324 Evolution? The Fossils Say YES!
rhinos. This group had not been updated in a century, but I published a comprehensive
monograph on the rhinos after more than 20 years of work (Prothero 2005).
Another part of the problem was conceptual: early work on many groups of ungulates
focused too often on trying to find primitive ancestors and link them to descendants, ignor-
ing shared derived characters. When cladistics came along, it made the study of ungulates
much more rigorous, especially when cladistic analyses were completed on many of the
major groups (see Prothero et al. [1986] on rhinos; Prothero et al. [1988] on the ungulates;
and the chapters in Prothero and Schoch [1989] on perissodactyls and in Prothero and Foss
[2007] on artiodactyls; and Prothero [2016] for the most recent information). Part of the prob-
lem had been a paraphyletic “wastebasket” group, the “Condylarthra,” which had been
used to cluster any archaic ungulates that were not members of the living orders. With the
aid of cladistics, this wastebasket has been broken up, and anyone who still uses the archaic
and obsolete term “condylarth” (such as creationists) reveals their ignorance of the current
state of research.
We can trace ungulates back as far as 85 million years ago in the early Late Cretaceous
to fossils known as zhelestids from Uzbekistan (Archibald 1996). By the latest Cretaceous
(67 million years ago), there were primitive ungulates known as Protungulatum that had
many distinctive features, such as larger body size than any other latest Cretaceous mammal,
more rounded cusps on the teeth, and a distinctive ankle region. After the extinction of the
nonavian dinosaurs, the ungulates quickly dominated the habitats on the ground beneath
the trees and radiated very rapidly in the Paleocene, so they are by far the most common
fossils in Paleocene strata from the Bighorn Basin of Wyoming or the San Juan Basin of New
Mexico. Most of these archaic ungulates (once called condylarths) are members of extinct
groups that look nothing like the living hoofed mammals and were only distantly related
to them (fig. 14.1). By the late Paleocene, we find the first fossils that clearly represent the
origins of the major living orders of ungulates. These include Radinskya from the Paleocene of
Mongolia, the primitive relative of the odd-toed perissodactyls, or horses, rhinos, and tapirs;
Phosphatherium, Eritherium, Daouitherium, and Minchenella from Africa and Asia, the earliest
members of the elephant-mastodont clan; Diacodexis and Dichobune, the earliest members of
the even-toed hoofed mammals, or artiodactyls; and the mesonychids, which are distantly
related to the whales. By the early Eocene, all of these ungulate groups were evolving rap-
idly into many different families and genera, most of which are now extinct. This story is
so detailed and interesting that we can review only a few highlights in a chapter like this,
but I recommend you take a look at my new book, The Princeton Field Guide to Prehistoric
Mammals (published by Princeton University Press in 2016).
Tiny Horses, Hornless Rhinoceroses, and Thunder Beasts
The geologic record of the ancestry of the horse is one of the classic examples of evo-
lution.
—William Diller Matthew, “The Evolution of the Horse: A Record and Its Interpretation”Let us start with the most familiar example of mammalian evolution, the origin of the horse
(figs. 14.2 and 14.3). This is one of the oldest cases used to support evolution in the fossil
record and still one of the best—and therefore the most distorted and misrepresented by