Joseph & Benjamin ye sons of our Sister thurston were baptised allso ye 15th of ye 7th 1640. [15 Sep 1640]
↑ Benjamin is listed in Dedham town records, but Joseph is not. Since they are baptized on the same day, the obvious conclusion is that Joseph was born before Benjamin, hence 1639 or earlier. If they were twins, one would have expected both births to be recorded in the town records.