ELECTRONIC SUPPLEMENTARY MATERIAL
When and how did the terrestrial mid-Permian mass extinction
occur? Evidence from the tetrapod record of the Karoo Basin, South
Africa
Michael O. Day1*, Jahandar Ramezani2, Samuel A. Bowring2, Peter M. Sadler3, Douglas H.
Erwin4, Fernando Abdala1 & Bruce S. Rubidge1
1
Evolutionary Studies Institute, School of Geosciences, University of the Witwatersrand, Private
Bag 3, Johannesburg 2050, South Africa
2
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139, USA
3
4
Department of Earth Sciences, University of California, Riverside, California 92521, USA
Department of Paleobiology, National Museum of Natural History, Washington, D.C. 200137012, USA
Contents:
Supplementary Methods
Supplementary Discussion
Supplementary References
Supplementary Datasets
Supplementary Figures
1. SUPPLEMENTARY METHODS
(a) Calculating biostratigraphic ranges for individual taxa
Initially, 1:250 000 geological maps for the relevant regions were consulted (map sheet:
Republic of South Africa 3122, Victoria West, 1:250 000 Geological Series 1992; map sheet:
Republic of South Africa 3120, Williston, 1:250 000 Geological Series 1992; map sheet:
Republic of South Africa 3220, Sutherland, 1:250 000 Geological Series 1983; map sheet:
Republic of South Africa 3220, Sutherland, 1:250 000 Metallogenic Series 1997; map sheet:
Republic of South Africa 3222, Beaufort West, 1:250 000 Geological Series 1979; map sheet:
Republic of South Africa 3224, Graaff-Reinet, 1:250 000 Geological Series 1994; map sheet:
Republic of South Africa 3326, Grahamstown, 1:250 000 Geological Series 1996). The outcrop
of geological units shown in Figure 2 is primarily taken from these published geological maps,
which vary in quality. West of 24°E only the members of the Teekloof Formation have been
mapped, while east of this meridian not even the contact of the Middleton and Balfour
formations is mapped.
A review of the existing sub-formational stratigraphic paradigms for the Abrahamskraal
Formation was conducted and the extensive fieldwork undertaken in this study allowed the
outcrop area of particular horizons within the Abrahamskraal Formation and Poortjie Member to
be identified [1]. Fossil localities were imported into Google Earth from the ArcGIS database
constructed by Nicolas [2] and it was therefore possible to determine, to varying degrees of
accuracy, the stratigraphic position of individual specimens. The division of the Karoo Basin into
sectors was necessary as the lithostratigraphy of the Lower Beaufort Group has long been known
to vary laterally in thickness and in architecture [3–5]. The stratigraphic sections used to
determine the number of sectors are presented in ref. [1].
(b) Fossil identifications
Numerous tetrapod genera have been described from the Tapinocephalus and Pristerognathus
AZs and the validity of these had to be addressed to ensure the most reliable analyses of
diversity. Of the most common taxa, only the dicynodonts, anteosaurid dinocephalians and
lycosuchid therocephalians have received relatively thorough taxonomic attention; however,
current projects are reviewing the taxonomy of tapinocephalid dinocephalians (S Guven),
titanosuchid dinocephalians (S Jirah), gorgonopsians (C Kammerer), and temnospondyls (C
Marsicano).
For this study, valid therocephalian genera and the most recent specimen identifications were
taken from Abdala et al. [6], Abdala et al. [7], van den Heever [8] and the personal observations
of FA. The identification of dicynodont specimens relied primarily on work by Angielczyk [9],
Angielczyk and Rubidge [10–12], Angielczyk et al. [13,14] and Kammerer et al. [15] and
observations made by K Angielczyk and J Fröbisch of ESI collections. Dinocephalian
identifications were mostly unchanged from collection catalogues with the exception of recent
collections, though relying on the review of Kammerer for anteosaurids [16]. The identification
of pareiasaur material was taken from the collection catalogues except for Embrithosaurus, for
which a revised description was provided by Lee [17]: only the referred specimens listed by Lee
and one recent specimen are accepted as Embrithosaurus. Eunotosaurus and Broomia
identifications were taken from collection catalogues and the literature [18]. Basal synapsid
identifications were taken from Botha-Brink and Modesto [19], Modesto et al. [20,21] and Reisz
and Modesto [22]. Some museum specimens and those collected during recent fieldwork by the
ESI in Johannesburg were identified by MOD, BSR or FA with reference to recent literature.
The specimens included in this study are listed in Supplementary Table 1.
