Simplify the following text for a Grade 11 native English-speaking person.
Original: 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 ) .