Complex sentence: Dorudontines were originally believed to be juvenile "Basilosaurus" as their fossils are similar, only smaller, but have since been shown to be a different species with the discovery of "Dorudon" juveniles.
Split into simpler sentences: Dorudontines were originally believed to be juvenile "Basilosaurus" as their fossils are similar but smaller. They have since been shown to be a different species with the discovery of "Dorudon" juveniles.