For three decades, paleontologists around the world were divided over a provocative discovery: did a dwarf Tyrannosaurus rex species really exist?
In 1988, paleontologist Robert Bakker and his colleagues at the Cleveland, Ohio Museum of Natural History reclassified a specimen first discovered in 1942 and exhibited at the museum.
They were, they said, the first known member of a new small species that they named Nanotyrannus.
Then, in 2001, another team discovered the nearly complete skeleton of a small tyrannosaurus near the town of Ekalaka, Montana, in the rich and intensely studied fossil formation known as Hell Creek.
They named the creature – little bigger than a draft horse – Jane and soon classified it as a juvenile tyrannosaurus rex.
But a minority of experts kept insisting that it was part of the newly classified species of Nanotyrannus. They pointed to the morphology of their skull and bones, which they said differed from T. rex adults.
Inside a study published Wednesday in the journal Science Advances, researchers led by Holly Woodward of Oklahoma State University, performed a microscopic analysis on samples from inside the tibial bones and femur of Jane, as well as from a less complete set of bones from an animal named Petey.
This technique, known as paleohistology, confirmed that the two were immature individuals – not adults, the scientists said.
By extension, the study authors said, the existence of Nanotyrannus seems very unlikely.
"The cool thing about fossil bones is that an entire bone fossilizes to microscopic size," Woodward told AFP.
"We can infer growth rate, age (and) maturity level."
The researchers took extremely thin slices of bone samples – so thin that light could pass through them – and then studied them under powerful microscopes.
The size of the blood vessel openings revealed that the two dinosaurs were still in a rapidly growing phase at the time of death. If they were adults, this vascularization would have been less prominent.
Only half a dozen specimens
The team was also able to count the growth rings in each animal's bones, just as you can determine the age of a tree: 13 years for Jane and 15 for Petey.
The study adds to the still limited knowledge of scientists of the 20-year period between a dinosaur hatching and its adulthood.
Jane, who weighed just one ton, died before reaching the exponentially rapid growth phase that would normally lead to an adult weight of just under 10 tons.
"Everyone loves T. rex, but we really don't know much about how he grew up," Woodward said. "It's probably the most famous dinosaur in the world, and most of the time we have really big skeletons."
This is partly due to the obsession of collectors and the public to find and display the largest T-rex skeletons possible – sometimes unearthed to the detriment of smaller specimens.
Unfortunately, Woodward said, only five to seven young T. rex dinosaur fossils are known worldwide, and some of them are in private collections not accessible to researchers.