The walls of the limb bones became much thinner in theropods, and thinner still in today’s birds.

Since all Theropods have thin-walled bones, it must have been present in one of their ancestors.

Click on the cladogram where you think thin-walled bones first evolved.

Click on the next circle on the feature bar to continue.

Oops!

Take a closer look to find the first appearance of thin-walled bones. Try again.