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.

references

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.

7 comments:

Mickey said...

Do you think Yunnanosaurus youngi is really Yunnanosaurus? And if not, what is it?

Bill Parker said...

Great post. The basal sauropodomorph record of the western U.S. is pretty interesting as they do not appear in the fossil record until the earliest Jurassic and then are done by the middle Jurassic. A paltry 24 million years in comparison to the longer record elsewhere (S. Africa, S. America, Europe)where they are common in the Late Triassic. Interesting concept that basal sauropodomorphs finally make their way over to North America to almost be wiped out immediately. Why bother? ;)

Adam Yates said...

Mickey - I don't really know what to make of Y. youngi just yet. It is fairly clearly a basal sauropodomorph of 'prosauropod grade' despite the sauropod-like characteristic of a longer ischium than pubis. There don't appear to be convincing uniquely derived characteristics linking youngi to huangi.

Bill - I'm really interested in your new blog and am going to add it to my blogroll immediately. The lack of sauropodomorphs in the Triassic of North America is weird. Then again consider that basal sauropodomorphs strangely vanish from Europe in the Early Jurassic (perhaps because it gets too wet and coastal?)

220mya said...

I have no particular problem with the idea of an end-Early Jurassic extinction. Nonetheless, I'm a bit skeptical about the evidence for it. As you point out, there is basically no Aalenian dinosaur record. However, the rest of the Middle Jurassic dinosaur record isn't much better, particularly when compared to the Early Jurassic and Late Jurassic records. I just don't think there is enough of a record to derive a reliable paleodiversity signal. This is also exacerbated by a lack of high-precision geochronological constraints for Early Jurassic assemblages.

I have no doubt that the faunal turnover you describe is real. Whether or not it happened right at the end of the Early Jurassic and was synchronous across Pangaea is unknown. My biggest caveat beyond the lack of a Middle Jurassic record is that you need to demonstrate that any turnover eclipsed background extinction/origination rates for dinosaurs.

Adam Yates said...

Randy,

The poor middle Jurassic signal works both ways. True it is well nigh impossible to demonstrate synchronous Pangaea-wide extinction event but by the same token gradual turnover cannot be demonstrated either. I find it rather suspicious that sometime during this 'blackhole' of the dinosaur record we have a major turnover in dinosaur faunas, and a marine mass extinction event and a massive continental flood basalt event. This may not be intractable. There seems to be good, fairly continuous deposition of terrestrial beds from the Early Jurassic through to the late Middle Jurassic in Yunnan and Sichuan in China. Perhaps some intensive sampling (similar to Roger Smith's work in the Karoo) could shed light on this question.

220mya said...

Adam;

I didn't mean to suggest that the problem will always be intractible. You are right, in some areas the potential for a good record is really exciting. But the combination of aridification and transgression means we are unlikely to ever have as good of record as we do in the Early Jurassic. For example, all of western North America was covered by either dunes or sabkhas - neither are great for preserving body fossils.

That said, I've always found diversity vs. rock bias studies very interesting. Because one can interpret them two ways: 1) the rock bias means we don't have a reliable paleodiversity estimate; or 2) the transgressions that reduce rock volume for terrestrial assemblages are actually driving diversity change during these times (habitat fragmentation, species area effects, etc). Of course, its probable that both work together in the fossil record.

I guess my earlier comment was that I'm a bit uneasy equating a faunal turnover that happened some time during a 15 million year period (OK, the turnover probably happened before the Bathonian, so we're talking about more like 7-10 Ma) with a mass extinction. I just don't think we have any handle on the age or tempo of this turnover. And I'm not quite ready to call it a *major* event. Faunal turnover is a regular evolutionary process - basal sauropodomorphs and basal theropods have to go extinct at some point - what makes their dissappearance in the Middle Jurassic anything above average? Its not like we're talking about the global extinction of all sauropodomorphs and theropods. I find it tricky to discuss, given that we're really talking about the extinction of some paraphyletic grades. We're not really talking about the extinction of all basal sauropodomorphs at once, just the few lineages that straggled on to the end of the Early Jurassic. That puts it into perspective for me as a less potent event.

Still, I don't want to seem like a cranky old man here. There is alot to discover, and people simply haven't crunched the numbers yet to look at this turnover in detail. Maybe it was a major event - we just need the studies to demonstrate it.

220mya said...

Well, this Nature news story is rather fortuitous:

Dalton, R. 2008. The new mother lode. Nature 455:153-155. DOI:10.1038/455153a
http://www.nature.com/news/2008/080910/full/455153a.html