In 1972, David Raup published an influential paper on taxonomic diversity during the Phanerozoic. In that paper, he estimated extinction rates based on the number of fossil families and genera for the period and before and after. The idea was to estimate the “kill rate” of major disruptions in earth’s history.
A new paper by Sarda Sahney and Michael J. Benton attempts to do this for the Permian extinction, arguably the biggest of all time. They attempt to reconstruct the “guilds”, or ecological
roles communities, of the Permian, and assess the biological diversity in terms of taxonomic diversity.
It is this that I have problems with.
Sahney and Benton use a Linnaean rank as a surrogate for diversity of species, or rather, two ranks: orders and families. The problem with this is that it is unclear how objective such ranks are. Linnaean ranks were even by Linneaus’ own standards, fairly arbitrary – in fact Linnaeus, who had five ranks only, said they were merely an aid to the student and the collector (and a very good aid they were too).
But apart – arguably – from species, none of these ranks represented anything real. Oh, Linnaeus thought kingdom did, but recent work shows us that the kingdoms are just arbitrary bits of the phylogenetic tree marked out because it suits us to do so. Plantae might be real, but Animalia seems to be artificial, as Fungi ought to be included with it.
The particular rank used by Sahney and Benton – family – isn’t even one of the original Linnaean ranks. It was a term of Michel Adanson, a younger contemporary of Linnaeus, and didn’t get included into Linnaean ranks until 1871. So what does it indicate in the natural world?
Arguably it represents some apprehended degree of difference expressed by taxonomists, but this is highly suspicious when discussing animals. Humans evolved to identify certain kinds of taxonomic resemblance, but the payoff was survival, not taxonomic exactitude. So we are very good at identifying “doggies”, “cats” and “horsies” but less good at identifying, say, deer (the name “deer” is related to the German Tier, which means “animal”).
Taxonomists, however, spend their lives learning about the organisms they study, so perhaps there’s something that is implicit or tacit that they are expressing by the ranks? Some people think that phylum, another rank that was not original to Linnaeus, but came from Cuvier, is “natural”, but at best only within animals. The botanical “equivalent”, division, is not commensurate with the zoological phylum, and neither phyla nor divisions are commensurate with anything in the classification of bacteria, algae and other single-celled organisms.
I have no doubt that there was a mass extinction at the end of the Permian. But the figures for species extinction are bogus, as they rely on these artificial taxonomic ranks. Moreover, there’s no guarantee that even if the ranks were objective, that they would indicate lower rank diversity.
This is the 700th post here.