The asaphid trilobite fauna: Its rise and fall in Baltica

The Baltica terrane is well known as the Asaphus Province of the Ordovician World. Being situated in intermediate latitudes and surrounded by fairly wide oceans, Baltica had a benthic fauna that developed in a relatively endemic direction. The most common trilobites were the asaphids. The present study discusses how they appeared, evolved in various environments around Baltoscandia and finally disappeared. The total range of the asaphids based on data from 381 species is estimated as extending from the Furongian, latest Cambrian to the end Ordovician. The remnant Cambrian Olenid Fauna, which was adapted to black bituminous shale facies, survived into the Tremadocian. This Cambrian fauna includes the first asaphids (Promegalaspides, Niobella, ?Eoasaphus). With a shift to lighter-coloured and carbonatic sediments in the middle Tremadocian, this fauna was replaced by the immigrating Ceratopyge Fauna. The latter contains the first five widely distributed asaphid genera Promegalaspides, Niobe, Niobella, Niobina, and Asaphellus. As carbonate sedimentation became more widespread in many areas from the Floian onwards, the asaphids increased in number and formed a stabilised Asaphid Fauna during the Dapingian. In Baltoscandia a gradual development of endemism related to facies belts is observed during the Darriwilian. Records of the mud-related niobines, Megistaspis and Ptychopyge faunas, together with some other benthic macrofaunas are not found beyond the boundary of the Kunda/Aseri regional stages (middle Darriwilian). Their disappearance may be due to a catastrophic event. In contrast Asaphus s.l. survived this interval. Drift of Baltica towards lower latitudes and fluctuations in sea-level influenced the Asaphid Fauna leading to its gradual collapse by late Sandbian time. Influx of the islotelines during the early Katian gave way to a set of illaenimorph species found together with corals that are related to a reefal environment. This morph disappears prior to the Hirnantian glaciation.