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



Hi David

I feel like a smart kid in a classroom. The error is a logical one as well as a failure of imagination. We only know xenacanths are in freshwater deposits when they're with freshwater species fossils. But living bull sharks can adjust to different levels of salinity so there's no reason why a xenacanth can't either. My hometown's river, the Brisbane River, has bull sharks all the way into the freshwater end of the river - as they made their way up the river their livers apparently adjust their salt management system. It's an ongoing research project as to just how they do that. I can't see any reason why xenacanths would be restricted to a narrow salinity range, especially if their habitat included seasonally flooded forest land like the Amazon basin. Would be an adaptive advantage to be flexible.

Adam

David Marjanovic wrote:

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.