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

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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

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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, and fully logged by Vajda et al. (2020, fig. 3). The uppermost coal seam in the detritus of unknown origin (McLoughlin et al. 2019). The ~ 1.5 m thick Permian succession (termed the Vales Point coal in the Newcastle coal seam is underlain by a pale gray mudstone (paleosol) containing coalfield) is typically 1.3–1.6 m thick in the Munmorah coastal exposures, abundant Vertebraria roots derived from woody glossopterids growing in and at Frazer Beach contains a parting of dark gray, carbonaceous siltstone forested mires. This interval corresponds to the pre-extinction (Changh- in its upper part. At the top of the coal, a discontinuous bed of singian) Dulhuntyispora parvithola Palynozone and is characterized by carbonaceous microbreccia up to 0.2 m thick was noted by Retallack typical Permian elements dominated by various taeniate bisaccate pollen (2005), Vajda et al. (2020), and in this study. This granule breccia consists taxa (mostly Protohaploxypinus and Striatopodocarpites species) pro- of fragments of plant debris (including charcoal), silicate mineral grains, duced by glossopterids, together with diverse spores. Key-taxa include D. and possible devitrified volcanic fallout (pumice) in a mud matrix. parvithola and species of Microbaculatispora, Neoraistrickia,and Overlying the breccia is a 0.45 m thick interval of medium to light gray Horriditriletes. The Glossopteris flora, as represented by the Dulhuntyis- siltstone rich in charcoal but barren of pollen and spores that is, in turn, pora assemblage, extends unchanged to the top of the Vales Point coal. overlain sharply by the fine-grained sandstone bed from which the burrows Plant macrofossils are absent from beds immediately overlying the documented herein descend. This sandstone bed is typically 0.25 m thick uppermost Permian coal seam and this change is mirrored in the and is sharp-bounded at its base and top. Above this bed is a further palynological record by an abrupt boundary between the Dulhuntyispora interval of light gray to green-gray siltstone with a discontinuous, thin tuff parvithola Zone assemblages and the ‘dead zone’ reported by Vajda et al. lamina near the top. Bounding this fine-grained package is a composite (2020). The change in the palynological and macrofloral signal is interval of mainly thin-bedded, fine-grained sandstone 1.20 m thick that expressed over a few centimeters of section and reveals the complete thickens southward into a 6 m thick cross-bedded sandstone body. and sudden demise of the glossopterid forests. Although the succeeding The Vales Point coal seam is interpreted as the product of a ‘dead zone’ is devoid of miospores, it contains sparse algal thalli and Changsinghian in situ peat accumulation in a coastal plain mire, affected fungal spores along with abundant charcoal and unidentifiable, seemingly by minor inundation by clastic sediment (Vajda et al. 2020). The transported (rather than degraded) organic matter. The fungi possibly grew microbreccia capping the seam is evidently the product of a similar flood on the decaying forest debris and may have included ‘phoenicoid’ or of slightly coarser material, that might have been stimulated by soil erosion ‘pyrophilic’ fungi that commonly proliferate in the immediate aftermath of (as postulated by Retallack 2005) or alternatively by volcanic activity wildfires and other catastrophic events (Carpenter et al. 2011; McMullin- (Fielding et al. in press). The overlying siltstone is interpreted as the Fisher et al. 2011). product of mud accumulation on a floodplain that was likely waterlogged, Abundant freshwater algae appear at ~ 1.5 m above the Vales Point at least seasonally. The top of this interval was evidently subjected to coal, i.e., capping the ‘dead zone’. These are represented chiefly by desiccation, judging by the abundance of large burrows extending leiospheres (Leiosphaeridia spp.), Circulisporites parvus, and Quadris- downwards from this level. The siltstone package is also rich in charcoal porites horridus, all affiliated with green algae. Importantly, this laminated and wood remains reflecting proliferation of wildfires and erosion of the bed is devoid of pollen and spores typical of the preceding D. parvithola landscape following the mass extinction. The overlying tabular sandstone Zone, but includes sparse Brevitriletes bulliensis, a key element of the bed is interpreted as a splay deposit that issued onto the floodplain from a succeeding Playfordiaspora crenulata Zone of uppermost Permian (but nearby channel that overtopped or breached its bank during a flood. This post-EPE) age. pattern is repeated above by alternating laminated and massive siltstones The first post-extinction plant macrofossils occur in a 1.20 m thick, (which take on stronger hues upward) and tabular sandstones. The overall weakly silicified, laminated, very fine grained sandstone package initiating impression is of a lowland alluvial plain crossed by rivers and flanked by ~ 1.6 m above the Vales Point coal. These deposits host Lepidopteris, floodplains that were periodically inundated by water and at other times Voltziopsis, Ginkgophytopsis, and isoetalean leaf fragments and striate exposed and desiccated. stem impressions characteristic of uppermost Permian to lowermost Triassic assemblages of eastern Australia (Retallack 2002). Acid- Age and Paleobiological Context macerated organic residues from these beds contain abundant and robust Lepidopteris and some Voltziopsis cuticle fragments characteristic of low- Age Constraints from Radiogenic Isotopes.—Recent absolute age diversity, post-extinction, sclerophyllous floras of eastern Australia. This estimates using U-Pb chemical abrasion-isotope dilution-thermal ioniza- level sees the inception of a relatively low-diversity Playfordiaspora tion mass spectrometry (CA-ID-TIMS) of zircons tied to detailed crenulata Zone palynoassemblage characterized by abundant bisaccate palynostratigraphic studies (Metcalfe et al. 2015; Laurie et al. 2016; Alisporites pollen grains attributable to Voltziales (Bomfleur et al. 2013) Fielding et al. 2019; Mays et al. 2020; Vajda et al. 2020) provide a high- and rare monosulcate pollen grains affiliated with Lepidopteris (Antevs resolution temporal framework for understanding the depositional, 1914). Pteridophytes are sparsely represented in the macroflora but vegetational, and environmental changes through the Permian–Triassic abundant in the palynoflora (constituting a spore spike). They are succession in the Sydney Basin. Several beds were sampled for zircons represented mainly by abundant acavate fern spores (e.g., Brevitriletes (CA-ID-TIMS dating) from exposures at Frazer Beach and adjacent areas. bulliensis, Cyathidites, Osmundacidites, and Apiculatisporites species)

