The fungi did it

[Christopher Taylor <ck.taylor@auckland.ac.nz>]

An article from earlier this year which I just came across:

Casadevall, A. 2005. Fungal virulence, vertebrate endothermy, and dinosaur
extinction: is there a connection? Fungal Genetics and Biology 42 (2):
98-106.

"Fungi are relatively rare causes of life-threatening systemic disease in
immunologically intact mammals despite being frequent pathogens in insects,
amphibians, and plants. Given that virulence is a complex trait, the
capacity of certain soil fungi to infect, persist, and cause disease in
animals despite no apparent requirement for animal hosts in replication or
survival presents a paradox. In recent years studies with amoeba, slime
molds, and worms have led to the proposal that interactions between fungi
and other environmental microbes, including predators, select for
characteristics that are also suitable for survival in animal hosts. Given
that most fungal species grow best at ambient temperatures, the high body
temperature of endothermic animals must provide a thermal barrier for
protection against infection with a large number of fungi. Fungal disease is
relatively common in birds but most are caused by only a few thermotolerant
species. The relative resistance of endothermic vertebrates to fungal
diseases is likely a result of higher body temperatures combined with immune
defenses. Protection against fungal diseases could have been a powerful
selective mechanism for endothermy in certain vertebrates. Deforestation and
proliferation of fungal spores at cretaceous­tertiary boundary suggests that
fungal diseases could have contributed to the demise of dinosaurs and the
flourishing of mammalian species."

    I'm sorry to say it's pretty much as silly as it sounds. The premise
goes like this:

    - Fungal diseases are rare in endotherms compared to ectotherms, perhaps
due to greater resistance.
    - Widespread deforestation after the K-T boundary led to a sizable
increase in fungi in the environment (all that lovely dead vegetation to
grow on), which would have resulted in greater risk of infection from
facultative parasites. (Note for the phylogenetically-retentive: Fungi seems
to be used in this paper in the old sense of the name, covering everything
from slime moulds to oomycetes to fungi proper :-S)
    - Endotherms survive better than ectotherms (see above), therefore birds
and mammals survive but ectothermic dinosaurs don't.

    Leaving aside the possibility that dinosaurs may have been endotherms
(as was recently re-iterated on the list, we can't rule the possibility of
ectothermic dinosaurs out), it doesn't take much effort to poke holes in
this speculation. Remember:
    Lizards! Some one on the list recently pointed out that there are more
living species of lizard than there are mammals, even leaving out the
snakes, which are phylogenetically speaking an idiosyncratic form of lizard.
So how come ectothermic lizards did so swimmingly? Not to mention
crocodiles, which aren't too different in size from at least a smaller
dinosaur.
    No mention, of course, of all the marine organisms that lost their place
at the table. No-one expects the foraminifera! Forams were (last I heard)
pretty heavily hammered by the K-T extinction. How were they affected by
air-borne fungi? Perhaps they didn't suffer from extinction at all - maybe
the styles simply changed as to what kind of test to wear? Did some kind of
protozoan Upper Cretaceous Yves Saint-Laurent inspire sweeping changes as
fashion-conscious forams tried their utmost not to look like they were
trapped in the Maastrichtian?
    Perhaps saddest of all, this article received a favourable editorial in
the lastest issue of Mycological Research (under the title "Did pathogenic
fungi contribute to dinosaur extinction?"). It concludes "While the
hypothesis is difficult to test... The idea merits floating in all basic
mycology courses, where it is sure to generate interest and debate".
Hopefully, debate on how this rubbish continues to get published.

    Cheers (and apologies for the slightly acerbic tone),

        Christopher Taylor