Does the fossil record present a true picture of the history of life, or should it be viewed with caution? The further back in time, the less well preserved are the fossils found in rocks, and they are harder to find. Estimates of the diversification of life through time would therefore seem to be plagued by an unavoidable bias. The problem is partially resolved by the observation that different fossil groups show similar patterns of diversity rising with time. Palaeontologists at the University of Bristol in England recently showed how new assessment methods, in which the order of fossils in the rocks (stratigraphy) is compared with the order inherent in evolutionary trees (phylogeny), provide a more convincing analytical tool (M.J. Benton, M.A. Wills, R. Hitchin, 2000. Quality of the fossil record through time. Nature, vol.403, pp.534-537). The two parameters, stratigraphy and phylogeny, are independent but relate to the same history. Their assessment of relationships between stratigraphy and phylogeny, for a sample of 1,000 published phylogenies, show no evidence for diminution in the quality of the fossil record going backwards in time. Although ancient rocks clearly preserve less information than more recent ones, if fossil information is scaled to the finest, global stratigraphic division, the stage, and the taxonomic level of the family, the fossil record of the past 540 million years provides uniformly good documentation of the course of evolutionary change..
TIMING THE END-TRIASSIC MASS EXTINCTION
The close of the Triassic Period marks one of the five biggest mass extinctions in Earth’s history. Until recently its age was not known sufficiently well to match the extinction with possible causes. A recent paper (J. Palfy, J.K. Mortensen, E.S. Carter, P.L. Smith, R.M. Friedman, H.W. Tipper 2000. Timing the end-Triassic mass extinction: First on land, then in the sea? Geology, vol.28, pp.39-42) reports a U-Pb zircon age of 199.6 +/- 0.3 Ma from a tuff layer in marine sedimentary rocks that span the Triassic-Jurassic transition. The dated level lies immediately below the last occurrence of conodonts and a prominent change in radiolarian faunas. Other recently obtained U-Pb ages connected with fossil time divisions based on ammonites confirm that the Triassic Period ended ca. 200 Ma. This is several million years later than suggested by previous time scales (208 Ma). Published dating of continental sections suggests that the extinction peak of terrestrial plants and vertebrates occurred before 200.6 Ma. The end-Triassic biotic crisis on land therefore appears to have preceded that in the sea by at least several hundred thousand years.
DELAYED BIOLOGICAL RECOVERY FROM MASS EXTINCTIONS
One of the most significant features of mass extinctions is the recovery and diversification of surviving life forms that follow them. Post-extinction recovery is seen by many as a major factor in biological evolution. However, palaeobiologists have worked mainly on recoveries following the “Big Five” mass extinctions. Palaeontologists from the Department of Geology and Geophysics at Berkeley, California have examined how fast life rebounds after extinctions throughout the geological record of the last 540 Ma. This general study (J.W. Kirchner & A. Well 2000. Delayed biological recovery from extinctions throughout the fossil record. Nature, vol.404, pp.177-180) shows that the rate of appearance of new species (origination) lags roughly 10 Ma behind extinctions, rather than replenishing diversity immediately after them. This applies to the “Big Five” as well as to minor, background extinctions
The Berekely scientists’ results suggest that there are intrinsic limits to how quickly global biodiversity can recover after extinction events, regardless of their magnitude.
They also imply that today’s anthropogenic extinctions will diminish biodiversity for millions of years to come.
WERE THE DINOSAURS FRIED BY ULTRAVIOLET LIGHT?
From Astronomy Now, 3 May 2000
BY NEIL ENGLISH
Over the past few decades, the rise and fall of the dinosaurs has captured the imagination of the public and the scientific community alike. While it is clear that the impact of a large asteroid straddling the coastline of what is the Yukatan peninsula in Mexico some 65 million years ago, may have wiped out these magnificent reptiles, the debate still rages as to precisely how they met their demise.
Many scenarios have been suggested, including a kind of nuclear winter in which enormous quantities of dust were ejected into the stratosphere, circling the globe and blotting out sunlight for weeks or months. But not everyone agrees that such a successful biological lineage as the dinosaurs could have been obliterated in this way.
Now, two American scientists – Charles Cockell of NASA’s Ames Research Centre In California, and Andrew Blaustein of Oregon State University, have worked out the events that occasioned themselves immediately after the KT impact.
In a recent paper communicated in Ecology Letters, they explain that the levels of nitrogen and sulphur oxides produced during the impact event would have all but destroyed the ozone layer, hereby doubling the levels of lethal UV radiation incident on the earth’s surface. This deluge of ionising radiation would have put additional stresses on the biosphere already stretched to the extreme by the impact.
What is even more remarkable though, is that significant sulphate deposits are only found over 1 percent of the earth’s surface, rendering the KT extinction event particularly lethal for the dinosaurs, but not for our kind – the small, furry, milk-suckling mammals.
Copyright 2000, Astronomy Now