THE SIGNIFICANCE OF FOSSIL SEMIONOTID FISH FROM THE OWL ROCK MEMBER, CHINLE FORMATION (LATE TRIASSIC), SAN JUAN COUNTY, UTAH

Fieldwork in the Owl Rock Member of the Chinle Formation, in the Lisbon Valley area, San Juan County, Utah, resulted in the collection of hundreds of fossil fish, including diverse semionotids. Like semionotids from the Newark Supergroup of eastern North America, specimens from this locality display a wide variety of body forms and dorsal ridge scale modifications. This diversity has resulted in taxonomic confusion within the genus Semionotus. It has been proposed that the formation of extensive lakes during the initial rifting apart of the North Atlantic Basin enabled founder semionotid species with distinct dorsal ridge scales to explosively diversify in the newly opened ecospace in these expanding lake systems, as have modern cichlid fishes in the East African Rift lakes. Two interesting specimens of moderately deep-bodied Semionotus from a locality in Lisbon Valley, deemed “Walt's Quarry,” are a maximum of 4.4-4.6 cm deep and approximately 10.5-10.8 cm long. They have “simple modified” dorsal ridge scales in which the lengths of the scale “body” and its posteriorly-directed spine are subequal. Both specimens are conspecific and resemble Semionotus redfieldii, Semionotus schaefferi, and Semionotus olseni, from the Early Jurassic Towaco Formation of the Newark Supergroup. The Chinle fish resemble these three eastern species in general body shape; however, the dorsal ridge scales of the two Walt's Quarry specimens do not resemble the dorsal ridge scales of any described semionotid species. Thus, even conservatively, these specimens are determined to represent a new species of Semionotus in an undocumented species group from the Triassic of Utah that are ecologically convergent with species from the Newark Supergroup.