A phylogenetic analysis and review of Paleozoic phyllocarid systematics is presented using morphology-based characters from Cambrian to modern taxa. The resulting cladograms of the Phyllocarida suggest that the suborder Ceratiocaridina as traditionally defined (families Ceratiocarididae and Caryocarididae) is paraphyletic. Caryocarididae is subsequently elevated to subordinal rank with the erection of Caryocaridina n. suborder, resulting in two monophyletic suborders. Haplocaris n. gen. is erected to contain Caryocarididae taxa without triangular spine-like projections of the anterior telson margin. Emended diagnoses, quantified with morphometrics where appropriate, are integrated into this analysis, and result in synonymy of many Cambrian—Silurian caryocaridids and ceratiocaridids with pre-existing taxa. Two representatives of the Leptostraca are included in this analysis. A visual key of well-established representatives of the Ceratiocaridina is presented and suggestions are offered that will help to improve the long-term stability of the Archaeostraca.

Three new types of arthropod are described from Cambrian intertidal lithofacies of the Elk Mound Group and St. Lawrence Formation of Wisconsin and the Potsdam Group of Quebec. These arthropods are preserved ventrally in sandstone in life position and in three dimensions, allowing detailed characterization of limb morphologies, labrums, and other organs such as eyes. A taphonomic model is presented, illustrating this unusual, uncompressed, three-dimensional style of preservation. Arenosicaris inflata n. sp., from the Terreneuvian-Furongian Elk Mound Group and the Furongian St. Lawrence Formation, is the earliest unambiguous occurrence of a malacostracan phyllocarid. This 3 cm long arthropod had ovate valves, five pairs of biramous pleopods, and at least 3 pairs of thoracopods. Mosinieia macnaughtoni n. sp., a large (>10 cm long) euthycarcinoid of uncertain affinity with flattened or paddle-like appendages also occurs in Elk Mound strata. Mictomerus melochevillensis n. sp. represents a new euthycarcinoid family and is the first known non-trilobite arthropod from the middle Cambrian-Furongian Potsdam Group of Quebec. M. melochevillensis n. sp. is large (8–10+ cm long), with as many as eleven pairs of well-preserved homopodous, uniramous, non-paddle-like limbs. Both M. macnaughtoni and M. melochevillensis differ substantially from previously known euthycarcinoids in limb morphology and represent the oldest known representatives of the group. Additionally, both M. melochevillensis n. sp. and M. macnaughtoni n. sp. possess morphologies that are consistent with abundant subaerial and subaqueous Diplichnites and Protichnites trackways known from these units, suggesting that these may be the earliest land-going animals.

Two species of well-preserved phyllocarid crustaceans, Ceratiocaris macroura n. sp. and Ceratiocaris papilio Salter, occur in the Eramosa Formation Lagerstätte of Ontario, Canada, represented by several dozen articulated fossils preserving limbs and thoracic segmentation. Ceratiocaris macroura n. sp. is distinguished by possession of an exceptionally elongate telson in relation to its body length. Ceratiocaris papilio preserves fine detail of the abdominal scale-like ornamentation, allowing reinterpretation and enhanced characterization of this key feature. Ceratiocaris papilio has a wide paleogeographic distribution across Laurentia but is restricted to shallow marine depositional settings within the middle Silurian.

The large euthycarcinoid arthropod Mictomerus melochevillensis from the middle Cambrian-Furongian Potsdam Group of Quebec occurs as threedimensional casts at the end of Cruziana-and Didymaulichus-like trace fossils. This association provides a rare opportunity to test functional morphological hypotheses about these animals, it provides a framework for understanding how arthropods can be sand-cast in three dimensions, and it suggests that euthycarcinoids may have burrowed into mud as an antidesiccation strategy. In the coeval Elk Mound Group of Wisconsin, the phyllocarid arthropod Arenosicaris inflata occurs on the same beds as Cruziana-like and Rusophycus traces; together with morphologically similar ropelike traces, they are interpreted as having been produced by phyllocarids. These traces preserve the earliest known ethological evidence of phyllocarid crustaceans, and imply that Cambrian phyllocarids employed antidesiccation and possibly feeding strategies still used by modern intertidal leptostracan phyllocarids.

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