The base of the Lystrosaurus Assemblage Zone, Karoo Basin, predates the end-Permian marine extinction

Abstract

The current model for the end-Permian terrestrial ecosystem crisis holds that systematic loss exhibited by an abrupt turnover from the Daptocephalus to the Lystrosaurus Assemblage Zone (AZ; Karoo Basin, South Africa) is time equivalent with the marine Permian-Triassic boundary (PTB). The marine event began at 251.941 ± 0.037 Ma, with the PTB placed at 251.902 ± 0.024 Ma (2σ). Radio-isotopic dates over this interval in the Karoo Basin were limited to one high resolution ash-fall deposit in the upper Daptocephalus AZ (253.48 ± 0.15 (2σ) Ma) with no similar age constraints for the overlying biozone. Here, we present the first U-Pb CA-ID-TIMS zircon age (252.24 ± 0.11 (2σ) Ma) from a pristine ash-fall deposit in the Karoo Lystrosaurus AZ. This date confirms that the lower exposures of the Lystrosaurus AZ are of latest Permian age and that the purported turnover in the basin preceded the end-Permian marine event by over 300 ka, thus refuting the previously used stratigraphic marker for terrestrial end-Permian extinction.

Fig. 1. Generalized stratigraphy of the Permian–Triassic Beaufort Group, Karoo Basin, South Africa, with vertebrate biozones.

U-Pb age assignments separating the Tropidostoma, Cistephalus, and Daptocephalus AZs from Day et al.. and Rubidge et al.; red arrow is lower Changhsingian age in the Elandsberg Member; yellow arrow is date reported, herein; and published postulated position of the Permian–Triassic boundary age (as based on the marine record). Vertebrate-range extensions follow re-analysis of original data set used to interpret the terrestrial response to the end-Permian event^,.

Fig. 2. Nooitgedacht stratigraphic section.

a Orthogonal outcrop image using drone technology (see Supplementary Note 1) in which the red arrow marks the horizon identified by other workers as the terrestrial PTB and the yellow arrow indicates the location of the ash-fall bed from which our U-Pb CA ID-TIMS age assignment originates. b Image from Botha-Brink et al. identifying their vertebrate-defined PTB; compare with (a). Note that Botha et al. lowered their vertebrate-defined PTB ∼8 m in the section. c Measured stratigraphic column beginning at an exposed, resistant sandstone body in the Loskop koppie. Red arrow marks the vertebrate-defined PTB as above with biozone boundaries as reported in 2014 and 2020 for the same reported stratigraphic section; yellow arrow marks the ash-fall horizon; P marks the palynological assemblages; a normal magnetic polarity zone is marked, accordingly; E identifies an erosional contact marking a phase of landscape degradation and missing section; and plots of Hg ppb:TOC % for two intervals are provided, with vertical scale in centimeters. Hg and TOC values are the average of triplicate analyses of each horizon sampled. Palynological zone assignments, correlated with Australia^,, appear against recovered pollen-and-spore assemblages. See supplemental information for legend. Vertical scale in 5 m intervals. d Field image of thin, very light gray (N8) ash deposit sampled in the current study. Scale in cm.

Fig. 3. A Wetherill concordia diagram showing U-Pb ID-TIMS data for single zircon crystals from the ∼1 cm thick ash bed, Nooitgedacht section, Karoo Basin.

A Wetherill concordia diagram plots U-Pb (uranium-lead) data, and associated error ellipses for Pb/U ratios based on 2-sigma errors, with ²⁰⁶Pb-²³⁸U ratios on the Y axis and ²⁰⁷Pb-²³⁵U ratios along the x axis. Our Fig. 3 does not include individual points (centroids to the ellipses) as these would be obscured due to the degree of overlapping of our ellipses.The weighted mean age obtained from data for 13 individual grains is 252.24 ± 0.11 Ma (2 σ; MSWD = 0.58). This excludes the data for one older xenocryst, which was omitted from the mean age. Lower right inset shows plot of ²⁰⁶Pb/²³⁸U dates, with horizontal gray bar indicating two-sigma error range of mean. Overlapping dates obtained with EARTHTIME tracer (²⁰⁵Pb-²³³U-²³⁵U) shown in dark gray bars and those from ROM tracer (²⁰⁵Pb-²³⁵U) in lighter gray bars. Lower left inset is an image of zircon grains representative of the dated grains and the population, in general.

Fig. 4. Diagram synthesis of late Permian and early Triassic global chronostratigraphic time scale in Ma; magnetostratigraphy and polarity intervals^, (black = normal, white = reverse); duration of Siberian Trap magmatic lava, pyroclastic, and sill emplacement activity; Australian palynological assemblage zones^, and geochronometric age placed on vegetation collapse in the Sydney Basin; compared with the results of the present study.

Ages of the Dulhuntyispora parvithola palynological assemblage are provided for the upper Daptocephalus^, and lower Lystrosaurus AZ^{(this paper)} against magnetostratigraphic context based on interpreted magnetic stratigraphy at Old Lootsberg Pass and Bethel farm^,. Striatopodocarpites fusus, a taxon of the D. parvithola zone, is illustrative of taeniate pollen grains. Biostratigraphically important Daptocephalus AZ and Lystrosaurus AZ taxa range extensions, up and down, follow an assessment of the original data set used to construct the end-Permian extinction model. Glossopteris leaves are preserved as macrofossils in the upper Daptocephalus^– and lower Lystrosaurus AZs. Global Stages: Wuch. = Wuchiapingian, Ind. = Induan; European Triassic stages: G = Griesbachian, D. = Dienerian.