Age of mammals

Figure 1. Skull of Deltatherium fundaminis, one of the first Paleocene mammals described from North America. Its relationships are still debated. From Williamson (1996). For a reconstruction, see the figure below -- it is climbing down a branch near the top center.

Mammals appear first in the late Triassic, at about the same time as dinosaurs. Throughout the Mesozoic, most mammals were small, fed on insects and lead a nocturnal life, whereas dinosaurs were the dominant forms of life on land. This situation changed abruptly about 65 million years ago, when dinosaurs disappeared with the exception of their descendants, the birds, probably as the result of a meteorite impact. The epoch right after the mass extinction of the dinosaurs is called the Paleocene. At the beginning of the Paleocene epoch, the world was practically without larger sized terrestrial animals. This unique situation was the starting point for the great evolutionary success of the mammals. Only ten million years later, at the end of the Paleocene, they had occupied a large part of the vacant ecological niches. By this time, the landscape was teeming with small insectivorous and rodent-like mammals, medium sized mammals were searching the forests for any kind of food they could cope with, the first large (but not yet gigantic) mammals were browsing on the abundant vegetation, and carnivorous mammals were stalking their prey.

Despite this impressive diversification, most Paleocene mammals are still on a primitive level of anatomy in comparison to mammals of today. Often they show only the first beginning of specializations that characterize their descendants from later epochs, such as optimization of the teeth for a special kind of food or adaptations of the limbs to fast running. The Paleocene mammalian fauna is therefore often called archaic. The beginning of the following epoch, the Eocene, brought about an important modernization of this fauna. Several groups of mammals with more modern appearance spread over the northern hemisphere at this time, whereas the decline of the archaic forms started.

The Paleocene is a crucial time in the history mammals. Unfortunately, mammal fossils from this epoch are either scarce or entirely unknown in many parts of the world. Thus we can only speculate how the fauna of whole continents looked after the extinction of dinosaurs. Even where fossils occur, most species are only known from their characteristic teeth, and skeletons are only known for a few forms. Nevertheless, knowledge of Paleocene mammals is steadily increasing. This website tries to summarize what we know about these first ten million years of the age of mammals.

The target of this section of the site is to compile the most interesting facts about Paleocene mammals. Paleontological literature can be a somewhat dry matter, especially if the subject is known from not more than fossil teeth and the entire animal remains largely unknown, as is often true for early mammals. Naturally, biological interpretations are quite limited in such cases. Nevertheless, there are exceptional cases where we can get a more complete idea of the animal and its behavior. I have tried to extract from the literature these highlights, besides giving an overview of the evolution and relationships of Paleocene mammals.

Reconstruction of a middle Paleocene habitat in New Mexico, USA. On the forest floor two mesonychids of the genus Ankalagon are eating the small crocodile Allognathosuchus. On the left the coati-like arctocyonid Chriacus is skillfully climbing up a plane-tree and is watching a Deltatherium on the small branch in the middle. Ankalagon and Chriacus are primitive members of the hoofed mammals, whereas the relationships of Deltatherium are still debated. Drawing by Gregory S. Paul, from Stanley (1989).

The order Multituberculata, informally also known as 'multis', is a diverse lineage of Mesozoic to early Cenozoic mammals that occupied a rodent-like niche. They appear first in the late Jurassic and are last known from the early Oligocene. The late Triassic haramiyids were sometimes considered as early multituberculates. More complete fossils have recently shown that haramiyids are a very different group of early mammals. But even without haramiyids, multituberculates existed for a time span of about 100 million years, the undisputed record for an order of mammals.

Multituberculates do not belong to any of the groups of mammals living today: the primitive egg-laying monotremes and the more advanced marsupials and placentals, both also known as therians. The relationships of the multituberculates to these groups are still debated. Multituberculates have been considered as either a group that branched off even before the monotremes, as close relatives of the monotremes or as sister group of the therian mammals. Anyway, multituberculates were clearly very mammal-like, both in details of the internal anatomy, like the structure of the middle ear with the three auditory ossicles, and in external appearance, like the recently demonstrated possession of hair (see below). The anatomy of the pelvis suggests that multituberculates did not lay eggs like monotremes but gave birth to very small, immature young like marsupials.

