Middle Cambrian agnostoids and trilobites from the Lower Allochthon, Swedish Caledonides

A middle Cambrian (Miaolingian) agnostoid-paradoxidid fauna from the Alum Shale Formation of the Lower Allochthon in the Caledonides of northwestern Sweden is described. More than 1000 specimens were collected from 46 localities in the Blaik Nappe Complex. This complex represents displaced strata, originally deposited on the distal outer shelf of Baltica. Strong tectonic overprint prevented systematic bed-by-bed sampling, but the collected fauna represents the Pentagnostus praecurrens, Triplagnostus gibbus, Acidusus atavus, Ptychagnostus punctuosus, Goniagnostus nathorsti and lower part of the Lejopyge laevigata agnostoid zones. The Hawke Bay hiatus encompasses the Kibartian and basal Bödan stages. Higher up, the upper part of the L. laevigata Zone appears to be absent, and the overlying Agnostus pisiformis Zone is found only at a few localities, developed as the Exporrecta Conglomerate. This is suggestive of local isostatic uplift in the late Miaolingian. One new species, Cotalagnostus greilingi n. sp., is described and Lejopyge calva Robinson, Hypagnostus clipeus Whitehouse, Doryagnostus deltoides Robinson and Lisogoragnostus confluentus (Rudolph) are for the first time reported from Sweden. All other taxa recorded are known also from the Miaolingian of southern Scandinavia, but the faunal composition in terms of relative frequencies differs. Agnostoid arthropods dominate (39 taxa) with trilobites (17 taxa) as a minor to rare constituent. The palaeoecology of Miaolingian faunas described from Scandinavia is discussed briefly. Three benthic biofacies are recognized, representing a depth transect from well-ventilated inner shelf environments dominated by a diverse trilobite fauna with sparse to infrequent agnostoids (southern Scandinavia) to outer shelf dysoxic environments with more frequent agnostoids and sparse to no trilobites (study area).