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together with cavate lycophyte spores (including P. crenulata, Densois- Ca to Kr on the periodic table. Concentrations of Na and Mg were also porites, and Lundbladispora species). Taeniate bisaccate pollen, probably measured under a helium flush to negate atmospheric interference. produced by conifers, occur in low numbers, although a few may represent reworked Glossopteris pollen. RESULTS The succeeding 15 m interval dominated by gray-green-red siltstones contains only dispersed and finely macerated plant remains (Vajda et al. Geochemistry.—The ~ 2 m of section that overlies the Vales Point coal 13 2020). Palynozones younger than the P. crenulata Zone were not detected seam is bracketed by two abrupt negative excursions in d Corg values, one in the studied interval. at the top of the coal seam to -31% VPDB (Fig. 2D) and the second approximately 2 m above it reaching -30% (Vajda et al. 2020, fig. 3). These excursions separate a recovery to higher values of -26%. The nature MATERIAL AND METHODS and timing of these excursions are consistent with widely documented Coastal cliff sections at Frazer Beach were logged and sampled in detail perturbations in the global carbon cycle associated with the EPE and in conjunction with an earlier study of the palynology, paleobotany, commonly linked to greenhouse gas emissions from the Siberian Traps sedimentology, and geochemical transition through the EPE (Vajda et al. Large Igneous Province (Korte and Kozur 2010; Williams et al. 2017; 2020). Two burrow fills were processed for palynological content using the Bagherpour et al. 2020). CIA values mirror the carbon isotope record. methods of Phipps and Playford (1984). Several dozen large burrows have Values of around 79 were obtained from beds underlying the uppermost now been identified in the Frazer Beach section and two were sampled for Permian coal. Above this are two maxima (90 at the EPE, and 89 thin sectioning and petrographic analysis using a Nikon Optiophot approximately 2 m above the EPE) coinciding with the negative excursions 13 polarizing microscope with a Lumenera Infinity1-2CB digital camera in d Corg values. Taken together, these trends indicate two distinct pulses attachment. A few additional burrows were identified in stratigraphically of warming that facilitated intense chemical weathering. The first coincides equivalent beds 340 m south of Frazer Beach at Wybung Head with the abrupt disappearance of the Glossopteris flora, whereas the (33811050.0200 S, 151837024.9700 E). Surface details of the burrows were second corresponds to the onset of ponding, characterized by a spike in photographed with a Canon 40D digital camera. A composite panorama algal remains in laminated sediments at this site (Vajda et al. 2020). A outcrop image was compiled from several photos using the ‘Photomerge’ trend towards lower CIA values (of around 85 to 86) between the maxima, function of Adobe Photoshop CC. Texture tracings of burrow surfaces and suggesting a decrease in the intensity of chemical weathering, is consistent cross-sections were produced using the ‘stylize–find edges’ filter in Adobe with sedimentological data indicating an interval of desiccation, drying and Photoshop CC. Digital images of the burrows in outcrop and hand a brief pulse of erosion following collapse of the glossopterid forest mires. samples, together with sampled portions and thin sections of two burrows, Moreover, this pattern appears to be represented consistently across the are stored with the registration prefix NRM X- in the collections of the Sydney-Bowen basin complex (T. Frank personal observation). Department of Palaeobiology, Swedish Museum of Natural History, Stockholm. Systematic Ichnology 13 13 The d C values of bulk sedimentary organic matter (d Corg) were analyzed using a Costech Elemental Analyzer connected to a Thermo Reniformichnus Krummeck and Bordy 2018 Finnigan MAT 253 Isotope Ratio Mass Spectrometer at the Keck-NSF Type Ichnospecies.—Reniformichnus katikatii Krummeck and Bordy Paleoenvironmental and Environmental Laboratory, University of Kansas. 2018. In preparation for analysis, samples were powdered using a Sibtechnik rock pulverizer. To remove inorganic carbon, samples were reacted with 1 N HCl for 24 h, rinsed three times in Milli Q water, dried at 408C, and Ichnospecies.—Reniformichnus australis n. isp. 13 repowdered with an agate mortar and pestle. The d Corg values are reported in per mil (%) relative to the Vienna Peedee Belemnite standard. Etymology.—From the Latin australis ¼ southern, denoting its high- Analyses of working standards (DORM, ATP) were used for quality paleolatitude occurrence. control, and values were reproducible to better than 6 0.13% (1r SD) for d13C. Holotype.—NRM X9101a, (plus thin section NRM X9101b). The Chemical Index of Alteration (CIA) of Nesbitt and Young (1982) is a widely used proxy that provides a means of tracking the conversion of Paratype.—NRM X9102a, (plus thin section NRM X9102b). feldspar minerals to clay minerals via hydration during the weathering of fine-grained siliciclastics. The CIA is calculated using the following Diagnosis.—Low-angle burrow fills of Reniformichnus type, differing equation: from the type species by the presence of crescentic excavation marks and small, irregularly distributed, raised, lobate fillings on the dorsal surface Al2O3 CIA ¼ 100 3 with oblique striae and punctae extending to the lateral walls, a smooth or Al2O3 þ CaO þ K2O þ Na2O scabrate ventral surface, and a broadly elliptical to (basally) asymmetrically where all oxides are in molar units and CaO* represents the CaO in the bilobate cross-section. silicate fraction following the method of McLennan et al. (1993). Fresh crystalline rocks are characterized by relatively low CIA values (45–55). Description.—Large sand-filled burrows are sparsely distributed along The concentration of Al2O3 during chemical weathering causes CIA values the cliff exposures at Frazer Beach and Wybung Head. They are the only to increase from this baseline. Elemental concentrations in mudrocks were obvious ichnofossils in the beds immediately above the EPE horizon at determined using a Bruker Tracer 5i portable X-ray fluorescence (XRF) these localities. They extend from the base of the 30–40 cm thick splay analyzer calibrated with a series of mudrock standards including USGS sandstone (Fig. 2, bed 4) downwards into the immediately underlying 40– shale standards SBC-1 and SCO-2, a suite of mudrock reference materials 50 cm thick gray siltstones (representing the post-EPE palynological ‘dead characterized by Rowe et al. (2012), and nine in-house reference materials. zone’) (Figs. 2 (bed 3), 3A). Some burrows appear to terminate within High-energy analyses were undertaken at 50 kV and 35 lA and a count these siltstones (Fig. 3C), but others continue lower in the succession time of 30 s, with a Cu 75 lm:Ti 25 lm:Al 200 lm filter for the elements through the 5–10 cm thick microbreccia bed (Fig. 2, bed 2) and end in the

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FIG. 2.—Typical distribution of burrows at Frazer Beach. A) Sedimentological log spanning the burrow-bearing interval. B) Panorama along a portion of the lower part of the cliff exposures at Frazer Beach. C) Same panorama with tetrapod burrows highlighted and key beds labelled. D) Stable carbon isotope trend through the studied interval showing sharp excursion at the end-Permian extinction (EPE) horizon. Explanations: VPDB ¼ Vienna Pee Dee belemnite; red dashed line marks the EPE defined by the top of the uppermost Permian coal; white dashed line marks the top of a microbreccia bed of granule-sized clasts; black dotted lines demarcate the splay sandstone; A and D simplified from Vajda et al. (2020).