The taxonomy of gorgonopsians has been reviewed over the last few decades [23–25] but no
consensus was reached on the genera from the Tapinocephalus AZ. Kammerer suggested that
most previously described gorgonopsian genera from the Tapinocephalus AZ are junior
synonyms of Eriphostoma [26] and this has been recently formalised, suggesting the presence of
only this one genus within the Tapinocephalus AZ and two (along with Gorgonops) in the
Pristerognathus AZ [27]. In the Tropidostoma AZ, there are reports of several gorgonopsian
taxa: Cyonosaurus, Lycaenops, Aelurognathus, Aelurosaurus, Aloposaurus, Gorgonops and
Scymnognathus. However, as in the older zones few of the specimens attributed to this AZ are
mentioned in the review studies. Only one specimen from this zone, of Aelurognathus, is widely
agreed to be correctly identified. Because most genera are represented by very few specimens
and the current understanding of gorgonopsian systematics has stalled at the level of the alpha
taxonomy, no gorgonopsians were included in the CONOP analysis. Only Gorgonops,
Aelurognathus and Eriphostoma are considered sufficiently secure to be included in Figure 1.
The temnospondyls of the Tapinocephalus and Pristerognathus AZs were also excluded from the
analyses as their taxonomy is uncertain; two valid genera from the Tapinocephalus AZ are
considered valid, Rhinesuchus and Rhinosuchoides [28], but this is based on the type specimens
only. No re-identifications were made and much of the material has been arbitrarily assigned to
Rhinesuchus.
(c) Uncertainty surrounding ages of biozone boundaries due to variable
biostratigraphic resolution
The uncertainty in the age assignment of biozones is affected by error associated with
uncertainty surrounding the exact lithostratigraphic position of biostratigraphic boundaries in the
southeast of the basin, from where the dated horizons of Rubidge et al. [29] are situated. Low
biostratigraphic resolution below the Cistecephalus AZ in this area introduces uncertainties in
the order of 300–500 meters with respect to the Tapinocephalus–Pristerognathus and
Pristerognathus–Tropidostoma assemblage zone boundaries. Time equivalents of this error were
calculated by estimating generalised time representation within the lower Beaufort Group in the
southeast of the basin using the oldest three dated horizons of Rubidge et al. [29], for which the
intervening stratigraphic distance is known (Fig. 4). This suggests that between horizon Bruce-1
(upper Koonap; 261.24 Ma) and horizon K220307-2 (uppermost Koonap; 260.41 Ma), 265 m of
rock represents deposition over approximately 0.83 m.y. Between K220307-2 and horizon
‘Tortoise’ (Middleton; 259.26 Ma), 420 m of rock was deposited over 1.15 Myr. This provides
an approximate time representation of 3132 years/metre in the upper Koonap Formation and
2738 y/m in the lower Middleton Formation. A 100 m uncertainty in the lithostratigraphic
position of a biozone boundary could therefore reflect a corresponding temporal uncertainty of
up to 0.274 Myr. Stratigraphic uncertainty calculations for individual boundaries are addressed in
the Suplementary Methods.
Only two known biozone boundaries are situated at or near a dated horizon: that of the
Tapinocephalus–Pristerognathus AZ (this paper) and the Cistecephalus–Dicynodon AZ [29].
Error due to the lithostratigraphic contraints on these biozone boundaries is low (<50 m), which
corresponds to an error of between 0.14 to 0.16 m.y. (Fig. 4). Previous to this study the age of
the Tapinocephalus–Pristerognathus AZ boundary was also poorly constrained, with an
stratigraphic error of perhaps 300 m (<0.94 m.y. based on upper Koonap Formation depostional
rate). An age estimate of ~256.75 Ma for the Cistecephalus–Tropidostoma AZ boundary can be
extrapolated from an age of 255.24 ± 0.16 Ma for the top of the Cistecephalus AZ and that of
256.25 Ma for the early-middle part of the AZ [29], with an error of around ~100 m (~0.27
m.y.). Uncertainty surrounding the age of the Pristerognathus–Tropidostoma AZ boundary is
highest, as no conclusive indicator taxa for the Tropidostoma AZ have been found in the southeast of the basin (see Supplementary Information). Assuming <3.5 Ma for the Pristerognathus
and Tropidostoma AZs combined using the ages posited above, an estimate of ~258.5 Ma can be
tentatively suggested for the age of the Pristerognathus-Tropidostoma AZ boundary; however,
until dates are obtained from better biostratigarphically constrained horizons in the southwest of
the basin this comes with high lithostratigraphic error of up to 500 m (~1.37 m.y.) (Fig. 4).
(d) CONOP
Not all genera present in each sector were included in the CONOP analysis. Genera for which
minimum stratigraphic ranges could not be calculated due to uncertainty in their provenance
were excluded to avoid the obfuscation of genuine diversity and range patterns. The CONOP
runs used a wider list of genera with known minimum ranges from the Eodicynodon,
Tapinocephalus, Pristerognathus and Tropidostoma AZs to minimize edge-effects within the
Tapinocephalus–Pristerognathus AZ transition (Supplementary Table 2). Specimen occurrences
within the Tropidostoma AZ are taken from published data [30, 31]. The exclusion of genera
from some sectors leads to a lower generic richness in the fully-resolved composite sequence
produced by CONOP, which in turn means our estimates of extinction rates and of severity are
conservative.
