Due to the discovery of the plant pupae of the same age as the dinosaurs in the primordial forest at the bottom of the largest tiankeng in the Yeye tiankeng-Dashiwei-it was speculated that Dashiwei should be formed in the dinosaur era, 65 million years ago. If the following are true, which one can best refute the above speculation? ['桫 椤 still exists.', 'No 桫 椤 found in other Leye tiankengs.', 'Dinosaurs cannot enter this extremely deep sinkhole.', 'No dinosaur fossils found in Dashiwei.']