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

摘要

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.