Rudists are a kind of sea animal that lived a long time ago and had a hard shell to protect their body.
Rudists are interesting because they are animals that lived a long time ago and are not alive now.
Examples:
The museum has a big fossil of a rudist.
Rudists are animals that lived in the sea.
The rudist's shell is very hard.
The rudist says, 'I'm an old sea animal.'
Rudists are like the superheroes of the sea animals.
The rudist's shell is as big as a car.