Palaeogeographic setting of a deep-water trilobite fauna from the Early Devonian (Pragian) of the Upper Yarra,central Victoria. Phacopidae of Victoria,Part 3

A Pragian (Early Devonian) trilobite fauna from the Norton Gully Formation in the Upper Yarra area of central Victoria consists exclusively of the phacopid Prokops moorei sp. nov., a species with a highly variable visual surface including greatly reduced and blind morphologies. Several trilobites are preserved as moult assemblages, but most occur as isolated tergites on bedding-planes crowded with dacryoconarids and small bivalves. The autecology and taphofacies of the fauna indicate a deep-water setting, with the biofacies associations closely resembling deep-water assemblages described from Devonian sequences elsewhere. The distributions of laterally equivalent late Pragian facies from eastern areas of the Melbourne Zone indicate an inclined shelf between Lilydale and the Upper Yarra area, deepening eastwards from shoreline to outer shelf settings. To the northwest and northeast of the Upper Yarra area, the shelf was bounded by tectonically active margins associated with the converging Benambra Terrain, and to the south by the Waratah Bay Platform.     

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 diagnoses are given for species previously described, including Maurotarion euryceps (McCoy, 1876; = Cyphaspis spryi Gregory, 1901), Raphiophorus jikaensis (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 (McCoy, 1876). A new phacopid genus, Orygmatos is described, represented by the species O. yanyeani gen. et sp. nov. Other species newly described include Cromus melbournensis sp. nov., Arcticalymene australis sp. nov., “Ananaspis” woiwurrungi sp. nov. and Kettneraspis hollowayi sp. nov.

Species composition of the trilobite fauna varies spatially, and a number of distinct assemblages can be defined. Abundant trilobite moult configurations 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 indicate the Melbourne Formation was deposited at relatively shallow depths on a broad, eastwardly deepening shelf, with deposition dominated by storm processes. The palaeoenvironment comprised a BA-1 community including the Arcticalymene australis trilobite assemblage, restricted to very shallow depths (～20 m) on the SW coastal margin of the shelf and preserved in proximal tempestite lithologies; and a BA-5 community group containing three distinct trilobite assemblages dominated by species of Cromus and a deeper water fauna, preserved in distal tempestite lithologies and ranging widely over the shelf at depths in the range of maximum storm wave base (～50 – 80 m).