A single, geographically and temporally restricted horizon at Bentiaba, Angola (14.3° S), preserves a concentration of skeletons and isolated elements representing sharks, rays, bony fish, at least three species of turtles, two species of plesiosaurs, at least five species of mosasaurs, and rare volant and terrestrial forms. The concentration, referred to as the Bench 19 Fauna, formed on a narrow continental shelf at paleolatitude 24°S as predicted by paleomagnetic data and confirmed by plate motion models. The shelf evolved as a transform passive margin along faults associated with the opening of the South Atlantic. Latitude 24°S falls today along the coast of northern Namibia, an area of intense upwelling and hyperarid coastal desert. The Namibe Basin in southern Angola is separated from the Walvis Basin of Namibia by the Walvis Ridge, and the continental shelf in northern Namibia is eight times the width of that at Bentiaba. However, the sediment entombing the fossils at Bentiaba is an immature feldspathic sand, shown by detrital zircon ages to be derived from nearby exposed granitic shield rocks, suggesting similar climatic and drainage conditions between the two regions. Temporal control of the Bentiaba section is provided by magnetostratigraphy and stable carbon isotope chemostratigraphy anchored by an Ar40/Ar39radiometric date on basalt. The age of Bench 19 is constrained to chron C32n.1n and thus falls between 71.4 and 71.64 Ma. Massive bedding without hummocky cross-bedding or other sedimentary structures indicates deposition in shallow water below wave base. δ18O analysis of bivalve shells indicates a water temperature of 18° C immediately below Bench 19. Nearest neighbor distance peaks at 5 m (n=192, range = 0 – 35 m, mean = 8.487, σ = 6.475). The areal distribution of specimens and their separation by meters indicates a scattered distribution over the sea floor, lacking linear orientation as in a strand or concentrated by transporting agents into a tight mass. Specimens have varying degrees of completeness and evidence of scavenging, but none shows evidence of prolonged exposure on the sea floor. The bonebed is clearly attritional, evidenced by rare and scattered dinosaur and pterosaur elements in an otherwise marine assemblage. Gut contents, scavenging marks, and associated shed shark teeth indicate biological association and attrition due to feeding activities among marine species. However, the geologically short time bin and taxonomic stability within the Bench 19 Fauna indicates the accumulation occurred in an ecological, as opposed to an evolutionary, time dimension. The ecological diversity of marine amniotes shown by tooth and body-size disparity and by δ13C values of tooth enamel indicates a variety of foraging areas and dietary niches. Nevertheless, relative abundance of shed teeth and carcasses contrasted with δ13C data indicates the Bench 19 area was used as a foraging ground for diverse species, including molluscivorousGlobidens phosphaticus, small piscivorous species expected near the coast (Halisaurussp.), abundantPrognathodon kianda, which fed on other mosasaurs at Bench 19, and other species that may have been transient opportunistic feeders in the area (Mosasaurussp.).
|Title of host publication||Proceedings of the Secondary Adaptation of Tetrapods to Aquatic Life|
|Place of Publication||Washington DC, USA|
|Publication status||Published - 1 Jan 2014|
|Event||Secondary Adaptation of Tetrapods to Aquatic Life, Washington DC, USA - |
Duration: 1 Jan 2014 → …
|Conference||Secondary Adaptation of Tetrapods to Aquatic Life, Washington DC, USA|
|Period||1/01/14 → …|