310 Evolution? The Fossils Say YES!
the Archonta (primates, colugos, and tree shrews, but not bats) as their sister group, forming
a group known as the Euarchontoglires. Thus nearly all the topology of the anatomical tree is
retained, a strong corroboration from these two completely independent sources of data that
we must be getting close to the true pattern of relationships of placental mammals. Eventu-
ally, these inconsistencies between anatomical and molecular data sets will be ironed out, but
for now we have at least most of the “big picture.”
What both the anatomical and molecular phylogenies of mammals show consistently is
that the great radiation was already underway before the nonavian dinosaurs vanished at
the end of the Cretaceous. We knew that the most primitive fossils of placentals and marsu-
pials dated back to the Late Jurassic, about 140 million years ago, but the branching history
of all the major orders of placentals was difficult to decipher with just teeth alone. There were
teeth in the Late Cretaceous–earliest Paleocene that were clearly ungulates (the zhelestids
and Protungulatum), primates (Purgatorius), and carnivore relatives (Cimolestes), but most of
the placental teeth of the Late Cretaceous were insectivorous mammals, both true Insec-
tivora (related to living moles, shrews, and hedgehogs) or members of unrelated groups that
just happened to have the same diet. Now that the new molecular phylogenies are out, they
suggest that nearly all the major placental orders differentiated in the Cretaceous, and a bit
earlier than the traditional phylogeny suggests. The discrepancy is still being resolved, but
the data point in the same direction: the explosive radiation was already underway with the
primitive members of the lineages before the nonavian dinosaurs disappeared. Only after
the landscape was cleared of large animals did these lineages then diverge in ecology and
body size and begin to occupy the newly available niches, ultimately specializing into things
as different as bats and whales.
Creationists, of course, don’t keep up with or understand any of this, and they misinter-
pret most of what they do bother to read. Once again, Gish (1995:184) decried the supposed
lack of transitional forms among the orders and supported his case with grossly out-of-date
quotations. Clearly, he has no idea how much progress we have made in the past 20 years.
In some cases, he is simply wrong. He claims (p. 188) that there are no transitional forms for
rodents and quotes Romer’s 60-year-old textbook as his source. If he had bothered to read
anything more recent (published in the 1970s and 1980s, long before his 1995 edition; most
recently summarized by Meng et al. [2003] and Meng and Wyss [2005]), he would have
learned about the amazing discoveries of the anagalids (fig. 13.13) and mimotonids from the
Paleocene beds of China. These fossils are now hailed as a classic primitive linking taxon
that apparently gave rise to both rodents and rabbits in Asia during the Paleocene. In addi-
tion to the transitional anagalids, the most primitive true rodents and rabbits also appeared
in Asia before they spread to other continents in the early Eocene—and the differences
between them are so subtle that only specialists can tell them apart. What Gish doesn’t dare
mention, of course, is that once rodents appear in the record in the Eocene of the rest of the
northern continents, they have an extraordinary fossil record, with hundreds of specimens
spanning every meter of section in places such as the Big Badlands of South Dakota. As
Martin (2004) points out, in many parts of the Cenozoic, you can get hundreds of distinctive
rodent teeth in every meter of sediment and use them to date the rocks very precisely. And
if Gish had read any recent works on rodent paleontology, there are many, many transitional
forms between the major groups of rodents. Not only that, but the molecular phylogenies of
rodents are now closely reflecting the traditional anatomical phylogenies, so we have a very
robust database of their evolution.