upper part of the 1.6 m thick Vales Point coal seam (Figs. 2 (bed 1), 3B, The burrows are filled predominantly by sandstone of identical 3H). At least 26 burrows are evident along the Frazer Beach exposure, appearance to the overlying splay sandstone (Fig. 2A, 2B, bed 4) except which extends laterally for about 37.5 m. A slightly greater number of that they tend to have a slightly higher silt content, especially around the burrows may be present because the cliff face is covered in some places by margins. The infilling sediment of the lower (near-horizontal) portions of scree and vegetation. A relatively well exposed 3.7 m wide portion of this the burrows is slightly coarser than the upper portions and, in some cases outcrop that appears to be representative of the succession (Fig. 2) contains internal horizontal laminations are preserved (Figs. 3F, 4A). The lower at least eight burrows of varying sizes and orientations. Burrows were not surface of the burrow fill has a scabrate appearance, lacking any consistent detected in equivalent strata at Snapper Point, 700 m to the north-northeast. ornamentation apart from indistinct scaly mud-chip indentations (Fig. 4B, However, sparse burrows of equivalent size and fill occur within coeval 4D). The lower surfaces of some burrows have a variably developed central strata 340 m to the south of Frazer Beach at Wybung Head (Fig. 3E). We groove (on casts) or ridge (on molds) that generates an inconsistently have not detected burrows of this or other types at an equivalent bilobed appearance in cross-section (Figs. 4A, 4B, 6F–6H). Margins of the burrows are characterized by ill-defined oblique striae and irregular stratigraphic level elsewhere in the Sydney Basin but the representation of punctae (Figs. 4E, 6C). Upper surfaces of the burrow fill are characterized sedimentary facies is not consistent at all sections embracing this interval. by weakly defined fine crescentic ridges and furrows (probably Burrows descend at up to 708 from the overlying sandstone but, after a representing excavation scratches; Figs. 4F, 4G, 6C) with their apices short distance, they curve to angles of , 208 to the horizontal near their directed towards the lower end of the burrow. The surface texture of the lower terminus (Figs. 2, 3A–3G). They are straight to very gently curved in burrow is otherwise scabrate or includes ill-defined, raised, lobate features plan view (Fig. 4F). The majority of burrows are represented by exposed that have pointed apices in the direction of the lower end of the burrow segments of low-angle sandstone casts of more-or-less straight trajectory (Fig. 6D, 6E). (Fig. 3A–3D, 3G). Complete burrows are not accessible but total lengths Two sampled burrow fills (Figs. 4A, 4B, 6B, 6H) studied in thin section appear to be ~ 1–2 m. The burrows are broadly elliptical to almost have equivalent textures and composition. The burrow fills consist of sub- rectangular in cross-section (Fig. 4A, 4C) with maximum widths of 40– litharenite (in the classification scheme of Folk et al. 1970) incorporating 150 mm (mean 87 mm, median 78 mm), heights of 11–100 mm (mean 37 thin horizontal laminae (, 3 mm thick) of lithic wacke (Pettijohn 1975). mm, median 36 mm) and aspect ratios of 1.5–3.9 (mean 2.3, median 2.3) The sub-litharenites consist of a poorly to moderately sorted framework of (Fig. 5A–5C). Burrow cross-sectional dimensions do not change around 75% quartz, 20% lithic fragments (mainly chert, foliated significantly along their length. Burrow termini are rarely preserved in metamorphic grains, and minor mudstone clasts), and , 5% corroded the rock face but they appear to represent simple blind endings lacking an feldspars, biotite, muscovite, and other mineral grains (Fig. 4J, 4K, 4M). enlarged chamber (Fig. 3C). Particles of charcoal or reworked coal and opaque oxide or sulfide minerals

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FIG. 3.—Photographs of individual sand-filled tetrapod burrows. A) Curved burrow (upper arrow) descending from overlying splay sandstone, and low-angle straight burrow (lower arrows) penetrating post-EPE microbreccia layer (Bed 2). B) Enlargement of lower end of low-angle straight burrow from A. C) Low-angle burrow in siltstone with rounded terminus. D) Gently curved burrow in siltstone. E) Oblique section of burrow descending from channel sandstone and penetrating uppermost portion of Vales Point coal seam. F) Cross-section of large burrow with bilobate base and internal lamination. G) Two burrows (arrowed) in siltstone. H) Large burrow (illustrated in F) penetrating uppermost portion of Vales Point coal seam (Bed 1). All burrows from Frazer Beach except E (from south side of Wybung Head). Bed numbers refer to those in Figure 2. All scale bars ¼ 100 mm.

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FIG. 5.—Details of Reniformichnus australis dimensions. A) Histogram showing the distribution of burrow widths. B) Histogram showing the distribution of burrow heights. C) Histogram showing the distribution of burrow aspect ratios in cross-section.

are present locally, especially along silty laminae (Fig. 4J). The matrix, Remarks.—Of the diverse array of burrows reported from Permo– representing 5–15% of the burrow fill by volume, is dominated by fine Triassic continental strata, the Sydney Basin examples are most similar to quartz and clays together with unidentifiable organic particles. The cement forms that Krummeck and Bordy (2018) attributed recently to Renifor- is predominantly siliceous with moderate overprinting by red-brown iron michnus. That ichnogenus was established with a single species oxide/hydroxide or organic stains (Fig. 4K, 4M). Silty laminae appear to (Reniformichnus katikatii) for large (100–150 mm maximum diameter) have essentially the same composition as the arenitic fill except for a higher tubular burrow fills with reniform cross-sections and various sharply proportion of clays, organic detritus and oxide/sulfide particles. Framework defined surficial scratches from the Lower Triassic Katberg Formation of quartz grains within the burrows are generally angular to sub-angular, the Karoo Basin, South Africa (Krummeck and Bordy 2018). The Sydney whereas chert particles are markedly more rounded (Fig. 4K). Most quartz Basin burrows (R. australis) share many architectural features with R. grains are monocrystalline, have even extinction in cross-polarized light, katikatii and clearly fall within the same ichnogenus. They are lack fluid inclusions, and several are euhedral or contain embayments, distinguished from the type species by their more variable shape in collectively indicating a local volcanic source (Fig. 4J, 4K). cross-section (almost circular to broadly elliptical, trapeziform, or Thin sections of two samples recovered from the splay sandstone reniform). They also have a greater range of maximum cross-sectional immediately overlying the burrow-bearing beds reveal almost identical widths (40–150 mm vs. 100–150 mm) and of heights (24–100 mm vs. 50– textural and compositional characters to the burrow fills (Fig. 4H, 4I). The 60 mm) and slightly higher cross-sectional aspect ratios (average 2.3 vs. only notable difference is the sparse occurrence in the splay sandstone of 2.16). More significantly, R. katikatii has a distinctive network of robust minute (1–1.5-mm-diameter) burrows or root casts that are elliptical in intersecting scratch features on the ventral surface and subparallel ridges cross-section and infilled with disorganized silty material (Fig. 4H, 4L). on the margins and dorsal surface. In contrast, R. australis bears subtle These apparent minute burrow structures lack linings and their length is excavation marks including crescentic ridges and grooves (Figs. 4G, 6C) indeterminable. and various roughly rhombic or lacriform lobate features on the dorsal The two burrow fills studied in thin section were also processed for surface (Figs. 4B, 6D, 6E) and only inclined scratches, irregular pitting, or palynological content. The organic residues extracted from these burrow scabrate textures on the lower surface and lateral walls (Fig. 4D, 4E). fills were devoid of pollen and spores. They contained only abraded Collectively, the excavator scratch marks are less extensive and more phytoclasts, fine charcoal, and amorphous organic matter of the same types subdued in R. australis burrow fills compared with R. katikatii. The represented elsewhere within the ‘dead zone’. textural differences might relate to either a different tracemaker or

