''Archeognathus'' is a fossilized jaw apparatus of a large predatory
conodont
Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...
from the
Ordovician period (
Darriwilian to
Sandbian 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. Complete articulated jaw apparatus of ''Archeognathus primus'' are common in the
Winneshiek Shale
The Winneshiek Shale (originally the Winneshiek Lagerstätte) is a Middle Ordovician (Darriwilian-age) geological formation in Iowa. The formation is restricted to the Decorah crater, an impact crater near Decorah, Iowa. Despite only being discove ...
lagerstätte of
Iowa, 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 of Ordovician conodonts. This strongly suggests that most specimens referred to ''Coleodus'' actually belong to ''Archeognathus''.
References
Enigmatic prehistoric animal genera
Ordovician animals
Fossil taxa described in 1938
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