There are a couple of ways we can try to tell how old a dinosaur was when it died.
If you cut open a fossil dinosaur bone, you can see lines, just like if you were looking at rings in a tree. Trees rings happen when a tree grows slowly in a tough season like an icy cold winter. You can count the rings to see how many winters that tree has lived through. And because there is only one winter each year, then you know how many years old the tree is. Easy!
Animals, like dinosaurs, formed similar lines in their bones whenever they slowed down their growing. But there’s a catch: this might not happen once each year like in a tree.
Why would a dinosaur slow down its growing? A dinosaur might not grow very fast if there is not enough food to eat. This might happen if there hasn’t been much rain and so there are not as many plants around to eat. Or there might be loads of food around, but the dinosaur is using all its energy to fight other dinosaurs, rather than using it to grow.
There might be lots of times each year when the dinosaur stopped growing, and each time would make a growth line in its bones. So if you find a fossil with lots of growth lines, you might not be looking at the bones of a really old dinosaur, but a very busy, stressed-out dinosaur! So this is quite a complicated way to try and guess its age.
Use your head
Another way to try to guess the age of a dinosaur is to look at how its skull bones connect to each other. Lots of baby animals don’t have a solid skull. Instead, their skull is made up of different bits that gradually stick together into one piece as it grows.
We’re not sure whether baby dinosaurs had skulls that grew like this. Some scientists have tried to find out by looking at skulls from baby emus and alligators, both of which are a bit similar to dinosaurs. They discovered that emu chicks have skull bones that stick together as they grow, but baby alligators don’t! So that doesn’t really give us a clear answer either.
Sometimes it’s really easy to tell how old a dinosaur was. If you find a dinosaur egg, you can use something called X-rays to look inside it and see if there is a baby dinosaur fossil inside. If there is, you know that dinosaur was 0 years old!
Then, if you find a bigger fossil from the same kind of dinosaur nearby, there is a chance that dinosaur was the baby dinosaur’s grown-up parent.
If you find a baby and a grown-up together, you can learn lots more things by looking at the differences between the two. It might tell you how the dinosaur changes size and shape as it gets older.
You might find a dinosaur that looks like a mixture between the two. That might be a “big kid” dinosaur that is well on its way to becoming a grown-up.
It’s still hard to tell exactly how old each dinosaur was. But scientists are like detectives, and they have lots of clever ideas that are helping them get better at it all the time.