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Last words on the cross

hjPlatypus is extinct.  After 223 continuous months, this issue’s Prize Crossword (print edition only) will be the last to appear in Geoscientist.  As he slithers back to his natal creek, he would like to thank all his faithful regulars and all who have ever sent in a completed grid.

The disappearance of the crossword, three months in advance of my own as Editor, has put me in mind of sudden extinctions, their signature in the record.  This was a very hot topic during my formative research years in the early 80s, the subject of an influential paper (see Further reading) by Phil Signor and Jere Lipps of the University of California, Davis, and an object lesson in how something that was actually blindingly obvious was only widely perceived thanks to a new and fruitful theory.

Before the discovery of the famous iridium layer at the top of the Cretaceous, and the realisation that a massive asteroid impact may have caused an instantaneous mass extinction, everyone believed in gradualism.  Until that moment, palaeontologists didn’t think any more about it when they saw taxa petering out, as expected, before the Paleocene.  But Signor & Lipps realised and proved it by use of the mathematicks) that even sudden disappearances can seem gradual, where sampling is rare.

Men who noticed things

As usual though, you don’t need maths.  Read this month’s obituaries.  Perhaps you knew one or more of these men, as I did.  Did you see them on the very day they became extinct?  Or did you reflect sadly, as their bell of quittance was heard in the gloom, on the last time you had met, phoned or received an email?  Just like a rare fossil taxon in the sedimentary record, they lived continuously, but only put in an appearance occasionally.  Their last appearance in the fossil record of your memory therefore happened some time before they actually drew their last breath.

The same phenomenon happens in reverse – gradual appearance, rather than disappearance  – under your sink.  One day there will appear (unbeknown) the first ‘last bottle’ of some cleaning product that you will ever buy.  I will probably buy more bleach before I snuff it, but – hand degreaser?  I’m not so sure.  Now that I am locked out of the engine compartment of my jalopy by its manufacturer, I never use it.  My last Swarfega may already have been bought.  Silver polish might be next.  These analogies illustrate what is now called the ‘Signor-Lipps Effect’. 

Sudden extinction only looks sudden among taxa that appear in every bed – like microfossils.  They all vanish together.  Rarer fossils’ – like dinosaurs’ - last appearance onstage, however, nearly always predates their actual ‘final curtain’. 

So, farewell, sweet monotreme.  Touch Signor-Lipps and part.

Further reading

  • Signor, P W and Lipps J H 1982: Sampling bias, gradual extinction patterns and catastrophes in the fossil record.  GSA Spec. Pap.  190: 291-96.
  • Gould, S J 1996: Dinosaur in a Haystack, in book of that name, Penguin Science p147.

DR TED NIELD NUJ FGS, EDITOR

ted.nield@geolsoc.org.uk, @TedNield @geoscientistmag