Bruce S. Rubidge

27th Du Toit Memorial LectureRe-uniting lost continents – Fossil reptiles from the ancient Karoo and their wanderlust

Fossil discoveries from South Africa have greatly expanded knowledge of the development of life on Earth. In particular, the enormous palaeontological wealth of the Karoo, covering a period of almost 100 million years from the Permian to the Jurassic, has enhanced understanding of the evolution of important tetrapod lineages, including mammals and dinosaurs. These fossils provide the best record of continental Permian to Jurassic faunal biodiversity, and have been crucial to studies of the global Permo-Triassic mass extinction in the continental realm, as well as giving insight into other extinction events. Recent collaborative interdisciplinary studies of stratigraphic and geographic distribution patterns of Karoo fossils have enhanced biostratigraphic resolution and global correlation of vertebrate faunas from the Permian to the Jurassic. This in turn has led to a better understanding of the biodiversity across Pangaea, and the places of origin and initial diversity of early tetrapod evolutionary lineages. Many of these originated in the southern African portion of the Gondwanan super-continent. The combination of palaeontological and sedimentological studies has led to new basin development models and solved problems which each discipline in isolation could not have achieved.

A lamprey from the Devonian period of South Africa

Lampreys are the most scientifically accessible of the remaining jawless vertebrates, but their evolutionary history is obscure. In contrast to the rich fossil record of armoured jawless fishes, all of which date from the Devonian period and earlier, only two Palaeozoic lampreys have been recorded, both from the Carboniferous period. In addition to these, the recent report of an exquisitely preserved Lower Cretaceous example demonstrates that anatomically modern lampreys were present by the late Mesozoic era. Here we report a marine/estuarine fossil lamprey from the Famennian (Late Devonian) of South Africa, the identity of which is established easily because many of the key specializations of modern forms are already in place. These specializations include the first evidence of a large oral disc, the first direct evidence of circumoral teeth and a well preserved branchial basket. This small agnathan, Priscomyzon riniensis gen. et sp. nov., is not only more conventionally lamprey-like than other Palaeozoic examples, but is also some 35 million years older. This finding is evidence that agnathans close to modern lampreys had evolved before the end of the Devonian period. In this light, lampreys as a whole appear all the more remarkable: ancient specialists that have persisted as such and survived a subsequent 360 million years.

High-precision temporal calibration of Late Permian vertebrate biostratigraphy: U-Pb zircon constraints from the Karoo Supergroup, South Africa

Therapsid and other tetrapod fossils from the South African Karoo Supergroup provide the most detailed and best studied terrestrial vertebrate record of the Middle and Late Permian. The resulting biostratigraphic scheme has global applicability. Establishing a temporal framework for these faunas has proven difficult: magnetostratigraphy has been hampered by a Jurassic overprint, and intercorrelation with Permian marine sequences has been equivocal. Here we report U-Pb zircon isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates for five volcanic ashes interbedded with fossils from the Pristerognathus , Tropidostoma , and Cistecephalus vertebrate biozones of the Beaufort Group. This temporal framework allows correlation to marine zonations and improves understanding of rates of faunal evolution and patterns of basin evolution. Our results identify no correlative vertebrate extinctions in the Karoo Supergroup to the marine end-Guadalupian mass extinction and raise the question of whether there is any record of a terrestrial extinction related to the Emeishan large igneous province.

