There are many misconceptions about what most biologists understand by evolution. By definition, describing any animal as primitive is not the same as saying that it has not undergone the same amount of adaptive change as everything else. Coelacanths are certainly very much like some fossils, but that does not mean that they have stopped evolving. In much the same way, modern crocodiles are very similar to fossil crocodiles. In both cases we can see that these animals are supremely adapted to their environments, but these environments have not changed recently and nor have the animals.
Mantis shrimps, as a group of animals, are twice as old as the dinosaurs. The dinosaurs’ environment changed too rapidly for them to adapt (maybe literally overnight if asteroid-impact theories are correct), but the mantis shrimps have obviously been able to cope with changes. Everything alive today is equally modern, and when biologists describe a creature as primitive they mean simply that it does not appear to have changed much recently. Fossils only give information about the harder parts of animals that existed in the past. Nothing about the physiology or behaviour of deceased animals is preserved in the rocks.
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mantis shrimps managed to overcome the challenges associated with the rapid changes in surrounding conditions, whereas dinosaurs perished.