Re: Fossil Records Show Biodiversity Comes and Goes

From: George (george_at_wtfiswrongwithyou.com)
Date: 03/16/05 

Date: Wed, 16 Mar 2005 22:11:50 GMT

<jtreat@cox.net> wrote in message
news:1111004999.087065.229890@f14g2000cwb.googlegroups.com...
>
> John Harshman wrote:
>>
>> Some observations on the published diversity curve.
>>
>> 1. It doesn't have the shape we would expect from "mass extinction
>> events". The curve is roughly a cycloid, i.e. the shape created by a
>> point on a rolling circle. From a trough, diversity rises sharply at
>> first, then slows as it comes to a peak, then decreases gradually,
>> accelerating into the trough, and suddenly reverses. What we would
>> expect from a mass extinction is a more saw-toothed shape: diversity
>> increases quickly from a trough, perhaps slowing as it reaches a peak
> or
>> perhaps not, then plunges instantly into another trough. This
> observed
>> gradual decline is not consistent with either periodic impacts or
>> episodes of flood vulcanism, which are geologically instantaneous
>> events. Instead it calls for an explanation in which some kind of
>> environmental stress begins slowly, increasing over millions of years
>
>> (starting some time before a peak and attaining a maximum at the
> trough,
>> so covering 30ma or more), and then suddenly vanishing. I have no
> idea
>> what that would be. It might be argued that this curve shape is an
>> artifact caused by Signor-Lipps effect, but I don't think that's
>> possible on this timescale, with marine invertebrates.
>
> I wondered why this should be an astronomical or geologic event. It
> looks very much like a predator/prey or epidimic cycle. It is not
> unreasonable to be looking at a ecological cause which would tend to
> have a wave form like this.

Indeed. If you look at the evolution of many species through the fossil record,
one thing stands out like a sore thumb to me. As an example, look at crinoids.
Crinoids most likely evolved from very simple animals found in the Burgess
shale. By their heyday, they had branched out into many ecological niches, and
had diversified into many very highly specialized species. The crinoids
demonstrate a record of diversity that at their heyday in the late Mississippian
can only be described as bizarre. Similarly, dinoaurs evolved from very simple
animals, and in their heyday in the Cretaceous had evolved into complex, and
equally bizarre, and very specialized forms. Such overspecialization tends to
make species vulnerable to factors such as environmental change. If a species
evolves to become a specialist in eating Eucalyptus trees, and all the
Eucalytpus trees die out in some environmental catastrophe (severe drought, for
instance), so do the Eucalytpus eaters.