Why are some dinosaurs huge and others small? The answer is unexpected

Theropod dinosaurs, more intriguingly known as meat-eating dinosaurs, inhabited the prehistoric Earth, ranging in size from the more giant, bus-sized T. rex to the smaller, dog-sized Velociraptor.

Scientists have long been perplexed as to why dinosaur sizes evolved to be so significantly different.

Now, according to a study published in the journal Science on February 23, surprisingly, dinosaurs didn't necessarily get the way they were by growing more slowly or more quickly.

Why is the biggest dinosaur so big?

Numerous animals, including dinosaurs, had bones that slowed or stopped growing yearly. Similar to tree rings, these marks reveal the animal's age and can be used to calculate the growth rate.

"Rings like these are called cortical growth marks," Michael D. D'Emic, a paleontologist at Adelphi University and the study's lead author, said in a press release.

"Widely spaced rings indicate faster growth, and narrowly spaced rings tell us that an animal was growing more slowly," he added. 

A total of 80 different theropod bones were examined by his team of international researchers, who measured roughly 500 such growth rings.

They discovered that there was no relationship between growth rate and body size.

"Some gigantic dinosaurs grew very slowly, slower than alligators do today. And some smaller dinosaurs grew very fast, as fast as mammals that are alive today," highlighted D'Emic.  

The results were particularly unexpected because it is generally believed that animals evolve to be larger by growing more quickly than their ancestors. This study demonstrates that it's equally likely that larger and smaller animals had growth spurts that lasted longer or less time.

Co-author Thomas Pascucci, whose graduate thesis contributed to the research, highlighted how awe-inspiring prehistoric animals like dinosaurs are due to how dissimilar these appear to be to our modern environment.

He also emphasized that they were animals that evolved in environments and under conditions similar to today.

Body size evolution in vertebrates

In O'Connor's opinion, the study's results pave the way for further research into how animals control their growth.

"Alteration of different growth control mechanisms, at molecular or genetic levels, likely accounts for the range of developmental strategies our team observed in theropod dinosaurs," he explained.

He also stated that further research into living things will allow us to better understand the factors underlying body size evolution in vertebrates in general. 

"This has really important implications because changes in rate versus timing can correlate to many other things, like how many or how large your offspring are, how long you live, or how susceptible to predators you are," D'Emic added.