What can exceptionally preserved fossils in a meteorite crater reveal about ancient conodont animals?

Details

This descriptive fossil study examined exceptionally preserved conodont apparatuses—their arrangements of elements—and compared their anatomy and size with other conodonts. Considerable numbers of apparatuses with hyaline elements occur in the Winneshiek Konservat-Lagerstätte in northeastern Iowa, dating to the middle–upper Darriwilian (Middle Ordovician, Whiterockian). The fossils occur alongside a restricted biota, including other conodonts, in fine-grained clastic sediments deposited in a meteorite impact crater.

The common apparatuses were identified as Archeognathus primus Cullison, 1938 and Iowagnathus grandis, a new genus and new species. The 6-element apparatus of A. primus consists of two pairs of archeognathiform (P) elements and one pair of coleodiform (S) elements. The 15-element apparatus of I. grandis is somewhat reminiscent of the prioniodinid type. It contains ramiform elements: one alate element and 7 pairs of digyrate, bipennate, or tertiopedate types.

Both taxa have giant elements, with both crowns and basal bodies preserved. Preservation of basal bodies had not previously been reported in Ordovician conodont apparatuses. Comparing the apparatus sizes of the Winneshiek specimens with those of Scottish Carboniferous conodont animals preserved with soft parts suggests that the Iowa animals were significantly larger than those Scottish animals.

The apparatus of A. primus differs conspicuously from the apparatuses of the prioniodontid Promissum from the Upper Ordovician Soom Shale of South Africa, whereas the apparatus architecture of I. grandis shows some similarity to Promissum. Based on the Winneshiek collections, the study proposes a new family, Iowagnathidae, within Conodonta.