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The Pancrustacean Conundrum: A Conflicted Phylogeny with Emphasis on Crustacea

Pancrustacea represents a diverse clade of arthropods and includes the crustaceans and insects. The morphological and ecological diversity is among the most impressive of all invertebrates, with representatives including barnacles, shrimps, crabs, copepods, ostracods, brine shrimp, dragonflies, grasshoppers, true bugs, butterflies, and beetles, among others. For decades, morphological and molecular studies have attempted to reconstruct the evolutionary relationships between the major lineages, but a consensus on pancrustacean phylogeny is still under debate. Much of the uncertainty surrounding the evolutionary history of this ancient group stems from uneven and unrepresentative sampling, conflicting data, and different analytical approaches. First, we will introduce the major lineages of Pancrustacea and discuss the evidence for monophyly within these groups. Next, we will review the pioneering morphological and molecular studies that have contributed to current understanding of higherlevel (class/ subclass) relationships within Pancrustacea, with an emphasis on Crustacea. Within these sections, we will start with a series of early studies and describe how the development of methods and technology allowed for the advancement of morphological and molecular phylogenetics. A summary of current knowledge of pancrustacean phylogeny will be followed by a brief discussion of new emergent methods in the field that can be applied to future phylogenetic and phylogenomic studies.