Bones Reveal that Dinosaurs May Have Been Warm-Blooded

By Chelsea Whyte on June 28, 2012 8:37 PM EDT

Dinosaur fossil
Using bone samples of current-day animals, researchers have debunked the long-held theory that dinosaurs were cold-blooded. (Photo: Creative Commons)

Though depictions of dinosaurs may make the ancient creatures seem like the cold-blooded reptiles we know today, new research proves they were warm-blooded.

A team of Spanish scientists studying markings on dinosaur bones known as lines of arrested growth - thin, dark rings that form during slow-growing seasons similar to the those found in trees - said their work "dismantles the key argument of the hypothesis that dinosaurs could have been cold-blooded reptiles".

Scientists have long thought that dinosaurs were cold-blooded because these lines of arrested growth (LAG) are only found in cold-blooded creatures today, brought on because these animals grow in short bursts when they can gather energy from sources of warmth outside their body. Warm-blooded animals, on the other hand, created their own heat and therefore grow continuously.

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The presence of LAGs in dinosaur was the biggest indicator that dinosaurs were cold-blooded, but after analyzing bones samples from mammals that hail from ecosystems all over the world, the scientists at Institut Català de Paleontologia Miquel Crusafont (ICP) found that these rings show up in both cold- and warm-blooded large mammals capable of living in hot and cold ecosystems as well as humid and dry environments, whether they are living at the North Pole or in the tropics.

Figuring out if dinosaurs were warm-blooded endotherms (made their own body heat) or were "cold-blooded" ectotherms that relied on outside sources of warmth could illuminate a lot about how they lived, grew and evolved, reports LiveScience. How warm an animal is has an impact on their metabolism, and therefore how quickly they can grow and have babies.

The ICP researchers said that the changes in bone represent a kind of internal clock that regulates an animal's needs according to the seasonal availability of resources. Rain and a limited supply of food or water have more effect on the LAGs than external temperature, they said.

"Some previous studies had already questioned this hypothesis and among the international scientific community there has been increasing consensus about the idea that LAGs were not necessarily indicators of ectothermy. Similarly, examples of mammals that seemed to have LAGs in their bones had emerged. This study conclusively closes the debate," said researcher Meike Köhler.

This discovery came as a bit of an accident. The team did not set out to prove that dinosaurs weren't cold-blooded. They undertook the systematic study as a way to understand how changing environments affect mammals in many climates.

"The study we have carried out is very powerful, both in terms of the amount of material and the diversity of species with which we worked, but we did not design it to find a response to the thermophysiology of dinosaurs," said Köhler, according to TG Daily. "We sought to better understand the physiology of extant mammals and how the environment affects them - how their growth changes as a result of external temperatures, rain and the availability of food and water."

But their findings refuted the long-held notion that dinosaurs were cold-blooded. Though the prehistoric beasts are still considered reptilian, we now know they weren't at the mercy of the outside environment to generate their own heat. 

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