In the picture to the left, the digits (fingers) of the right hand of a dinosaur are numbered.

To make it easier to compare the lengths of the second and third fingers, we have highlighted the second finger in blue.

Notice that the second finger is the longest in the hand of Plateosaurus, but the third finger is the longest in Herrerasaurus and Triceratops.

Since all of the Saurischians shown have a long second digit, it is likely that this feature evolved in the lineage just before the Saurischians.

The blinking icon shows us where this feature most likely evolved.

Again, this is an inference we can make based upon the fossil evidence.

Click on the next circle in the feature bar and we'll figure out when thin-walled bones first evolved.