Huaibao Liu

A new Lagerstätte from the Middle Ordovician St. Peter Formation in northeast Iowa, USA

A new fossil fauna has been discovered from a recently recognized shale unit within the middle Ordovician St. Peter Formation in northeast Iowa. It contains a variety of invertebrates and vertebrates, including soft body tissues, impressions, and 3-dimensionalpreservations. The exceptional preservation reveals a new Konservat-Lagerstatte, the Winneshiek Lagerstatte, and opens a unique window into the community that inhabited the margins of the Laurentian cratonic seaway during Middle Ordovician transgression. Among the fossils, several conodont assemblages, including the apparatus of enigmatic coleodontids, are preserved. Some conodont assemblages associated with soft body tissues are particularly noteworthy.

A new Ordovician arthropod from the Winneshiek Lagerstätte of Iowa (USA) reveals the ground plan of eurypterids and chasmataspidids

Euchelicerates were a major component of Palaeozoic faunas, but their basal relationships are uncertain: it has been suggested that Xiphosura—xiphosurids (horseshoe crabs) and similar Palaeozoic forms, the synziphosurines—may not represent a natural group. Basal euchelicerates are rare in the fossil record, however, particularly during the initial Ordovician radiation of the group. Here, we describe Winneshiekia youngae gen. et sp. nov., a euchelicerate from the Middle Ordovician (Darriwilian) Winneshiek Lagerstätte of Iowa, USA. Winneshiekia shares features with both xiphosurans (a large, semicircular carapace and ophthalmic ridges) and dekatriatan euchelicerates such as chasmataspidids and eurypterids (an opisthosoma of 13 tergites). Phylogenetic analysis resolves Winneshiekia at the base of Dekatriata, as sister taxon to a clade comprising chasmataspidids, eurypterids, arachnids, and Houia. Winneshiekia provides further support for the polyphyly of synziphosurines, traditionally considered the stem lineage to xiphosurid horseshoe crabs, and by extension the paraphyly of Xiphosura. The new taxon reveals the ground pattern of Dekatriata and provides evidence of character polarity in chasmataspidids and eurypterids. The Winneshiek Lagerstätte thus represents an important palaeontological window into early chelicerate evolution.

The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa

BackgroundEurypterids are a diverse group of chelicerates known from ~250 species with a sparse Ordovician record currently comprising 11 species; the oldest fully documented example is from the Sandbian of Avalonia. The Middle Ordovician (Darriwilian) fauna of the Winneshiek Lagerstätte includes a new eurypterid species represented by more than 150 specimens, including some juveniles, preserved as carbonaceous cuticular remains. This taxon represents the oldest described eurypterid, extending the documented range of the group back some 9 million years.ResultsThe new eurypterid species is described as Pentecopterus decorahensis gen. et sp. nov.. Phylogenetic analysis places Pentecopterus at the base of the Megalograptidae, united with the two genera previously assigned to this family by the shared possession of two or more pairs of spines per podomere on prosomal appendage IV, a reduction of all spines except the pair on the penultimate podomere of appendage V, and an ornamentation of guttalate scales, including angular scales along the posterior margin of the dorsal tergites and in longitudinal rows along the tergites. The morphology of Pentecopterus reveals that the Megalograptidae are representatives of the derived carcinosomatoid clade and not basal eurypterids as previously interpreted.ConclusionsThe relatively derived position of megalograptids within the eurypterids indicates that most eurypterid clades were present by the Middle Ordovician. Eurypterids either underwent an explosive radiation soon after their origination, or earlier representatives, perhaps Cambrian in age, remain to be discovered. The available instars of Pentecopterus decorahensis suggest that eurypterids underwent extreme appendage differentiation during development, a potentially unique condition among chelicerates. The high degree of appendage specialization in eurypterids is only matched by arachnids within chelicerates, supporting a sister taxon relationship between them.

