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Re: Extinct vs Disappeared

"But you could argue this for ANYTHING. Hence, "disappeared from the
fossil record" is effectively synonymous to "extinct": a hypothesis
falsifiable by
discovery of new specimens younger than the previous extinction date."

True, but I think the key here is "effectively" synonymous; I see his
semantic point. In the strictest sense, absence of evidence is not
evidence of absence, so the more honest statement is indeed
"disappeared from the record," rather than "went extinct." We can't
actually confirm extinction (or the first appearance of a clade, by
the same token) through *lack* of fossils.

But your point is also well-taken: parsimony is ultimately more useful
to us in science than this kind of semantics. Absence of evidence is,
in this instance, both a strong proxy for evidence of absence - based
on prior knowledge of evolutionary history - and the more truthful
statement about what the available evidence permits us to claim (i.e.,
there's no prima facie reason to presume they're extant somewhere
else, unless and until we have positive evidence for such).

So I think you're both right: it's true to say that "disappears from
the record" is more strictly accurate, and that we also can't assume
survival without evidence. The absence of a clade is simply ambiguous
until we have clear cause for a conclusion - environmental evidence,
transitions with abrupt changes in flora and fauna, etc. At the least,
the timescale over which extinction took place is unclear, but
parsimony does let us arrive at reasonable interpretations of current
observations either way.