Middle Cambrian (Drumian) trilobites and agnostid arthropods from the Manuels River Formation, Avalonian New Brunswick: bracketing the green–black facies boundary

First described by G.F. Matthew 140 years ago, the trilobites and agnostid arthropods of the Manuels River Formation of New Brunswick remain essentially unstudied in modern terms, and most species are known only from his line drawings. Across Avalonian North America, southern Britain, and Belgium, the black mudstone-dominated Manuels River Formation is a distinctive depositional sequence that unconformably overlies greenish mudstone of the Fossil Brook Member of the Chamberlains Brook Formation. This unconformity marks a persistent change from green oxic to black dysoxic marine facies. New collections from limestone nodules at the classic locality at Porter Road yield a low-diversity assemblage dominated by trilobite species of the solenopleurid genera “ Jincella” Šnajdr, 1957 and “ Balticoglaucus” Geyer, 2024. The fauna belongs to the upper Paradoxides abenacus Zone, and associated species of such agnostid arthropods as Goniagnostus Howell, 1935 b, “ Tomagnostus” Howell, 1935 b, and “ Onymagnostus” Öpik, 1979 point to a correlation with the traditional Hypagnostus parvifrons Zone of the Drumian Stage in Avalonian Wales, and the upper “ Acidusus” atavus Zone of Baltica. This interval likely falls in the upper Mawddachites hicksii to lower Paradoxides davidis trilobite zones of Avalonian Newfoundland and Britain. The fauna provides biostratigraphic constraints on a newly recognized bentonite in southern New Brunswick, and on the Avalonian "green–black" boundary. New species are Hypagnostus porterensis, “ Jincella” arenata, and “ Balticoglaucus” avalonensis. Paradoxides abenacus Matthew, 1886, “ J.” acadica (Whiteaves in Matthew, 1886), H. umbo (Matthew, 1886), and “ Euagnostus” tessella (Matthew, 1886) are revised from type and (or) new material.