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I used to think any particular fossil was very unlikely to have been the
direct ancestor of any other, but all of a sudden I'm not so sure.

For a "single gene pool" species (which most will be unless they're about to
subdivide), a typical breeding individual will have two children, four
grandchildren etc until in the tenth generation, in the simple model, it has
1024.  In the twentieth generation there might be expected to be, in the
simplest case, a million descendants of the original.

Of course, in practice, many of those will be duplicated, having extra
inheritance from the original individual, and a dwindling "tail" of the
population will still avoid any of the ancestry.  Nonetheless, if there were
a million individuals in the species, then after 20 generations, a pretty
big fraction of the species will be descended from the original.

But 20 generations might be only a century.  After a thousand years, if an
individual left any breeding progeny at all, it would be very likely to be
ancestral to almost the entire species (as would most of the other breeding
members of its generation), as is the way with sexual reproduction.  In
parthenogenic types individual fossils are much less likely to to be any
other fossil's ancestor.

If one species gave rise directly or indirectly to another, the chances of
ancestry from the earliest to the latest might really be quite high, since
the longer the time went on, the more likely the ancestry would be to have
spread throughout the entire species.