Complex sentence: In the middle of the Jurassic, very large Pliosauridae evolved, characterized by a large head and a short neck, such as "Liopleurodon" and "Simolestes".
Split into simpler sentences: In the middle of the Jurassic, very large Pliosauridae evolved. These were characterized by a large head and a short neck, such as "Liopleurodon" and "Simolestes".