On the Origins of New Forms of Life
9.7: Ungulates
EUGENE M. MCCARTHY, PHD GENETICS
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| The right-rear foot of a Masai Giraffe. Image: C. Burnett, Wikimedia, license cc-sa-3.0 |
The characteristic feature of ungulates is the hoof. In addition, many bear either horns (cattle, deer, rhinoceroses) or tusks (hippos, chevrotains, musk deer, pigs). A few have both (guemals, muntjacs). Many others, such as horses and camels, have neither. All living hoofed mammals are included in one of two categories, perissodactyls (those forms with odd number of toes) and artiodactyls (those with an even number). In the traditional account of evolution, perissodactyls and artiodactyls first appear in the Eocene. Perhaps for this reason, paleontologists tend to place the fossils of earlier hoofed animals into other "extinct" orders to indicate they are somehow basically different from, and not ancestral to, taxa included in the two extant ungulate orders Perissodactyla and Artiodactyla. This practice tends, in turn, to reinforce the story told about hoofed mammals “first appearing” in the Eocene. Moreover, as we have seen, fossiliferous terrestrial deposits of Eocene age are far more common and widespread than those dating from the preceding epoch (i.e., the Paleocene). This discrepancy might easily lead a paleontologist interested in ungulates alone to suppose they suddenly appeared or became abundant in the Eocene.
Nevertheless, earlier animals with hooves are known from the Paleocene, some of them with tusks and/or horns. These forms seem to constitute reasonable ancestors for hoofed mammals of later date. Several ancient mammalian orders that supposedly left no descendants include fossil forms that seem to qualify as hoofed mammals: Order Condylartha (e.g., Arctocyonidae and Phenocodontidae; Order Litopterna (e.g., the horselike Proterotheriidae and Notohippidae, as well as the camellike Macraucheniidae); orders Notoungulata (e.g., rhinoceroslike toxodontids and tapirlike isotemnids); and Amblypoda (e.g., Xenungulata, the rhinoceroslike Dinocerata, and the members of Family Coryphodontidae). The small, horselike Hyracotherium is known from the Paleocene of North America. Van Valen (1988) describes early South American ungulates that were the contemporaries of dinosaurs. In the late Cretaceous of North America, there existed Protungulatum, an ungulate that is abundant in the Montanan Hell Creek formation, where it is found alongside Tyrannosaurus rex. Thus, the idea seems plausible enough that the various early ungulates could have been the ancestors of the various hoofed mammals of later times.
Of course, many of the early ungulates just listed are considered to be unrelated forms that evolved “in parallel” in response to similar “environmental demands.” The similarity is, again, supposed to be due to convergence. In defense of this viewpoint, it should be said that until the 1960s it must have seemed much more reasonable to suppose highly similar types of terrestrial animals could come into being on different continents in a completely independent fashion. Up to that time, convincing evidence (in particular, paleomagnetic evidence of sea-floor spreading) documenting continental drift was unavailable. Moreover, religionist attacks on evolutionary theory claimed only special creation could explain the presence of similar terrestrial animals on widely separated, fixed landmasses. Proponents of evolution perhaps felt compelled to address these objections with any argument they could bring to bear, however implausible it might seem today.
If we still believed continents were immobile, it would be easier to believe that the rhinoceroslike “leontinids” and "toxodontids", and the elephantlike “pyrotheres” of ancient South America might have nothing to do with the rhinoceroses and elephants of Africa and Eurasia. The paleontologists and taxonomists who created our system for classifying fossils had no idea that the oft-noted similarities of these animals could actually be the result of a common descent from precursors living at the time that Africa was still in contact with South America. They could not picture horses swimming from North to South America, which in their minds had always been immobile and distantly removed continents. So horselike prototheriids could have nothing to do with horses. With our knowledge of plate tectonics, however, we can imagine prototheriids evolving from various earlier forms with horselike traits whose descendants later became isolated by water barriers. But we are burdened with a system of classification tainted by the biases of a less-informed era. It's time to re-examine the fossils and reconsider traditional mammalian taxonomy in light of modern knowledge concerning the former positions of the continents.
Under stabilization theory's assumption that similar organisms should share similar ancestors, knowledge of the geographic distribution of extant (or, at least, relatively recent fossil) flora and fauna can be used to patch up gaps in the fossil record. For example, in the present case, the worldwide distribution of fossil ungulates suggests an origin at least as early as the lower Cretaceous, even though a vast gap exists in the terrestrial fossil record (on the order of 60 million years) prior to the late Cretaceous. Continental Cretaceous deposits are abundant only late in the period and even these are severely limited in geographic extent (Mongolia and western North America). Angiosperms were not widespread until the middle Cretaceous. The oldest reliable grass fossils apparently are pollen grains of the grass Monoporites annulatus, dated to the uppermost Cretaceous (although grasses might have begun to proliferate somewhat earlier). Hoofed mammals, which feed largely on grasses, would be relatively rare until the vegetation favoring their expansion became common. The ready availability of grass (which can much better withstand the effects of continual grazing than can most other plants) then, may explain the rapid expansion of ungulate populations in the early Tertiary. For all these reasons, we might expect it to be difficult to trace the earlier course of Cretaceous ungulate evolution through fossils alone. But the presence of a variety of hoofed mammals in the Tertiary of both South and North America prior to the unification of those two continents (at about 5 mya) suggests ungulate evolution may eventually be traced back at least to the early Cretaceous.
Moreover, it may be worthwhile to reconsider the reptile status of certain ornithischian "dinosaurs" of the Jurassic and early Cretaceous with ungulatelike traits (such as the heterodontosaurids and hypsilophodontids). Potential precursors of hoofed mammals existed even in synapsid times, in the late Paleozoic/early Mesozoic. In the shape of its skull, and in the general appearance of its skeleton, the large synapsid herbivore Kannemeyeria (early Triassic) was reminiscent of the much later rhinoceroses (as were the Permian herbivores Pareiasaurus and Scutasaurus). The skull of their contemporary, Estemmenosuchus, resembles that of the much younger uintathere ungulates. Only theory excludes these alleged reptiles from consideration as ungulate ancestors.
Even if these earlier “reptiles” are excluded from consideration, there is certainly no reason to suppose the earliest fossils officially accepted as hoofed mammals are indeed the earliest hoofed mammals. There is absolutely no evidence that the various ungulate forms of the late Cretaceous and Paleocene evolved gradually from the "small, primitive, generalized" Cretaceous placental mammal embraced by orthodox biology. These earliest “officially approved” hoofed mammals are by no means identical to the various hoofed creatures of more recent strata. But hybridization among such forms (and of these forms with other ungulate forms as yet unknown from fossils) may well have given rise to the various and numerous hoofed forms of later times. Presumably the production of such forms would in many cases be as straightforward as was the production of the various modern breeds derived from hybridization of the Zebu (Bos indicus) with European domestic cattle (Bos taurus), a cross that produces partially fertile hybrids of both sexes.
8.1: Dubious Assumptions - © Macroevolution.net