Primate fossils record the history of a group that showed a precipitous rise in diversity and abundance at the beginning of the Cenozoic. Plesiadapoid primates also demonstrated disparity—as many as 12 zoological families are currently recognized. The plesiadapoid primates were an overwhelming presence during the Paleocene because they were occupying a series of open niches for small-bodied generalized herbivores. After the origin and dispersal of true rodents in the Late Paleocene, primates decline, and they never again achieve the abundance and diversity that they had during the Paleocene. After the extinction of the first major primate radiation (the plesiadapoids), the second radiation of euprimates also suffers an extinction event at the end of the Eocene.

The primate order never recovers from these two major extinction events. Its subsequent history has been a history of decline, even though primate groups that fascinate the public (monkeys and apes) had not yet evolved. Given the prominence of non-human primates on lists of endangered species, one could argue that most living primates are doomed to extinction. Both paleontology and historical records illustrate how rapid the decline in primate numbers can be.

Gelada baboons (Theropithecus gelada) exemplify the dire threat of extinction to non-human primates. At many Pleistocene African sites, they constitute half of all the vertebrate fossils discovered. Beginning 1 mya, they suffer a precipitous decline. Geladas now are found only in the mountains of Ethiopia, and they are fast diminishing in numbers even within this last stronghold. During the 1970s, researchers estimated their numbers as being between 100,000 and 200,000 animals; their current numbers in the wild are estimated to be 20,000—a decline of as much as 90 percent in about 40 years (Tucker, 2009)

The origin of fossils

After an animal dies, its behavior immediately stops. Of course! Thus, behavior is the first component of the phenotype to be lost. After this, DNA and soft tissues are also rapidly lost. Large and small animals may eat or scavenge the carcass, dismembering the body, stripping away flesh, and breaking open bones that are rich in marrow. Bacteria and fungi alter soft tissues as decay takes place. Nevertheless, soft-bodied organisms may be preserved as flattened carbon films, preserved as calcium phosphate, or altered by early mineralization. For example, in the 425 mya Eramosa Formation of Canada, animal tissues containing melanin were altered by sulfur early after death; this caused resistance to bacterial decay (von Bitter et al., 2007). Exceptionally well-preserved material from this formation disproved an idea that shallow marine fossils after the Cambrian would be unlikely to fossilize in great detail. It had previously been thought that an increase in burrowing organisms after the Cambrian would irretrievably alter sediments. Specimens from the Eramosa Formation show that this is not necessarily the case.

However, it must be understood that the likelihood of any single ancient organism being preserved is miniscule. It is only the multitude of organisms living over vast reaches of geological time that allows these faint probabilities to emerge as recognizable fossils. A special sub-discipline of paleontology has been created to study all of the processes that affect an organism immediately after death until its discovery as a fossil. This is taphonomy. Techniques used by paleontologists to study taphonomy and taphonomic processes affecting fossils are also used by archaeologists when analyzing archaeological materials and sites. They are also used by forensic anthropologists when analyzing material from a crime scene, especially when considering events around the time of death and after death (Klepinger, 2006).

Major climatic oscillations that are characteristic of the Pleistocene actually begin far earlier, in the Late Cenozoic. Seasonality becomes more pronounced. Temperature and precipitation are no longer equably distributed throughout the year. Aridity increases, and tree cover is lost in many places. Plants that use the C4 photosynthetic pathway (largely tropical grasses that can tolerate prolonged drought) spread very widely, heralding the shift to a “C4 world” (Cerling & Ehleringer, 2000). This cooler and drier global climate foreshadows the outright appearance of continental glaciation during the Pleistocene.