Bivalved arthropods from the Middle Ordovician Winneshiek Lagerstätte, Iowa, USA

Abstract. The Middle Ordovician (Darriwilian) Winneshiek Lagerstätte of northeast Iowa is preserved in a meteorite crater. Besides conodonts, the fossils are dominated by arthropods, particularly eurypterids and phyllocarids. Here we describe the bivalved forms, which include at least seven different taxa. The small phyllocarid Ceratiocaris winneshiekensis, new species, is the most abundant; it is the oldest representative of the Ceratiocarididae. A single incomplete abdomen and telson bearing furcal rami is reminiscent of notostracan branchiopods but its affinities are unknown. Decoracaris hildebrandi, new genus and species, is a rare form with a shield that extends anteriorly into a swollen horn and reaches lengths of 9 cm: it may represent a thylacocephalan crustacean but this cannot be confirmed without soft parts. Iosuperstes collisionis, new genus and species, is represented by suboval valves 10–25 mm long: its affinities are unknown. A probable leperditicopid, which ranges in length from 8 to 18 mm, is commonly preserved in a ‘butterflied’ configuration. It does not preserve the scars and sinuses characteristic of three-dimensionally preserved leperditicopids from elsewhere. Finally the fauna includes at least three ostracods, including a palaeocope with a granular surface and relief similar to Lomatopisthia, and a smooth ?podocope. The Winneshiek fauna differs from those of other Ordovician Lagerstätten from restricted settings such as Airport Cove and William Lake in Manitoba (Katian) where, apart from ostracods, bivalved arthropods are absent, and Silurian examples such as Brandon Bridge (Telychian), which lacks eurypterids, and the Williamsville Member of the Bertie Formation (Pridoli) where conodont assemblages are absent.

Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat-Lagerstätte, Iowa, USA

Abstract. Considerable numbers of exceptionally preserved conodont apparatuses with hyaline elements are present in the middle-upper Darriwilian (Middle Ordovician, Whiterockian) Winneshiek Konservat-Lagerstätte in northeastern Iowa. These fossils, which are associated with a restricted biota including other conodonts, occur in fine-grained clastic sediments deposited in a meteorite impact crater. Among these conodont apparatuses, the common ones are identified as Archeognathus primus Cullison, 1938 and Iowagnathus grandis new genus new species. The 6-element apparatus of A. primus comprises two pairs of archeognathiform (P) and one pair of coleodiform (S) elements. The 15-element apparatus of I. grandis n. gen. n. sp. is somewhat reminiscent of the prioniodinid type and contains ramiform elements of alate (one element) and digyrate, bipennate, or tertiopedate types (7 pairs). Both conodont taxa are characterized by giant elements and the preservation of both crowns and basal bodies, the latter not previously reported in Ordovician conodont apparatuses. Comparison of the apparatus size in the Winneshiek specimens with that of the Scottish Carboniferous soft-part-preserved conodont animals suggests that the Iowa animals were significantly larger than the latter. The apparatus of A. primus differs conspicuously from the apparatuses of the prioniodontid Promissum from the Upper Ordovician Soom Shale of South Africa although the apparatus architecture of I. grandis n. gen. n. sp. shows some similarity to it. Based on the Winneshiek collections, a new family Iowagnathidae in Conodonta is proposed.

TAPHONOMY AND BIOLOGICAL AFFINITY OF THREE-DIMENSIONALLY PHOSPHATIZED BROMALITES FROM THE MIDDLE ORDOVICIAN WINNESHIEK LAGERSTÄTTE, NORTHEASTERN IOWA, USA

Abstract The Winneshiek Lagerstätte occurs within an Ordovician meteorite impact structure beneath part of the city of Decorah, Iowa. The Lagerstätte has yielded an atypical marine fauna including phyllocarid crustaceans, eurypterids, conodonts, linguloid brachiopods, and jawless fish. Associated with these taxa are vermiform fossils: elongate, morphologically variable, and often three-dimensionally preserved bromalites of uncertain organisms. The preservational state of these bromalites is significantly different from that of other components of the Winneshiek biota. Here we present a compositional and microstructural analysis of the vermiform fossils in order to elucidate their taphonomy and biological affinities. The majority of studied specimens are preserved three-dimensionally and composed of calcium phosphate, while a minority are preserved as carbonaceous compressions. Winneshiek bromalites exhibit important similarities to examples documented from both older and younger sediments. They provide independent evidence of predation in the Winneshiek assemblage during the Great Ordovician Biodiversification Event.

The Winneshiek biota: exceptionally well-preserved fossils in a Middle Ordovician impact crater

The Winneshiek Shale (Middle Ordovician, Darriwilian) was deposited in a meteorite crater, the Decorah impact structure, in NE Iowa. This crater is 5.6 km in diameter and penetrates Cambrian and Ordovician cratonic strata. It was probably situated close to land in an embayment connected to the epicontinental sea; typical shelly marine taxa are absent. The Konservat-Lagerstätte within the Winneshiek Shale is important because it represents an interval when exceptional preservation is rare. The biota includes the earliest eurypterid, a giant form, as well as a new basal chelicerate and the earliest ceratiocarid phyllocarid. Conodonts, some of giant size, occur as bedding plane assemblages. Bromalites and rarer elements, including a linguloid brachiopod and a probable jawless fish, are also present. Similar fossils occur in the coeval Ames impact structure in Oklahoma, demonstrating that meteorite craters represent a novel and under-recognized setting for Konservat-Lagerstätten.