Iowagnathus

Located near the town of Decorah in northeastern Iowa, the Winneshiek Shale is a Lagerstätten fossil site dating to the Darriwilian stage of the middle Ordovician, and underlies the larger St. Peter Sandstone. The only known area that exposes the site (the upper strata at least) are thin outcrops along the Upper Iowa River, located in Winneshiek County. == Discovery and naming ==

Iowagnathus was originally described by Liu et al., 2017, on the basis of multiple well preserved apparatuses including the holotype and paratype specimens (SUI 139888 and SUI 139887 respectively). Iowagnathus is one of the most commonly found conodont genera known from the Winneshiek biota, alongside the contemporary species Archeognathus primus. In the original description, this genus was recognized as distinct enough from other conodont genera, including Erismodus, Paraprioniodus and Erraticodon, due to differences in element placement, and overall apparatus morphology, to be placed in a new genus and species. A new family, the Iowagnathidae, was also erected to include it. This genus is one of the few members of the Winneshiek Biota not definitively known from other deposits, with the only other potential occurrence of it outside of Iowa being similar conodont elements found within middle Ordovician strata of Siberia. The conodonts genus name, Iowagnathus, is derived from the state of Iowa, and the Greek word gnathos, meaning "jaw. the species name, grandis is derived from the Latin word grandis, meaning "big, or "large" due to the exceptional size of the apparatus and elements belonging to this species. == Description ==

The apparatus of Iowagnathus is relatively complex compared to those of other conodonts, with fifteen elements total, including nine S elements, four P elements, and two M elements in total. The elements themselves are very large, with a length of around 16 mm (1.6 cm) long usually being recorded, however, one S0 element (belonging to the specimen SUI 139895) has a lateral width alone of around 16 mm, placing it among the largest known recorded conodont elements. However, using size comparisons between the apparatus of I. grandis compared to those of other conodonts, such as Clydagnathus (which is known from body fossils) suggests a potential size of up to 1.0 m (3.3 ft) long for the full animal, which would make I. grandis one of the largest conodonts so far described. == Classification ==

Alongside designating I. grandis as a new genus and species, Liu et al., 2017 also erected a new family, the Iowagnathidae, to include it. Conodonts as a group have had an interesting taxonomic history, as for more than hundred years after their initial discovery in 1856, they were only known from their phosphatic elements. It wouldn't be until the late 20th century when body fossils from a handful of species were discovered, confidently placing conodonts as agnathan vertebrates. == Paleoecology ==

Iowagnathus possesses several unique traits compared to other conodonts, including an exceptionally large apparatus, and the elements within the apparatus, as well as the potential large size of the full animal. This suggests that I. grandis occupied a high trophic level in its environment, and the common occurrences of bromalites containing conodont elements within the Winneshiek Shale, which may or may not have been produced by I. grandis, indicates that at least some form of predation was occurring within the area at the time. Because of the anoxic, low pH conditions of the benthic section of the Winneshiek Shale during the Darriwilian, and the overall lack of benthic taxa, I. grandis would've most likely have been nektonic (actively swimming in the water column), or nekto-benthic (swimming close to, or just above the seafloor). Living alongside I. grandis were a number of other large macropredators, including the Archeognathus primus, another large macropredatory conodont. The main difference between the two species is that the apparatus of A. primus is significantly less complex then that of I. grandis, with only six elements total making up the apparatus, compared to the fifteen in I. grandis. Another contemporary macropredator in the Winneshiek Shale was Pentecopterus decorahensis, a large eurypterid belonging to the megalograptid family that was the largest organism in the biota. The large number of macropredators suggests that the environment at the time was nutrient rich, however it is not known how long the predatory species were living in the area, or if they were seasonal visitors. == References ==