The Perissodactyls
The perissodactyls, or the odd-toed hoofed mammals, are not very diverse today. There are currently only four living species of tapirs, five species of rhinos, and a handful of species of horses, asses, and zebras. Most of these are endangered in the wild, and several have gone extinct in the last century. However, perrissodactyls were much more diverse in the Eocene and Oligocene, with a number of families and other lineages that are now extinct (e.g., brontotheres, palaeotheres, chalicotheres, lophiodonts, other tapiroids, hyracodonts, amynodonts) and even a higher diversity of extinct genera and species of horses, rhinos, and tapirs than are living today (Prothero and Schoch 1989, 2002). Each of these groups is easily fossilized and found in nearly all the Holarctic continents since the early Eocene, so they tend to have an excellent fossil record. Even though horse evolution has received the lion’s share of the publicity, the record of rhinos, tapirs, and brontotheres is also excellent, and each deserves more frequent mention as exemplars of evolution to replace the overused examples of horse evolution.
The most striking thing about perissodactyl evolution is that we can see the very earliest stages of their diversification preserved in the fossil record. For many years, paleontologists have focused on the archaic hoofed mammal (“condylarth”) group known as phenacodonts as the sister taxon of perissodactyls (Radinsky 1966, 1969; Thewissen and Domning 1992). These creatures were widespread around the Holarctic region of Eurasia and North America in the Paleocene and early Eocene and do indeed share many characters in common with perissodactyls. Phenacodonts, in turn, provide a link between perissodactyls and the most primitive clades of ungulates (Prothero et al. 1988). Moving even closer to true perissodactyls, we have the late Paleocene Chinese fossil known as Radinskya, which is a close sister group to almost all the earliest perissodactyls (McKenna et al. 1989). Known from a partial skull and a few other fragments, its teeth are more primitive than any bona fide perissodactyl, yet it shows some derived characters that make it a good sister taxon to that order. However, it is so primitive in most of its characters that McKenna et al. (1989) were unsure about its taxonomic assignment.
From these Asian Paleocene roots, there was a rapid diversification of perissodactyls in Europe and North America in the early Eocene. The earliest members of the horse, rhino, tapir, and brontothere lineages in North America are so similar to one another that only subtle features of the teeth and the skull allow us to tell them apart (Fig. 1). If you look at their fossils today, you would never guess that they would eventually diversify into such disparate groups as horses, rhinos, and tapirs, yet this is the evidence from the fossil record. This point was driven home to me while working on an undergraduate research project on early Eocene mammals from the Bighorn Basin of Wyoming. The specimens of the earliest horses (now called Protorohippus, according to Froehlich 2002) and the earliest tapiroids (Homogalax) were virtually identical, except that the Homogalax molars had slightly better-developed cross-crests, a signature of the teeth of all later tapiroids. This incredible degree of similarity is also found in their skulls and skeletons (Fig. 1). In addition, the earliest relatives of the brontotheres look much like early horses and tapiroids. By the late early Eocene and middle Eocene, all of these lineages had diverged enough that tapiroids are much easier to distinguish from horses, and brontotheres are distinct from both. This is powerful evidence about how lineages can be traced back to common ancestors that are virtually indistinguishable from one another.
Horse Sense
Of these lineages, the story of horse evolution is most familiar. Ever since Marsh’s work of the 1870s, it was clear that the earliest horses (formerly called “Eohippus” or “Hyracotherium,” but now referable to Protorohippus and several other genera—Froehlich 2002) were beagle-sized creatures with simple low-crowned teeth, relatively short limbs and toes, and four toes on the hand and three toes on the hind foot. From this ancestry, horses are well documented to have become larger, longer-limbed, with a reduced number of side toes, and with higher-crowned teeth in most lineages (MacFadden 1992). By the 1920s, this simple idea of horse evolution was codified into diagrams that showed a single lineage of horse evolution from “Eohippus” to Equus (Fig. 2). This is the image that has become iconographic in nearly every textbook treatment of evolution since then.
One of the beauties of science (and particularly paleontology) is that it never stands still or rests on its laurels but continually builds and changes and revises its ideas as new material and data emerge. Since the 1920s, a huge number of additional horse fossils have been found, and many more species and genera described. By the time of Simpson’s (1951) book on horses, it was clear that their evolution was much more bushy and branching than the old diagrams suggested, and the work of the late twentieth century only added to the bushiness of their family tree (Fig. 3). In addition, studies of individual parts of this bush show surprising things about their diversity. For example, the classic “gradual” transition from Mesohippus to Miohippus was actually a bushy branching event, with as many as three species of Mesohippus and two of Miohippus occurring in the same late Eocene beds of Lusk, Wyoming, at exactly the same level (Prothero and Shubin 1989). Multiple species of horses were also documented from the same beds in the early Eocene (Froehlich 2002), and there were 12 different species of horses in the Railroad Quarry A in the upper Miocene Valentine Formation of Nebraska. Thus, we have begun to appreciate that horse evolution is extremely bushy and branching, in contrast to the oversimplified “single lineage” models of a century ago.
