The event that was dated using the most valid and reliable source is millipede appears.
In science, reliable sources include those that provide strong evidence to prove a hypothesis is false or true. The most common reliable sources in science are:
Based on this, the event that was dated using the most valid and reliable resource is the millipede appearance because this event was dated using fossils, which are a type of reliable source in science. Moreover, other sources such as history books or green plants are not reliable.
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