PALAIOS, 2020, v. 35, 342–357 Research Article DOI: http://dx.doi.org/10.2110/palo.2020.007 DWELLING IN THE DEAD ZONE—VERTEBRATE BURROWS IMMEDIATELY SUCCEEDING THE END- PERMIAN EXTINCTION EVENT IN AUSTRALIA 1 1 1 2 3 3 STEPHEN MCLOUGHLIN, CHRIS MAYS, VIVI VAJDA, MALCOLM BOCKING, TRACY D. FRANK, AND CHRISTOPHER R. FIELDING 1Swedish Museum of Natural History, Svante Arrhenius v. 9, SE-104 05, Stockholm, Sweden 2Bocking Associates, 8 Tahlee Close, Castle Hill, NSW, Australia 3Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0340, USA email: [email protected] ABSTRACT: A distinctive burrow form, Reniformichnus australis n. isp., is described from strata immediately overlying and transecting the end-Permian extinction (EPE) horizon in the Sydney Basin, eastern Australia. Although a unique excavator cannot be identified, these burrows were probably produced by small cynodonts based on comparisons with burrows elsewhere that contain body fossils of the tracemakers. The primary host strata are devoid of plant remains apart from wood and charcoal fragments, sparse fungal spores, and rare invertebrate traces indicative of a very simplified terrestrial ecosystem characterizing a ‘dead zone’ in the aftermath of the EPE. The high-paleolatitude (~ 65–758S) setting of the Sydney Basin, together with its higher paleoprecipitation levels and less favorable preservational potential, is reflected by a lower diversity of vertebrate fossil burrows and body fossils compared with coeval continental interior deposits of the mid-paleolatitude Karoo Basin, South Africa. Nevertheless, these burrows reveal the survivorship of small tetrapods in considerable numbers in the Sydney Basin immediately following the EPE. A fossorial lifestyle appears to have provided a selective advantage for tetrapods enduring the harsh environmental conditions that arose during the EPE. Moreover, high-paleolatitude and maritime settings may have provided important refugia for terrestrial vertebrates at a time of lethal temperatures at low-latitudes and aridification of continental interiors. INTRODUCTION lifestyles and environmental preferences of the animals that lived in the immediate aftermath of the EPE. In most regions of Pangea, continental strata immediately post-dating Here we document numerous large sand-filled burrows in strata the end-Permian extinction (EPE) are devoid of macroscopic animal immediately overlying and, in some cases, transecting the palynologically remains. In the well-studied Karoo Basin, South Africa, diverse vertebrate defined EPE at Frazer Beach in the northern Sydney Basin, Australia. The body and trace fossils generally are re-established in beds at least 5 m paleoenvironmental context of the burrows is constrained by high- above the apparent EPE, which is equated with the boundary of the resolution palynological, sedimentological and geochemical investigations. Daptocephalus and Lystrosaurus zones (Gastaldo et al. 2019, 2020; Botha et al. 2020). These first few meters of strata overlying the EPE horizon GEOLOGICAL SETTING AND BIOSTRATIGRAPHY equate to a depositional interval during which very simplified terrestrial ecosystems prevailed. Tectonic and Depositional Setting High-resolution palynological studies of the end-Permian extinction event (EPE) within continental strata from various parts of the world have Basin Setting.—The Sydney Basin is the southernmost component of identified a recovery succession that commonly initiates with a ‘dead zone’ the large meridional Sydney-Gunnedah-Bowen foreland basin complex that is essentially devoid of recognizable plant remains (Looy et al. 2001; located in eastern Australia (Fig. 1A, 1B). The basin system developed to Bercovici et al. 2015; Zhang et al. 2016; Vajda et al. 2020). This is the west of a continental volcanic arc, the New England Orogen, which commonly associated with, or succeeded by, a pulse of fungi (representing originated in association with subduction of Panthalassan oceanic crust ‘disaster taxa’; Visscher et al. 1996; Rampino and Eshet 2018) followed by along the eastern margin of Gondwana during the Permian (Fig. 1D). The a spike in the abundance of algal cysts and acritarchs (representing Sydney Basin was situated at high latitudes during the Permian–Triassic proliferation of ‘opportunist’ primary producers; Vajda et al. 2020). transition, although estimates of its precise location vary from ~ 65–758S Several meters above the EPE horizon, there is a gradual return of plant (Veevers 2006; Fig. 1D) to ~ 85–908S (Klootwijk 2016) depending on the spores and pollen, representing the initial stages of a protracted re- criteria used for pole-path reconstruction. The basin has an onshore areal establishment interval of herbaceous and woody plant communities. extent of . 