Paper No. 12
Presentation Time: 8:00 AM-12:00 PM
CHONDRICHTHYANS FROM THE ARKADELPHIA FORMATION (LATE MAASTRICHTIAN) OF HOT SPRING COUNTY, ARKANSAS
Erosion of the upper Arkadelphia Formation by the Ouachita River in Hot Spring County, Arkansas reveals a diverse and abundant Late Maastrichtian chondrichthyan fauna representing at least fourteen species. To date, the assemblage consists of: Squatina hassei, Ginglymostoma lehneri, Carcharias cf. samhammeri, Serratolamna serrata, Squalicorax pristodontus, Squalicorax kaupi, Palaeogaleus sp., Galeorhinus giradoti, Ischyrhiza avonicola, Ischyrhiza mira, Sclerorhynchus sp., Ptychotrygon cf. vermicula and Rhombodus binkhorsti, all of which are widely known from the Upper Cretaceous of North America. An extremely rare species, Schizorhiza cf. weileri, currently known only from Texas and Africa, is also present. In addition, the chondrichthyan deposits contain osteichthyans and reptile remains as well as molluscan steinkerns, echinoid spines, and fragments of branching corals. The fauna occurs almost entirely as teeth scattered by modern river activity across the surface of the dark gray marl and clay of the Arkadelphia Formation. Fossils are collected from within localized accumulations of river sediments. Although no in situ specimens were recovered, the absence of any chondrichthyan material of clearly Paleocene age suggests that all of these fossils originally derive from the Arkadelphia Formation rather than from outcroppings of the Paleocene Midway Group which occur upstream from the collection site. This interpretation is also supported by the delicate, unabraded crown, root and side cusp elements, and other pristine features of these specimens. The Arkadelphia fauna of Arkansas expands the known geographic distribution of late Maastrichtian chondrichthyans in North America and provides an important biostratigraphic connection between faunas of the Gulf Coastal Plain and Western Interior Seaway.