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Re: Cladistic Idea



To put it even more simply, the fossil record is not merely incomplete, it
is wildly incomplete.

It's not that we're missing half of it. We're missing almost all of it. We
have far less than 1% of all the vertebrate species that ever lived known
from fossils.

Fossils are a tiny, random, or even skewed, sample of the fauna at the time.

Thus, if we took a sample of animals that have just been deposited and
begun fossilizing in the year 2011, and compare it to an equal sized
sample from the Pleistocene, we might get a large number of mice and
voles, bony fish, an opossum, and a snapping turtle from this year in one
deposit. The Pleistocene sample might have a mastodon, a big cat, and a
bunch of mice and voles and bony fish, and a painted turtle. We should
never conclude from this that opossums are more advanced than mastodons.
Nor that painted turtles evolved into snapping turtles. The more primitive
animals are just as likely to show up in the later horizons, especially if
they are common, even if they have lower diversity, so long as they aren't
extinct.

The stratigraphy should not be used to bend the phylogeny for this reason.



>
>> It seems to me that it would be interesting to approach a cladistic
>> analysis of
>> fossil taxa one age at a time. As an oversimplified example, imagine an
>> analysis of ten fossil taxa, named "A" through "J". Imagine that
>> stratigraphically, they are arranged like this:
>>
>> epoch #5: J
>> epoch #4: G,H,I
>> epoch #3: E,F
>> epoch #2: D
>> epoch #1: A, B, C
>>
>> The standard way to do a cladistic analysis would be to take all ten
>> taxa, score
>> them for all the characters, and away you go. My suggestion is to
>> consider the
>> fauna of each age only in light of what has come before.
>
> But the biota of epoch 1 didn't consist only of A, B and C. _At the
> minimum_, it consisted of A, B, C, and the ancestor of D through J!
>
> By including D through J in the analysis, you effectively include that
> ancestor. By excluding them, your taxon sample is incomplete, and this
> can very easily result in a wrong tree topology. And if you include only
> some of them, you get a lopsided representation of that ancestor.
>
> Naturally, it gets worse if the fossil record is worse and D through J
> isn't a clade that exclude A, B and C.
>
> A bad fossil record also means you can't necessarily trust the
> stratigraphic ranges of the taxa you know. What if G, known only from
> epoch 4, actually existed from 2 to 5?
>


Jason Brougham
Senior Principal Preparator
Department of Exhibition
American Museum of Natural History
81st Street at Central Park West
212 496 3544
jaseb@amnh.org