One prehistoric creature alive today is the crocodile, and there are others. If there was such a thing as Evolution, then why are the prehistoric life forms today still the same. There is no real evidence of a species becoming another. Divergence by mutation disproves Evolution. Only by merging shows that species are able to cross over to form new species. Divergence limits a species by becoming to dissimilar. Each species has it's own limits for mutation. Most every mutation is bad for a species. Very seldom is a mutation favorable. And even with favorable mutations do we ever see a species leaving it's family group. No!
Thank you for using the example of the crocodile! It makes this a little easier.
The evolution of the crocodile (and it near-relative, the alligator) is actually fairly-well documented since the evolutionary differences can be seen in the fossil records. As I will point out in a moment, you have two distinct species (alligators and crocodiles) that clearly evolved from the same genetic ancestor. (You can read more about this
HERE and in the links on that page)
As for why the crocodile has not evolved further, we don't know that it actually hasn't! Evolution is a ever-present process... but it still takes much longer than we could possibly hope to observe with our own eyes. The question is, what type of trait mutation or variance would help a crocodile live longer or reproduce more? I'll see if I can clarify this below.
Now, let's clear up a few things....
1) I'm a little concerned that we are talking about different things when you say "divergence by mutation". Genetic mutation and variance causes small trait differences within a species. Let's consider birds, and let's narrow down the list of possible variables so that we are just focusing on their beaks. Let's say a particular species of bird has a beak that is currently best-suited for puncturing the ground to dig for bugs and worms. Now, if some small mutation of variance in their "beak-gene" were to occur, their beak might turn out ever-so-slightly curved. This could make the beak a little less ideal for dirt-poking, but more effective and flesh-tearing. If circumstances were just right, the improved ability to tear flesh might make birds with this mutation more likely to survive.
Now we've got out variance, so all we add is time. The birds with straight beaks continue to thrive, migrating if neccessary to stay "on the bugs"... while the birds with slightly curved beaks figure out the additional food supply available to them by scavenging dead animals. Gradually, as they begin to rely more on scavenging, those with more curved beaks are able to more easily tear flesh and eat... to the point where the curve of the beak becomes a very distinguishing characteristic.
We now have a different species of bird! We still have the old species of "straight-beaks", and we have the new species of "curved beaks".
Do you now see how divergence is proof of evolution?
2) Most mutation is neither good, nor bad, for a species. It all depends on how pronounced the mutation or variance is, and if it makes the affected-members of the species more or less able to survive and propogate.
3) I'm not sure where you got the idea that evolution caused a species to leave its scientifically-classified family. If you think the Theory of Evolution postulates this, please provide an example.
