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

Abstract

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.

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

“…; Liu et al . ). Among the recovered specimens, not counting the algae and non‐conodont microfossils such as acritarchs, 51% represent conodonts, 7% phyllocarids, 6% eurypterids and 1.6% bivalved arthropods.…”

“…However, the lowermost St Peter Sandstone in drill cores from northwestern Iowa has produced a few conodonts, suggesting a late Darriwilian age (Witzke & Metzger ; Liu et al . ), but whether this date is applicable also to the basal St Peter Sandstone in the Decorah region remains questionable.…”

“…) 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 . ). Shelly fossils, such as trilobites, corals and articulate brachiopods, as well as graptolites, have not been found.…”

“…; Liu et al . , , , ), was of more than usual interest. Subsequent palaeontological research (Lamsdell et al .…”

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

“…Apart from this, unusually large conodonts (elements up to 16 mm in length) with complete apparatuses (Liu et al . ), possible jawless fishes, linguloid brachiopods, molluscs, algae and acritarchs have been found (Liu et al . ; P.A.…”