CRUSTACEANS AND CHELICERATES IN CATCHMENTS AND CRATERS: SOFT-BODIED PRESERVATION OF THE LOWER PALEOZOIC WAUKESHA AND WINNESHIEK BIOTAS

Resolving how the taphonomic peculiarities of soft-bodied konservat lagerstätten relate taxa and histology to preservation quality and pathways is critical to establishing coherent histories of clades and ecosystems. This study addresses this challenge by characterizing the preservation of arthropods from the Waukesha and Winneshiek biotas using light microscopy, SEM, and EDS.

The Waukesha biota was deposited in a sediment trap in the intertidal Brandon Bridge Formation from the Silurian (Llandoverian) of Wisconsin, USA. It is unusual among Silurian Michigan Basin soft-bodied deposits for its diversity that nonetheless lacks eurypterids. Arthropod fossils are preserved as discolorations staining the matrix (likely diffuse kerogen), thin sheets or blisters of black kerogen, and via phosphatized morphologies. Phosphatization takes the form of thin veneers, thicker sheets with subtle impressions of tissue detail, or sub-3D deposits that leave substantial impressions in the matrix.

The Winneshiek lagerstätte is a low-diversity biota from the Middle Ordovician (Darriwilian) Winneshiek Shale, deposited in a meteorite crater. The preservation of this biota is unusual: once dry, the shale falls apart when rewetted, thicker kerogenized tissues readily peel off the shale, and the phyllocarids and eurypterids bear a superficial resemblance to the insects and plants of many Cenozoic lake deposits. Hence, Winneshiek Shale fossils were compared to those from the latest Eocene Florissant Formation to look for structural similarities.

The preservation quality of two relatively common fossil groups – Acheronauta n. gen. from the Waukesha and phyllocarids from the Winneshiek – was quantified and analyzed to observe relationships between the preservation quality of different histologies and preservation pathways. While sub-3D phosphatized Acheronauta fossils certainly stood out more, specimens phosphatized with kerogen or thinner phosphate sheets could show substantial preservation quality. Similarly, there was little connection between the mineralization of the carapace or gut in Winneshiek phyllocarids and the retention of intricate details like thoracopods or pleopods. This highlights the importance of close observation of soft-bodied fossils, even those that appear decidedly unspectacular.