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Re: The fungi did it
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.
The prepublished online abstract was mentioned onlist, and I contacted the
author in a heroic ;-) attempt to prevent publication. Arturo Casadevall was
so kind as to send me a complete pdf so that I could understand his points
better. A short discussion ensued. In sum, it seems plausible to me that the
fungal spike, a known consequence of the impact, could well have contributed
to the extinction of dinosaurs and could even help explain why dinosaurs
were hit harder than mammals... even though it's practically impossible to
test.
"Fungi are relatively rare causes of life-threatening systemic disease in
immunologically intact mammals despite being frequent pathogens in
insects,
amphibians, and plants.
Often the same species of mould can produce disease in all those and more
victims.
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.
An interesting finding!
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.
Hurray! A selective advantage of endothermy plus homeothermy! :-)
Fungal disease is relatively common in birds but most are
caused by only a few thermotolerant species. [...]
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."
If we assume that all dinosaurs were as susceptible as modern birds to
infections with large amounts of fungal spores.
- Fungal diseases are rare in endotherms compared to ectotherms,
perhaps
due to greater resistance.
First and foremost due to higher body temperature.
- 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)
Really? Slime moulds and oomycetes don't infect animals, do they? (And isn't
there only one clade of the polyphyletic slime moulds that is pathogenic at
all?)
- Endotherms survive better than ectotherms (see above), therefore
birds
and mammals survive but ectothermic dinosaurs don't.
Here there is a problem. I haven't been able to convince Arturo of the
evidence for widespread endothermy in nonavian dinosaurs. (Should have
mentioned sauropods, for instance.) Still, the higher susceptibility of
birds than mammals is interesting.
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.
Oh yes. I fear I forgot them, too. How easily do they get fungal infections?
What about turtles (which have a higher metabolic rate -- they can't run
away, so they can't thermoregulate as easily by behaviour)?
No mention, of course, of all the marine organisms
that lost their place at the table.
Eh, all that mould isn't supposed to _replace_ the impact. As we've read in
Science a year or two ago, it's a _consequence_ of the impact.
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".
By people who know even less about dinosaurs!?! :-S