LATE MESOZOIC STRATIGRAPHY AND VERTEBRATES OF THE GOBI BASIN

Repeated intervals of block-faulting across the Gobi Basin produced a complex mosaic of grabens and semigrabens which were infilled with vertebrate-bearing strata of late Mesozoic age. Although it is nowhere preserved in its entirety, superpositional relationships between the major lithostratigraphic units permit the reconstruction of a composite stratigraphic section. Lower through Middle Jurassic volcanics and coarse-grained clastics deposited on alluvial fans occur near the margins of the Gobi Basin and contain few vertebrate fossils. Upper Jurassic and lowermost Cretaceous strata (Sharilinian and Tsagantsabian time) contain coarse-grained fluvial deposits and some fossiliferous fine-grained alluvial plain and lacustrine sediments. Large, perennial lakes prevailed during Shinkhudukian (Aptian) time, to be replaced by vast, fluvially-influenced plains with smaller lakes during Khukhtekian (Aptian-Albian) time. Vertebrate assemblages of Early Cretaceous age are depauperate and characterized by endemic taxa, while during Khukhtekian time they became more diverse owing to immigration from Europe/North America. Fluviolacustrine and lacustrine environments of Baynshirenian (Turonian through early Campanian) time were inhabitated by shamosuchids, segnosaurids and diverse chelonians. The Gobi region was then gradually transformed into semi-arid alluvial plains interrupted by seasonal lakes and dune fields with ephemeral interdune streams and ponds. Semi-arid environments were widespread during Barungoyotian (Campanian) time and supported vertebrate assemblages dominated by lizards, gobiosuchids and protoceratopsids. Increased pluviosity during Nemegtian (Maastrichtian) time is indicated by meandering stream facies containing abundant remains of ornithomimids, tyrannosaurids and hadrosaurids. Available evidence points to a steady increase in dinosaurian diversity to the end of the Cretaceous. © 1991 Academic Press Limited.