Mammoths, Sabertooths, and Hominids

65 Million Years of Mammalian Evolution in Europe

Jordi Agustí, Mauricio Antón

• Publisher: Columbia University Press
• Year: 2005
• ISBN:
  9780231516334
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Mammoths, Sabertooths, and Hominids takes us on a journey through 65 million years, from the aftermath of the extinction of the dinosaurs to the glacial climax of the Pleistocene epoch; from the rain forests of the Paleocene and the Eocene, with their lemur-like primates, to the harsh landscape of the Pleistocene Steppes, home to the woolly mammoth. It is also a journey through space, following the migrations of mammal species that evolved on other continents and eventually met to compete or coexist in Cenozoic Europe. Finally, it is a journey through the complexity of mammalian evolution, a review of the changes and adaptations that have allowed mammals to flourish and become the dominant land vertebrates on Earth.

With the benefit of recent advances in geological and geophysical techniques, Jordi Agustí and Mauricio Antón are able to trace the processes of mammalian evolution as never before; events that hitherto appeared synchronous or at least closely related can now be distinguished on a scale of hundreds or even dozens of thousands of years, revealing the dramatic importance of climactic changes both major and minor. Evolutionary developments are rendered in magnificent illustrations of the many extraordinary species that once inhabited Europe, detailing their osteology, functional anatomy, and inferred patterns of locomotion and behavior. Based on the latest research and field work, Mammoths, Sabertooths, and Hominids transforms our understanding of how mammals evolved and changed the face of the planet.

CHAPTER 1

The Paleocene: The Dark Epoch

ACOMMON SCENARIO TENDS TO POSIT THE EARLY EVOLUTIONARY radiation of placental mammals as occurring only after the extinction of the dinosaurs at the end of the Cretaceous period. The same scenario assumes a sudden explosion of forms immediately after the End Cretaceous Mass Extinction, filling the vacancies left by the vanished reptilian faunas. But a close inspection of the first epoch of the Cenozoic provides quite a different picture: the “explosion” began well before the end of the Cretaceous period and was not sudden, but lasted millions of years throughout the first division of the Cenozoic era, the Paleocene epoch. Following the partition of the Tertiary period by Charles Lyell into the Eocene, Miocene, and Pliocene epochs, the paleobotanist W. P. Schimper added the Paleocene in 1874 to include a number of fossil floras from the Paris Basin that preceded the first Eocene levels. These Paleocene strata were characterized by the presence of primitive mammals preceding those of the Eocene epoch, in which the first members of the modern orders were already recognizable (perissodactyls, artiodactyls, rodents, bats, and others).

The term dark has been applied to the Paleocene epoch in the title of this chapter for two main reasons. First, our knowledge of this remote time of mammalian evolution is much more obscure and incomplete than our understanding of the other periods of the Cenozoic. Second, compared with our present world, and in contrast to the succeeding epochs, the Paleocene appears to us as a strange time, in which the present orders of mammals were absent or can hardly be distinguished: no rodents, no perissodactyls, no artiodactyls, bizarre noncarnivorous carnivorans. In other words, although the Paleocene was mammalian in character, we do not recognize it as a clear part of our own world; it looks more like an impoverished extension of the late Cretaceous world than the seed of the present Age of Mammals. But the seeds were there.

THE AFTERMATH OF A MASS EXTINCTION

Spanning no more than 10 million years, between 65.5 and 55.5 million years ago, the Paleocene began after the great event at the end of the Cretaceous that ended more than 150 million years of reptilian domain over the continents and seas. Despite its catastrophic biotic effects, this event seems not to have deeply affected the long-term, overall conditions of the planet. On a global scale, there appear to have been few variations between the late Cretaceous and the early Paleocene climates. The analysis of plant distribution in Canada immediately after the crisis supports this statement, indicating a quick recovery of species of palms and screw pine, in proportions similar to those existing now in Southeast Asia (Nichols et al. 1986).

The distribution of landmasses during the Paleocene was quite different from the arrangement of land today. From the breaking away of the supercontinent Pangaea at the beginning of the Triassic, new marine domains opened throughout the Mesozoic, such as the Atlantic and Indian Oceans. At the beginning of the Paleocene, there were still some remnants of the broken Gondwana, the southern Pangaean continent. Thus Australia and South America were still attached to Antarctica (which was about where it is today). Africa and India, the other original Gondwanan “fragments,” were far from both these southern continents and their present position close to Eurasia. At this point in the Paleocene, they “floated” isolated from the other continental landmasses, surrounded by the Atlantic and Indian Oceans and the Tethys Sea. The Tethys Sea formed an east–west continuous marine belt that separated North America from South America and Eurasia from Africa and India. This means that there was a continuous equatorial current, from the eastern to the western Pacific, throughout the Indian and Atlantic Oceans and the Tethys Sea. Along the coasts irrigated by this warm current, tropical reefs and corals recuperated after their near demise during the End Cretaceous Mass Extinction. Giant protists called alveolines, in symbiosis with green microscopic algae, formed massive banks of limestone, now embodied and pushed up into mountain ranges such as the Pyrenees and the Alps as a consequence of the Alpine orogeny. To the north of this Tethyan equatorial current, North America and Europe were still connected through Greenland, as shown by the similarity of the Paleocene and early Eocene mammalian faunas of both continents. North America was also connected to eastern Asia through the Bering corridor. In contrast, Europe and Asia were separated during most of the Paleocene and Eocene by a shallow sea that extended through the eastern margin of the Urals (the Turgai Strait) and connected the Arctic Ocean to the Tethys Sea.