Geochemical signatures of atmospheric carbon dioxide have been followed over the last 20 million years using boron/calcium ratios from fossil foraminifera. During the Middle Miocene (14–10 mya), atmospheric carbon dioxide was roughly similar to that of the present, even though global temperatures were about 3–6°C warmer, and sea levels were about 25–40 m higher (Tripati et al., 2009). A major Late Miocene land mammal extinction event termed the “Vallesian Crisis” occurred in Western and Central Europe at the end of the Vallesian Land Mammal Stage beginning 9.6 mya. Many rhinoceroses and tapirs disappeared, and pig diversity declined. A turnover occurred among the rodents. After the Vallesian Crisis, murid rodents, which include modern mice and rats, become the dominant rodents in Late Miocene communities. Atmospheric carbon dioxide fell during the Late Miocene. Ice sheets in West Antarctica and Greenland grew when atmospheric carbon dioxide fell significantly below modern levels. Glacial conditions thus intensify during the Late Miocene (after ~10 mya) and Late Pliocene (3.3–2.4 mya).

Because anthropoids are higher primates, much research has focused on their origins. However, there are a number of factors that make anthropoid origins problematic. To begin with, there are a number of purported “first” specimens. Fossils from Burma (Myanmar), North Africa, and Germany have been advanced as the first anthropoids. This diverse material indicates that there are differences in opinion about the proto-anthropoids. Were they tarsiiform or adapiform primates? If they were tarsiiforms, were they close relatives of the living tarsiers or an extinct member of the tarsiiform omomyids? A concomitant consideration is this: what is the position of the tarsier lineage? Is it a sister group to the anthropoids, which would be reflected by sorting both tarsiers and anthropoids into the Suborder Haplorhini? Alternatively, did anthropoids arise from adapiform primates?

Resolving the phyletic position of the living tarsiers (Infraorder Tarsiiformes) is crucial before these questions can be answered. During the Eocene, tarsiiform primates had a widespread distribution throughout the northern hemisphere. Living members of this Infraorder (tarsiers) are now found only in some Indonesian islands (Borneo, Sumatra, and Sulawesi) and in the Philippines. Genomic sequences from living primates, spread across about 90 percent of the living genera, demonstrate that tarsiers are an ancient relict lineage distantly related to anthropoids (Perelman et al., 2011). The living tarsiers have the longest lineage documented by fossils of all primates. The earliest member of the Family Tarsiidae is Xanthorhysis, found in the Middle Eocene of Shanxi Province, China, at about 45 mya (Beard, 1998a). But a number of phenotypic traits found in living tarsiers occur in fossil primates with no close relationships to the family. For example, 50 mya crania of Shoshonius cooperi from Wyoming superficially resemble the cranium of Tarsius (Beard et al., 1991).

The Eocene is a critical turning-point in primate evolution. During this epoch, the archaic plesiadapoid primates begin to go extinct, and primates of modern aspect, the euprimates, first appear. The Eocene expansion of the euprimates results in the founding of the major extant primate lineages: lorisiforms, lemuriforms, tarsiiforms, and anthropoids. These modern lineages are all traceable to Eocene ancestors. And, at the end of the Eocene, primates suffer a major decline—something that is also true for other mammalian orders. What was the Eocene crucible of primate evolution like? Details of Eocene climate and geography are well known (Figure 9.1).

The first 30 million years of the Cenozoic were very different from the present climate. Global temperatures were much higher than they are now, and the poles had no or virtually no ice. After the extreme temperature excursion at the Paleocene/Eocene boundary—the Paleocene/Eocene Thermal Maximum (PETM)—Eocene temperatures remained high and stable for a long time. Besides the PETM (Chapter 8), the Eocene experienced a less extreme rise in temperature at 53 mya, and temperatures remained constantly very high until 51 mya. In fact, the very warm and steady global climate between 53 and 51 mya is known as the Early Eocene Climatic Optimum (Zachos et al., 2008). The temperature of the earth slowly declined after this, but tropical habitats were widespread. The Gulf Coast at Laredo, Texas, was ringed with tropical rainforest and mangrove swamp, and therefore resembled the modern mangrove-laden coasts of Southeast Asia (Westgate, 2009). Global temperature peaked again at 42 mya, during the Middle Eocene (Zachos et al., 2008:Fig. 2). By the Late Eocene, temperatures had fallen enough to remove much of the tropical forests from Southern California, and about 40 percent of the non-carnivorous mammals—including primates—go extinct (Tomiya, 2009). Genera over 100 g in size with crushing, bunodont teeth increase in number, and the rodents proliferate. The terminal Eocene witnessed a drastic plummeting of temperature that severely affected mammal evolution, including the evolution of the primates (Chapter 11).

