Dinosaur: Fossils Reveal Evolution of 'Pocket Sauropods'

By Elizabeth Palermo

Newly unveiled models of a group of Europasaurus holgeri dinosaurs at the Dinopark Munchehagen. Copyright and Credit: Dinopark Münchehagen.

Sauropods are best known for being the largest dinosaurs ever to roam Earth. But a new study of these ancient creatures focuses on a surprising fact: Some sauropods were actually quite small.
The conclusion is based on the discovery of the fossil remains of the smaller-than-average sauropod dubbed Europasaurus holgeri in 2006 in a quarry in northern Germany.The specimens were approximately 20 feet (6 meters) long and are believed to have supported dinosaurs weighing less than a ton each. While these dimensions may seem large by today's standards — the animals were bigger than the average horse — they belonged to animals that were significantly smaller than other sauropods.
Scientists originally thought the fossils may have belonged to juvenile dinosaurs. But the new study determined that the fossils actually belonged to adult dwarf dinosaurs, said lead researcher Martin Sander, a professor of vertebrate paleontology at the Steinmann Institute of Geology, Mineralogy and Paleontology at the University of Bonn in Germany.

 The dwarfism exhibited in this rare discovery of sauropod fossils is a result of what's known as island or insular dwarfism. This gradual shrinking of a large species over several generations has also affected other animals — like elephants and hippopotamuses — living in isolated and cramped quarters.
This particular group of sauropods, Europasaurus holgeri, lived about 150 million years ago in what is now Europe. But during the Late Jurassic period, Europe was submerged in a shallow sea, and most of the animals that lived there inhabited small islands. Over time, Europasaurus evolved to better survive in its island habitat by shrinking, the researchers said.
To make their case, the researchers focused on the details of the anatomy of these diminutive dinosaur specimens. They found that, in the case of Europasaurus, two different sizes of dwarf dinosaurs — a small dwarf and a large dwarf — evolved during the Late Jurassic, Sander told Live Science in an email.
"Bone microstructure tells us that the largest of the two kinds of Europasaurus was fully grown," Sander said. "To find this out, we had to grind samples of Europasaurus bones into thin slices, about one-twentieth of a millimeter in thickness."
At this thickness, Sander explained, the bone becomes translucent and can be studied with a microscope, allowing researchers to examine the bones' microstructure. The researchers also examined the shapes of the skull bones to determine each specimen's morphological ontogenetic stage (MOS), or where that animal is over the course of its development.
Sander said both the MOS and the specimen's microstructure help researchers determine how old a dinosaur was when it died.
Once the researchers determined that the specimens they were studying did, indeed, belong to the dwarf dinosaur Europasaurus and not juvenile sauropods, one important question remained: How did Europasaurus get so small?
"To be a dwarf as a dinosaur, your ancestors have to have been giants," Sander said. "In the case of Europasaurus, this is not difficult to check because, with very few exceptions, all of those long-necked sauropods were giants. The question then becomes how to shrink your dinosaur."
Sander said there were two ways dinosaurs could shrink over the course of evolution: Either a dinosaur could stop growing earlier than its ancestor — after five years instead of 20, for instance — or a dinosaur could grow for the same time period (say 20 years), but did so more slowly, at half the speed.
In both cases, a dinosaur would end up being significantly smaller than its ancestor, Sander said. In the case of Europasaurus, both processes seem to have been at work. However, his team was not able to determine which process was dominant.
Another mystery left unresolved by the University of Bonn study is that of the origins of the two different "forms" of Europasaurus — what Sander refers to as "a small dwarf and a large dwarf." These two sizes of Europasaurus could represent an instance of sexual dimorphism, Sander said, in which males and females of the species are formed or sized differently. However, scientists aren't ruling out another possibility: that the fossils from the 2006 discovery represent two distinct Europasaurus species, separated either by time or by distance.
http://www.livescience.com/46384-dwarf-sauropod-dinosaur-evolution.html