Complex sentence: Ferugliotheriidae is one of the two known families in the order Gondwanatheria, an enigmatic order of extinct mammals that lived when the dinosaurs were the dominant terrestrial vertebrates.
Split into simpler sentences: Ferugliotheriidae is one of the two known families in the order Gondwanatheria, an enigmatic order of extinct mammals. Members of the Ferugliotheriidae family are called Ferugliotheriid gondwanatheres, or simply Ferugliotheriids, which lived when the dinosaurs were the dominant terrestrial vertebrates.