South America has had a strange biogeographic history. It was literally an island continent for most of the Tertiary. This is because the last land connection between Africa and South America was lost in the Middle Cretaceous (about 105 mya), and the South American connection with Antarctica—and hence the South American connection with Australia—was lost in the Late Cretaceous (about 80 mya). Central America did not yet exist, and only gradually came into being after a complicated succession of increasingly larger volcanic island arc systems finally culminated in the current land mass. The Isthmus of Panama did not form a complete land bridge between North and South America until 3.5 mya. The “splendid isolation” of South America yielded a unique fossil mammal history with peculiar ecosystems in which carnivorous marsupials and giant flightless birds preyed on a unique array of placental ungulates. Deciphering the fossil history of South America became a major research subject for vertebrate paleontologists investigating convergent evolution within long-separated lineages of marsupial and placental mammals (Scott, 1937; Simpson, 1980, 1984; Flynn, 2009).

Habitats in South America are also affected by strange types of forest vegetation, such as the narrow, tall Araucaria trees (Family Arucariaceae), whose leaves form tight spirals at the end of branches (Figure 13.1). These trees are otherwise found in Australia. Forests of southern beech (Nothofagus) are also characteristic of tropical and temperate forests in southern South America. There are almost 40 species in the genus, which is classified as a separate family (Family Nothofagaceae). Members of this genus are found throughout the South Pacific Rim, including Australia, Tasmania, New Guinea, and New Zealand. Fossils of southern beech are found in Antarctica. These Nothofagus forests were once found throughout the ancient landscape of the Gondwana continent, before plate tectonic movements tore the continent apart. The lowland tropical rainforests of Amazonia are now a major feature of the modern Neotropics, but their range has drastically expanded and contracted with Pleistocene climatic fluctuations. Archaeological evidence indicates that pre-contact Amerindians extensively altered the Neotropical rainforests through sophisticated agro-forestry, and so the current expanse of lowland tropical rainforests was apparently much different even as late as A.D. 1491.

What is taxonomy?

Taxonomy is the study, identification, and sorting of organisms. It is sometimes satirized as mere “beetle-collecting,” but both Darwin and Wallace began as ardent hunters of beetles and emerged as co-discoverers of natural selection (Berry & Browne, 2008). At its simplest level, taxonomy is a highly practical exercise. Taxonomists identify things. For example, exterminators examining insects that are infesting a house are doing taxonomy when they identify the culprit species; fish biologists are doing taxonomy when they identify fish species in a freshwater lake. A more complex level of taxonomy is classification. This is the formal, scientific identification and categorizing or sorting of organisms. A scientifically accepted group of organisms of any rank is then called a taxon (plural, taxa). The dataset of organisms being studied is provisionally called an operational taxonomic unit (OTU), until a decision is made about the final sorting.

Members of the public often consider taxonomy to be merely mindless and obsessive collecting, saving, and cataloguing, but it is much more than this. For example, the correct identification of pests or vermin afflicting domesticated plants and animals is necessary for the preservation of an adequate food supply; the correct identification of pathogens responsible for human disease is necessary for the maintenance of public health. Conservation biologists working within nature reserves must be able to identify hundreds of species, assess biodiversity, and predict the likelihood of maintaining biodiversity within a reserve. Some recent taxonomic revisions have major effects on both human health and mammal conservation. Anopheles mosquitoes with no discernible physical distinctions have been newly separated into multiple species based on DNA differences. Some of these species spread malaria to humans, while others do not (Paskewitz, 2011). The giraffe has recently been separated into six or more species, and the African elephant has been separated into two distinct species. The African lion exists in two separate subspecies, and one of these has only 500 individuals dispersed across eight West and Central African nations. Instead of being widespread and abundant, these large, well-known, and beloved mammals may therefore be highly endangered taxa (Conniff, 2010; Patterson, 2013).

