Geological setting and paleoecology of the Upper Cretaceous Bench 19 Marine Vertebrate Bonebed at Bentiaba, Angola

Abstract

The Bench 19 Bonebed at Bentiaba, Angola, is a unique concentration of marine vertebrates preserving six species of mosasaurs in sediments best correlated by magnetostratigraphy to chron C32n.1n between 71.4 and 71.64 Ma. The bonebed formed at a paleolatitude near 24°S, with an Atlantic width at that latitude approximating 2700 km, roughly half that of the current width. The locality lies on an uncharacteristically narrow continental shelf near transform faults that controlled the coastal outline of Africa in the formation of the South Atlantic Ocean. Biostratigraphic change through the Bentiaba section indicates that the accumulation occurred in an ecological time dimension within the 240 ky bin delimited by chron 32n.1n. The fauna occurs in a 10 m sand unit in the Mocuio Formation with bones and partial skeletons concentrated in, but not limited to, the basal 1-2 m. The sediment entombing the fossils is an immature feldspathic sand shown by detrital zircon ages to be derived from nearby granitic shield rocks. Specimens do not appear to have a strong preferred orientation and they are not concentrated in a strand line. Stable oxygen isotope analysis of associated bivalve shells indicates a water temperature of 18.5°C. The bonebed is clearly mixed with scattered dinosaur and pterosaur elements in a marine assemblage. Gut contents, scavenging marks and associated shed shark teeth in the Bench 19Fauna indicate biological association and attrition due to feeding activities. The ecological diversity of mosasaur species is shown by tooth and body-size disparity and by d 13 C analysis of tooth enamel, which indicate a variety of foraging areas and dietary niches. The Bench 19 Fauna was formed in arid latitudes along a coastal desert similar to that of modern Namibia on a narrow, tectonically controlled continental shelf, in shallow waters below wave base. The area was used as a foraging ground for diverse species, including molluscivorus Globidens phosphaticus, small species expected near the coast, abundant Prognathodon kianda, which fed on other mosasaurs at Bench 19, and species that may have been transient and opportunistic feeders in the area.

Post-Gondwana Africa and the vertebrate history of the Angolan Atlantic Coast

“…The younger South Atlantic magmatic event of Torsvik et al (2009) is recorded in Angola by the Ombe Basalt at Bentiaba in Namibe Province, dated at 84.6 ± 1.5 Ma (Strganac et al, 2014b). Predicted paleolatitude of the Ombe Basalt at its emplacement determined from oriented paleomagnetic samples was 24.5° S. The width of the South Atlantic was some 2700 km at that time and latitude, roughly half that of its current width (Strganac et al, 2014a;Reeves, 2014).…”

“…7). Strganac et al (2014a) argue that Bentiaba was formed in a setting similar to that of the Skeleton Coast, between 20º and 24ºS paleolatitude. An inference of coastal upwelling is based on the prevalence and abundance of marine top-consumers and coeval petroleum source rocks with type II kerogen derived from marine plankton (Zimmerman et al, 1987).…”

“…Africa's drift can be followed especially well because conjugate basins shared between Africa and South America provide fixed starting points (Brownfield and Charpentier, 2006), and because of seafloor magnetic stripes (Cande et al, 1989;Müller et al, 1997;He et al, 2008), hot spot traces (O'Connor and le Roex, 1992;O'Connor et al, 1999), predicted paleolatitudes determined from igneous rocks (Strganac et al, 2014a), and fault zones that can be followed from the Mid-Atlantic Ridge to the coast of Africa ( fig. 4) (Eagles, 2007;Guiraud et al, 2010).…”

“…Biostratigraphic uniformity of the Bench 19 fauna, evidence of interspecies interactions, including gut contents and scavenging marks, and the presence of pterosaur and dinosaur bones in a marine setting, indicate that the accumulation was attritional but occurred in an ecological time dimension within the 240 ky bin delimited by chron 32n.1n (71.40-71.64 Ma). The spatial distribution and taphonomy of fossils suggest a rich feeding area for diverse top consumers in waters 50-100 m in depth and at a water temperature based on δ 18 O from bivalve shells of 18.5° C (Strganac et al, 2014a(Strganac et al, , 2015.…”

“…An inference of coastal upwelling is based on the prevalence and abundance of marine top-consumers and coeval petroleum source rocks with type II kerogen derived from marine plankton (Zimmerman et al, 1987). Coastal desert is suggested by short intermittent rivers draining into the sea inferred from provenance of detrital zircons in sediments from mosasaur excavations (Strganac et al, 2014a). If these interpretations are correct, they can be extended to Iembe because of its paleolatitude at the time of formation.…”