STRATIGRAPHY AND PALEONTOLOGY OF A NEW NEARSHORE DINOSAUR-BEARING SUCCESSION FROM THE LATE CRETACEOUS OF THE MORONDAVA BASIN, WESTERN MADAGASCAR

The well-known terrestrial vertebrate record from Cretaceous sediments of the Mahajanga Basin of northwestern Madagascar has significantly enhanced our understanding of evolution and biogeography of the southern continents but is currently limited to terminal Cretaceous horizons. Recent and ongoing field reconnaissance forays into the Morondava Basin of southwestern and western Madagascar are beginning to extend the scope of these efforts, seeking to recover terrestrial fossils from the Late Cretaceous horizons dating to the interval during, and immediately following, the isolation of Madagascar from the other Gondwanan landmasses. Notably, the Ampolipoly region of western Madagascar has yielded a spectacular assemblage of marine and terrestrial fossils from a nearshore succession tentatively dated to the Coniacian. This nearshore record consists of fine-grained sediments, discrete channel bodies, and cross-bedded heterolithic units indicative of fluvially-dominated deposition; shallow marine sands and deeper marine mudstones comprise the superjacent units. To date, recovered fossils from the lower fluvially-dominated horizons include wood and the first definitive remains of Cretaceous plant compression fossils from Madagascar. The compression flora is composed of gymnosperms, sphenopsids, and angiosperm leaves and seeds. Abundant remains of marine invertebrates including ammonites, brachiopods, bivalves, gastropods, echinoids, and worms are found in the upper, marine-dominated strata. Vertebrate fossils recovered from one particularly fossiliferous horizon of concretionary marine sandstone include fish, abundant sharks including at least two taxa of the durophagous Ptychodus, and large associated remains of mosasaurs. Interestingly, among the best preserved vertebrate remains are large sections of articulated or very closely associated skeletons of medium-sized to very large titanosaurian sauropods. The presence of well-preserved terrestrial fossils demonstrates the potential of these marginal marine settings, often overlooked as sources of information about terrestrial evolution.