Old World monkeys are called cercopithecoids, after the Superfamily Cercopithecoidea. The earliest cercopithecoid dates to the Oligocene at 25.2 mya. This is Nsungwepithecus gunnelli from the Rukwa Rift in Tanzania (Stevens et al., 2013). The species is represented by a partial mandible with a single tooth. However, this M3 is a bilophodont tooth (Figure 14.5). The bilophodont molar crown pattern of this species heralds both its cercopithecoid status, and its presumed ability to masticate tough, mature leaves. However, microwear on the teeth of the widespread Eurasian Late Miocene colobine Mesopithecus pentelicus indicates that it was a hard-object feeder, like the modern platyrrhine tufted capuchin Cebus apella (Merceron et al., 2009). Hard seeds or fruit may have constituted the preferred diet of Mesopithecus, rather than mature leaves. Folivory could therefore initially have been a fallback dietary strategy for cercopithecoid monkeys. Analysis of the dentition of Victoriapithecus, the most abundant and well known of early cercopithecoids, indicates that it was probably more frugivorous than folivorous (Benefit, 1999).

Detailed analysis of stomach contents of cercopithecoids and hominoids demonstrates that the bilophodont teeth of cercopithecoids allow them to shred leaves into fine strips (Walker & Murray, 1975). Leaves appear to have been pulled across a grater. The bunodont teeth of hominoids merely rumple the leaves—they enter the stomach relatively intact, with only a few puncture marks. The cercopithecoid ability to shred leaves during mastication hastens digestion once leaves enter the stomach. The leaf-eating cercopithecoids or colobines (Subfamily Colobinae) have higher-crowned molar teeth and more pronounced bilophodonty than the cercopithecines. This allows for heavier mastication. Colobines also possess a sacculated stomach in which endogenous bacterial colonies can break down cellulose. This bacterial activity is known from many experiments on economically valuable domesticated cattle. In cattle, anaerobic bacteria in the rumen chamber of the stomach are responsible for breaking down otherwise indigestible plant cellulose while releasing calories and vitamins. The rumen is a center for biofermentation, and the anaerobic bacteria that it contains are therefore indispensable for digesting plant foods. Similar bacterial colonies in the sacculated stomachs of colobines could not only break down indigestible plant fibers, but also de-toxify natural plant alkaloids that would be poisonous to other primates.

While reading websites, blogs, newspapers, or popular magazines, one frequently encounters a statement like this: “The discovery of new human fossil X completely rewrites the textbooks!” Many editors would set this entire sentence in bold capital letters. Or, “New fossil primate is the first monkey …” or, “New higher primate is the first human ancestor.” Such hysteria has become a normal part of press hyperbole. One expects that virtually every new primate or human fossil will completely rewrite the textbooks. But is it true? Dinosaur paleontology also receives a great deal of attention from both the public and the press. Do new dinosaur fossils mandate a complete rewriting of the textbooks?

