Abstract
Undichna gosiutensis isp. nov. is described from the Middle Jurassic (Callovian) Arapien Shale of central Utah. The new ichnospecies consists of three regularly sinuosidal intertwined grooves and is interpreted as a swimming trail produced by a fish with a caudal fin and a double anal fin (or an anal fin with an anterior spine) whose tips reached lower than the rest of the body. This is the first reported Mesozoic occurrence of Undichna in North America and the first evidence of fish in Jurassic marine strata of Utah. The occurrence of Undichna in marine strata confirms that the ichnogenus is not restricted to freshwater environments as was once thought. Undichna gosiutensis occurs in fine‐grained, laminated limestones associated with a small, generalistic, and shallow‐tier invertebrate ichnofauna. The overall trace fossil assemblage suggests that the boundary between dysoxic and oxygenated waters was located very close to or at the sediment‐water interface.
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