- The Paleontological Society
Trilobites from the Missisquoia Zone and the Symphysurina brevispicata Subzone of the Symphysurina Zone (Ibexian Series, lowest Ordovician) were collected from measured sections in the uppermost Deadwood Formation in the Black Hills of South Dakota and Bear Lodge Mountains in northeasternmost Wyoming. These collections were made by Christina Lochman-Balk and her students, and turned over to the author to complete the project. They are compared with previous reported occurrences of this fauna from this area. No trilobites from the underlying Sunwaptan Stage (Upper Cambrian) occur with the lowest Ordovician trilobites, suggesting that the sharp faunal extinction at the base of the Ordovician (North American sense = Eurekia apopsis Zone, Ibexian Series) occurred in the Deadwood Formation as it did over all of the North American continent.
The Black Hills is a domal uplift cored by Precambrian granites and quartzitic and schistose metamorphic rocks that was formed during the Laramide orogeny (Stitt, 1998). The Deadwood crops out as an oval belt of siltstones, sandstones, shales, and limestones unconformably overlying the Precambrian rocks. Specimens used in this study were collected from a series of measured sections located in the northernmost outcrop belt of the Deadwood.
Hu (1973) identified lowest Ordovician trilobites from one section (Bridal Veil Falls) in the northern Black Hills and from three sections (Reuter Canyon, Bearlodge Ranch, and Sheep Mountain) in the nearby Bear Lodge Mountains. He also described one new genus and four new species. His composite faunal list, and subsequent taxonomic reassignments in brackets, include the following trilobites: Euloma cordilleri [=Highgatella cordilleri Winston and Nicholls, 1967], Euptychaspis trematocus, Highgatella facila [=Apoplanias rejectus Ludvigsen, 1982], Hystricurus sp., Missisquoia graphica [=Missisquoia typicalis Westrop, 1986], Missisquoia cyclochila [=Missisquoia typicalis Ludvigsen, 1982], cf. Missisquoia typicalis, Olenaspella elongata, Symphysurina brevispicata, and the brachiopod …