Enigmatic worm-like fossils from the Silurian Waukesha Lagerstätte, Wisconsin, USA

Abstract: The lower Silurian Waukesha Lagerstätte of Wisconsin, USA, is a Konservat-Lagerstätte which has yielded a diverse assemblage of exceptionally well-preserved soft-bodied organisms. This study comprises first-hand descriptions and assessments of 23 enigmatic worm-like specimens from the Waukesha Lagerstätte, alongside a literature review. The study focuses on the depositional environment in which the strata of the Waukesha Lagerstätte was formed, as well as which morphologies, mode of preservation and taxonomic affinities of the fossils. In total, eight different worm-like taxa have been identified, not only including annelids, but probably also arthropods and a poriferan. This suggests that worms and worm-like organisms formed an important and diverse component of the past biotas that are now preserved in the Waukesha Lagerstätte. Despite their exceptional preservation, the worm-like fossils of Waukesha Lagerstätte are difficult to interpret and unambiguously assign to specific taxa. Specimens previously interpreted to represent leeches are herein rather considered to be palaeoscolecid worms, based on detailed scanning electron microscopy (SEM)-studies which revealed the presence of external plates and nodes, closely similar to those reported from other palaeoscolecidans. One of these taxa could belong to Wronascolex because of its raspberry-like plates. SEM-based elemental mapping of one specimen shows enrichment of calcium and phosphorous in the fossil compared to the surrounding host rock, suggesting that the Waukesha fossils might be preserved as calcium phosphate. Future studies that could be of interest regarding this collection of fossils include more detailed investigations of fine structures in SEM in the specimens interpreted as palaeoscolecidans.