Friday, September 5, 2008
The Drakensburg Lavas and the First Great Dinosaur Dying
What you are looking at is a thick pile of basalt, that was extruded onto the Earth’s surface some 183 million years ago, during the latter part of the Pliensbachian stage (or at the Pliensbachian-Toarcian boundary, depending on whose timescale you follow) of the Early Jurassic. They are part of a 2 km thick sheet that is centred on the mountainous nation of Lesotho in Southern Africa. They take their name, the Drakensburg Group, from the Drakensburg Range, a ragged row of peaks said to resemble the back of a dragon that runs along the border of Lesotho and the South African province of Kwazulu-Natal. This large pile of basalt is an erosional remnant of a truly enormous volcanic province. Other parts of what was once a continous sheet of lava extend north to Zimbabwe, Botswana and Zambia, and westward into Namibia. What is even more jaw dropping is that if Gondwana is reassembled, then these southern African lavas (generally called the Karoo flood basalts) are just part of one enormous province that extends into the Southern tip of South America (the Chon Aike Province) and across Antarctica (the Ferrar Province) and into southern Australia. Taken together the total volume of magma that was either extruded onto the surface, or emplaced as intrusions below it, would come to more than two and a half million cubic kilometres (Wignall 2001). This volume actually exceeds the estimated original volume of the famous Deccan Traps of India, which were extruded at the end of the Cretaceous (when most dinosaur lineages famously kicked the bucket). Given that these vast volcanic outpourings seem to be linked with mass extinction events with disturbing regularity it seems odd that the truly enormous Karoo-Ferrar province is not linked to a big extinction event – or is it?
An extinction event amongst marine molluscs (yay! see molluscs have much to teach us!) in the late Pliensbachian has been recognised in Europe and South America and this has been tied to the Karoo-Ferrar eruptions (Hallam 1961, Aberhan and Fürsich 1996). But the general consensus is that this was a weak mass extinction, well below the level of the ‘big five’ mass extinctions.
But how sure can we be? One thing that is clear to dinosaur aficionados is that the early Middle Jurassic has an abysmal record of terrestrial faunas and this may well be masking the effects of a terrestrial mass extinction. Indeed the first stage of the Middle Jurassic Epoch, the Aalenian, is the only Mesozoic stage that does not have its own valid, diagnostic dinosaur taxon (or at least it didn’t a few years ago, maybe there is one now). Another thing that the dinosaur record shows is that prior to the middle Jurassic, dinosaur faunas were rather uniform the world over with a community structure dominated by basal sauropodomorphs (usually a massospondylid) with small coelophysid and larger dilophosaurids representing the theropod contingent and much rarer small basal ornithischians. This type of fauna can be found in Southern Africa (Elliot, Clarens and Forest Sandstone Formations), North America (Kayenta, Navajo and Portland Formations), Antarctica (Hanson Formation) and China (Lower Lufeng Formation). It is interesting that the two dominant components of this faunal association, the basal sauropodomorphs and the coelophysids are basically holdovers from the Triassic. However once the record picks up again higher up in the middle Jurassic things have changed a great deal. Gone are the coelophysoids and basal sauropodomorphs*. In their place we find ceratosaurs and tetanurans filling the large predator niches while eusauropods and eurypods (that is ankylosaurs and stegosaurs) occupy the large herbivore niches. This combination of taxa remained dominant around the world to the end of the Jurassic. So was this turnover a gradual affair? Maybe not, and I have suggested that there was actually a terrestrial mass extinction event that cleared away the coelophysoids and basal sauropodomorphs in my so far unpublished chapter in the upcoming Complete Dinosaur II. If so, it would seem very likely that this event was the same one that killed those poor little clams in the late Pliensbachian. In other words the Drakensburg and associated lavas really were significant for dinosaur evolution. Perhaps without them we may never have got such majestic beasts as Apatosaurus and Brachiosaurus. Quite independently Ronan Allain and Najat Aquesbi came to the same conclusion in their monograph on Tazoudasaurus, which I featured here. Ronan and myself must think alike for this isn’t the first time we’ve come up with the same idea more or less simultaneously. Earlier we both published the connection between the dating of the Karoo-Ferrar volcanics and the age of Vulcanodon at more or less the same time (Allain et al.2004, Yates et al. 2004).
Nevertheless there exists an alternative explanation. A team of French geologists led by Fred Jourdan have suggested that the late Pliensbachian extinction event was really mild because the Karoo-Ferrar basalts were extruded over an extended 8 million year period (Jourdan et al. 2005). Other continental flood basalt provinces show a pattern where 90% or more of their volume is extruded in a brief spell of less than 600 000 years. Jourdan et al. clearly demonstrated that the lavas to the north of South Africa were extruded over a period extending from 182 to 177 million years ago. Does this spell the end of the late Pliensbachian dinosaur extinction hypothesis? Perhaps but I’m not ready to discard this idea just yet. Note that the long duration of eruptions is restricted to regions north of South Africa. The Drakensburg (an erosiaonal remnant of a truly vast area shown by the intrusions that riddle the rest of the Karoo Basin) still yields a tight cluster of dates, while palaeomag indicates the whole pile experienced just one magnetic reversal (Duncan et al. 1997). What we need is a comprehensive sampling of the Antarctic, South American and Australian lavas to see whether they also extruded rapidly at the same time the Drakensburg lavas were extruded.
*There is one recorded Middle Jurassic basal sauropodomorph, Yunnanosaurus youngi, but I would like to see a better stratigraphic control on its age.
Aberhan M, Fürsich FT (1997). Diversity analysis of Lower Jurassic bivalves of the Andean Basin and the Pliensbachian-Toarcian mass extinction. Lethaia 29: 181-195
Allain R,Aquesbi N, Dejax J, Meyer CA, Monbaron M, Montenat C, Rechir P, Rochdy M, Russell DA and Taquet P (2004). A basal sauropod dinosaur from the Early Jurassic of Morocco. Comptes Rendus Palevol 3(3):199-208
Duncan RA, Hooper PR, Rehacek J, Marsh JS, Duncan AR (1997) The timing and duration of the Karoo igneous event, southern Gondwana. Journal of Geophysical Research 102 (B8): 18127-18138.
Hallam A (1961). Cyclothems, transgressions and faunal change in the Lias of North West Europe, Transactions of the Edinburgh Geological Society 18: 132–174.
Jourdan F, Féraud G, Bertrand H, Kampunzu AB, Tshoso G, Watkeys MK, Le Gall B (2005). The Karoo Large Igneous Province: brevity, origin and relation with mass extinction questioned by new 40Ar/39Ar age data. Geology 33: 745-748.
Wignall PB (2001) Large Igneous provinces and mass extinctions. Earth Science Reviews 53: 1-33.
Yates AM, Hancox PJ, Rubidge BS (2004). First record of a sauropod dinosaur from the upper Elliot Formation (Early Jurassic) of South Africa. South African Journal of Science 100: 504-506.