In the late Cretaceous multituberculates were widespread and diverse in the northern hemisphere, making up more than half of the mammal species of typical faunas. Although some lineages became extinct during the faunal turnover at the end of the Cretaceous, multituberculates managed very successfully to cross the K/T boundary and reached their peak of diversity during the Paleocene. They were an important component of nearly all Paleocene faunas of Europe and North America, and of some late Paleocene faunas of Asia. Multituberculates also were most diverse in size during the Paleocene, ranging from the size of a very small mouse to that of a beaver.

Figure 1: Reconstruction of the Paleocene multituberculate Ptilodus, about 50 cm in length. From Cox (1998).

Ptilodus is a typical medium-sized multituberculate from the Paleocene of North America. It is known from a nearly complete skeleton discovered in Canada, which shows that Ptilodus can best be compared with today's squirrels. The skeleton of Ptilodus shows several adaptations for life in trees, for instance sharp claws. Like in squirrels, the foot of Ptilodus was very mobile and could be reversed backward, which allowed the animal to climb down trees with its head pointing downward. A marked difference from squirrels is the long prehensile tail, which Ptilodus used like a fifth limb when climbing in trees.

The dentition of Ptilodus and other multituberculates is most similar to rodents in the enlarged front teeth, especially the pair of lower incisors, which are followed by a gap (called diastema) in the lower jaw. The last lower premolars of most multituberculates form large, serrated blades. This type of teeth is called plagiaulacoid after the Mesozoic multituberculate genus Plagiaulax. Several groups of mammals have independently developed plaugiaulacoid teeth, although not to the same degree as multituberculates like Ptilodus:

The back of the multituberculate jaw was occupied by a battery of molars that formed a grinding mechanism. These molars carried several longitudinal rows of many small cusps (or tubercles), which is the origin of the name Multituberculata.

Figure 2: Skull of Ptilodus montanus from the middle Paleocene of Montana, U.S.A, showing the blade-like lower premolar. After Miao (1993).

The diet of multituberculates is a long debated issue. Following the analogy to rodents, it could be assumed that multituberculates were herbivores. However, even today's rodents do not all feed on vegetable material only. In addition, the blade-like lower premolar must be taken into account. The recent rat-kangaroos, which share this feature, include not only herbivores but also omnivores that feed on plants, insects or even carrion. A similar diet can be imagined for multituberculates, too. In this scenario, the enlarged incisors would be have been used for picking up and killing insects or other prey. The blade-like premolars could have served both for biting hard shelled seeds and for chopping up small prey.

Members of a specialized family of multituberculates, the Taeniolabididae, can be considered more confidently as herbivores. Like rodents, taeniolabidids (and some related multituberculate families) had developed gnawing teeth with a self-sharpening mechanism. Only the front side of the incisors is covered with hard enamel. The rest of the tooth consists of softer material which is worn away more easily. Therefore the tooth has always a sharp cutting edge at the front. The blade-like premolars are strongly reduced, whereas the molars form enlarged, complex grinding surfaces with an increased number of cusps. The type genus of the family is Taeniolabis from the early Paleocene of North America. This beaver-sized animal is the largest known multituberculate. Taeniolabis had a massive skull, low in profile, with a short, blunt snout. Its well-developed gnawing teeth and complex molars are strong evidence that Taeniolabis, unlike most other early Paleocene mammals, had specialized on vegetable diet.

Figure 3: Skull of Taeniolabis taoensis from the early Paleocene of New Mexico, U.S.A, about 180 mm in length. Note development of gnawing teeth. From Sloan (1981).

Remarkable fossils from the late Paleocene of China are known for a second taeniolabidid, Lambdopsalis. The skeleton of Lambdopsalis shows specializations for digging, so this multituberculate was probably living in burrows. Well-preserved skulls indicate that the ear of Lambdopsalis was inefficient for hearing high-frequency airborne sound, but well-adapted for hearing low-frequency vibrations, which would be useful for a burrowing animal. Lambdopsalis was obviously a favorite prey of carnivorous mammals, as evidenced by its occurrence in coprolites (fossilized excrements) of carnivores at a Chinese locality. Absolutely unique is the fact that these coprolites preserve fossilized hair of the prey, including that of Lambdopsalis, which is very similar in structure to hair of living mammals. This is the only direct evidence that the extinct multituberculates had hair, and at the same time the oldest known occurrence of fossil mammalian hair.

After the diversity of multituberculates had reached a maximum in the Paleocene, it started to decline towards the end of this epoch. This may be due to competition with an increasing number of placental herbivores, especially with the archaic hoofed mammals called condylarths and with primates. In the latest Paleocene true rodents appeared as additional competitors that were particularly close in ecology. A number of multituberculate genera survived into the Eocene, but few new genera developed, and multituberculates became extinct in the early Oligocene.

