Exceptionally preserved arthropodan microfossils from the Middle Ordovician Winneshiek Lagerstätte, Iowa, USA

Abstract

The Middle Ordovician (Darriwilian) Winneshiek Shale from Winneshiek County, Iowa, USA, hosts a Konservat‐Lagerstätte that has yielded a diverse fauna including soft‐bodied fossils. The shale is rich in organic content; in particular, algal material and fragmentary cuticular remains. Palynological acid treatment alongside modified, low‐manipulation processing enables the extraction of these ‘small carbonaceous fossils’ (SCFs) from the matrix, allowing a more detailed view of their morphology. Together these methods have yielded exceptionally well‐preserved crustacean‐type setae and a population of distinctive microfossils which we identify as the feeding appendages of a small‐bodied arthropod. We present two hypotheses for their identity: as either pancrustacean mandibles, or euchelicerate coxae. Overall, the detailed topological similarities and implied functional equivalence to the coxae of xiphosurid chelicerates, in particular, outweigh the resemblance to certain branchiopodan and hexapodan mandibles. In turn, however, the restricted size range and lack of associated limb or carapace fragments pose a taphonomic conundrum, suggesting an extreme biostratinomic bias. By comparison with previously described arthropodan SCFs from the Cambrian of Canada, the Winneshiek fossils extend the geographic, palaeoenvironmental and temporal range of this taphonomic window and provide a complementary tool for reconstructing the diversity and ecology of the Winneshiek biota.

High-resolution δ 13 C org chemostratigraphy links the Decorah impact structure and Winneshiek Konservat-Lagerstätte to the Darriwilian (Middle Ordovician) global peak influx of meteorites

“…; Nowak et al . ,b; Hawkins et al . ) shows that this deposit, which is known as the Winneshiek Shale, contains a fauna of some unique, bedding‐plane preserved fossils, such as up to 1.7‐m‐long eurypterids, other unusual arthropods and giant conodont elements, some of which occur in conodont apparatuses of previously unknown types (Liu et al .…”

Early Cambrian small carbonaceous fossils (SCFs) from an impact crater in western Finland

“…fig. 3 of Nowak et al 2018;Bicknell et al 2018); however, the arrangement and shape of the terminal serrations do not closely resemble the gnathobasic spines of any arthropod we know of. We deem it more likely that this complex structure is a chaeta, but that the finely detailed signatures of microvillar construction have been obliterated due to taphonomic fusion with the associated leiosphaerid acritarch.…”

“…of Nowak et al . ; Bicknell et al . ); however, the arrangement and shape of the terminal serrations do not closely resemble the gnathobasic spines of any arthropod we know of.…”

Contextualizing the Onset of the Great Ordovician Biodiversification Event

“…In Laurentia, Nowak et al . () explore the diversity and significance of small carbonaceous fossils (SCFs) associated with the Middle Ordovician Winneshiek Lagerstätte. This study extends the geographical, palaeoenvironmental and temporal range of arthropod microfossils (pancrustacean mandibles or euchelicerate coxae), so far only known from the Cambrian.…”