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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.