Simplify the following text for a Grade 8 native English-speaking person.
Original: One of the earliest known terrestrial ( land-dwelling ) gastropods is Maturipupa , which is found in the Coal Measures of the Carboniferous period in Europe , but relatives of the modern land snails are rare before the Cretaceous period , when the familiar Helix first appeared .
Simplified: Relatives of the modern land snails are rare before the Cretaceous period . The familiar Helix first appeared in that period .