Temperatures at the beginning of the Paleocene were 2 or 3°C lower than those of the late Cretaceous. From the early Paleocene to the middle Eocene, the average surface temperature of the oceans underwent a gradual increase, reaching levels between 2 and 4°C warmer than today. However, evidence indicates that during the Paleocene, temperatures fluctuated from cooler to warmer and again to cooler by the end of this epoch. The climate was humid, and the dominance of arboreal taxa indicates the extension of canopy rainforests over most of the continents. There is also geological evidence for the existence of dry–wet fluctuations, as shown by the huge amounts of gypsum and salts that were deposited subsequent to the desiccation of ancient lakes and basins, such as those of Ager and Tremp to the south of the Pyrenees.

THE CRETACEOUS INHERITANCE OF THE PALEOCENE

The first evolutionary radiation that led to Paleocene mammalian diversity began well before the end of the Cretaceous, and by the late Cretaceous a varied fauna of placental mammals already existed, including several insectivores and some primitive ungulate and primatelike species. These eutherian taxa joined other previously successful mammalian groups, such as the multituberculates and the marsupials. With the exception of this last group, which was severely affected by the End Cretaceous Mass Extinction (they declined from nine to one marsupial genus), all the remaining groups traversed this boundary without major variations. Therefore, the basal Paleocene mammalian faunas were not so different from those of the late Cretaceous and were basically composed of a number of families rooted in the Mesozoic. Among them, the most successful and diversified one was the multituberculates.

The multituberculates were a peculiar group of primitive mammals whose origins can be traced back to the early Jurassic and, perhaps, even the late Triassic. They have been so far the most successful and longlasting order of mammals, having survived for more than 100 million years until their extinction during the Oligocene. They were not truly therian mammals and were probably closer in biological terms to the living monotremes than to marsupials and placentals. The cranial and dental anatomies of multituberculates looked like those of rodents, with long, chisel-like incisors separated from the cheek-teeth by a large space without teeth (diasteme). The term multituberculate refers to their peculiar dental morphology, each cheek-tooth displaying a number of parallel rows of small cusps that operated against a similar counter-row in the upper or lower jaw (multituberculate means “several cusps”). Altogether, the masticatory apparatus of the multituberculates formed, as in present rodents, an efficient chopping device.

From this basic rodentlike design, the multituberculates radiated into a variety of morphotypes during the Cretaceous and the Paleocene. Some of them, like the ptilodonts, were squirrel-like arboreal forms. The most outstanding feature of the ptilodonts was the peculiar shape of their last lower premolar, larger and much more elongated than the other cheek-teeth, its occlusive surface forming a serrated slicing blade. This was perhaps used for crushing and opening hard seeds and nuts. However, most of the small multituberculates like the ptilodonts probably supplemented their diet with insects, worms, and fruit.

Thanks to the well-preserved specimens of the ptilodont Ptilodus recovered from the Bighorn Basin in Wyoming, we know that these multituberculates could abduct and adduct their hallux and had the foot mobility characteristic of such arboreal mammals as present-day squirrels, which descend trees headfirst.

A group of European multituberculates developed a dental design like that of the ptilodonts, with an enlarged bladelike lower premolar. These were the kogaionids, first known from the late Cretaceous beds of Hateg in Romania. The most successful genus of this family, Hainina, was once thought to be a ptilodont because of its large, bladelike premolar in the lower jaw. However, further analysis has shown that Hainina and its late Cretaceous ancestor Kogaionon were primitive multituberculates, with molars bearing a smaller number of cusps and retaining a fifth premolar, a characteristic relating them to some Jurassic genera and not to the late Cretaceous ptilodonts. This unique combination of archaic and advanced characteristics indicates that a large, bladelike premolar was independently acquired at least twice during the late Cretaceous, by both the North American ptilodonts and the European kogaionids.

The taeniolabids were quite different from Hainina and the ptilodonts. They were a group of multituberculates that, unlike the latter, had a much heavier, more massive anatomy. They reached the size of a beaver and probably had a fully terrestrial lifestyle. While the ptilodonts succeeded in North America and the kogaionids in Europe, the highest taeniolabid diversity was found during the late Cretaceous and Paleocene in Asia, suggesting that taeniolabids originated there.

