Complex sentence: Other species, some of them reaching a length of up to seventeen metres, had the "pliosauromorph" build with a short neck and a large head; these were apex predators, fast hunters of large prey.
Split into simpler sentences: Other species, some of them reaching a length of up to seventeen metres, had the "pliosauromorph" build with a short neck and a large head. They were apex predators, fast hunters of large prey.