The best-fit composite sequences show an increase in generic richness in the upper Moordenaars
Member that reaches a peak in the Karelskraal Member (Fig. 3C). As any diversity pattern
reconstructed from fossil remains is susceptible to collecting bias, this was preliminarily assessed
by considering the number of specimens recovered from each stratigraphic bin. Because the
number of specimens retrieved from each bin increases up to the top of the Abrahamskraal
Formation during which the diversity crash begins, we do not consider our results to be a
consequence of sampling intensity. Similarly, although the sampling intensity for the lower
Poortjie member is lower than that of the upper Abrahamskraal Formation, this is bracketted by
the much better sampled mid-Poortjie Member.
2. SUPPLEMENTARY DISCUSSION
(a) Age of the Pristerognathus and Tropidostoma AZs
Tuff sample K1202-B1 reported here was recovered from the farm Puntkraal in the Sutherland
district at coordinates S 32° 22.519, E 21° 05.857. In situ, this tuff appeared as a white layer
within blue and maroon overbank fines in a horizon 3.5 m above the basal sandstone bed of the
Poortjie Member (7 m above the base of the member itself). This is remarkably close to the
traditional lithostratigraphic placement of the Tapinocephalus–Pristerognathus AZ contact and
fossil collecting in the vicinity supports this.
Previously published geochronology for the lower Beaufort Group in the Eastern Cape Province
[29] was based on collecting efforts conducted in the Jansenville district. In this part of the basin
there is a paucity of fossils but the presence of Endothiodon in the lower Middleton Formation,
close to a horizon dated at 259.262 ± 0.092 Ma, suggested that these strata belonged to the
Tropidostoma AZ, from where this genus is most commonly known [31]. However, our recent
collecting has demonstrated that Endothiodon is relatively abundant in the upper Poortjie
Member (below the range of Tropidostoma and thus within the Pristerognathus AZ) and occurs
rarely even in the middle Poortjie Member. Previous work by Day [32] indicated that
Endothiodon occurred as low as the uppermost Abrahamskraal Formation, but during preparation
of this paper it was found that these specimens had dubious locality data. The presence of
Endothiodon is therefore not a reliable indicator for the presence of the Tropidostoma AZ, thus
the age of ~259.26 Ma could correspond with the upper Pristerognathus AZ.
The biostratigraphically well constrained age of 260.259 ± 0.081 Ma for the position of the
Tapinocephalus–Pristerognathus AZ boundary does not cast doubt on the reliability of the
existing ages but rather reflects the comparatively lower biostratigraphic resolution of the lower
Beaufort Group in the south-east of the Karoo Basin. The acknowledgement of stratigraphic
uncertainty surrounding biozone boundaries, particularly in the south-east of the basin, accounts
for the difference between the biozone ages presented here and those suggested by Rubidge et al.
[29].
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SUPPLEMENTARY DATASETS
Dataset S1. Stratigraphic position of all fossil specimens used in this study.
Dataset S2. Ranges of each genus in each sector of the Main Karoo Basin for CONOP.
Dataset S3. U-Pb isotopic data for analyzed zircons. (a) Thermally annealed and pre-treated
single zircon. Data used in date calculation are in bold. (b) Total common-Pb in analyses. (c)
Measured ratio corrected for spike and fractionation only. (d) Radiogenic Pb. (e) Corrected for
fractionation, spike and blank. Also corrected for initial Th/U disequilibrium using radiogenic
208Pb and Th/U[magma] = 2.8. Mass fractionation correction of 0.25%/amu ± 0.04%/amu
(atomic mass unit) was applied to single-collector Daly analyses. All common Pb assumed to be
laboratory blank. Total procedural blank less than 0.1 pg for U. Blank isotopic composition:
206Pb/204Pb = 18.42 ± 0.35, 207Pb/204Pb =15.36 ± 0.23, 208Pb/204Pb = 37.46 ± 0.74. Corr.
coef. = correlation coefficient. Ages calculated using the decay constants λ238 = 1.55125E-10 and
λ235 = 9.8485E-10 (Jaffey et al. [33]).
Dataset S3
SUPPLEMENTARY FIGURES
Figure S1. Date distribution plot of analyzed zircons from tuff sample K1202-B1. Bar heights
are proportional to 2σ analytical uncertainty of individual analyses. Horizontal line signify
calculated sample date and the width of the shaded band represents internal uncertainty in
weighted mean at 95% confidence level. Arrows point to additional analyses (outliers) plotting
outside the diagram. See Extended Data Table 1 for complete analytical data and text for details
of date uncertainties. MSWD is mean square of weighted deviates.
Figure S1