CHONDRICHTHYAN ASSEMBLAGES FROM THE SURPRISE CANYON (LATE MISSISSIPPIAN, SERPUKHOVIAN) AND WATAHOMIGI (LATEST MISSISSIPPIAN/EARLY PENNSYLVANIAN, SERPUKHOVIAN/BASHKIRIAN) FORMATIONS OF THE WESTERN GRAND CANYON, NORTHERN ARIZONA

The Surprise Canyon Formation consists of a series of channel fills and karstic cave deposits of Late Mississippian (Serpukhovian) age in Grand Canyon representing a considerable hiatus between the Redwall Limestone and the overlying Supai Group. The formation is nowhere continuous and consists of isolated, lens-shaped outcrops scattered over Grand Canyon and Marble Canyon to the east. It represents three major depositional events: a nearshore fluvial infilling of karstic channels in the lower member; a middle shallow marine, bioherm reef member; and an upper member of deeper open water deposition. In the late Meramecian or early Chesterian shallow deltaic and tidal drainage channels formed during the westward retreat of the Redwall Limestone sea. In the late Chesterian/Serpukhovian a period of subsidence allowed marine waters to gradually flood the eroded valleys forming local estuaries. As the sea transgressed the estuaries also moved eastwards, their deposits forming the marine middle and upper units of the Surprise Canyon Formation.

The chondrichthyan assemblage from the Latest Mississippian Surprise Canyon Formation is remarkably diverse with a total of thirty-one taxa, identified from teeth and dermal elements. These include four new euselachian taxa; two protacrodontids, a group best known from the late Devonian, and two anachronistids, with ties to the modern sharks. This assemblage best compares with the Bear Gulch Limestone assemblage from central Montana. The Surprise Canyon assemblage provides a range extension of the paraselachians Srianta and Heteropetalus, which previously were only known from the Bear Gulch Limestone assemblage. However, the Surprise Canyon assemblage differs from the Bear Gulch assemblage in its greater number of euselachian taxa and the presence of the xenacanthimorph Bransonella nebraskensis and the elasmobranch Clairina sp.

The succeeding early Bashkirian Watahomigi Formation represents open marine deposition and contains only two taxa: a new xenacanth, and the holocephalan Deltodus. The relationship between the Surprise Canyon and Watahomigi chondrichthyan assemblages and other significant coeval chondrichthyan assemblages suggest that there may have been an eastern and western distinction between the Euamerican assemblages during the Serpukhovian.