Complex sentence: It is thought that these ancient tombs were constructed from 6th to 7th century, and there were about 600 ancient tombs by the survey of Taisho period (1912-1926).
Split into simpler sentences: It is thought that these ancient tombs were constructed from the 6th to 7th century. Some 600 ancient tombs had been discovered by the survey of Taisho period (1912-1926).