Isolated teeth from the Middle Permian (early Guadalupian) Kaibab Formation of Arizona are described as a new species of the xenacanth shark genus Bransonella. Bransonella tribula n. sp. is a small tooth in which the intermediate cusp is 65% of the length of the principal cusps and the cristae on the labial face extend down over the base, covering it, and bifurcating to form distinctive double crested ridges. Fin spines from the same localities in the Kaibab Formation show the characteristic xenacanth feature of a double row of large thorn-like denticles along the posterior margin. Bransonella tribula n. sp. is the only xenacanth shark known from the Kaibab Formation at present, however, due to the lack of articulated material the fin spines are attributed to ?Bransonella tribula n. sp. The ecomorphology of Bransonella suggests a primitive, small, gracile, marine xenacanth that fed near the sea floor like the modern catsharks (Scyliorhinidae).
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