Complex sentence: Fossils indicate that before evolving baleen, Mysticeti also had teeth, so defining the Odontoceti via teeth alone is problematic, and paleontologists have instead identified other features uniting fossil and modern odontocetes that are not shared by Mysticetes.
Split into simpler sentences: Fossils indicate that before evolving baleen, Mysticeti also had teeth, so defining the Odontoceti via teeth alone is problematic. Instead, paleontologists have identified other features uniting fossil and modern odontocetes that are not shared by Mysticetes.