Late Cambrian and Early Ordovician Faunas from Wilberns Formation of Central Texas

The San Saba Member of the Wilberns Formation of central Texas is mostly coarse-grained, trilobitic limestone that grades eastward to dolomite and contains sandstone intervals in its westernmost exposures. Faunas from the limestone place the middle part of the San Saba within the Trempealeauan Stage (Saukia zone) of the Upper Cambrian, but the uppermost part belongs to the Lower Ordovician. Three subzones within the Saukia. zone are described. In ascending order they are: the Saukiella junia subzone containing Bayfieldia, Briscoia, Corbinia, Euptychaspis, Eurekia, Saukia, Saukiella, Stenopilus, Triarthropsis, and Finkelnburgia; the Saukiella norwalkensis subzone containing Bayfieldia, Bowmania, Briscoia, Calvinella, ?Dikelocephalus, Eurekia, Euptychaspis, Heterocaryon, Idiomesus, Keithiella, Leiocoryphe, Macronoda, Plethometopus, Prosaukia, Saukiella, Stenopilus, Theodenisia, and Finkelnburgia; the Corbinia apopsis subzone containing Acheilops, Apatokephaloides, Corbinia, Eurekia, Idiomesus, Leiobienvillia, Leiocoryphe, Plethometopus, Theodenisia Triarthropsis, Apheoorthis, Finkelnburgia, Glyptotrophia, and Nanorthis. Trilobites of the Saukiella junia and Saukiella norwalkensis subzones are closely allied to those of the Trempealeauan of the upper Mississippi Valley, whereas the trilobites of the Corbinia apopsis subzone represent an influx of the Hungaia magnifica fauna. The low Ordovician faunas are subdivided into two zones. The lower zone is named the Missisquoia zone and contains Highgatella, Homagnostus, Hystricurus, Missisquoia, Parabolinella, Symphysurina, Apheoorthis, Conotreta and Syntrophina. It correlates closely with a low Ordovician fauna from Vermont described by Shaw. The upper zone is the Symphysurina zone and contains Clelandia, Homagnostus, Hystricurus, Jujuyaspis, Symphysurina, Apheoorthis, and Syntrophina. It correlates most closely with the Symphysurina zone of Utah and Nevada. New trilobite species are Acheilops masonensis, Bayfieldia simata, Bowmania sagitta, Briscoia llanoensis, Calvinella procera, Clelandia texana, Corbinia apopsis, Corbinia implumis, Euptychaspis jugalis, Homagnostus reductus, Keithiella patula, Leiobienvillia leonensis, Leiocoryphe halei, Missisquoia inflata, Missisquoia nasuta, Saukiella planata, and Symphysurina bubops.