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Re: On the subject of mysterious absences...The Answer

The error involves Premise 1 and Premise 2. Premise 1 is using the word "amphibean" to describe a specific group of creatures including frogs, toads, salamanders, and various other existing groups. Premise 2 is using the term in a much broader sense, to include lineages which have long been extinct- basically, any tertrapod which is/was not an amniote (is that correct? I'm no scientist). There's no reason why a trait shown to be near-universal among the group reffered to in Premise 1 should be similarly common among members of the broader group.

The same premises also contain further, related errors, assumed within the error described:

1: The assumption that traits nearly universal among living representatives of a group must have been near-universal among extinct members of that group (most living dinosaurs can fly, therefor most dinosaurs must have been able to fly).

2: Defining a group in part by exclusion- as taking amphibeans to mean, as stated, any tetrapod which is not an amniote, and then expecting any member of this group to by more like another memebr of the group in every way than like any member of the excluded group (both apatosaurs and dromeaosaurs were non-avian dinosaurs, therefor the must have both walked on the same number of legs).

Am I on the right track?

----- Original Message ----- From: "David Marjanovic" <david.marjanovic@gmx.at>
To: "DML" <dinosaur@usc.edu>
Sent: Friday, April 25, 2008 7:02 AM
Subject: Re: On the subject of mysterious absences...The Answer

"Thecodontia" is a useful term for all those archosauriform taxa that
do not belong to any of the "major" archosauriform groups (Saurischia,
Ornithischia, Crocodylia, Pterosauria).

Thecodontia is a _misleading_ term.

Premise 1: Amphibians don't tolerate saltwater (with very few clearly derived exceptions that can live in somewhat brackish water).
Premise 2: Anthracosaurs were amphibians.
Conclusion: Therefore anthracosaurs lived in freshwater.

Premise 3: Anthracosaurs lived in freshwater.
Premise 4: Xenacanthids always occur together with anthracosaurs or other amphibians (premise 2).
Conclusion: Therefore xenacanthids lived in freshwater.

Premise 5: Xenacanthids lived in freshwater.
Premise 6: When we find reasonable numbers of well-preserved xenacanthid remains, we are dealing with a place where they lived and died, as opposed to carcasses washed into the sea or something.
Conclusion: Therefore, deposits with reasonable numbers of xenacanthids are freshwater deposits.

Spot the error.