Trilobite faunas and palaeoenvironmental setting of the Silurian (early Ludlow) Melbourne Formation, central Victoria

Trilobites are common faunal elements in the Melbourne Formation, a unit of early Ludlow (upper nilssoni Biozone) age, which crops out extensively in the Darraweit Guim Province of the Melbourne Zone, central Victoria. New di agnoses are given for species previously described, including Maurotarion e uryceps (McCoy, 1876; = Cyphaspis spryi Gregory, 1901), Raphiophorus jikaen sis (Chapman, 1912; = Ampyx yarraensis Chapman, 1912), Cromus simpliciculus (Talent, 1964), Cromus spryi (Chapman, 1912), Sthenarocalymene kilmorensis (Gill, 1945; = Gravicalymene hetera Gill, 1945) and Trimerus harrisoni (Mc Coy, 1876). A new phacopid genus, Orygmatos is described, represented by th e species O. yanyeani gen. et sp. nov. Other species newly described includ e Cromus melbournensis sp. nov., Arcticalymene australis sp. nov., "Ananasp is" woiwurrungi sp. nov. and Kettneraspis hollowayi sp. nov. Species composition of the trilobite fauna varies spatially, and a number o f distinct assemblages can be defined. Abundant trilobite moult configurati ons are conclusive for interpretation of the benthic fauna as autochthonous , inferring depth estimations based on benthic community distribution to be valid. A depth-related succession of communities is recognised and indicat e the Melbourne Formation was deposited at relatively shallow depths on a b road, eastwardly deepening shelf, with deposition dominated by storm proces ses. The palaeoenvironment comprised a BA-I community including the Arctica lymene australis trilobite assemblage, restricted to very shallow depths (s imilar to 20 m) on the SW coastal margin of the shelf and preserved in prox imal tempestite lithologies; and a BA-5 community group containing three di stinct trilobite assemblages dominated by species of Cromus and a deeper wa ter fauna, preserved in distal tempestite lithologies and ranging widely ov er the shelf at depths in the range of maximum storm wave base (similar to 50 - 80 m).