The evolutionary history of shrimps can be determined or inferred by various techniques. This history is best expressed by their classification and systematics. A good fossil record shows history directly, but that of shrimps, with their flimsy exoskeletons, is scanty because of poor preservation. The oldest reliable shrimp fossils are dendrobranchiates from the Triassic Period with pleocyemates showing up later in the Jurassic and later periods. Phylogeny (genealogy) has been traditionally studied by morphological techniques, using evolutionary polarities of anatomical characters with cladistic analysis. However, in recent decades, analysis of molecular RNA and DNA sequences have produced molecular phylogenies often considered superior to morphological ones in hypothesizing the splitting of ancestral clades into derived taxa. Both morphological and molecular phylogenies show that the Decapoda had a shrimp-like ancestor from which dendrobranchiates and pleocyemates evolved, each going their separate evolutionary paths. The decapod and dendrobranchiate ancestral shrimps were most likely pelagic shrimps with some penaeoids later invading benthic habitats. The primitive nektobenthic procaridideans are most like a caridean ancestor, always showing up as the most basal lineage in both morphological and molecular phylogenies. Relationships among caridean families are still controversial, as is the origin of the Stenopodidea and its families. |