FIG. 4.—Details of burrow morphology and composition of infill. A) Large burrow in cross-section showing bilobate base and weakly laminated sub-litharenite/lithic wacke infill. B) Underside of large burrow illustrated in A showing median central groove (¼ ridge in original burrow) and irregular scale-like mudflake patterning. C) Small burrow in cross-section showing weakly graded infill. D) Ventral view of small burrow showing mainly scabrate texture. E) Lateral view of small burrow showing sparse pockmarks. F) Dorsal side of small burrow cast showing weakly defined crescentic scratch marks. G) Enlargement of F showing crescentic scratch marks. H) Thin section of the sandstone bed 60 cm above the EPE horizon in cross-polarized light showing the dominance of angular quartz grains and small dark zone of silty infill (upper center) representing a possible invertebrate burrow or root cast. I) Thin section of the same bed illustrated in H, showing dominance of angular quartz and lesser proportion of rounded chert grains. J) Thin section of large burrow in cross-polarized light showing the dominance of angular quartz grains and accessory chert, mica, and organic grains. K) Thin section of large burrow in cross-polarized light showing euhedral and angular quartz together with accessory chert, and organic grains. L) Enlargement of the possible invertebrate burrow or root cast from H in plane-polarized light showing distinctly finer infilling compared to the surrounding sandstone. M) Thin section of small burrow in plane-polarized light showing angular to sub-angular quartz and chert grains together with rounded mudclasts set in an iron-stained matrix. Direction towards lower termination of burrow is upwards in B, and to the left in D–G. Samples: A, B ¼ NRM X9101a; J, K ¼ NRM X9101b (thin section taken from ¼ NRM X9101a); C–G ¼ NRM X9102a; M ¼ NRM X9102b (thin section taken from ¼ NRM X9102b); H, I, L ¼ X9103b (thin section of block from overlying sandstone bed X9103a). Scale bars ¼ 10 mm for A–G; 1 mm for H, J; 100 lm for I, K–M.

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FIG. 6.—Line drawings, surface details and schematic reconstruction of R. australis burrows. A) Schematic diagram showing descent of curved sand-filled burrows from a splay sandstone and variably terminating in siltstones, microbreccia or coal above and below the EPE horizon. B) Elliptical cross-section of relatively small burrow. C) Small burrow in dorsal view showing crescentic scratch marks (dashed lines) and line of cross-section in B (solid line). D, E) Enlargements of burrow cast surface texture outlined in C showing small, pointed, lobate infillings that may represent excavational features. F–H) Cross-sections of medium- to large-diameter burrow casts showing variable levels of compaction and development of a ventral groove. Samples: B–E ¼ NRM X9102a; F, G ¼ burrows in outcrop; H ¼ NRM X9101a. Scale bars ¼ 10 mm.

contrasting physical properties of the iron-rich muddy (Karoo Basin) and for sporadic reports of typical Triassic palynomorph species within organic-rich silty (Sydney Basin) host deposits. uppermost Permian beds (Foster 1979, 1982; Helby et al. 1987). Potential Burrows similar to Reniformichnus australis reported from Lower down-section reworking of palynomorphs in burrow fills places an Triassic strata of South Africa, Antarctica, and China commonly occur as additional caveat on the interpretation of apparent plant diversity trends strikingly pale sandy casts in deeply reddened or mottled muddy paleosols across the EPE based on uncritical tabulation of published taxon records (e.g., Bordy and Krummeck 2016). In contrast, the Australian examples (Nowak et al. 2019). are represented by buff sandy infillings within a recessive medium to pale gray, moderately organic-rich siltstone package that has been interpreted to DISCUSSION represent a weakly developed paleosol (redoxic hydrosol or dystric gleysol) in laminated floodplain or lacustrine sediments (Retallack 1999). Subtleties Comparison with Other Gondwanan Permian–Triassic Burrows in color or textural contrast, together with preservational and interpretative Large (. 20 mm diameter) burrows have long been recognized in biases have meant that some fossil burrow systems have been overlooked uppermost Permian and Lower Triassic continental strata of southern in early studies of Permian–Triassic fluvial systems (Voorhies 1975). The Africa and the Transantarctic Mountains (Kitching 1977; Stanistreet and weak contrast in color between the Frazer Beach burrows and their host Turner 1979; Smith 1987; Groenewald 1991, 1996; Babcock et al. 1998). rock makes the traces difficult to detect (Fig. 2) and this may account for In the past two decades the documented diversity of Permian and Triassic their omission from previous reports of this well-studied section (Retallack burrow forms from these and other regions has expanded greatly 1999, 2005; Williams et al. 2017; Mishra et al. 2019). (Groenewald et al. 2001; Retallack et al 2003; Hasiotis et al. 2004; Reniformichnus australis burrows have been identified only in a single Gastaldo and Rolerson 2008; Sidor et al. 2008; Liu and Li 2013; Bordy package of strata immediately overlying (and transecting) the EPE horizon and Krummeck 2016; Krummeck and Bordy 2018; Yang et al. 2018; Guo in the northern Sydney Basin. Given their subtle expression in outcrop, et al. 2019). Moreover, the identity of the excavators has been advanced by further studies are likely to uncover additional examples of tetrapod the recovery of skeletal remains within some burrow casts and by careful burrows in Lower Triassic strata of the Sydney Basin and this may comparison of burrow architecture and morphology with those produced complement the meager body fossil record for interpreting the diversity of by modern animals (Hasiotis et al. 1993, 2004; Groenewald et al. 2001; terrestrial vertebrates in the high paleolatitudes of eastern Australia in the Damiani et al. 2003; Smith and Botha 2005; Modesto and Botha-Brink wake of the EPE. 2010; Fernandez et al. 2013). Large burrows in these deposits range from Passive infilling of R. australis-type burrows that initiate in the ‘dead relatively simple tubes, a few tens of centimeters long and ~ 20 mm in zone’ but terminate in the uppermost Permian coals by sediment from diameter (Gastaldo and Rolerson 2008), to complex spiral or branched splay sands derived from up to 1 m above the EPE horizon might account excavations with enlarged terminal chambers reaching several meters long

