text: They are distinguished in two respects : they were the earliest known jawed vertebrates , and they had stout spines supporting their fins , fixed in place and non-movable ( like a shark 's dorsal fin ) .
simplified: They are distinguished in two respects : they were the first known jawed vertebrates , and they had stout spines supporting their fins , fixed in place and non-movable ( like a shark 's dorsal fin ) .