A diverse vertebrate assemblage from Lower Cretaceous high latitude continental deposits at Lightning Ridge, southeastern Australia
|Location||International Geological Congress,oslo 2008|
|Holding Date||08 September 2008|
The Lower Cretaceous (Albian) high latitude deposits of Lightning Ridge in southeastern Australia have yielded a diverse range of fossils uniquely preserved in opal. The currently defined assemblage includes primarily aquatic vertebrates such as teleost fish, the dipnoans Metaceratodus and presently extant Neoceratodus forsteri, rare lamniform sharks, the primitive non marine eucryptodiran turtle Otwayemys plus a potential early meiolaniid, semi-procoelous and ziphodont crocodyliforms, pliosauroid plesiosaurs, and the platypus-like monotremes Steropodon and durophagous Kollikodon. Sympatric continental vertebrates comprise small pterodactyloid pterosaurs, ornithomorph birds, and dinosaurs: titanosauriform sauropods, theropods including Rapator, thyreophorans, and mainly ornithopods - Muttaburrasaurus and Fulgrotherium.
The Lightning Ridge vertebrate fossils derive exclusively from claystone sediments of the middle Albian Griman Creek Formation. This unit is famous for commercial opal mining and is reconstructed as a coastal fluvial/estuarine to lagoonal depositional environment on the basis of molluscan fossils (non-marine unionoid bivalves, thiarid and viviparid gastropods), planar cross-bedding (implying low-energy, unidirectional stream flow), and plant root impressions. Vertebrate taphonomy also indicates extensive winnowing of palaeochannel sediments. Estimates of palaeolatitude place southeastern Australia at between 70° and 85° South during the late Early Cretaceous. Sedimentary structures (including glacial erratics, glendonites, cryoturbated terrestrial sediments and tillites), densely growth-banded fossil wood, isotope data and climatic modelling additionally infer highly seasonal, cool to very cold conditions possibly with winter freezing during the Aptian and early Albian. Terrestrial faunas from this time preserve distinctive low-temperature tolerant forms (e.g. temnospondyl amphibians); however, by the middle Albian the onset of warmer conditions at high-latitude marks a shift towards groups favouring more equable climates (e.g. crocodilians, sauropods). The Lightning Ridge fossils thus represent an Early Cretaceous high latitude ’post-freeze’ assemblage, and display a taxonomic diversity consistent with lower latitude Gondwanan landmasses.