Simplify the following text for a Grade 11 native English-speaking person.
Original: This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design .
Simplified: ( The ichthyosaur ) converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design .