''Archeognathus'' is a fossilized jaw apparatus of a large predatory

conodont Conodonts, are an extinct group of marine jawless vertebrates belonging to the class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning " cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known from their hard ...

from the

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

period ( Darriwilian to

Sandbian The Sandbian is the first stage of the Upper Ordovician. It follows the Darriwilian and is succeeded by the Katian. Its lower boundary is defined as the first appearance datum of the graptolite species '' Nemagraptus gracilis'' around million ...

stages). Its large size has made classification difficult, and it has historically been compared to conodonts and gnathostomes (jawed fish) since its remains were first discovered in

Missouri Missouri (''see #Etymology and pronunciation, pronunciation'') is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it border ...

. Complete articulated jaw apparatus of ''Archeognathus primus'' are common in the Winneshiek Shale

lagerstätte A Fossil-Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that preserves an exceptionally high amount of palaeontological information. ''Konzentrat-Lagerstätten'' preserv ...

Iowa Iowa ( ) is a U.S. state, state in the upper Midwestern United States, Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Ill ...

, allowing its identity as a conodont to be secured. The jaw apparatus of ''Archeognathus primus'' is much more simple than that of most conodonts, with only six elements in its jaw. These include a pair of long, small-toothed, sawblade-like "coleodiform" S elements at the front of the jaw, followed by two pairs of smaller "archeognathiform" P elements with large teeth. The S elements are indistinguishable from those of many species of ''Coleodus'', a

wastebasket taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the purpose of classifying organisms that do not fit anywhere else. They are typically defined by e ...

of Ordovician conodonts. This strongly suggests that most specimens referred to ''Coleodus'' actually belong to ''Archeognathus''.

References

Conodonts Ordovician animals Fossil taxa described in 1938 {{conodont-stub