Biostratigraphic correlation in the Karoo : The case of the Middle Permian parareptile Eunotosaurus

The richness of fossil tetrapods from the Beaufort Group of South Africa has enabled biostratigraphic subdivision of this Permo-Triassic succession, with global applicability. Despite being the thickest of the seven biozones recognised, attempts at further subdivision of the Middle Permian Tapinocephalus Assemblage Zone (Abrahamskraal Formation) have not been successful, largely because the exact stratigraphic ranges of fossil taxa are unknown. This gap in knowledge has limited stratigraphic correlation of the Abrahamskraal Formation and hindered understanding of Middle Permian Karoo basin development. Currently, the lowermost Beaufort Group is split between an eastern and a western stratigraphic scheme and, because of poor outcrop and the relative paucity of fossils in the east, stratigraphic correlation between the two areas has been uncertain. Recent fossil discoveries of the parareptile Eunotosaurus africanus in the Eastern Cape and Free State provinces have extended its known geographic range in the east. An additional specimen from the lower Middleton Formation in the Eastern Cape has, for the first time, enabled the biostratigraphic correlation of this unit with the Poortjie Member of the Teekloof Formation in the west. These finds confirm the diachroneity of the boundary between the marine Ecca Group and the terrestrial Beaufort Group.


Introduction
The taxonomic affinity of the enigmatic Permian parareptile Eunotosaurus africanus, easily recognised in the field by its distinctive broad ribs (Figure 1), has long intrigued palaeontologists. The genus has been placed in the Parareptilia as the sole member of a sister taxon to the Millerettidae 1,2 and, most recently, as a sister taxon to modern turtles. 3 All specimens of Eunotosaurus come from the upper Tapinocephalus and Pristerognathus Assemblage Zones of the Permian Beaufort Group of South Africa 4,5 and most specimens have been recovered from the western part of the Main Karoo Basin (Table 1, Figure 2) where the genus has already been used for biostratigraphic correlation. 6

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Source: Council for Geoscience, Pretoria

Materials and methods
The Beaufort Group fossil tetrapod geographic information system (GIS) database 8 was used to project the localities of all specimens of Eunotosaurus onto Google Earth satellite imagery (both use the WGS84 coordinate system) whence their stratigraphic position was deduced from geological maps.
The quality of locality information for each specimen varies, with many earlier discovered specimens being assignable only to the area of a farm. The range of strata that outcrops in the area of a locality, or across the farm as a whole, therefore has a direct influence on the accuracy with which each specimen could be placed stratigraphically (Table 1). Extensive fieldwork allowed refinement of the stratigraphic provenance of all specimens of Eunotosaurus whose stratigraphic position, as calculated from the geological maps, could potentially form one of the extremes of the stratigraphic range of the genus.
For the purposes of this study, because of the heterogeneous nature of the stratigraphy, the basin was divided into four regions: the northwestern, southwestern, central and southeastern Karoo (Figure 2). Stratigraphic sections were either measured by the authors or taken from the literature 9-12 and drawn up for each region, after which each section was split into 50-m intervals. Such intervals represented the greatest provenance accuracy which could be determined for most of the fossils. The stratigraphic position of each Eunotosaurus specimen was then plotted on the stratigraphic section for its respective region, spread across the number of 50-m intervals each could possibly occupy. This approach compensated for the level of uncertainty accompanying all fossils, either because of the lack of precise locality data or because of the complexity of the geology or local topography. The range ultimately calculated is the minimum range for the genus based on the existing data with a maximum error of 50 m.

Results
Compilation of all the locality data relating to Eunotosaurus indicates that the stratigraphically lowest specimens are SAM-PK-K07611 from   In the Free State, the specimens of Eunotosaurus are associated with scylacosaurid therocephalians and, as no dinocephalians have been discovered, these specimens are considered to fall within the Pristerognathus Assemblage Zone 9 ; however, the lithostratigraphic context is uncertain here. 9,11 The scarcity of fossil material in the eastern part of the Basin is problematic. The difference in stratigraphic regime, confounded by the absence of the Eodicynodon Assemblage Zone and an unknown proportion of the Tapinocephalus Assemblage Zone near Grahamstown, 7 makes subdivision of the eastern formations uncertain. Although BP/1/6218 may prove to be the lowest occurrence of Eunotosaurus, until collecting efforts create a larger sample size, the stratigraphic ranges of specimens from the eastern Karoo Basin have limited use unless compared to the west.

Age
Traditionally, the age of the Tapinocephalus and Pristerognathus assemblage zones has relied upon comparisons to the Russian Permian, and more specifically to the Ischeevo fauna. 5,14 This comparison places them in the Capitanian stage, dated at 265.8-260.4 million years ago. 15 From existing material, the stratigraphic range of Eunotosaurus corresponds to the upper third of the Tapinocephalus Assemblage Zone and the entirety of the Pristerognathus Assemblage Zone. The upper end of the Eunotosaurus range is therefore dated to about 260 million years ago, at the Capitanian-Wuchiapingian boundary. 14 The lower extent of its range is more difficult to determine, but given the data available from biostratigraphic correlation with Russia, the first appearance of the genus would probably fall within the middle Capitanian.

Conclusion
Our study demonstrates that Eunotosaurus has a restricted stratigraphic range of about 450 m within the fluvial lower Beaufort Group of the Main Karoo Basin and can therefore be useful for biostratigraphic correlation. The presence of this genus in only the upper part of the Koonap Formation and lowermost Middleton Formation supports the lithostratigraphic correlation of the Poortjie Member in the western Karoo Basin to the base of the Middleton Formation in the East. 16 As the distribution of Eunotosaurus corresponds to a fixed time period, the recognition that the genus occurs in the upper but not the lower Tapinocephalus Assemblage Zone in the south has further utility; the presence of Eunotosaurus with a Tapinocephalus Assemblage Zone fauna on the Ecca-Beaufort boundary in the Carnarvon district, Northern Cape, 6 and its presence with a Pristerognathus Assemblage Zone fauna immediately above the Ecca-Beaufort boundary at Philippolis, southern Free State, 9 confirm the younging of the Karoo palaeoshoreline towards the north.