| dc.description.abstract | A new species of semionotiform fish is described based on specimens collected from the Upper Triassic Church Rock Member of the Chinle Formation. This new species of Semionotus is characterized by a deep body with a large post-cranial hump, and dense tuberculation on the posterodorsal margin of the skull that continues into the dorsal ridge and dorsolateral flank scales. Semionotus n. sp. has unique cranial suspensorium morphology compared to other Semionotus taxa, with a vertical preoperculum with a short and broad paddle-like ventral process. The infraorbital series expands ventrally to the suborbital and contacts the anterior ramus of the preoperculum, which has only previously been observed in Semionotus kanabensis. This shared characteristic, observed only in Semionotus taxa from the western United States, may provide some insights into the evolutionary relationships of taxa within Semionotidae, which currently remain unresolved. New specimens of Semionotus kanabensis were also collected from the Chinle Formation of Lisbon Valley. Previously, specimens of S. kanabensis were only known from the Lower Jurassic Moenave Formation of southwestern Utah. The Moenave formation specimens are preserved three-dimensionally, but are missing several anatomical regions and important morphological characters due to preservation. Specimens described in this thesis from the Chinle Formation lend new insights into the morphology of S. kanabensis. Because of the excellent preservation of the Lisbon Valley specimens, many anatomical regions that are missing or poorly preserved in previously studied specimens (e.g., fins, circumorbital series, neurocranium) are described in detail, allowing for a revised differential diagnosis of this species. In addition, the known age and distribution of S. kanabensis is also expanded from the Early Jurassic to the Late Triassic with the addition of the older Chinle Formation specimens, indicating that this species persisted through the Late Triassic mass extinction event. | |