One would think an improving record of horse evolution should impress creationists with all the new data. Instead, they quote old ideas out of context to deny that horse evolution occurred at all, or use outdated quotations about the replacement of the simplistic linear model with the complex bushy model to deny the reality of horse evolution (Gish 1995: 189–197; Wells 2000:195–207). Others like Sarfati (2002: 132–133) claim that all these fossil horses are within the range of variation of modern horses. Clearly, he has never actually looked at the fossils, since primitive horses like Protorohippus do not even remotely resemble the smallest modern ponies of the genus Equus. Every single comment on horse evolution from the creationists’ literature betrays their complete lack of any firsthand knowledge of horse anatomy or fossils and shows that they cannot tell one bone from another. Instead, they criticize scientists for changing our ideas about horse evolution as we learned more from more and better fossils. Maybe this makes sense in their mindset of unchanging truths, but in the real world (and in science), more data are better, and change is good when the data demand it!
Rhinos Without Horns, Tapirs Without Snouts
If the evolution of horses were not enough, we now have excellent examples of the evolution of rhinos, tapirs, and brontotheres to add to the total evidence. My particular favorite is the evolution of the rhinoceroses, which I have studied for over 30 years (Prothero et al. 1986, 1989; Prothero 1998, 2005). The earliest rhino relatives like Hyrachyus are barely distinguishable from contemporary tapiroids (Fig. 4) in the early middle Eocene. By the late Eocene, they had diversified into three branches: the hippo-like amynodonts, the long-legged running hyracodonts, and the true rhinoceroses, family Rhinocerotidae. Each family shows considerable diversification and evolution, with the hyracodonts evolving into the gigantic indricothere Paraceratherium (formerly called Baluchitherium or Indricotherium), the largest land mammal that ever lived. It was a hornless rhino from the Oligocene of Asia that reached 7 meters tall at the shoulder and weighed at least 20 tons, larger than any elephant. Yet despite its huge size, it retained the relatively long slender limbs and toes of its hyracodont ancestry and did not develop the stubby graviportal toes seen in elephants and larger dinosaurs. The living family Rhinocerotidae also shows an incredible array of diverse body forms and interesting evolutionary patterns. Most extinct rhinos were hornless, since they do not show the roughened area on the top of the skull to which the horn (made out of compacted hairs) attached. Others exhibited many different horn combinations (single nasal horn, paired nasal horns, two tandem horns, single frontal horn), four independent episodes of dwarfing, independent development of high-crowned teeth in lineages adapted to grazing, and at least three instances of rhino lineages developing into short-legged, barrel-chested hippo-like forms. Indeed, the evolution of rhinoceroses is fully as interesting and complex as the story of the horses, but has been underappreciated and underpublicized because it was harder to simplify into the “linear” model once applied to horses (e.g., Fig. 2) and also because until recently, rhino systematics were so confused and outdated that nothing could be done with them (Prothero 2005).
Closely related to rhinos are the tapirs and their kin, including the chalicotheres. We have already seen that the earliest tapiroid, Homogalax, is barely different from the earliest horse (Fig. 1). From this ancestry, tapiroids rapidly developed the specialized molars with two strong cross-crests for chopping up their leafy diet and the retracted nasal bones that were the attachment area for their prominent proboscis (Fig. 5). The continual transformation of their teeth and skulls can be seen throughout their evolution, so that although Homogalax bears only the tiniest resemblance to the modern tapir, it can be linked with numerous transitional fossils that show every step in their evolution (Fig. 5).
Thunder Beasts
Extinct perissodactyls provide many good examples of evolutionary transitions as well. The brontotheres (“thunder beasts”) or titanotheres (Fig. 6) were long portrayed by the paleontologist Henry Fairfield Osborn (1929) as a continuous gradual lineage that got larger and eventually developed huge paired battering-ram horns on their noses. This outdated notion has been completely revised with modern taxonomy (Mihlbachler 2008), but the general trends are still apparent on their bushy family tree (Fig. 7). Brontotheres evolved from creatures such as Lambdotherium that looked much like contemporaneous early Eocene horses and tapiroids, but with tiny differences in their teeth. By the middle Eocene, brontotheres had become much more diverse in size and anatomy, with multiple lineages coexisting at the same time. In the Chadronian (late Eocene, formerly thought to be early Oligocene), they reached the culmination of their evolution, becoming elephant-sized beasts whose impressive blunt battering rams on their noses have invited so much speculation. Although we now reject Osborn’s (1929) simplistic linear model of evolution and bad taxonomy (Fig. 6), the overall trends in brontothere evolution are still real, even as their taxonomy changed and the phylogeny became more bushy and branching.