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and over 0.5 m in diameter (Smith 1987; Groenewald 1991; Groenewald et G’ (giant) burrows from the Fremouw Formation that are distinctly larger al. 2001; Bordy et al. 2011). We observed no evidence of branching, (80–160 mm in greatest diameter) but otherwise similar in form and coiling, discrete wall linings, or strongly enlarged terminal chambers in the distribution to the Type L burrows and the largest of the Sydney Basin Frazer Beach burrows. burrows. Although the dimensions of these burrow types in the Fremouw Krummeck and Bordy (2018) tentatively assigned several burrows Formation have a bimodal distribution, their general similarities and described from Lower Triassic strata of Antarctica and South Africa to overlap in dimensions with the Sydney Basin examples suggest that the Reniformichnus, including Scoyenia isp. of Groenewald (1991, p. 21), Antarctic burrow categories might represent excavations by individuals in ‘type G burrows’ of Miller et al. (2001, p. 221), unassigned non- different age cohorts or separate sexes of a single tetrapod species. Based mammalian cynodont burrows of Damiani et al. (2003, p. 1748), and on their similar size, architecture, occurrence, and surface morphology, we ‘ichnogenus A burrows’ of Sidor et al. (2008, p. 278). Of these, and the consider that it is likely that the Antarctic ‘Type L’ burrows of Miller et al. diverse array of other Permo–Triassic burrow forms, we note that the (2001) and R. australis from the Sydney Basin were produced by Sydney Basin examples are most similar to those documented by Miller et conspecific or at least congeneric trace makers. al. (2001) from the Fremouw Formation (Lower Triassic) of Antarctica, Differences in dimensions caused by pre-fill compression of tunnels casts described by Sidor et al. (2008, figs. 2–4) from the lower Fremouw excavated in soft sediments or later diagenetic compaction can hinder Formation (Lower Triassic) and Lashly Formation (lower Middle Triassic) comparisons between burrow assemblages (Sidor et al. 2008). Indeed, of Antarctica, and forms illustrated by Bordy and Krummeck (2016, fig. 3) several of the burrows at Frazer Beach appear to be significantly from the Lower Triassic of the Karoo Basin, South Africa. compressed (Fig. 6F, 6G). We regard such examples in the Frazer Beach Tetrapod ichnogenus A of Sidor et al. (2008, fig. 2) is represented by a section to be primarily the result of pre- or syn-fill compaction of the single reference specimen but is notably similar to the Sydney Basin tunnels. The uppermost Permian coal seam in the Frazer Beach to

burrows in its roughly consistent width, two ventral parasagitally aligned Newcastle area has vitrinite reflectance (Ro max) values of around 0.7% lobes separated by a central ridge, ill-defined marginal scratch marks, and (Diessel 1975; Middleton 1989). This can equate to an original burial weak crescentic furrows and ridges on the dorsal surface of the cast. The depth of between 1.4 and 5 km depending on the local geothermal gradient illustrated example of this burrow is slightly broader (157 mm) than the (Suggate 1998) and later thermal overprinting during the breakup of largest of the Sydney Basin burrows (150 mm) but, at 85 mm high, is eastern Gondwana (Och et al. 2009). Since the framework grain within the range of heights expressed by the Australian population (Fig. 5). organization in the rock masses of both the burrow fill and the overlying Tetrapod ichnogenus B of Sidor et al. (2008, figs. 3, 4) is represented by splay deposit never progresses beyond tangential and long grain contacts burrow casts that are more similar in size to the Sydney Basin examples (i.e., lacking obvious sutured boundaries, fractured corners, or micro- (43–67 mm wide and 24–39 mm high), with consistent dimensions stylolitization), we interpret the burial compaction of the sandstones and throughout, and weakly defined arcuate ornamentation on the dorsal burrow fills to have been no more than a few percent, based on surface. They differ only in having more strongly developed lateral scratch comparisons with experimental compression of sands (Allen and marks and a very pronounced ventral ridge (groove on the cast), generating Chilingarian 1975). The notable flattening of some burrows (e.g., Fig. a more distinctly reniform cross-section. Scratch marks, particularly on 6F, 6G) probably relates to some plasticity in the host soil, especially in ventral and lateral surfaces, may become subdued or entirely worn away by portions close to the water table, before or during passive filling of the regular traffic through the burrow (Groenewald et al. 2001), so the burrow by flood-transported sands. prominence of these features should not be considered strongly diagnostic Burrows illustrated by Liu and Li (2013) from Guadalupian or of an ichnotaxon. Lopingian strata of China are larger (up to 230 mm wide) than R. Bordy and Krummeck (2016) identified multiple burrow types in Lower australis but have a similar reniform cross-section, shallow excavation Triassic strata of the Katberg and Burgersdorp formations of the Karoo angle and sandy infilling within a siltstone host rock. Upper Permian Basin. Burrows they identified as ~ 40 mm in average diameter are continental strata of the Sunjiagou Formation in South China host inclined especially similar to the Sydney Basin examples. The South African forms burrows around 20 mm in diameter but these lack the surface are simple cylinders with a slightly curved lower portion, circular to ornamentation or ventral groove of the Sydney Basin examples (Guo et slightly elliptical in cross-section, diameters of 35–60 mm (aspect ratios of al. 2019). Middle Triassic inclined burrows from both Argentina (up to 270 1.0–1.6), lengths of at least 0.76 m, lacking a terminal chamber or mm wide) and China (up to 300 mm wide), similarly, are larger than R. branching, and having a knobbly to scabrate surface texture with only ill- australis but share many of the other morphological features outlined defined crescentic undulations on the dorsal surface. Bordy and above including the median ventral ridge (Krapovickas et al. 2013; Yang et Krummeck (2016, fig. 2G–2I) also identified slightly smaller and larger al. 2018). burrows referred to type I and type II burrows, respectively. These traces Although no vertebrate burrows have been confirmed from post-EPE appear less well preserved and somewhat compressed compared to their ~ strata of the Prince Charles Mountains (Flagstone Bench Formation), East 40 mm diameter burrows, and the larger forms have a weakly developed Antarctica, sand-filled tubes previously interpreted as ‘root channels’ ventral groove (on casts), but are otherwise similar and may represent small (Webb and Fielding 1993) or ‘root traces’ (McLoughlin and Drinnan 1997) and large end-members, respectively, of the same burrow type. The Sydney occurring in floodplain red-beds rich in carbonate nodules and desiccation Basin examples also show the development of a central groove only in the cracks deserve re-investigation as potential burrow forms. largest examples (Fig. 6F–6H); small burrows are typically circular to In uppermost Permian and Lower Triassic strata of the Karoo Basin, elliptical (Fig. 6B), or have only a weakly developed ventral groove, but tetrapod burrows are commonly found in association with smaller (, 20 there is generally a continuum in burrow form and dimensions within the mm) but structurally similar, gently inclined, cylindrical, silt- or sand-filled local assemblage (Fig. 5A–5C). burrows typically attributed to Katbergia (Gastaldo and Rolerson 2008; Casts identified as ‘Type L’ (20 to . 80 mm in largest diameter) burrow Bordy et al. 2011). Larger and more complex coiled or branching burrows fills from the Lower Triassic Fremouw Formation, Antarctica (Miller et al. of Gyrolithes type are also present in the Karoo Basin (Smith 1987; 2001; Hasiotis et al. 2004) are similar to the Sydney Basin burrows in size, Groenewald et al. 2001). Although we did not detect Katbergia or straight to gently curved low-angle orientation, cylindrical form with Gyrolithes in the Sydney Basin, elements of the Reniformichnus- circular to elliptical cross-section, sporadic development of a ventral Katbergia-Gyrolithes association, either together or in isolation, appear groove, and in having chevron-shaped dorsal scratch marks and otherwise to be widely distributed across middle and high latitudes of Gondwana mostly irregular surface markings. Miller et al. (2001) also identified ‘Type around the Permian–Triassic transition (Smith 1987; Groenewald et al.