A study has been conducted on both Old World higher primates (catarrhines) and dinosaurs, testing to see whether new fossils result in a complete re-vamping of evolutionary history—that is, do new fossil finds repeatedly rewrite the evolutionary history of a group? Tarver et al. (2010) discover that this is not true for catarrhine primates over the last 200 years of study. The basic outline of catarrhine evolution has remained the same since the early twentieth century. New dinosaur fossils, on the other hand, do continually and radically shift our understanding of dinosaur evolutionary history. Many new lineages have been discovered, and new fossils expand our understanding of the geographic expansion of dinosaurs. Our understanding of dinosaur evolution changes rapidly and wildly. Yet, fossils of new catarrhine primates result in virtually no change in the understanding of their fossil record and evolutionary history. Clearly, the mass media is unduly fixated on catarrhine primates. The principal reason for this is that humans are catarrhine primates, and the merest scrap of a new human fossil generates hysteria in the popular press. This also reflects a funding bias. Funding agencies are more apt to focus on primate (including human) paleontology, than paleontological work on other animal groups. Dinosaurs are clearly an exception—major dinosaur research programs have been funded by private donations alone. This is why a test of whether new catarrhine primate or dinosaur fossils truly do rewrite evolutionary history is important. As a physical anthropologist, I am irreverent in pointing this out: dinosaur discoveries trump those of primates in terms of the advance of knowledge. Why study primates at all? Is this just stubborn single-mindedness, or a simple exercise in human vanity?

A major goal of this book is to show that fossil primates (including fossil humans) fit within evolutionary patterns seen among other mammals. What is a mammal? Mammals are a class of animals. That is, they are technically arranged in the zoological Class Mammalia, which is formally characterized by a number of distinctive traits. With the exception of birds, most of the animals with which we are most familiar are mammals. There are about 5,000 living mammal species, although they comprise only about 5 percent of all known animal species. The term “crown species” is often used to refer to living species, in contrast to fossil species, because the living animals appear at the top or crown of an evolutionary tree. Mammals have a very ancient and complicated evolutionary history. Figure 1.1 presents a simplified version of this evolutionary past. A complete and continuous fossil record documents the transition from first reptiles to first mammals. Although a number of “mammal-like reptile” or proto-mammal groups independently evolved mammalian traits, the first creatures identified as true mammals emerge about 200 mya (million years ago). Living mammals occur in three main groups: the ancient monotreme mammals of Australasia, and the closely related placental and marsupial mammals, whose origins are much more recent.

The Cretaceous world

The Cretaceous Period is notable for documenting the evolutionary success of dinosaurs as dominant land animals. The end of the Cretaceous provides abundant evidence of this, particularly in North America. Novacek (2007) refers to the terminal Cretaceous as a “Dinosaur Camelot,” which poignantly invokes lost glories in forgotten landscapes. Flying and aquatic reptiles were also dominant at this time. The rise of angiosperms or flowering plants also occurred during the Cretaceous. As related later in this chapter, at least one researcher, Robert Sussman, believes that the origin of primates was linked to the rise of angiosperm plants. The paleogeography of the earth during the Late Cretaceous and at the Cretaceous/Tertiary boundary is well known (Figures 7.1 and 7.2)

Morphological, molecular, and genetic data have been used to study the relationships of living and fossil placental mammal orders. The plate tectonic separation of Africa and South America that began about 100 mya has been used to explain the divergence of placental mammal orders. However, a recent analysis using both morphology and molecular evidence refutes the notion that the creation of the Atlantic Ocean had anything to do with the divergence of placental mammal orders (Asher et al., 2003).

Introduction

Georges (Baron) Cuvier (1769–1832), who worked in the late eighteenth century, is often identified as the Father of Vertebrate Paleontology. He achieved this eminence by virtue of two things. First, he recognized that some organisms had become extinct. The idea that species were not immortal was a revolutionary one. Species were traditionally held to have remained unchanged and immutable since the point of Creation. Cuvier believed that episodic global catastrophes accounted for these extinctions. A new creation would account for subsequent species. The history of life on earth thus became a series of revolutionary extinctions and creations.

Second, Cuvier recognized that fossil bones and teeth resembled those of living animals, and that these bones and teeth could be used to reconstruct the lifeways of extinct creatures. In fact, it was the fond expectation of Baron Cuvier that future anatomists would discover morphological rules or relationships that would allow an absolute knowledge of the lifeways of an extinct species from even a single bone or tooth. Animals were machines whose bodies worked through mechanical principles. It was the duty of functional morphologists to discover these principles. Cuvier seems to have expected that there would be an inevitable relationship between an animal’s niche and its anatomy. In the same way that mathematicians have discovered the equations for an ellipse or a circle, Cuvier believed that future anatomists would discover the absolute relationships, as rigorous as equations, between form and function. To this end, Cuvier articulated a “law” of correlation of parts.