THE AMPHIBAMID MICROPHOLIS FROM THE LYSTROSAURUS ASSEMBLAGE ZONE OF SOUTH AFRICA

Abstract The small temnospondyl Micropholis stowi from the Lower Triassic Lystrosaurus Assemblage Zone (Karoo Basin, South Africa) is redescribed on the basis of a larger sample of specimens, revealing a range of previously unknown anatomical features. Micropholis is recognized as a member of the Amphibamidae, representing both the last occurrence and the only Gondwanan member of this clade. (1) The largest specimens have an elongated narrow snout, larger otic notches, prominent quadrate processes, and wide cheeks giving the skull a marked lateral curvature. (2) Palpebral ossifications include a fabric of numerous polygonal ossicles contouring the size and position of the eyeballs. (3) The palate houses large fangs with inwardly curving crowns, an elaborate basicranial region resembling closely that of Amphibamus and Doleserpeton, and an extremely reduced rod-like palatine and ectopterygoid approaching the derived condition seen in other amphibamids. (4) In the postcranial skeleton, the greatly extended transverse processes, the morphology of the interclavicle and scapulocoracoid, and the structure of the pelvis and tail are characteristic features of Micropholis some of which are quite similar to the condition in Eoscopus and Platyrhinops. It is evident that two morphs of Micropholis, which differ in skull width and palatal dentition, are present in the Karoo Basin. Phylogenetic analysis suggests Micropholis to be nested within the amphibamid dissorophoids as the basal-most taxon. Within the Amphibamidae, Doleserpeton and Amphibamus are sister groups, nested with successive sister taxa Platyrhinops and Eoscopus. In particular, the miniaturized genera Doleserpeton and Amphibamus share a range of derived character-states with Micropholis, but the latter lacks the pedicellate dentition and has a more plesiomorphic posterior skull table and parasphenoid, suggesting that some of the shared derived states may have been acquired independently during the long isolated evolution of the Micropholis lineage in the Southern Hemisphere. In the present analysis, Micromelerpeton is the most basal offshoot of the dissorophoids, followed by a grade towards amphibamids formed by the following successive sister groups (from the base crownwards): (1) the trematopids, (2) Ecolsonia, and (3) the Dissorophidae, the latter being the sister taxon of all analyzed amphibamids.

Breakthroughs in the biodiversity, biogeography, biostratigraphy, and basin analysis of the Beaufort group

Over the past decade researchers working on the rocks of the Beaufort Group in the main Karoo Basin of South Africa have vastly increased our understanding of this important Permo-Triassic sequence. Many new fossil forms have been discovered, allowing for breakthroughs into the biodiversity, biogeography and biostratigraphy of the group. Taxonomic and phylogenetic advances are many and varied, and cover most of the vertebrate taxa, but with emphasis placed on the temnospondyl amphibians, archosauriforms and non-mammalian synapsids, in particular the anomodontia. Biostratigraphic breakthroughs have centered on the Middle Permian Eodicynodon and Tapinocephalus assemblage zones, the Late Permian Dicynodon Assemblage Zone, and the Triassic Lystrosaurus and Cynognathus assemblage zones. Correlation of these biozones with better dated sequences in Europe, Russia and China has allowed for many chronostratigraphic refinements, which are in turn vital for sequence stratigraphical analysis of the basin fill. Based on fossil data, both the lower (Ecca–Beaufort) and upper (Beaufort–Molteno) contacts of the group have been proved to be highly diachronous. The refined chronostratigraphic framework has also allowed for a better analysis of the basin evolution through time, particularly in terms of the correlation of external stimuli that affect basin sedimentation patterns.

The most basal anomodont therapsid and the primacy of Gondwana in the evolution of the anomodonts

A new specimen from the base of the Beaufort Group, Upper Permian of South Africa, represents a new therapsid (‘mammal–like reptile’) which has been identified as the most basal (‘primitive’) member of the Anomodontia. Anomocephalus africanus gen. et sp. nov. is based upon a partial skull that exhibits several characteristic anomodont synapomorphies including the presence of isodont marginal teeth and a dorsally bowed zygoma, but is distinguished from other anomodonts by the possession of peg–like marginal dentition with oblique wear facets on the tips of the teeth. Anomocephalus is excluded from a clade comprised of all other anomodonts as (i) the snout is relatively long, (ii) the vertically aligned zygomatic process of the squamosal is blade–like, and (iii) the squamosal does not contact the ventral tip of the postorbital. The basal position of Anomocephalus, together with its South African occurrence, strongly supports the postulate that a Gondwanan distribution was ancestral for anomodonts.

Foredeep submarine fans and forebulge deltas: orogenic off-loading in the underfilled Karoo Basin

Abstract Third-order sequence stratigraphic analysis of the Early Permian marine to continental facies of the Karoo Basin provides a case study for the sedimentation patterns which may develop in an underfilled foreland system that is controlled by a combination of supra- and sublithospheric loads. The tectonic regime during the accumulation of the studied section was dominated by the flexural rebound of the foreland system in response to orogenic quiescence in the Cape Fold Belt, which resulted in foredeep uplift and forebulge subsidence. Coupled with flexural tectonics, additional accommodation was created by dynamic loading related to the process of subduction underneath the basin. The long-wavelength dynamic loading led to the subsidence of the peripheral bulge below base level, which allowed for sediment accumulation across the entire foreland system. A succession of five basinwide regressive systems tracts accumulated during the Artinskian (∼5 My), consisting of foredeep submarine fans and correlative forebulge deltas. The progradation of submarine fans and deltaic systems was controlled by coeval forced and normal regressions of the proximal and distal shorelines of the Ecca interior seaway respectively. The deposition of each regressive systems tract was terminated by basinwide transgressive episodes, that may be related to periodic increases in the rates of long-wavelength dynamic subsidence.