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2001; Sidor et al. 2008; Bordy et al. 2011; Bordy and Krummeck 2016). A dicynodont Diictodon (Smith 1987) and the Early Triassic cynodont similar and coeval trace fossil association also appears to be present in Trirachodon (Groenewald et al. 2001; Smith and Swart 2002) or China (Liu and Li 2013; Yang et al. 2018; Guo et al. 2019) reflecting Langbergia (Abdala et al. 2006). Skeletal and paleoecological evidence common environmental regimes and animal behaviors across a broad range also suggest a fossorial lifestyle for the Permian dicynodont Cistecephalus of latitudes in both hemispheres. (Cluver 1978), latest Permian to Early Triassic therocephalian Bauria Terrestrial animals occupying equivalent floodplain habitats and having (Groenewald et al. 2001), various Early Triassic cynodonts (Smith and a similar burrowing style clearly proliferated across southern Gondwana Botha 2005), and possibly Early Triassic theriodonts (Modesto and Botha- through the latest Permian to Middle Triassic. Some may have already been Brink 2010). Other potential vertebrate burrowers also include procolo- present in China by the Guadalupian or early Lopingian (Liu and Li 2013) phonid parareptiles (Groenewald 1991). However, all of these taxa have and others persisted as far afield as Poland until at least the Late Triassic body sizes, excavation architectures or stratigraphic ranges that are (Tałanda et al. 2011) and, in some regions, into the Jurassic (Bordy et al. inconsistent with the Sydney Basin burrows. Rather, small cynodonts, such 2017). as the Early Triassic Thrinaxodon, are more likely to have been the excavators of the Frazer Beach burrows based on the preservation of Potential Tracemakers skeletal remains in burrows architecturally similar to R. australis in beds of the Palingkloof Member, Karoo Basin (Damiani et al. 2003) that are The sandstone infillings of the burrows are lithified, they originate from known to be close to the level of the EPE (Gastaldo et al. 2015, 2018, a single stratigraphic interval, and petrographic analysis shows them to be 2019, 2020). Slightly expanded terminal chambers of Thrinaxodon compositionally equivalent to the overlying splay sandstone, so there is no burrows described by Damiani et al. (2003) are not evident in R. australis possibility that these burrows represent the activities of modern animals. but the size, roughly elliptical or bilobate cross-section, inclined surficial We cannot identify a unique tracemaker for R. australis because skeletal scratches, and ventral median ridge are features in common. The furrows remains are not preserved within the burrows. However, we exclude flanking the median ridge likely represent the limb positions of the invertebrates as potential tracemakers of R. australis. Insects and other vertebrate excavator in both burrow forms. terrestrial arthropods typically produce burrows of smaller size with We cannot exclude small-statured representatives of other reptilian or distinctive architectures and surface morphology (Bryson 1939) that differ amphibian groups as potential excavators or modifiers of R. australis from the Sydney Basin burrows. Freshwater crayfish can produce burrows burrows. However, the rare case of a temnospondyl co-preserved in a of similar size to R. australis but they commonly have a more vertical Karoo Basin Early Triassic burrow with Thrinaxodon appears to represent entrance tunnel, clearly defined wall linings, variable branching that opportunistic use of an existing (cynodont-constructed) burrow by the extends to the water table, and many retain distinctive bioglyphs of amphibian for avoidance of harsh environmental conditions (Fernandez et appendages (chelipeds, pereipods, pleopods) on the burrow walls (Hasiotis al. 2013). Various extant salamanders (similar in form and lifestyle to and Mitchell 1993; Hasiotis et al. 1993; Hembree and Hasiotis 2006; temnospondyls), especially the mole salamander (Ambystoma talpoideum), Martin et al. 2008; Bordy and Krummeck 2016). Burrows of marine and Congo eel (Amphiuma means) and Siberian salamander (Salamandrella brackish-water crayfish can be even more complex incorporating multiple keyserlingii) construct burrows in soil or leaf litter for hibernation, branches and chambers (Einarsson et al. 2016). Although some burrows aestivation, or pursuit of prey (Dunn 1923; Martin 2013). Extant similar to R. australis from the Lower Triassic of Antarctica have been salamander burrow architectures have not been well documented and it attributed to crayfish in earlier studies (Babcock et al. 1998), these have is not known whether they include a median ridge, although at least some subsequently been reinterpreted as tetrapod burrows (Hasiotis et al. 2004). have widths greater than heights, orientations from horizontal up to 358 to We also exclude aestivation and nesting behavior of lungfish as the the surface, and some excavations extend to the water table (Dunn 1923; origin of R. australis burrows. Although lungfish aestivation burrows are Dodd 1991; Martin 2013). known from the Permian of Gondwana (Francischini et al. 2018), they are typically simple vertical or subvertical lacriform structures , 0.5 m deep. Eastern Australian Late Permian–Early Triassic Vertebrates Lungfish nesting burrows commonly have more complex U- or T-shaped architectures with distinctive terminal chambers (Greenwood 1986; Vertebrate body fossils are scarce in the Changhsingian–Induan interval Hasiotis et al. 1993, 2007; Miller et al. 2001; Hembree 2010; McCahon of the Sydney-Gunnedah-Bowen basin complex. Dana (1849) and and Miller 2015; Bordy and Krummeck 2016). Woodward (1931) reported palaeoniscoid fish assigned to Urosthenes Permian–Early Triassic tetrapods tended to produce relatively simple from the Lopingian Illawarra Coal Measures of the Sydney Basin. burrows consisting of an unbranched (or sparsely branched) inclined or Woodward (1909), Welles and Estes (1969) and Warren (1997) reported helical shaft with or without a terminal chamber and commonly possessing skull, teeth and other body parts of several temnospondyl amphibians from a median ventral ridge (¼ groove on the burrow cast; Groenewald et al. coeval strata in this basin. Warren (1997) considered fragmentary skeletal 2001). The elevated ridge on the ventral side of some R. australis burrows remains recovered from the lower Newcastle Coal Measures (Wuchiapin- provides particular support for a tetrapod tracemaker. Median ventral gian–Changhsingian) of the northern Sydney Basin to be the only possible ridges have been found in several Lower Triassic burrows from Gondwana, Permian reptilian body fossils yet discovered in Australia. At least 13 some with the presumed tracemaker preserved at the lower terminus of the genera of bony fish and two genera of freshwater sharks (mostly juveniles) burrow (Groenewald 1991; Groenewald et al. 2001; Damiani et al. 2003), have been reported from the Changhsingian Rangal Coal Measures of the and some modern small fossorial mammals produce burrows with similar adjacent Bowen Basin (Duy Phuoc 1980; Campbell and Duy Phuoc 1983; architectures (Melchor et al. 2012). Leu 1989) but, thus far, only two species have been formally described. Diverse terrestrial vertebrates are known from Lower Triassic strata of One of these, Ebenaqua ritchiei, was a deep-bodied bony fish about 50– the Karoo Basin and many of these groups include known or potential 110 mm long (Campbell and Duy Phuoc 1983). The other, Surcaudalus fossorial taxa based on preservation of body fossils within the burrow, rostratus, was a xenacanth shark with prominent dorsal spines (Leu 1989). skeletal size, gait, and anatomical adaptations for digging (Bordy et al. Originally considered to be freshwater assemblages (Campbell and Duy 2011). Lystrosaurus, a common dicynodont therapsid of latest Permian and Phuoc 1983), the strata hosting these fish have been reinterpreted recently Early Triassic times, was fossorial but even juveniles of this taxon as estuarine deposits (Fielding 2015). Lungfish were not recorded. The produced burrows substantially larger (~ 300 mm diameter; Botha-Brink lower Changhsingian Burngrove Formation of the Bowen Basin hosts a 2017) than R. australis. Burrowing is known also for the Permian tuffaceous lacustrine bed bearing a diverse array of trails and tracks

