Bossies and Blowholes 329whole range of primitive anatomical features (three functional digits on their hands and feet,
shorter snout, much lower crowned teeth, and many other features) that clearly distinguish
them from any living horse, no matter how small.
In fact, if creationists had bothered to spend any time at all looking at real fossils, they
would have been amazed by how subtle the transition is from phenacodontids and Radin-
skya to the early perissodactyls. Even more surprising, the earliest Eocene horses, rhinos,
and tapirs are also very hard to tell apart—yet modern horses look nothing like the liv-
ing tapirs or rhinos today. This fact struck me when I was working on my undergraduate
research project on early Eocene mammals from the Bighorn Basin of Wyoming. Although
the literature on the subject was clear, it was a major challenge trying to tell the earliest horse
teeth from the teeth of Homogalax, the earliest member of the rhino-tapir lineage. They are
virtually identical in size and in cusp-by-cusp detail (fig. 14.5), except that Homogalax tends
to have slightly better connections of the crests between the cusps. Anyone without a sharp
eye would miss this difference completely, and the same is true of the skulls and skeletons.
All of the early perissodactyls (horses, rhinos, tapirs, and brontotheres) look so similar when
they begin their evolution that only a trained eye can tell them apart. Yet we can trace the
evolution of each of these distinct lineages through time, and they soon begin to look very
different, so that by the late Eocene, they are dramatically distinct in size and body shape,
and even a kid could tell them apart. This is one of the best examples of how we can docu-
ment the origin of many modern distinct lineages back to ancestors that converge to the
point of being virtually indistinguishable.
But if the evolution of horses is not convincing enough, let’s look at my favorite group,
the rhinoceroses. They have just as long and dense and detailed a fossil record as horses, yet
they have received almost no attention because their systematics was a mess for decades and
nothing could be concluded until the valid species were determined using the new collec-
tions (Prothero 2005). Once that was done, however, we again see a highly bushy, branching
family tree of rhinos (fig. 14.6) in North America (and a similar pattern in Eurasia), with
many different families, species, and genera spanning almost 50 million years. The earliest
relative of rhinos was the early Eocene form known as Homogalax, which also gave rise to
tapirs, and yet Homogalax is virtually identical to the early Eocene horses (fig. 14.5). By the
middle Eocene, we see the split between the tapiroid lineage and the lineages that lead to
the three main families of rhinocerotoids. Unlike horses, which evolved mainly in North
America with occasional emigrations to Eurasia, rhinos evolved on both hemispheres and
immigrated back and forth, so their family tree is much more bushy and dominated by sud-
den immigration events than that of the horse (fig. 14.6). Although most of the species are
distinct, we can still see evolutionary trends, particularly in the front of the snout (fig. 14.7A),
where the primitive forms have many incisors and small canines, and as rhinos evolve, they
lose most of the incisors and develop sharp, short tusks between their remaining incisors.
In addition, their cheek teeth (fig. 14.7B) also show evolutionary trends, such as the modi-
fication of the primitive premolars into molar-like crests that resemble the Greek letter “U.”
My rhino monograph (Prothero 2005) documented many other changes, both gradual and
punctuated, such as the size changes in many lineages, and the gradual development in
horns within the genus Diceratherium. Like horses, rhinos also get larger and more spe-
cialized throughout their evolution. They started out with four toes on the front foot, and
reduced it down to three by the middle Eocene—but unlike horses, they remained three-toed
even today and never became highly specialized one-toed runners like living horses.