The Oligocene epoch begins at 33.9 mya. Plate tectonic reconstructions demonstrate that, for the first time, the geography of the earth largely resembled that seen today. In particular, the Atlantic and Mediterranean basins had finally achieved a modern aspect (Figures 11.1 and 11.2). Discussions of anthropoid primate origins and dispersal (Chapter 12) center on these two ocean basins and sweepstakes or rafting events that cross these basins.

During two episodes that straddled the Eocene/Oligocene transition, the world abruptly shifted from a warm to a cold phase. These two episodes lasted a total of 300,000 years. The cold phase that began after this shift has persisted ever since. Antarctica, which had previously been ice-free, except for some areas in the high interior mountains, acquired a permanent continental ice sheet. The Oligocene itself was an epoch of global aridity. So harsh were conditions that global sea level was drastically lowered, and signs of blowing aeolian dust are found in ocean sediment cores worldwide.

Mammals have never recovered from the harshness of the Oligocene. For the first time since the beginning of the Cenozoic, the extinction rate of mammalian families exceeded their origination rate. At the beginning of the Oligocene, 95 families of mammals exist. These fall to 77 families by the end of the epoch. Mammal diversity was thus reduced by 19 percent in only 11 million years—a severe reduction from which the class has never recovered. In Europe, the mammal turnover during the Eocene/Oligocene transition was so marked that it was noted over a century ago by the paleontologist Hans Stehlin (1909), who labeled it "La Grande Coupure" (“The Big Cut”). There are also significant mammal extinctions during the Eocene/Oligocene boundary in Asia. Here the extinction event is labeled “The Mongolian Remodeling” (Meng & McKenna, 1998; Hartenberger, 1998).

The origin of continents and oceans

The title of this section is a translation of a book first written by the meteorologist Alfred Wegener in 1915, and revised and translated many times afterwards. Wegener proposed that a giant supercontinent (Pangaea) had once existed. Portions of this supercontinent split apart and moved away, creating ocean basins and the modern geography of the earth. This hypothesis was called continental drift. However, although Wegener amassed data on the similarity of continental coastlines (especially South America and South Africa), geological strata, and animal and plant distributions to support his hypothesis, he had no workable mechanism to explain the movements of the fragmented supercontinent. Most geologists considered Wegener to be an eccentric crank, although biologists appreciated the fact that continental drift eliminated the necessity for invoking land bridges to explain animal and plant distributions; and biologists further applauded the fact that continental drift rendered massive parallel evolution unnecessary. Wegener’s hypothesis was resurrected in the 1960s, which witnessed a revolution in geological thought. Using new evidence about earthquake and volcanic belts, paleomagnetism, and sea-floor spreading, geologists outlined a new mechanism of continental movement called plate tectonics.

Movements (tectonics) affect plates of lithosphere. Convection currents deep within the mantle of the earth are the ultimate cause of these movements. These convection currents are driven by heat. Radioactive materials exist within the mantle. Their natural decay processes emit heat, and thus a molten iron core lies underneath the mantle. Movements within the mantle drag the overlying crust, and upwelling mantle plumes create local hotspots of volcanic activity, as observed in the Hawaiian Islands. These deep mantle plumes and hotspots can be tracked by volcanic islands and submarine sea mounts as plates pass over the plumes. These plumes can also affect the speed of plate movements and the intensity of volcanic eruptions. For example, the Réunion plume in the Indian Ocean is responsible for the simultaneous rapid motion of the Indian plate and slowing of the African plate beginning 67 mya (Cande & Stegman, 2011). The enormous Indian Deccan flood basalts are adjacent to the Réunion plume, and begin erupting at the same time. These massive volcanic eruptions contribute to the major extinctions that occur at the K/T boundary.