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attributable to fish, possible tetrapods, and various invertebrates (Malone et reasons for burrowing by tetrapods. Even shallow burrows (of 0.5 m depth) al. 1969; Warren 1972; McLoughlin 1990; Shi et al. 2010), but no can produce temperature and humidity levels that are relatively mild and comprehensive trace-fossil analyses have been undertaken on the consistent on a diurnal and seasonal basis enabling survival of some assemblage. None of these Lopingian animals shows evidence of a vertebrates under extremely harsh (both torrid and frigid) surface fossorial lifestyle. conditions and facilitating expansion of their potential geographic A trackway of an animal with five digits on each fore- and hind-limb distribution (Voorhies 1975; Kinlaw 1999). was recovered from the roof shales (basal Narrabeen Group) of the Bulli Up to 50% of modern mammal species generate burrows or excavations coal seam at Bellambi Colliery to the south of Sydney in 1913, and a of some form (Voorhies 1975) and this life strategy may have had its second trackway was recovered from the same stratigraphic level at origins among early synapsids. Although tetrapod burrows are known from Oakdale Colliery in 1995. These trackways, assigned to the ichnotaxon strata extending back to at least the Guadalupian–Lopingian (Liu and Li Dicynodontipus bellambiensis (Retallack 1996; Warren 1997), are 2013), they are especially common and diverse around the Permian– positioned 25–50 cm above the top of the Bulli coal seam, which marks Triassic transition and throughout Lower to Middle Triassic successions the end-Permian extinction event in the southern Sydney Basin (Fielding et (Groenewald et al. 2001; Sidor et al. 2008; Bordy et al. 2011; Bordy and al. 2019; Mays et al. 2020). On the basis of palynostratigraphic data from Krummeck 2016; Yang et al. 2018). Many synapsids appear to have the nearby Coalcliff-27 bore, these beds are referable to the Play- adopted a fossorial lifestyle from the Lopingian to Middle Triassic to avoid fordiaspora crenulata Zone of latest Changhsingian to early Griesbachian the harsh conditions that developed in continental interiors at this time age (Mays et al. 2020) and are roughly contemporaneous with the R. associated with intensification of the Pangean megamonsoon (Parrish australis burrows at Frazer Beach, located within the same palynozone 1993; Damiani et al. 2003; Krapovickas et al. 2013). Given that Sun et al. (Vajda et al. 2020). Tracks and other trace fossils of this age from the (2012) inferred that low paleolatitudes experienced lethally hot conditions Karoo Basin are assigned to the FA III ichnoassemblage of Marchetti et al. around the EPE, burrowing likely had a strong selective advantage as the (2019) and include footprints of small dicynodonts, cynodonts, procolo- climate of the Pangean interior deteriorated towards hotter and more arid phonids, and non-archosauriform neodiapsids. The trackways from the conditions through the latest Permian and Early Triassic (Groenewald et al. southern Sydney Basin were produced by an animal, possibly the 2001; Smith and Botha 2005; Botha-Brink 2017). dicynodont Lystrosaurus, with an estimated body width of ~ 320 mm Several tetrapod burrows outlined above that are broadly similar to R. and length of 8406200 mm (Retallack 1996), which is clearly too large to australis are known from South Africa (45–608S paleolatitude), the have produced R. australis burrows. We did not detect any fossil footprints Transantarctic Mountains /Victoria Land (75–858S), and Shaanxi, China of vertebrates at Frazer Beach or neighboring sites owing to a lack of (, 208N), in Lopingian to Middle Triassic strata. These burrows occur in extensive bedding plane exposures. equivalent mud-dominated floodplain facies of ephemeral river systems Skeletal remains and coprolites of vertebrates are common in slightly experiencing seasonal or episodic flash flooding under inferred hot and younger (Olenekian) successions of the Sydney and adjacent basins semi-arid to arid paleoclimates (Groenewald et al. 2001; Miller et al. 2001; (Cosgriff 1969; Howie 1970; Warren 1980; Warren and Black 1985; Smith and Ward 2001; Hasiotis et al. 2004; Sidor et al. 2008; Bordy and Thulborn 1986; Damiani and Warren 1996; Damiani 1999; Northwood Krummeck 2016). Such burrows are consistently associated with 1997; Yates 1999, 2000; Warren et al. 2006; Rozefelds et al. 2011; Haig et sedimentary features indicative of seasonal drought and episodic flooding. al. 2015; Niedz´wiedzki et al. 2016). These high-paleolatitude faunas are These features include extensive desiccation cracks, prominent rhizoliths, dominated by temnospondyls, but a few proterosuchid archosauriforms deeply reddened or mottled paleosols with carbonate, iron oxide/ and therapsids have also been documented (Thulborn 1979, 1983; hydroxide, or other concretionary nodules, scarce plant remains, playa- Northwood 1997; Rozefelds et al. 2011; Niedz´wiedzki et al. 2016). lake sedimentary facies, and weakly developed paleosols (calcisols and Although the paleoecology of these animals is not well understood, none of gleysols) intercalated with laterally extensive splay sandsheets (Miller et al. the vertebrates yet known from Lopingian–Lower Triassic body fossils or 2001; Retallack et al. 2003; Gastaldo et al. 2005; Gastaldo and Rolerson trackways of the Sydney or Bowen basins represents a strong candidate for 2008; Smith and Botha-Brink 2014; Bordy and Krummeck 2016). The the constructor of R. australis burrows. A dedicated search for body fossils strata at Frazer Beach were also deposited in floodplain and lacustrine in the basal beds of the Narrabeen Group offers the best chance of finding settings (at least 340 m from the nearest laterally equivalent channel body the excavators of R. australis burrows. at Wybung Head) but lack signs of intense desiccation. However, we did identify robust desiccation cracks a few meters higher in the succession on Paleoecology and Paleoenvironment the south side of Wybung Head, and Diessel et al. (1967) illustrated large- scale mudcracks preserved in coeval strata above the topmost Permian coal Burrowing is employed for diverse purposes by many modern terrestrial seam in the southern part of the Sydney Basin. Moreover, negative mammals, and also by considerable numbers of amphibians, reptiles, and excursions in d13C values and low Chemical Index of Alteration values that birds. Protection from harsh environmental conditions, avoidance of we identified in strata above the EPE at Frazer Beach are consistent with a predators, creation of nesting sites, and storage of food resources are phase of warming and drying. We assume that the R. australis burrows at among the most important reasons for adopting a burrowing lifestyle Frazer Beach were constructed above the water table and that the host (Voorhies 1975; Kinlaw 1999; Groenewald et al. 2001). Burrows that are sediments were sufficiently cohesive to prevent complete collapse of the excavated primarily for shelter by mammals are commonly simple burrows. They remained open until passive filling by sandy material during shallowly inclined tubes with slightly expanded terminal chambers located a single succeeding flood event. consistently above the water table (Reichman and Smith 1987). Burrows High water tables of the peat-accumulating floodplains that developed in constructed for other purposes, especially by social mammals, commonly the Sydney Basin during the Permian may have been unfavorable for have more complex meandering, coiled or branching tunnels, in some fossorial lifestyles. The loss of forested peatlands at the EPE may have cases with specialized chambers (Mukherjee et al. 2017). opened the landscape, and fluctuating water tables following this event We assume that the relatively simple R. australis burrows were mainly may have provided opportunities for tetrapod burrowers. The paleoclimate protective in function, but we cannot determine whether they were of the Sydney Basin immediately after the EPE was seasonal but had not excavated for the purposes of aestivation, avoidance of diurnal environ- reached the degree of aridity experienced in mid-paleolatitude regions of mental extremes, predator evasion, or as simple nurseries for young. Gondwana. Indeed, both a more complete plant macrofossil record and Avoidance of extremes of temperature and drought are the dominant geochemical (Chemical Index of Alteration) evidence suggest persistence

