Downloaded By: [La Trobe University Library] At: 08:06 28 July 2008 Estuarine deposition of a mid-Vise ´ an tetrapod unit, Ducabrook Formation, central Queensland: implications for tetrapod dispersal K. E. PARKER 1 * AND J. A. WEBB 2 1 Department of Zoology, La Trobe University, Vic. 3086, Australia. 2 Environmental Geoscience, La Trobe University, Vic. 3086, Australia. At Ducabrook property, central Queensland, the mid-Vise ´ an Ducabrook Formation has yielded a diverse vertebrate fauna (fish and one tetrapod taxon) from a thin unit among siltstone interbedded with sandstone, minor oolitic limestone and conglomerate. Five lithofacies can be distinguished: the Oolitic Facies, distinguished by oolitic limestone and straight parallel ripple crests; the Sandy Facies, composed of plane-laminated and current-rippled sandstones; the Conglomeratic Facies, represented by pebble conglomerate displaying planar cross-bedded megaripples; the Silty Facies of siltstone with abundant calcrete nodules or sand/silt/clay interlaminations; and the Lime-Flake Facies, characterised by abundant locally derived lime flakes. The last includes the fossiliferous tetrapod unit. The Oolitic Facies was deposited in the inner (proximal) and outer (distal) zones of an estuary, based on identification of tidal sedimentary structures (e.g. mud drapes) and estuarine oolitic fabrics; the Lime- Flake Facies and Silty Facies were deposited in the estuary and lower reaches of a river and its surrounds; and the Sandy and Conglomeratic Facies represent braid-river deposits. Overall, the sequence represents intermittent deposition throughout an estuary, both within the tidal channel and the surrounding tidal flats, with additional deposition from the feeder river. The tetrapod unit, from the Lime-Flake Facies, represents a twin-peaked storm-induced flood event onto the tidal channel floor. The vertebrate bones have a shared taphonomic history and have undergone only local transport. The tetrapod and fish were spatially and temporally concurrent, probably in a shallow tidally influenced proximal estuarine habitat experiencing monsoonal conditions. Estuarine adaptations of these vertebrate taxa can explain migration along shallow-water continental shelves between the supercontinents during the Late Devonian and Early Carboniferous. KEY WORDS: Carboniferous, depositional environment, Drummond Basin, Ducabrook Formation, estuary, paleoecology, Queensland, taphonomy, tetrapod, Vise ´an. INTRODUCTION At Middle Paddock, on Ducabrook cattle property in central Queensland (Figure 1), Lower Carboniferous (mid-Vise ´ an) tetrapods have been collected from a richly fossiliferous unit (¼tetrapod unit) that also contains abundant fish (Thulborn et al. 1996; Warren & Turner 2004). Upper Devonian and Lower Carboniferous fossil assemblages containing stem-group tetrapods are rare, and between the last aquatic tetrapods of the Famennian and the earliest terrestrial and aquatic tetrapods of the late Vise ´an is a 20 million year hiatus from which only a few tetrapod fossils have been recorded, all previously from the Northern Hemisphere (Coates & Clack 1995; Clack & Carroll 2000; Clack 2002a, b; Clack & Ahlberg 2004). The Southern Hemisphere Middle Paddock site lies within this crucial period in tetrapod evolution. Upper Devonian and Lower Carboniferous stem- group tetrapods and associated fish faunas are generally believed to have inhabited freshwater environments, but their non-marine sedimentary settings were fre- quently assigned using only the absence of strikingly marine evidence (Clack 2002b). Few of these faunas can be confidently determined as freshwater (Cressler 2001), and in fact many of the oldest tetrapod-bearing localities in the Northern Hemisphere may represent estuarine (Tibert & Scott 1999; Lebedev 2004) or even marine facies. The Upper Devonian tetrapod Tulerpeton was deposited in a marine environment (Lebedev & Clack 1993) but, despite the partial articulation of the remains, has been interpreted as a non-marine organism (Long & Gordon 2004). To understand early tetrapod evolution requires specific assessments of tetrapod habitats and paleobiogeography, in particular whether at least some stem-group tetrapods inhabited estuaries or marginal marine environments, so knowledge of the depositional settings and taphonomic attributes of tetrapod sites is essential. The Middle Paddock tetrapod unit lies in the Ducabrook Formation in the southern Drummond *Corresponding author: k.parker@latrobe.edu.au Australian Journal of Earth Sciences (2008) 55, (509 – 530) ISSN 0812-0099 print/ISSN 1440-0952 online Ó 2008 Geological Society of Australia DOI: 10.1080/08120090801888644