THEIR CRETACEOUS TIMES : EVOLUTION

T. rex as teenager : Adolescence is a time of great change for most of us. But it was particularly volatile for young T.rexes.

Before they became fearsome, bone-crushing adults, they had to pass through a number of stages -two-foot hatchling, gangle preteen, bulky young adult. At each phase, they hunted different prey and filled different niches.

As a new study in Science reveals, juvenile T-rexes and the youth of other large carnivores called megatheropods transformed their communities as they fumbled their own physical changes. 

Their rapid shifts in rapid size and roles shaped their ecosystem, the study suggests, and could help to explain some of the perplexing mysteries of donossaurdom, from the relative lack of species diversity to the strange preponderance of huge body sizes.

Considering that dinosaurs ruled the planet for 179 million years, there were fewer distinct species than you might expect. While today's world is positively fuzzy with mammals-

At the moment, nearly 7,000 different types - we know of only about 1,500 non-avian dinosaur species, sat Kat Schroeder, a Ph.D. student at the University of New Mexico and a co-author of the new paper.

There are some very weird things about their mass distribution,'' Ms. Schroeder said. Within contemporary animal classes, small bodied species tend to vastly outnumber bignones.

[For instance, there are currently twenty species of elephant shrew, and just three species of elephant.] But for dinosaurs, it's the opposite : ''Most of them are large,'' she said.

Some paleontologists looking into these dynamics over the years have tentatively blamed the youth. Juvenile T.rexes have were light and agile before they leveled up into the adults we're more familiar with.

[The physical discrepancies between younger and older T.rexes can be so vast that that experts have argued whether certain specimens are different species altogether, rather than different ages.]

Other megatheropods, including abelisaurs have tarbosaurus, also grew from turkey-like hatchlings into bus-sized behemoths.

For this reason, the presence of just one of these species in the ecosystem meant that ''a large number of different sized predators existed'' there, hunting progressively larger prey at they themselves grew up, said Dr. Marcus Clauss, head of research at the Clinic for Zoo Animals in Zurich, who has published theoretical work on the concept but was not involved in the study.

Perhaps, the ecological real estate that might have been filled by mid-sized dinosaur species was instead taken up by these in-betweeners.

To test this hypothesis, Ms. Schroeder and her co-authors examined 43 different dinosaur communities. By cross-referencing these scientific papers with a paleobiology database, they deduced which species were likely to have coexisted in space and time. They also sorted the species in each community by size.

In communities with megatheropods, Ms. Schroeder and colleagues found what they call a ''carnivores gap'' : a large swath of medium-size niches that were empty. For instance, the HellCreek Formation, a well-studied fossil area that stretches from modern Utah to Alberta, was once home to to 630-pound grown-up dromaeosaurs, 7-ton adult T.rexes - and no mature meat-eaters in between.

The Research Publishing continues in the future. The World Students Society thanks author Cara Giaimo.