Marine Vertebrates from the K-P Boundary in Eastern Texas

The ~1.5 m section of K-P Boundary sediments exposed along the Brazos River near Rosebud, Texas have been studied extensively, and interpreted as storm or tsunami deposits. The boundary sediments are composed of two superimposed sequences of a granular sand bed (0-10 cm thick) capped by a hummocky cross-bedded sandstone (up to 20 cm thick) containing trace fossils. The granular sand beds contain glass spherules and abundant vertebrate and invertebrate fossils recovered through screen-washing. These vertebrate-bearing layers are high-energy deposits that contain only latest Maastrichtian taxa of foraminifera, ammonites, and marine vertebrates.

The thousands of fossil teeth and bone specimens recovered so far from the boundary complex are from a diversity of species of bony fish, sharks, rays, and possibly reptiles. The faunal assemblage from the boundary is typical of latest Cretaceous seas and includes species of Rhombodus (Rhombodontidae), Rhinobatos (Rhinobatidae), Sclerorhynchus (Sclerorhynchidae), Pararhincodon (Parascylliidae), Ischyrhiza (Sclerorhynchidae), Enchodus (Enchodontidae), Hadrodus (Pycnodontiformes), and Squalicorax (Anacoracidae). The majority of the teleost fish fossils remain to be identified. However, over 70% of the identified specimens belong to Enchodus. This assemblage of chondrichthyan and osteichthyan fishes is most similar to assemblages from other Maastrichtian sites in Texas, but it also shares taxa with sites in New Jersey. No vertebrates have been found in the lowermost Paleocene rocks of the Littig Member above the boundary complex. All of the species identified so far are not known from Paleocene sediments anywhere, and those taxa presumably became extinct at the K-P Boundary. In addition, the presence of a diversity of Maastrichtian vertebrate species (including two species of Squalicorax) in the boundary complex rocks seems to indicate that those species survived to the very end of the Cretaceous, and there may not have been a gradual extinction among marine vertebrates.