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