SCIENCE LAB SCALAR : T. REX COUSIN OR FAMILY

In The Shadow Of A Giant Predator : Distant cousin to T. rex or part of the immediate family?

It is only 23 inches long, but one tyrannosaur skull has been a bone of serious contention among paleontologists for decades.

In 1998, a team of researchers named it Nanotyrannus lancensis, suggesting that the 58-centimeter fossil represented a distinct animal that lived in the shadow of Tyrannosaurus rex. In 1999, another group argued that the skull and similar specimens were from T. rex as a teenager before the species underwent a growth spurt preceding adulthood.

For years, the teenage T. rex hypothesis gained traction. '' Most people bought into it, including me,'' said Nick Longrich, a paleontologist with the University of Bath in England.

But Dr. Longrich has changed his mind. In a new study in the journal Fossil Studies, he and colleagues argue that enough evidence exists to resurrect Nanotyrannus as a distinct species, though part of the larger Tyrannosaur family.

On the basis of anatomical features, they argue that it isn't even very closely related to T. rex.

Dr. Longrich's team studied the original skull and most recent finds, all of which have been argued as representing adolescent T. rex, Dr. Longrich said. But his team said it had found about 150 differences in their anatomies, including skull details, a bladelike snout; and longer arms and claws that were possessed by an adult T. rex.

He also said the specimens had features consistent with mature animals, not adolescents.

Growth rings inside the bone from three specimens suggest slowing growth rates. The animals were on track to weigh over a ton, the researchers estimated ; a T. rex would weigh four to five tons.

'' We have three individuals, which basically rules out an individual variation or aberrant growth pattern,'' Dr. Longrich said. '' What we're seeing is that the growth patterns are inconsistent with these animals being juveniles.''

Other paleontologists objected strongly.

The specimens in question do show features in common with adult T. rex - among them the forehead, snout and braincase, said Thomas Carr, a paleontologist at Carthage College of Wisconsin who first made the case that Nanotyrannus was a young T. rex.

Moreover, he disagrees with the claim that the specimens don't match the growth patterns in other tyrannosaur skulls. '' With T. rex and tyrannosaurus in general, differences between juveniles and adults are quite extreme, and people are easily thrown,'' he said.

Holly Woodward, a paleontologist at Oklahoma State University, also raised objections. The spacing of the innermost growth ring in the bone tissue of nearly full-grown adult T. rexes suggests '' lower growth rate at younger ages before the big growth spurt,'' she said.

Dr. Longrich responded that no other dinosaur developed in the ways suggested by the young T. rex theory. ''For Nanotyrannus to turn into T. rex, this requires an extraordinary number of transformations,'' he said. [ Asher Elbein ]