The growth ring bone tissue, found in some dinosaur bones and modern, cold-blooded reptiles, look similar to growth rings found in trees. Tree rings grow each year, responding to changing seasonal conditions. By counting the rings, it is sometimes possible to tell the average age of a tree.

The presence of similar structures in certain dinosaur bones suggests the animals’ growth rates slowed down later in life. One interpretation is that the animals became more reptile-like. One major problem is interpreting a dinosaur’s growth ring bones: unlike a growth ring on a tree, no one knows the amount of time represented by each growth ring in a dinosaur bone.