This casts doubt on previous theories that assumed rapid and short-lived growth. The scientists' conclusions were included in an article published in the scientific journal PeerJ.
Professor Jack Horner of Chapman University in the United States said: "The four decades of continuous growth allowed the young tyrannosaurs to occupy a large number of ecological niches and perform multiple roles in their environments, which explains how tyrannosaurs were able to occupy the position of the dominant predators at the end of the Cretaceous period."
In recent years, paleontologists have disagreed about the growth rate of young tyrannosaurs; while some believed that these ancient predators grew very rapidly in their early years before their growth slowed down later, others assumed that they grew slowly over many decades.
The new study, which analyzed the bones of 17 tyrannosaurs from juveniles to fully grown adults, provided support for the second hypothesis. In the past, scientists had estimated the growth rate of tyrannosaurs based on bones from a single age group, which could have led to inaccuracies in the measurements.
Scientists corrected this deficiency by studying the "annual growth rings" in the bones of young and adult tyrannosaurs using four different methods, then analyzed the results using a new statistical method that takes into account the age characteristics of the bone structures, and drew a diagram that reflects the growth rate of these ancient predators.
Calculations have shown that tyrannosaurs grew much more slowly than previously thought; they reached a mass of about 8 tons and typical adult sizes not after 25 years of age, but by the end of their fourth decade. This discovery fundamentally changes scientists' understanding of the role that small and medium-sized tyrannosaurs played in the ecosystems at the end of the Cretaceous period.
The royal tyrannosaurs were the largest predators of the late Cretaceous period, living in what is now North America and several other regions. In the last two decades, scientists' understanding of their appearance has changed; they have discovered that these reptiles were warm-blooded and that many of their relatives possessed feathers.
