A missing main verb

However, bronze became very important in ceremonial and elite life from 300 BC. Additionally, iron tools are increasingly found in Southern Korea after 300 BC These factors clearly differentiate the time period 300 BC - 0 from the cultural, technological, and social scale that was present in the Mumun pottery period. The unequal presence of bronze and iron in increased amounts from a few high status graves after 300 BC as sets this time apart from the Mumun pottery period. It is thus that, as a cultural-technical period, the Mumun was finished by approximately 300 BC.

I'd like to know why the underlined sentence doesn't have a main verb.
Thank you in advance for your help.