Three new types of arthropod from Cambrian intertidal lithofacies of the Elk Mound Group and Lodi Member of Wisconsin, and the Potsdam Group of Quebec are described. These arthropods are preserved ventrally in three dimensions – allowing detailed characterization of morphology. Arenocaris inflata, from the Furongian Elk Mound Group and St. Lawrence Formation, is the earliest occurrence of a phyllocarid. Mosineia macnaughtoni, a large (>10 cm long) euthycarcinoid arthropod, also occurs in Elk Mound strata. Mictomerus melochevillensis represents a new family of early euthycarcinoids, and is a large (8–10+ cm long) arthropod with eleven pairs of homopodous, uniramous limbs. Phylogenetic analyses and reviews of Paleozoic phyllocarid systematics are presented, using morphology-based characters from Cambrian to Recent taxa. Resulting cladograms place Arenocaris inflata into a systematic context, and reveal that the families Ceratiocarididae and Caryocarididae, as traditionally defined, are paraphyletic. Caryocarididae is elevated to subordinal rank (Caryocaridina n. suborder), resulting in two monophyletic suborders. Emended diagnoses are integrated into this analysis, and result in synonymy of 30 Cambrian – Silurian caryocaridids and ceratiocaridids into 11 pre-existing species. The phyllocarid Arenocaris inflata from the Elk Mound Group of Wisconsin and the large enigmatic arthropod Mictomerus melochevillensis from The Potsdam Group of Quebec are both directly associated with trace fossils. Direct association between these arthropods and their traces allows functional morphological details of the animal to be assessed, provides a framework for understanding how arthropods can be sand-cast in three-dimensions, and helps provide insight about subaerially-produced traces from the Potsdam Group.

The Cambrian – Lower Ordovician Potsdam Group is a mostly siliciclastic unit that provides important insight into the paleoenvironmental, geologic and tectonic history of Early Paleozoic Laurentia. Nevertheless, in spite of 178 years of study the Potsdam in the Ottawa Embayment and Quebec Basin remains poorly understood. Also poorly understood is how the Potsdam relates with coeval strata regionally. In this work six siliciclastic paleoenvironments are recognized: (a) braided fluvial, (b) ephemeral fluvial, (c) aeolian, (d) coastal sabkha, (e) tide-dominated marine and (f) open-coast tidal flat. Fluvial strata were examined in particular detail and interpreted to consist of two end-member kinds. Braided fluvial deposits are dominated by low-relief bars formed in wide, shallow channels; however where basement structures limited the lateral growth of channels, flows were deeper and bar deposits thicker and higher angle. In contrast, ephemeral fluvial strata are dominated by sheetflood splay sedimentation with rare preservation of scour-filling supercritical bedform strata – all later subjected to aeolian reworking. In the upper Potsdam, alternating ephemeral and braided fluvial strata provide a record of climate change, which, respectively, correlate with documented global cool (arid) and warm (humid) periods during the Late Cambrian and Early Ordovician. Three allounits are recognized in Potsdam strata, recording regional episodes of sedimentation and facilitating correlation with coeval strata throughout eastern North America. These correlations, aided with provenance data from detrital zircons, show that changes in the areal distribution of sediment supply, accommodation and deposition/erosion were principally controlled by episodic reactivation of the Neoproterozoic Ottawa graben, which then periodically modified the stratigraphic expression of the ongoing Sauk transgression. Specifically, episodes of tectonic reactivation occurred during late Early to Middle Cambrian (allounit 1), late Middle to early Late Cambrian (allounits 2 – 3 unconformity), and Earliest Ordovician (allounits 3 – 4 unconformity). The earliest episode is correlated to regional extension of southern Laurentia, whereas the latter two are linked to peri-Laurentian accretion events that triggered reactivation of the Ottawa graben via the Missisquoi oceanic fracture zone.