[Reading Science] 'Large↔Mini' Freely... The Secret of Dinosaur Evolution

US Adelphi University Research Team's 10-Year Dinosaur Fossil Study
Diverse Strategies in Growth Rate and Duration
"Dispelling the Myth of Early-Rapid Growth Like Modern Animals"

How did dinosaurs evolve to become as large as Tyrannosaurus or as small as birds? Until now, scientists believed that modern animals and dinosaurs were similar in this regard. Large dinosaurs grew bigger by growing earlier and faster compared to smaller ones. However, recent research has attracted attention by showing that dinosaurs adjusted their body size through various growth strategies during evolution.

A research team from Adelphi University in New York, USA, recently published a paper in the international journal Science, presenting findings that dinosaurs appear to have used various evolutionary tricks to increase or decrease their size during the evolutionary process.

According to the paper introduced by the international journal Nature on the 27th, the scientific community has long held the notion that large animals like dinosaurs grew bigger by growing earlier and faster than smaller ones. This concept also applied to modern birds and mammals. The reason why elephants and ostriches are much larger than their genetic cousins, such as Chihuahuas or sparrows, is that they grow earlier and faster. Of course, there are exceptions. Crocodiles and alligators, which belong to the same crocodilian family, show similar growth patterns by growing continuously over a long period to increase their size. However, paleontologists had believed that large theropod dinosaurs, including the 'idol' of the dinosaur world, Tyrannosaurus, which are the ancestors of modern birds, grew bigger through early and rapid growth like modern animals.

Haenam Dinosaur Museum. Archive photo.

However, the Adelphi University research team presented results that challenge this conventional wisdom. First, they examined fossils of Majungasaurus, discovered on the island of Madagascar in southeastern Africa, dating back about 66 million years. This dinosaur, about 7 meters in size, is a large dinosaur related to Tyrannosaurus. The team investigated the growth rings in the shinbone fossils of this dinosaur and found many narrow rings indicating slow growth over a long period, rather than wide rings that indicate rapid growth. In contrast, fossils of Ceratosaurus, another large dinosaur of similar size, showed the opposite result. Many wide rings were found in the early growth stages, suggesting rapid growth at a young age.

Over the past decade, the research team measured the bone growth rings of 42 theropod dinosaur species. The dinosaurs studied lived from about 230 million years ago to the end of the Cretaceous period, 66 million years ago. By analyzing the growth rings engraved in each dinosaur's bones, the team confirmed when, how, and to what extent the dinosaurs grew larger or smaller. The results varied. Among the 42 theropod species, 31% increased their growth rate and became larger than their ancestors, and 28% evolved into larger species by extending their growth period. Conversely, 21% reduced their size by shortening their growth period, and 19% did so by slowing their growth rate.

Vera Weisbecker, a professor of evolutionary biology at Flinders University in Australia, described the study as "an amazing result analyzing evolution over an incredibly long period" and commented, "It is fascinating that there were various pathways for dinosaurs to become smaller or larger during their evolution."