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Actually I don't think a common ancestor has been discovered, meaning that for all we know they are closer to crabs, or even amphipods. However, given their elongated bodies they could be related closer to lobsters than others from the group malacostraca. The current theory is that stomatopods evolved from small crustaceans that sifted through sand looking for tiny food items such like pods. When they stirred up the sand a few of them would escape into the water. That is why we believed they evolved the spearing appendages. When they could catch small prey that floated into the water column they eventually got larger (large enough to take down fish) and then some evolved calcified clubs to smash open clams and such. eventually a group relied on smashing more than spearing...well you can put the rest together. Correct me if i am wrong though....
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