Most people think of dinosaurs as slow creatures that lumbered through the world like the stoic cold-blooded lizards we know today. However, there is an increasing body of evidence that dinosaurs had more in common with dynamic warm-blooded mammals — like us!
Specifically, researchers are looking at the "growth lines" found in dinosaur bones. Growth lines are similar to growth rings found in trees: they are darker and thinner in "slow growing" times like winter, and lighter and wider in more active times.
Scientists previously thought only the bones of cold-blooded animals, which are known to grow in fits and starts, had growth lines, while warm-blooded creatures are thought to grow continuously throughout the year. However, a recent paper published in the journal Nature examined the bones of more than 100 wild ruminants (mammals with four stomachs like cattle and goats) and found that these decidedly warm-blooded ruminants also had growth lines, which formed between favorable and unfavorable growing seasons.
The growth lines are not only very similar between the dinos and the ruminants, but also suggest that dinosaurs must have grown very fast, which would mean they needed to eat a lot to maintain their internal heat, which also points in the direction of endothermy (warm-bloodedness).
"The argument we are giving in our paper, rather in favor of endothermy in dinosaurs, is that between the growth and rest lines, there's always a big region of highly vascularized [infiltrated with blood vessels] tissue that indicates very high growth rates," researcher Meike Köhler, of the Autonomous University of Barcelona in Spain, commented to LiveScience. "This is typical in dinosaurs and very different from reptiles, which have slow growth between the rest lines."
It should also be noted that warm-blooded creatures need to have the ability to move quickly and possess enough lung volume to pump oxygen into muscles needed to support this movement, both of which researchers haven't yet been able to confirm.
"There are a lot of arguments in favor and against endothermy in dinosaurs," Kohler told LiveScience. "It could be that they have some traits that are clearly endothermic," but others may be muddled.