MARINE VERTEBRATES FROM THE K-P BOUNDARY COMPLEX, BRAZOS RIVER, TEXAS

The latest Cretaceous and earliest Paleogene rocks exposed in and along the Brazos River Valley near Rosebud, Texas have been studied extensively for their lithologic composition, micropaleontological assemblages, and elemental content. The approximately 1.5 m of sediment associated with the contact of Mesozoic and Cenozoic rocks along the river has been interpreted as storm or tsunami deposits. Within this boundary complex there are two horizons that are rich in vertebrate fossils. The vertebrates are preserved in granular sands (0-10 cm thick) that are capped by hummocky sandstones (up to 20 cm thick) that have trace fossils. In the Brazos River there are two superimposed sequences of these beds. The granular sands contain glass spherules, clay clasts, and abundant fossils. Mollusc shells (ammonites, nautiloids, and pelecypods) and vertebrate remains are the most common fossils within the two granular sand layers. Fossil vertebrates are represented by teeth, fin spines, pharyngeal teeth, vertebrae, jaw fragments, skull fragments, and other pieces of bone. Screen-washing of sediments from the localities RB-1, 2, and 3 have resulted in several thousand vertebrate fossils. The fragmentary nature of the specimens points to the transportation prior to deposition.

The fossils present in these layers are from a diversity of species of bony fish, sharks, rays, and possibly reptiles. Specimens have been identified to a wide variety of taxa including Rhombodus (Rhombodontidae), Rhinobatos (Rhinobatidae), Hybodus (Hybodontidae), Sclerorhynchus (Sclerorhynchidae), Pararhincodon (Parascyllidae), Ischyrhiza (Sclerorhynchidae), Enchodus (Enchodontidae), Hadrodus (Pycnodontiformes), and Squalicorax pristodontus (Anacoracidae). The majority of the teleost fish fossils remain to be identified. This assemblage of chondrichthyan and osteichthyan fishes is most similar to assemblages from other Maastrichtian sites in Texas, but it shares taxa with sites in New Jersey. No vertebrates have yet to be located in the lowermost Paleocene rocks of the Littig Member above the boundary complex. The presence of the Maastrichtian shark species in the boundary complex rocks seems to indicate that those species survived to the very end of the Cretaceous and may not have suffered a gradual extinction.