An opalised mid-Cretaceous flora from the Griman Creek Formation at lightning ridge, eastern Australia

Abstract

A rich Albian–Cenomanian opalised plant fossil assemblage is described from the Griman Creek Formation at Lightning Ridge in the Surat Basin, northern New South Wales, Australia. The fossils are preserved predominantly as opal casts that retain few anatomical or micromorphological epidermal details. For this reason, most fossils are difficult to assign with higher taxonomic resolution than to plant families. Nevertheless, the assemblage appears to be dominated by scale-leafed cupressacean foliage and cones, with lesser proportions of araucariacean, podocarp and possibly cheirolepidacean conifers that likely constituted the middle and upper storeys of the palaeovegetation. Understorey ferns and sphenopsids are sparse. Angiosperms are notably absent but unusual pyramidal seeds may indicate the presence of the Bennettitales-Erdtmannithecales-Gnetales group in the palaeoflora. The plant fossils are co-preserved with a broad range of terrestrial and aquatic vertebrate and invertebrate remains that collectively attest to a coastal plain setting fringing the Surat Basin embayment of the epeiric ‘Eromanga Sea’, which flooded the epicratonic Great Artesian Basin complex during the Early Cretaceous. Several plant groups are shared with broadly coeval fossil floras from eastern Australia and New Zealand reflecting a fairly typical mid-Cretaceous middle- to high-latitude austral flora, but the Lightning Ridge assemblage notably lacks delicate and broad-leafed taxa, possibly owing to preservational and sampling biases. We highlight opportunities for the study of opalised plant fossils from various other assemblages in eastern Australia and note that analysis of future collections of carefully extracted specimens from the Lightning Ridge deposits offer the potential to yield micromorphological details.