Besides the multituberculates, a diversified fauna of relatively nonspecialized placental mammals existed at the end of the Cretaceous and persisted into the Paleocene. Most of them were originally included in the order Insectivora because of their archaic dentition, which indicates insectivorous habits. Insectivorousness requires the crushing of hard but fragile exoskeletons, made possible with individual tapering points rather than long blades, as in the carnivores. However, other than sharing a number of archaic, nonspecialized features, there is little positive evidence for assembling these archaic groups into a natural phylogenetic category. They took part in the first placental evolutionary radiation during the late Cretaceous, which continued during the Paleocene.

The leptictids best exemplified the characteristics of this group. The leptictids were archaic “insectivorous” placental mammals that originated during the late Cretaceous and became extinct during the Oligocene. Their cranial and dental anatomy was so archaic and basic for a placental mammal that establishing their close relationship to other groups is difficult. Most of the postcranial anatomy and the lifestyle of this group have been inferred from the best preserved specimens of Leptictidium from the middle Eocene of Messel, Germany (figure 2.7). According to this evidence, the leptictids were small placentals, with a body length between 60 and 90 cm, that bore a complete, archaic dentition including incisors (two or three), canines (one), and V-shaped premolars (four) and molars (three). The head ended in a long, slender snout that probably displayed a short trunk. This trunk likely was used for scratching in the undergrowth in search of insects and worms. According to the middle Eocene specimens of Leptictidium, the forelegs were extremely shortened, while the hind legs were elongated. This suggests a kind of locomotion similar to that of small kangaroos or jerboas, which use their elongated hind limbs for jumping. However, the leptictids’ tarsal anatomy contradicts this supposition, indicating a specialization for running on the ground. Most probably, they were capable of both kinds of locomotion, running slowly on the ground in search of food and jumping quickly in case of danger. A surprising feature of the Messel specimens is the extraordinarily long tail, formed of about forty vertebrae—a unique feature among modern placental mammals. They probably used the long tail for balance while jumping or running quickly.

Like the leptictids, the palaeoryctids are known from the late Cretaceous of North America and had a generalized placental anatomy. From a nearly complete skull of Palaeoryctes from the Paleocene of New Mexico, we know that they were probably small, shrewlike insectivores with a long snout like that of the leptictids. In contrast to our knowledge of that of the leptictids, we know little about the palaeoryctids’ postcranial anatomy. Unlike the short-lived leptictids, the palaeoryctids seem to have been ancestors of one of the groups that was going to succeed in the Eocene. Thus although the dentition of the palaeoryctids still indicates a mainly insectivorous diet, some details of their dental morphology relate them to the creodonts, the carnivorous order that filled the predator guild during the Eocene.

Another archaic, “insectivorous” placental group was the pantolestids. As with the leptictids, the best evidence of the pantolestids’ full anatomy and lifestyle comes from the beautifully preserved specimens of the middle Eocene of Messel. According to the data provided by Buxolestes from this locality, as well as other, less complete specimens, the pantolestids were semiaquatic fish predators with a body of about 50 cm that ended in a long tail of about 35 cm. They bore moderately strong canines and multicusped cutting teeth, which were supported by a strong jaw musculature. The forearms were powerful and ended in large bony claws. The ulna and the radius were free, allowing wide rotational movement. Both features probably indicate its ability to dig and build underground dens. The hind limbs were also powerful but could not be rotated in the same way as the forelimbs. The first vertebrae of the tail presented strong transverse expansions, or processes—which suggests that the tail was moved powerfully in the water, with movements reminiscent of those of other semiaquatic mammals like otters.

A fourth group of small archaic placentals that are often included among the insectivores was the apatemyids. Unlike the leptictids and pantolestids, the apatemyids were rather specialized forms bearing a complex dentition. In relation to their relatively small body, they had a large skull armed with two extremely large, curved incisors. These operated against a similar pair of strong procumbent incisors in the lower jaw and were followed by a serrated, scissorlike battery of premolars. In contrast to this specialized frontal dentition, the molars were relatively small. From the evidence available from another middle Eocene genus from Messel, we know that the apatemyids used their sophisticated dentition to tear open the bark of wood in their search for insect larvae. The apatemyids were common in North America during the Paleocene, being represented in Europe by Jepsenella...

Table of contents

• Cover 
• Half title
• Title
• Copyright
• Contents 
• Preface
• Acknowledgments
• Chapter 1. The Paleocene: The Dark Epoch
• Chapter 2. The Eocene: Reaching the Climax
• Chapter 3. The Oligocene: A Time of Change
• Chapter 4. The Early To Middle Miocene: When the Continents Collide
• Chapter 5. The Late Miocene: The Beginning of the Crisis
• Chapter 6. The Pliocene: The End of a World
• Chapter 7. The Pleistocene: The Age of Humankind
• Bibliography
• Color Plates
• Index