Fish Out of Water 231that they cannot have been ancestors of tetrapods. But once again, they are thinking of lad-
ders when we are talking about branching bushes. None of the living species are ancestral
to tetrapods, and no paleontologist ever made that claim. The lungfish, coelacanths, and
rhipidistians are distinct side branches, or sister taxa, to the tetrapods, and they share many
unique anatomical characters that support that relationship, but they branched off back in
the Devonian and have had their own unique history ever since then (fig. 9.1). We can see
how close they once were when we compare Devonian fossil forms (fig. 10.3), and it is irrel-
evant that the peculiar modern descendants have specializations that are not found in tetra-
pods. Evolution is a bush, not a ladder, and organisms continue to evolve and change, long
after they have branched off from their sister groups that lead to humans. It’s hard to tell
whether creationists can’t get this point straight because they are clueless, misinformed, or
just don’t want to face the truth.
We will leave lungfish and coelacanths aside for now and focus on the fossils that docu-
ment the transformation from rhipidistians to tetrapods.
FIGURE 10.3. Evolutionary transformation series of lungfish, from the primitive Devonian fossil Dipterus,
which closely resembles the earliest coelacanths and rhipidistians, to the highly specialized living forms.
(Reproduced by permission of the Royal Society of Edinburgh and P. Ahlberg; from Ahlberg and Trewin 1995)
ProtopterusNeoceratodusFleurantiaDipterus