Bossies and Blowholes 337Yet once again, a look at the first known artiodactyls from the early Eocene reveals a
form that no one would connect with a cow, giraffe, or camel (fig. 14.12). Diacodexis and
Dichobune were tiny creatures, about the size of a rabbit, with long delicate limbs and
simple primitive teeth and skulls. Some of them had very long hind limbs and were
apparently well adapted to hopping and leaping as well. Both anatomical cladograms of
the ungulates (Prothero et al. 1988) and the molecular phylogenies (Murphy et al. 2001a,
2001b) place their branching point at the very base of the ungulate radiation (fig. 14.1). Ken
Rose (1987) has suggested that the Paleocene archaic ungulate Chriacus looks very much
like a possible transitional form between the earliest ungulates and artiodactyls. Once
artiodactyls appeared in the early Eocene, they became very stereotyped and distinctive
in many features. Their simple cusped teeth soon evolved into half-moon-shaped crests,
giving them their signature selenodont teeth. Their legs and feet were long and delicate
from the very beginning and soon began to lengthen the central toes and reduce the side
toes (but in a different way than horses). All artiodactyls have a distinctive double-pulley
bone in their ankle known as the astragalus, which made their legs very efficient for front-
to-back running motions (but prevented them from rotating their foot or hind leg out of
the front-to-back plane).
By the middle Eocene, artiodactyls were undergoing an explosive radiation with many
different families, most of which are now extinct (fig. 14.11). However, the primitive rela-
tives of pigs were present, as were the first camels and the first ruminants. In the Oligocene,
another explosive evolutionary radiation occurred, this time with mostly living families,
such as the camels and early ruminants, and by the Miocene, the pronghorns, giraffes, ante-
lopes, and cattle were beginning to diversify as well. Some of these families have extraordi-
nary fossil records, and we can trace their lineages back to the Eocene, then look forward to
FIGURE 14.12. The earliest even-toed hoofed mammals, or artiodactyls, such as Diacodexis and Dichobune,
were small primitive forms that looked nothing like their living descendants, the pigs, hippos, camels, deer,
giraffes, pronghorns, cattle, sheep, antelopes, and goats. Nevertheless, they had the characteristic teeth found
in all primitive artiodactyls and distinctive features of the skull and ankle region that mark them as ancestors
of this great order of mammals. (Drawing by Carl Buell)