As seeds are easily radiocarbon-dated, researchers such as Janet Wilmshurst have looked at fossil seed cases that show rats’ tooth marks, and those that do not. If a rat-gnawed seed case was dated much earlier than 1250–1300 CE (the accepted date range for human settlement) this would support the theory that there was earlier, transient human arrival in New Zealand, via which the rat also arrived. So far, no gnawed seed cases have been found that date older than 1244 CE (± 70 years), suggesting that rats were not present before then. The seed case dates were confirmed by the presence of dated volcanic ash layers above and below the seed cases.