Sequence analysis of the Ecca—Beaufort contact in the southern Karoo of South Africa

Sequence analysis of the Ecca—Beaufort boundary in the southern Karoo Basin has revealed three separate facies associations spanning the stratigraphic interval between the top of the Fort Brown Formation and the lowermost maroon mudrocks of the Beaufort Group. This sequence was deposited in prodelta, delta front, and delta plain environments respectively. The lithological contact between the rocks deposited in the delta front and delta plain represents the palaeoshoreline, and occurs only once in the stratigraphic sequence, suggesting a continuous normal regression from Ecca to Beaufort times. This diachronous shoreline is associated with deltaic progradation within ahighstand systems tract. Sediment deposition was mainly a result of ephemeral flash floods, but perennial rivers also flowed from melting ice-capped highlands to the palaeosouth. Two separate fossil associations have been recognized and correlate with the lithological subdivisions. The lower fossil assemblage occurs in the Fort Brown andWaterford formationsand is characterized by silicified wood, comminuted plant material, and fish scales. The upper association occurs only in the lower Beaufort Group and includes in situ equisitalean, and well-preserved Glossopteris plant fossils and tetrapods of the Eodicynodon Assemblage Zone. The Beaufort—Ecca boundary coincides with the position of the palaeoshoreline. This lithological contact also marks achange in depositional style and palaeontological character between the two groups. The new placementof the boundary is some 300— 650m below the presently mapped contact in the south of the basin.

When and how did the terrestrial mid-Permian mass extinction occur? Evidence from the tetrapod record of the Karoo Basin, South Africa

A mid-Permian (Guadalupian epoch) extinction event at approximately 260 Ma has been mooted for two decades. This is based primarily on invertebrate biostratigraphy of Guadalupian–Lopingian marine carbonate platforms in southern China, which are temporally constrained by correlation to the associated Emeishan Large Igneous Province (LIP). Despite attempts to identify a similar biodiversity crisis in the terrestrial realm, the low resolution of mid-Permian tetrapod biostratigraphy and a lack of robust geochronological constraints have until now hampered both the correlation and quantification of terrestrial extinctions. Here we present an extensive compilation of tetrapod-stratigraphic data analysed by the constrained optimization (CONOP) algorithm that reveals a significant extinction event among tetrapods within the lower Beaufort Group of the Karoo Basin, South Africa, in the latest Capitanian. Our fossil dataset reveals a 74–80% loss of generic richness between the upper Tapinocephalus Assemblage Zone (AZ) and the mid-Pristerognathus AZ that is temporally constrained by a U–Pb zircon date (CA-TIMS method) of 260.259 ± 0.081 Ma from a tuff near the top of the Tapinocephalus AZ. This strengthens the biochronology of the Permian Beaufort Group and supports the existence of a mid-Permian mass extinction event on land near the end of the Guadalupian. Our results permit a temporal association between the extinction of dinocephalian therapsids and the LIP volcanism at Emeishan, as well as the marine end-Guadalupian extinctions.

Basin development during the deposition of the Elliot Formation (Late Triassic - Early Jurassic), Karoo Supergroup, South Africa

The integrated results of a facies analysis and provenance study of the Late Triassic to Early Jurassic Elliot Formation (Karoo Supergroup) provide some new insights into the development of the main Karoo foreland system of South Africa. Based on changes in the fluvial style, palaeocurrent pattern, provenance, isopach trends and fossil content, a regional lithostratigraphic subdivision of the Elliot Formation is proposed. In addition, the boundary between the Lower and Upper Elliot formations appears to be a second order sequence boundary. This unconformity was probably generated by the last stage of orogenic loading of the Cape Fold Belt, which interrupted the overall, first order orogenic unloading of the system, suggesting that tectonically controlled flexural subsidence existed in the main Karoo Basin until at least the end of Triassic. The magnitude of this pre-Upper Elliot tectonic event is signified by the presence of outsized quartzite pebbles and boulders, believed to have originated in the Cape Fold Belt. A number of tectonic structures, e.g. pene-contemporaneous normal faults and large-scale convolute bedding, coupled to sandstones with basement uplift/craton interior provenance, and easterly palaeocurrent direction for the Upper Elliot Formation suggest that the first stages of inversion from a compressional to extensional tectonic regime began only in the Early Jurassic.