Dwelling in the Dead Zone—vertebrate Burrows Immediately Succeeding the End-Permian Extinction Event in Australia

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 (∼ 65–75°S) 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.

Sedimentology of the continental end‐Permian extinction event in the Sydney Basin, eastern Australia

“…, 2020) have been referred to Reniformichnus australis by McLoughlin et al. (in press) and attributed to the activity of small, fossorial cynodonts.…”

Age and Paleoenvironmental Significance of the Frazer Beach Member—A New Lithostratigraphic Unit Overlying the End-Permian Extinction Horizon in the Sydney Basin, Australia

“…We find no evidence for marked aridification during deposition of the FBM but periodic desiccation of lake floors is evidenced by the development of large-scale mud cracks in lacustrine facies at Bulli Colliery (Diessel et al, 1967, figure 4) and sporadic tetrapod trackways and burrows in equivalent beds elsewhere in the basin (Harper, 1915;Retallack, 1996;McLoughlin et al, 2020). Isopach maps of the FBM indicate thicknesses of 2-5 m through central parts of the southern Sydney Basin (Figures 8A,B) indicating sediment accumulation in quiet waters over a broad region.…”

“…These burrows, assigned to Reniformichnus australis, descend from an overbank sandstone into the silty FBM and, in some cases, penetrate the underlying Vales Point coal seam. There is a continuum (normal distribution) of burrow sizes with maximum widths of 40-150 mm and heights of 11-100 mm (McLoughlin et al, 2020). They typically have a bilobate base, and bear weak bioglyphs on their surface ( Figure 15C).…”

“…The burrows are similar to forms interpreted as small cynodont burrows described by Sidor et al (2008) from the Lower Triassic of Antarctica, and by Damiani et al (2003) and Krummeck and Bordy (2018) from coeval strata in the Karoo Basin, South Africa. The Sydney Basin burrows can not be assigned definitively to any tracemaker in the absence of entombed skeletal remains but are inferred to have been produced by a Thrinaxodon-like cynodont-the range of sizes possibly being the result of production by a population of different sexes and age cohorts, together with some post-excavation compression of the burrow forms (McLoughlin et al, 2020). There is no strong evidence in terms of size distribution, shape or excavation traces to segregate these burrows into more than one ichnotaxon.…”

“…Recently, our team and others have noted that this shale/mudstone/siltstone unit represents a distinctive lacustrine facies association (Facies Association G of overprinted by paleosol development (Retallack, 2005), and is characterized by excursions in the values of carbon stable isotopes and chemical index of alteration (CIA) (Williams et al, 2017;Vajda et al, 2020). The unit hosts distinctive vertebrate ichnofossil (Retallack 1996;McLoughlin et al, 2020), and palynofloral assemblages initiating with a post-extinction "dead zone" primarily incorporating coalified wood and charcoal fragments. Higher in this interval, and in overlying units, abundant fungal, algal and sparse fern palynomorphs characterize post-extinction communities of opportunistic plants and the initial stages of a recovery flora Mays et al, 2020;Vajda et al, 2020).…”