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of at least seasonally wet conditions after the EPE in the high-paleolatitude trackways and body fossils are otherwise scarce in the uppermost Permian Sydney Basin (Retallack 2002; Fielding et al. 2019). Moreover, and lowermost Triassic floodplain successions of eastern Australia sedimentological and palynological evidence for extensive ponding in suggesting either lower faunal abundances and diversities or less favorable the landscape is expressed in Griesbachian strata throughout the Sydney- preservational conditions in the more humid, high-latitude, coastal regions Gunnedah-Bowen basin complex (Vajda et al. 2020; Wheeler et al. 2020), of Gondwana compared to the drier, mid-latitude, continental-interior whereas signatures for extremely hot and dry conditions in this region did landscapes of the Karoo Basin, South Africa. not develop until the Olenekian (Mays et al. 2020). Retallack (1999) Fossorial lifestyles appear to have conferred a strong selective advantage estimated paleoprecipitation rates of 1189 and 1210 6 174 mm/annum during the shift to harsher climates at the Permian–Triassic transition. based on the molecular ratios of bases/alumina of two samples from the Bt Moreover, burrowing may have also favored survivorship during later horizon of the Wybung clay paleosol above the EPE horizon at Wybung Mesozoic crises. The Sydney Basin burrow maker lived during an interval Head. These values are significantly higher than equivalent estimates of of very simplified ecosystems. The palynoflora of this interval consisting pre-EPE (346 6 141 mm) and immediate post-EPE (732 6 141 mm) mostly of charcoal, woody phytoclasts and fungi, implies a floodplain precipitation obtained for the burrow-rich Karoo Basin succession ecosystem dominated largely by saprotrophs. The scarcity of other trace (Retallack et al. 2003). Vajda et al. (2020) estimated that the entire 140 fossils and macro-plant remains suggests that an opportunistic lifestyle, cm thick ‘dead zone’ at Frazer Beach equates to approximately 10,000– potentially involving fungivory, detritivory, and aestivation within burrows, 20,000 years after the EPE. The entrance level of the R. australis burrows might have been advantageous for survival during this interval. Tetrapod occur approximately 50 cm above the EPE horizon suggesting their burrows are notably more abundant in the Karoo Basin (45–608S excavation occurred within the first few thousand years following the paleolatitude) in the Gondwanan interior compared to the Sydney Basin biotic crisis. (65–708S) and Victoria Land, Antarctica (75–858S) located along the The Sydney Basin’s higher paleolatitude and apparently delayed shift to Panthalassan margin. Selective pressure favoring a fossorial lifestyle may a regime of extreme drought and high temperatures may have reduced the have been lesser at high latitudes lending support to the hypothesis that advantages of a fossorial lifestyle, explaining the general paucity of polar and coastal regions may have provided important refugia for tetrapod burrows in this region. Nevertheless, a milder climate at the terrestrial vertebrates during the Permian-Triassic greenhouse crisis. Gondwanan continental margin might have endowed the region with habitats that acted as potential refugia from the extreme environmental ACKNOWLEDGMENTS conditions that were apparently experienced at low latitudes and in This research was funded by the Swedish Research Council (VR grant 2015- continental interiors (Sun et al. 2012; Bomfleur et al. 2018). Localized 4264 to V.V., and VR grants 2014-5234 and 2018-04527 to S.M.) and by a burrowing may have also provided micro-habitats conducive to the survival collaborative research grant from the National Science Foundation (EAR- of taxa opportunistically utilizing the dens of fossorial animals (Fernandez 1636625 to C.R.F. and T.D.F.). We thank two anonymous reviewers and the et al. 2013). Habitat engineering by fossorial animals may have contributed Chief Editor for their constructive comments on the manuscript. to survivorship of various opportunistic small tetrapods and invertebrates during the extreme conditions of the Early Triassic (Bordy et al. 2011; REFERENCES Botha-Brink 2017). 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