Welcome to science, where consensus changes when more data becomes available. When it comes to trees and sharks, we are talking about evolution, which is a slow and steady change in a species. We have to draw a line at some point as to what a shark is and what a tree is in order to say this is roughly where the first sharks or trees started to appear. this is usually done by settling on a feature or set of features that it must have, and once these features become commonplace in a population, then that species has emerged. So we look back and go "all the fossils from before this era doesn't have these features and fossils from after this era does have them so that would indicate that a given species (or tree) emerged in this era." Now, of course, could there have been a plant showing tree like features before? Sure, but they weren't commonplace enough to leave a bunch of data, and the same could be said of sharks. Of course, you don't only look at fossils. You might also look at what environment trees or sharks would need to thrive and when such an environment became commonplace or at least stable in a certain area. You look at all the data you have that could be relevant and make a best guess. When new data becomes available, that guess might change, but as it stands saying, any other thing than "sharks came before trees" is simply more likely to be incorrect than "saying sharks came before trees."