Evolutionary rhythms

The late Jack Sepkoski did a lasting service for those who study life’s record by combing the literature to compile the first and last appearance of each marine fossil genus.  It is from this archive that we have been able to visualise mass extinctions and those less in magnitude numerically.  As well as the “Big Five” there are other die-offs, particularly through the Mesozoic and Cenozoic record.  To some extent the extinction patterns also appear among terrestrial taxa that have been less well documented, partly because few have had Sepkoski’s determination and partly because land organisms leave fewer traces.  It quickly became apparent to him and other palaeontologists that extinction occurred sharply, which is why the biologically-determined division of Phanerozoic time since 542 Ma is so well defined world-wide.  What also emerged from inspection of the time series of genus and family numbers was a pulse in the timing of significant extinctions, which appears to have been between 25 and 30 Ma.  That struck a chord with specialists in volcanic activity, and there is a good correlation between the occurrence of flood-basalt outpourings and extinctions.  But at least one of the five largest extinctions, at the K-T boundary, coincides with abundant evidence for a major impact by an extraterrestrial body.  Planetary scientists then began looking for a pulsed variation in the intensity of bombardment of the Inner Solar System.  There is no tangible evidence of that, although there are theoretical arguments that suggest that the Sun in its ~250 Ma orbit around the galactic centre wobbles through dust arranged in bands close to the galactic plane every 30 Ma.

Extinctions are not, of course, the only features of the fossil record.  Primarily it charts variations in diversity, of which suddenly lowered numbers are one aspect in broader fluctuations.  Each extinction eventually precedes an increase in diversity as adaptive radiation from surviving taxa fills ecological niches left vacant or under-populated.  That part of the record has its fascinations, as complexity seems to have emerged in three great pulses, through the Palaeozoic, Mesozoic and Cenozoic Eras, each producing more diverse forms than its predecessor.  There are also slackenings in the pace and periods of apparent stasis.  Getting to numerical grips with the full record requires analysis that uses similar mathematical techniques to that which unlocked proof of Milankovich’s theory of astronomical pacing of climate from finely calibrated oceanic-sediment records.  It is possible to analyse time series in terms of discrete frequencies from which the curves can be reconstructed.  Physicists Robert Rohde and Richard Muller of the University of California have used this Fourier analysis on the 36 thousand strong catalogue published after Sepkoski’s death, with some recalibration of the time scale and some pruning of data – they removed genera with only a single record or whose age is poorly known (Rohde, R.A. & Muller, R.A. 2005.  Cycles in fossil diversity.  Nature, v. 434, p. 208-210).  There are definitely distinct frequencies that dominate the record, and they cannot be present by chance, although that is a purely statistical view.  But to their surprise, and everyone else’s, they are completely unexpected ones at 62 and 140 Ma.  It is proving exceedingly difficult to come up with plausible Earthly or extra-terrestrial explanations.  There are two interesting features: the 62 Ma periodicity dominates the record of relatively short-lived genera; and the “Big Five” seem to fit neatly into the patterns of diversity, albeit at unequally spaced intervals, when the effects of background fluctuations have been removed.  That filtering may allow for increasing preservation towards recent times.  One major control over diversity is, logically, a mixture of the number of potential niches and their geographic isolation, and both are probably related to plate tectonic activity.  Unfortunately, fluctuations in 2 and even 3 geographic dimensions have only the broadest calibration to time.  Added to that is the complex way in which global sea level has changed with time.  So we can expect a great deal of head scratching, and it may come as a relief that the crowing of some volcanologists and impact theorists may have been silenced at a single stroke!

See also:  Kirchner, J.W. & Weil, A. 2005.  Fossils make waves.  Nature, v. 434, p. 147-8.

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