SEMIONOTID FISH FOSSILS FROM THE OWL ROCK FORMATION, CHINLE GROUP [LATE TRIASSIC (MIDDLE NORIAN; LATE REVUELTIAN LAND VERTEBRATE FAUNACHRON)], SAN JUAN COUNTY, UTAH

Fieldwork in the Chinle Group, particularly the Owl Rock Formation, in the Lisbon Valley area of San Juan County, Utah, resulted in the collection of hundreds of specimens of fishes, mollusks, conchostracans, tetrapods, plants, invertebrate traces, and tetrapod tracks. Exploration was undertaken to obtain some of the well-preserved fish fossils for the Utah Museum of Natural History in Salt Lake City described by Bob Schaeffer in 1967. A significant in situ fish locality in the Owl Rock Formation, now coined “Walt's Quarry”, has resulted in four productive horizons, that yield exceptionally well-preserved specimens. The following taxa have thus far been identified: Synorichthys stewarti, Cionichthys dunklei, Cionichthys sp., Turseodus? sp., Lasalichthys sp., Semionotus n. sp., Semionotus sp., Hemicalypterus weiri, Chinlea sorenseni, and Redfieldiidae n. gen. et n. sp.

The confusing taxonomic state of the Semionotidae, particularly Semionotus, has caused many researchers to avoid this particular group all together. This genus is almost certainly oversplit, and the clade requires extensive reexamination. Like semionotids from the Newark Supergroup of eastern North America, specimens from the Chinle Group also display a wide variety of body forms. Two interesting specimens of moderately deep-bodied Semionotus from Walt's Quarry have “simple” dorsal ridge scales in which the lengths of the scale “body” and its posteriorly-directed spine are subequal. Both are close in overall dimensions (respective skull lengths=2.4 and 2.5 cm; skull heights=2.5 cm and 2.4 cm; maximum body heights=4.4 and 4.6 cm; and body heights from anterior dorsal fin to ventral surface, perpendicular to body midline ~4.3 and 4.2 cm; total dorsal ridge scale lengths=0.4 cm for both; and dorsal ridge scale heights=0.2 cm for both). Both specimens are conspecific and resemble Semionotus redfieldii McCune, 1987 and Semionotus olseni McCune, 1987, from the Early Jurassic Towaco Formation of the Newark Supergroup. The Chinle fish resemble these two eastern species in both dorsal ridge scales shapes and body form.