Since the discovery of the buried Chicxulub impact crater off the Yucatán Peninsula, Mexico, many geologists have regarded it as the “smoking gun” for the end-Cretaceous mass extinction. Such is the heft of K-T studies that money has been raised to drill into the crater and its overlying sediments. That began in late 2001 at an onshore site on the flank of the structure, and results are starting to emerge. However, research has been slow in getting underway on the crucial part of the core that goes through the boundary itself. That section was taken from the project’s headquarters in Mexico City to the Free University of Amsterdam, by Jan Smit, one of the pioneers of K-T boundary studies. Samples began to reach other researchers in December 2002, 6 months after the boundary section arrived in Amsterdam. For many, this was a little too slow and suspicions have been raised. Everyone wanted to get abstracts into the AGU/EGS/EUG bun fight in Nice in April 2003, where a conference session on Chicxulub had been scheduled. One report presented there seems set to stun the pro-impact school. Gerta Keller of Princeton University studied foraminifera in the samples immediately above the impact breccia – there were plenty. She claimed that they represented a period of about 300 thousand years of sedimentation that followed the impact. Moreover, they occurred below the level of a thin glauconite-rich horizon, which seems to represent the K-T extinction event itself. Not surprisingly, Keller concluded that the impact could not have caused the extinction. Smit dismisses the allegation of “hogging” the core samples, and also suggests that the foram-rich layers represent sediment that was washed back into the crater soon after it formed. It has always struck me as odd that whenever something startling emerges from scientific research, a sort of preciousness overwhelms supposed scientific “objectivity”. Counter claims and new variants of ideas rapidly evolve on the periphery of the discovery. There are reputations to be built, and defended, and of course “sexy” themes attract cash. The initial work that led to the recognition of a global layer of mass destruction, carried out by the Alvarez father and son team in the late 1970s, was a purer form of science – driven by curiosity and little else.
Sources: Dalton, R. 2003. Hot tempers, hard core. Nature, v. 425, p. 13-14. McKie, R. 2003. I’ve got a bone to pick with you, say feuding dinosaur experts. The Observer, 7 September 2003, p. 22.
Gamma-ray bursts and mass extinctions
There is a Gaelic saying, which roughly translated goes: There are more ways of killing a cat than drowning it in butter. It seems to apply to mass extinctions. A team of astrophysicists and palaeontologists from the University of Kansas and NASA, headed by Adrian Melott of the University of Kansas, has found peculiarities in the trilobite record after the Late Ordovician mass extinction (443 Ma) that are difficult to explain by the usual culprits. Planktonic trilobites were decimated, but those living in deeper water largely came through the extinction. Graptolites too incurred major changes, only the monograptids surviving until the Silurian. Many palaeontologists link the end-Ordovician extinctions to global cooling, evidenced by glacial rocks mainly in Africa. Melott and colleagues suggest that a realistic reason for a depth-related extinction pattern could be due to intense gamma rays emitted by the collapse of a nearby giant star into a black hole. Although most would be blocked by the Earth’s atmosphere, that would be at the expense of nitrogen oxides being created in large volumes from oxygen and nitrogen molecules. Nitrogen dioxide, the yellow colorant in photochemical smog would prevent solar radiation reaching the surface and trigger cooling. Also acid rain would lower the pH of surface water. Such a process could also explain the Late Ordovician glaciation of Africa.
Source Hecht, J. 2003. Did a gamma-ray burst devastate life on Earth? New Scientist, 27 September 2003, p. 17
Fossil oddities – a golfing trilobite and the ox-sized rodent
Gamblers and golfers do not like distractions, and many wear eye shades of some design or other. So it is intriguing to learn that a Devonian trilobite, Erbenochile, found in Morocco evolved a similar device. Richard Fortey and Brian Chatterton, of the British Museum of Natural History and the University of Alberta, respectively, analysed the peculiar eyes of this phacopid trilobite, and found that their tops had a sort of rim. Light shining down on the beast put the compound facets in shadow (Fortey, R. & Chatterton, B. 2003. A Devonian trilobite with an eyeshade. Science, v. 301, p. 1689). Not only would this arthropod have been undistracted from its activities by goings on above, but it could also see over its back.
Not since the discovery of the Late Miocene Bullockornis in Australia (see The Ducks of Death in EPN June 2000) have Neogene palaeontologists come up with a record beater. But now they have (Sanches-Villagra, M.R. et al. 2003. The anatomy of the world’s largest extinct rodent. Science, v. 301, p. 1708-1710). The Late Miocene of Venezuela has yielded a rodent (Phoberomys), whose bones suggest that it weighed in at about 0.7 tonnes. It is related to modern guinea pigs, and probably had much the same herbivorous habits. Its teeth suggest that it was grazer too, and like the modern capybara (one tenth the size of Phoberomys) it lived in swamps. Rodents now rank as the mammalian order with the greatest range of sizes. Because the digestive systems of mammals cannot efficiently break down the high cellulose content of grasses without the aid of internal bacteria, the bigger their gut, the more efficient they are as herbivores. So giant rodents make sense as regards their metabolism. However, they are not as well known for galloping as many other grazers, which is why smaller rodents prefer to escape predation by diving into burrows or among boulders. That would be difficult for a creature as big as an ox. Swamp dwellers, like the capybara and Phoberomys, can get away with not being fleet of foot, but would not do well on open grassland.
The compiler of EPN welcomes news of odd and awesome fossils, and hopes soon to learn of mighty hamsters and their adaptation to natural treadmills.
See also: Alexander, R.M. 2003. A rodent as big as a buffalo. Science, v. 301, p. 1678-1679).