Scientists say they have discovered new details about young Tyrannosaurus rex dinosaurs.Researchers examined fossilized skeleton samplesfrom two small Tyrannosaurus rex – also known as T. rex.The skeletons were discovered in the western American state of Montana in the early 2000s.Researchers named the two dinosaurs Jane and Petey.Each T. rex was estimated to have stood a little taller than a horse and was about twice as long.The researchers say the fossilized bone sampleshave helped them better understand the history and development of this famous dinosaur.The findings are described in a study recently published in Science Advances.The examination team was led by Oklahoma State University's Center for Health Sciences.Holly Woodward was a lead writer of the study.She said in a statement that it was unusual to study such small fossils from the T. rex family.This is because researchers usually seek to collect the largestfossil samples they can find to study and to show in public."The problem is that those smaller fossils may be from younger animals," Woodward said."So, for a long while we've had large gaps in our understandingof how dinosaurs grew up, and T. rex is no exception."The team removed thin pieces from the leg bones of Jane and Peteyand examined the samples under a microscope.Woodward said the study of the bones' "microstructure" can provide valuable informationabout the animals' growth rate, age and metabolism.Researchers said they were able to use this method to count ringswithin the fossil bone to estimate age.They said Jane lived to be age 13, while Petey was 15.The size of the blood vessel openings showed the two dinosaurswere still experiencing growth at a fast rate at the time of death.It takes a T. rex about 20 years to reach adulthood.Fossil evidence shows that when fully grown,the T. rex was about 12 meters long and about 5 meters tall.The researchers say the samples have already providedimportant details about young T. rex's growth and development.The fossil examinations suggest the small T. rex grew as fastas modern-day warm-blooded animals such as mammals and birds, the team reported.The researchers said the findings suggest that the teenage T. rex animalswere very quick on their feet and had knife-like teeth for cutting.By comparison, the adult T. rex had teeth meant to crush large bones.The researchers said they found a clear link between T. rex growth rates and the availability of food.This was discovered in spacing between the yearly growth rings of the fossil bones."The spacing between the rings within Jane, Petey,and even older individuals is inconsistent," Woodward said."Some years the spacing is close together, and other years it's spread apart."This finding suggests the T. rex grew less during years in which food supplies were limited.If a large supply of food was available, the T. rex experienced far more growth.Some scientists have questioned whether the two small skeletons really came from a T. rex.Those scientists said they believed the two represented evidenceof a newfound kind of dinosaur species called Nanotyrannus.The researchers said they believed Nanotyrannus was similar to T. rex,but was smaller and showed different skull and bone development.But Woodward and her team said the latest evidence from the two examined skeletonssuggests they belonged to teenage T rex dinosaurs – not a new species with a smaller body size.