I can't answer that but I am sure that there is something wrong with the record, or in fact there are more advanced plants at the bottom or the "geo column". Sorry if this seems "answer" insuffcient but its all I can say.
The creation theory predicts that almost all plants should be found at the bottom of the column - primitive and advanced, ALL of them mixed up together. In fact, advanced plants have NEVER been found there. If advanced plants had ever been found there, the theory of evolution would come crashing down. Instead, advanced plants are only found higher in the column, exactly as the theory of evolution predicts.
A theory is no good if its predictions are the reverse of what we see before our eyes. Thinking people discard theories that can't make predictions and don't explain the facts.
prophecy, you will find that creationists talk a lot about the animals when it comes to the fossil record and the flood. They don't talk much about the plants because (1) they can't explain the distribution of plant fossils, and (2) they hope no one will notice - because after all, animals are cuddly and cute, while plants are boring. (I know this for a fact because I studied Botany for one year, then dropped it for Zoology.)
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o--greyline--o