The Winneshiek Lagerstatte, Iowa, USA and Its Depositional Environments

Fossil Lagersttten are deposits containing abundant and/or exceptionally preserved fossils,often including soft-bodied tissue.Of these,Konservat-Lagersttten are especially important because they provide not only a much more complete record of the diversity and paleoecology of ancient communities,but also more detailed information on the taxonomy and anatomy of the biota compared to the normal shelly fossil record.However,Konservat-Lagersttten are rare because they require exceptional physical and chemical depositional conditions.Environmental conditions during the Cambrian may have been more favorable for the preservation of soft-bodied organisms than later in the Paleozoic,and such Lagersttten provide evidence of biodiversity changes during the Cambrian explosion.Although the Ordovician Radiation or Great Ordovician Biodiversification Event(GOBE) has been well documented,this is based mainly on the shelly fossil record because soft-bodied organisms are poorly represented during the 45 million years of the Ordovician.Only three Lagersttten,Beecher's Trilobite Bed in upper New York State,the Soom Shale in South Africa,and the recently recognized biota in Manitoba,Canada,approach the extent of soft-tissue preservation in Cambrian deposits.All the three Lagersttten are of Late Ordovicianage. The Winneshiek fauna was discovered recently near Decorah,northeast Iowa,USA(Fig.1) .It is preserved in a new stratigraphic unit which consists of greenish-brown to dark-gray sandy laminated shale with abundant organic carbon and pyrite(Fig.2) .The new fauna contains both invertebrate and vertebrate fossils;current recovery includes eurypterids and other chelicerates,phyllocarid crustaceans,ostracods,linguloid brachiopods,mollusks,isolated conodont elements and bedding plane assemblages,skeletal elements of jawless fish,various indeterminate fossil forms,and bromalites and other trace fossils(Fig.3) .Most of these fossils are extraordinarily preserved,some with soft tissues or body impressions,establishing the status of this deposit as a Konservat-Lagersttte,the only significant example of Middle Ordovician(Whiterockian) age. The importance of studies of the Winneshiek Lagersttte is manifold.The fauna contains several very rare taxa in the Ordovician fossil record.Phyllocarids,for example,are an important but poorly known group of Paleozoic crustaceans,and the resolution of controversies surrounding their taxonomic position has been frustrated by the rarity of information on their morphology other than the carapace.The new examples preserving trunk and appendages from the Winneshiek Lagersttte(Plates ①,②) are valuable for testing hypotheses of phyllocarid relationships and their significance for crustacean evolution.Eurypterids are the most diverse order of chelicerates in the Paleozoic,and the earliest specimens described in the literature are from the Late Ordovician.The Winnishiek eurypterids(Plate③) are not only the oldest but they occur on Laurentia,which is thought to be where the clade originated.This study will provide important new data to test ideas about eurypterid taxonomy,evolution,and their phylogenetic relationships to Xiphosura and Scorpiones. Vertebrates from the Winneshiek Lagersttte include conodonts and jawless fish.Conodonts are abundant and extraordinary in the fauna.Although more material is necessary to confirm that conodont soft tissues are preserved,the occurrence of several types of bedding plane assemblage,including the first known natural apparatus of Coleodontid conodonts(Plate④) ,has improved our knowledge of their classification and affinity.Many complete elements,comprising both basal body and crown,will also provide important information on conodont phylogeny and evolution.The evolutionry history and paleoenvironmental context of Ordovician fish remain poorly known.Articulated jawless fish fossils(Plate⑤) from the Winneshiek fauna certainly will provide significant new information on primitive vertebrates.Some 3-dimensionally preserved vermiform fossils(Plate⑥) and indeterminate fossils with special structures(Plate⑦) are also common.Their nature and paleoecologic importance need to be determined. The Winneshiek fauna preserves a remarkable range of non-biomineralized tissues.These materials provide important information on paleoecology,depositional environments,taphonomy,and diagenesis.Most of the eurypterids from the fauna,for example,are preserved as black or brownish cuticle remnants(Plate③) .Preliminary analysis indicates they contain a fixed-carbon component as high as 81%.Cuticular preservation of arthropods this old is rare.Organic analyses of such cuticular materials will provide further information on organic diagenesis,thermal maturity,affinities and potentially on conditions of deposition.In addition,the conodont elements range in color from amber through pearl to white.Along with the color alteration,conodont elements were mineralogically altered:corresponding dissolution occurs(Plate⑧) ,and dense denticle becomes an aggregate of water soluble fibrous material(Plate⑨) .Because elements with different colors occur in the same fauna,they must represent the result of different diagenetic or biologic processes,and the conodont color alteration index(CAI) of thermal maturity is not directly applicable to the Winneshiek Lagersttte.Bromalitic materials(Plate⑩) and other trace fossils frequently occur in the Lagersttte. The Winneshiek Lagersttte provides remarkable information on the depositional environment.Fine-scale lamination,in part sandy,in conjunction with the limited geographic extent of the shale,suggests a marginal to nearshore marine setting,possibly with tidal influence.The preservation of abundant organic matter and pyrite indicates low-oxygen conditions with restricted bottom circulation.Many animal groups that typify Ordovician open-marine shelf faunas,such as corals,bryozoans,crinoids,cephalopods,trilobites,and graptolites,were not found in the Winneshiek fauna.The restricted biota suggests deposition in an area that was separated from the open sea,possibly in a nearshore/estuarine environment with slightly brackish-water within Laurentia(Fig.4) . Preliminary considerations suggest that the Winneshiek fauna may have lived and been preserved within a meteorite impact basin.Firstly,the origin of the Lagersttte-bearing shale and underlying conglomerate/breccia succession in the study area is equivocal.The conglomerate/breccia unit consists of angular,unsorted,and multi-sourced clasts(Fig.5) ,some of which show boudinage-like structures(Fig.6) .The sediments differ significantly from those expected in normal deposits,but are usually found in impact structures.Secondly,results from analysis of water wells indicate that this abnormal succession is limited to a small area(Fig.7) .More than 400 wells have been drilled in the study area,and 182 of them penetrated to a depth below the Lagersttte-bearing shale.However,the abnormal succession occurs only in 29 wells(red points in Fig.7) ,which are distributed in a circular area with a diameter of about 6 km.Field investigations and a review of available well data indicate that the contact of the circulararea with surrounding strata is abrupt.In addition,faulting of early Paleozoic bedrock has been reported in the subsurface of the same area,although its extent and mechanics have remained poorly understood.More importantly,microdeformation features have been recognized in quartz grains from the conglomerate/breccia unit(Fig.8) ;such features have been widely considered as unequivocal evidence of impact.Two contemporaneous impact structures in the central United States,the Rock Elm structure in Wisconsin and the Ames structure in Oklahoma,provide comparisons with the Iowa feature:a similar sedimentary succession of,in ascending order,breccia,black shale,and pure sandstone;similar conodont faunas from the shale units;and similar paleolatitude settings in tropical Laurentia during the Middle Ordovician(Fig.4) . The importance of the investigation of the depositional environment is not limited to explaining the local geological history,but will improve our understanding of the GOBE,which is documented primarily from the study of faunas that inhabited open-marine shallow-shelf environments.By contrast,the Winneshiek fauna lived in a nearshore to estuarine(likely brackish) environment where faunal diversity has not been well documented.The possible impact structure which hosts the Lagersttte may also provide a test of the hypothesized link between the GOBE and asteroid breakup during the Ordovician.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.