Complex sentence: In fact larger primes were discovered (though without proofs provided) in 1851 (999,999,000,001) and 1855 (67,280,421,310,721) and 3,203,431,780,337 was proven to be prime in 1867.
Split into simpler sentences: In fact larger primes were discovered (though without proofs provided) in 1851 (999,999,000,001) and 1855 (67,280,421,310,721). Furthermore, 3,203,431,780,337 was proven to be prime in 1867.