There's not much solid about taxonomy of animals. Nature doesn't really have clearly defined classes, just rough clusters of similar organisms. It's the humans that like classifying things because it's easier for us to think about them and make predictions when we see them as classes rather than as individuals or even the whole global ecosystem as a single integrated super-organism. The more common features we can find in a class, the less thinking we have to do but it's OK if there are exceptions as long as it's not so many that it makes the classes useless.