Why do some extinct animals resemble live animals of today?

Answer 1

All animals and organisms have been evolved from one same thing, a cell. As it evolved and reproduced, it created variability even within its own type, creating genetic diversity. This eventually created various forms of them, sometimes so drastic, that made them completely different from the original.

As you look through the Biology related books, it's likely that you'll be able to see these trees. Kingdoms, phylum, class, etc are all generated due to so many variants of organisms.

Back to the extinct animals- These extinct animals resemble some of our live animals today, because most likely, they were evolved within the same tree of class. However, due to certain problems such as environmental changes, lack of survival, etc, some animals do become extinct. While they may become extinct, animals that were generated within the same tree may still live somewhere else around the world, still able to live through. This can be related to survival of the fittest.