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

[Adam <adam@crowlspace.com>]

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.