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Re: The Drunken Sailor's Stumble (long)



----- Original Message -----
From: "Tim Williams" <twilliams_alpha@hotmail.com>
To: <kinman@hotmail.com>; <dinosaur@usc.edu>
Sent: Wednesday, March 07, 2001 12:04 AM
Subject: Re: The Drunken Sailor's Stumble (was Re: March of "progress")


> Ken Kinman wrote:
>
> >     I'm not going to argue with you about Gould's ideas.  He's a bright
> >scientist, but some of his ideas are too simplistic in my view (Simon
> >Conway-Morris rightly criticizes some of these; see Crucible of
Creation).
>
> For once Ken, we're in agreement.  :-)  I find Gould's idea of drift being
> the major theme of evolution a little troubling.

Drift? If I have understood the book correctly and there aren't too many
gaping holes in my English, then Gould writes that the major theme of
evolution is diversification into all directions, not only that of "higher
complexity", and that there is no necessity for complex (to whatever degree)
life to evolve once life has appeared.

> In fact, Gould's own
> discussion on the evolution horses in the book (which correctly rebuts the
> view that equid evolution was *not* governed by forward progress) tends to
> undermine his assertion that diversity is not due to adaptive advantage.

Is there one negation too much in this sentence?

> >The most troublesome is that the so-called Cambrian "explosion" was
> >supposedly a phyletic explosion of body-plans, when it is more
> >realistically
> >regarded as an explosion of hard parts scattered here and there on an
> >already well-diversified Metazoan tree.
>
> I respectfully disagree: I think the Cambrian explosion was real, not an
> artifact of preservation.  But Gould (in "Wonderful Life") tends to
> overstate the diversity of Cambrian forms; some "unique" forms are now
> believed to be members of hitherto known forms (e.g. _Hallucogenia_ is
> likely to be a spine-backed onychophoran, not a "phylum" unto itself.)

Indeed, many Burgess et al. taxa have turned out to be successive sister
groups of known phyla (_Anomalocaris_ and _Opabinia_ are near-arthropods,
_Wiwaxia_ and the other halkieriids are near-mollusks...) and thus help to
resolve the basic radiation of Coelomata.

> Also, Gould's notion that certain Cambrian lineages survived while others
> didn't was due to chance (Dumb Luck) is a little difficult to swallow, in
my
> very humble opinion.

Well, he calls for a catastrophic mass extinction, IIRC -- "survival of the
most fortunate" at the Cambrian-Ordovician boundary just like at the K-T.
Just like HP Ken Kinman answered:

> Tim,
>      Well, I think there can be some element of "dumb luck" in what
survives
> and what doesn't, but Gould just overstates that as well.  There was
> probably some element of dumb luck in what survived the K-T extinction.
> There were probably birds in North America that could have survived if the
> asteroid had smashed into Southern Africa instead.  But most of the bird
> survivors were apparently in the far south, just because the asteroid
impact
> was far enough away to spare them (so there was some element of dumb
> luck---how much is debatable), but the bird extinction would have been
> devastating even down there (perhaps fewer than a dozen bird species
> worldwide? [quite probable IMHO -- DM]).

>      There may well have been some kind of extinction event at the
beginning
> of the Cambrian as well (I'm not saying that it was a total nonevent), but
I
> still believe most of extant Metazoan phyla were present, and the
explosion
> would be better termed a radiation of survivors (similar to K-T).

I have read an article by Adolf Seilacher on the Vendian-Cambrian
transition. Seilacher writes that the main phenomenon at this time was an
"agronomic revolution": Before it all known shallow seafloors were covered
by leathery biomats. There were mat encrusters/stickers (enormous
unicellular "vendobionts" [*Dickinsonia*, *Charniodiscus* etc.] and
shield-shaped sponges [Trilobozoa, which have been interpreted before as odd
echinoderms or odd jellyfish or odd...]), mat scratchers (proto-mollusks
like *Kimberella* and the trace fossil *Radulichnus*) and undermat miners
("worms" scratching the mats from below or eating the decaying lower layers
of the mats -- he accepts the 1.1-Ga-old trace fossils from Chorhat/India;
the "small shelly fossils" that were found there have turned out to be
etching patterns from acid preparation) -- the Garden of Ediacara. After it
there was bioturbation -- trilobites that ploughed the sediments several
centimeters deep, "worms" and crustaceans that dug in 3 rather than 2
dimensions -- and predation, both apparently driven by the evolution of hard
parts. The chemistry of the seawater must have changed because water and
sediments were no longer isolated. This "revolution" (vive l'évolution...)
apparently took quite some time (vendobionts are known from the Early
Cambrian, so it doesn't look like an impact was involved).

> In other
> words, the "explosion" or radiation would be at class, order and families
> levels, but most of the phyla were already in place (and not sure that
there
> were that many phyla that went extinct near the beginning of the
Cambrian).

I'm afraid you have fallen into the same trap as Gould -- ranks. This is
semantics.

>       Lots of vertebrate clades made it through K-T, and it looks like a
lot
> of metazoan phyla made it through whatever happened at the beginning of
the
> Cambrian.  We are just less certain whether the early Cambrian event was
> caused by a fairly quick physical event (like K-T) or a more drawn-out
> biological event (like an arms race of hard parts), or a bit of both.

However, basal mollusks or close relatives of mollusks were present in the
Vendian, while
arthropods were AFAIK not, so I'd say the basal radiations of Protostomia
and Deuterostomia took place before and at the Cambrian explosion.

> Even the extinct Phylum Petalonamae made it into the Cambrian.

What is Petalonamae? (damned Alzheimer ;-( )