60,000 km2 and hosts a Cisuralian–Middle Triassic Despite the dearth of skeletal remains and plant macrofossils in most sedimentary succession over 5000 m thick (Tadros 1995). Excellent immediate post-EPE (uppermost Changhsingian–lowermost Induan) coastal exposures of the Permian–Triassic transition beds occur in the continental successions, it is clear that a broad range of terrestrial northern (near Catherine Hill Bay) and southern (near Wollongong) parts vertebrate and vascular plant clades survived the mass-extinction event and of the basin and numerous fully cored boreholes penetrate the succession diversified later in the Triassic. In the absence of body fossils, tracks, trails in intervening areas. Detailed palynostratigraphic surveys of the sedimen- and other traces of animal behavior offer opportunities to unravel the tary succession (Helby 1970; Foster 1979, 1982; Helby et al. 1987; Mays et Published Online: August 2020 Copyright Ó 2020, SEPM (Society for Sedimentary Geology) 0883-1351/20/035-342 Downloaded from http://pubs.geoscienceworld.org/sepm/palaios/article-pdf/35/8/342/5153060/i0883-1351-35-8-342.pdf by Kathleen Huber on 25 September 2020 PALAIOS POST END-PERMIAN EXTINCTION VERTEBRATE BURROWS 343 FIG. 1.—Locality details for Frazer Beach and Wybung Head: A) Map of eastern Australia showing the Sydney-Gunnedah-Bowen foreland basin complex. B) Enlargement of the Sydney Basin showing the locations of Frazer Beach and Wybung Head. C) Uppermost Permian and lowermost Triassic stratigraphy of the Frazer Beach area. D) Permian global reconstruction showing the location of the Sydney Basin and other key regions mentioned in the text (Siberian Traps Large Igneous Province, Karoo Basin, Allan Hills). Maps modified from Blakey (2016) and Brunker and Rose (1969). al. 2020; Vajda et al. 2020) and radiogenic isotope dating of tuffs (Metcalfe correlative Newcastle and Illawarra coal measures in the northern and et al. 2015; Ayaz et al. 2016; Laurie et al. 2016; Phillips et al. 2018; southern Sydney Basin respectively, is characterized at Frazer Beach Fielding et al. 2019) have provided the Sydney-Gunnedah-Bowen basin (Munmorah State Conservation Area, New South Wales, Australia: complex with the best biostratigraphically and geochronologically 33811037.2100 S, 151837022.3400 E) and adjacent areas, by a transition from constrained Permian–Triassic succession in Gondwana. marginal marine to fully nonmarine coastal plain deposits and incorporates In broad terms, the Sydney Basin hosts a Permian transgressive- numerous thick seams of bituminous coal (Agnew et al. 1995). The Lower regressive sedimentary succession that records the waning phase of the Triassic succession, represented by the Narrabeen Group (Fig. 1C), is Late Paleozoic Ice Age. The Lopingian succession, represented by the characterized by laterally extensive, sandstone-dominated, fining-upward Downloaded from http://pubs.geoscienceworld.org/sepm/palaios/article-pdf/35/8/342/5153060/i0883-1351-35-8-342.pdf by Kathleen Huber on 25 September 2020 344 S. MCCLOUGHLIN ET AL. PALAIOS alluvial plain deposits lacking coals (Emerson and Branagan 2011). Where The results are currently under review in a companion study, but indicate a dominated by sandstone and conglomerate (in northern parts of the basin), very latest Permian to earliest Triassic age for the 1.5 m interval above the strata immediately overlying the uppermost Permian coal are assigned to Vales Point coal. Aggregate sediment accumulation rates calculated using the Munmorah Conglomerate. Further south and west in the Sydney Basin, available geochronological constraints within the basin have provided a the correlative part of the succession is dominated by the finer grained means to estimate the timing of events in the immediate aftermath of the Dooralong Shale. The two formations represent apparently coeval facies on EPE (Vajda et al. 2020). a broad scale but, locally, wedges of the Munmorah Conglomerate rest above the Dooralong Shale (Uren in Herbert and Helby 1980). Sediment Palynology and Paleobotany.—The Vales Point coal is composed of transport in the Sydney Basin through the Lopingian and Early Triassic tightly compacted plant remains dominated by glossopterid gymnosperms was mainly southwards via an axial drainage system through the foreland (Vajda et al. 2020). Quantitative analyses of upper Permian permineralized basin complex (Ward 1972; Fielding et al. 2001, in press). peats elsewhere in the Sydney Basin confirm that . 65% of the typical peat volume represents glossopterid tissues, with additional components Sedimentological Context.—The studied section at Frazer Beach was derived from an array of herbaceous ferns and lycopsids, fungi,