An odd group of late Mesozoic to early Cenozoic mammals from the ancient landmass of Gondwana is worth mentioning here since it may represent a southern radiation of multituberculates. These poorly known animals, appropriately called gondwanatheres, were first found in the late Cretaceous (Gondwanatherium, Ferugliotherium) and early Paleocene (Sudamerica) of South America. Rare fossils have recently also turned up in the Late Cretaceous of Madagascar and India, all former parts of disintegrating Gondwana. At least Gondwanatherium and Ferugliotherium had enlarged incisors for gnawing. But the most striking feature of advanced gondwanatheres like Gondwanatherium and Sudamerica are the very high-crowned molars. In the later Tertiary many groups of plant-eating mammals have developed high-crowned teeth to cope with the spreading grasses that strongly wear down teeth. However, such teeth are unique for the Mesozoic and the earliest Cenozoic, and we do not know what triggered their early evolution in gondwanatheres. Grasses did not appear in South America until much later, and fossils of Sudamerica were probably deposited in a mangrove swamp.

Some gondwanatheres were previously considered as early member of the Xenarthra (armadillos, anteaters and sloths) due to somewhat similar teeth and their geographic occurrence in South America, the home of the xenarthrans. Detailed studies of the dentition, including the microstructure of the enamel covering the teeth, have suggested that gondwanatheres may instead be the first 'multis' known from southern continents. More complete fossils will hopefully resolve the debate on the relationships of this enigmatic group from Gondwana.

Figure 4: High-crowned tooth of the gondwanathere Sudamerica ameghinoi from the early Paleocene of Patagonia, Argentina. vertical bar to the left is 1mm in length; the tooth is about 8mm high.The From Scillato-Yané & Pascual (1985).

Marsupials are often associated with the Australian continent where such popular animals as kangaroos and koalas live. Less widely known is that a number of marsupials occur in South America today. These include the opossums, the only marsupials that also occur in the northern hemisphere today.

Surprisingly, Mesozoic marsupials are mainly known from North America, and they may have originated there during the Cretaceous. Primitive opossum-like marsupials are among the most common and most diverse mammals in the late Cretaceous of North America. They include cat-sized forms like Didelphodon, one of the largest Mesozoic mammals - and one of the most widely known thanks to its appearance in "Walking with Dinosaurs". Didelphodon had teeth specialized for crushing, and it has been suggested that it fed on hard shelled food like clams, snails, turtles or even young ankylosaurs.

The diversity of North American marsupials was dramatically reduced at the end of the Cretaceous. Only one single lineage, represented by late Cretaceous Alphadon and Paleocene Peradectes, survived the faunal turnover. Thus marsupials fared hardly better than dinosaurs in North America, and worse than reptiles on the whole. Peradectes survived into the Eocene, and a few other opossum-like marsupials appear in North America during the Paleocene. However, marsupials never became a significant component of northern faunas again and finally became extinct there. Opossums reentered North America from South America only a short time ago, after the Isthmus of Panama had formed in the Pliocene.

Figure 5: Reconstruction of Late Cretaceous Alphadon, based mainly on the appearance of today's opossums since only the dentition of this animal is known. This reconstruction should be equally valid for Peradectes, the Paleocene to Eocene successor of Alphadon. From Cox (1988).

Marsupials were much more succesful on the southern continents. They arrived in South America relatively late in the Cretaceous, but by early Paleocene time this group of mammals was flourishing there. Marsupials make up more than 50% of the mammal species in some South American Paleocene faunas, and adaptive types include insectivores, omnivores, carnivores and small herbivores. Paleocene mammals are not yet known from Antarctica and Australia. However, these continents were connected to each other and to South America at that time, and Eocene marsupials occur there. So it is highly probable that marsupials lived there during the Paleocene as well.

Many Paleocene marsupials of South America are closely related to todays opossums, which are appropriately called "living fossils". Opossums and their relatives belong to the order Didelphimorphia (named after the recent Virginia opossum Didelphis). Pucadelphys, a primitive opossum from the early Paleocene of Bolivia, is known from the best fossils for any early marsupial. Nearly complete skeletons have been found in pairs, which are interpreted as male and female. They were probably trapped in their burrows during a flood. Pucadelphys was an agile animal with some capabilities for bounding and digging, but without strong adaptations for climbing in trees. Like most recent opossums it was probably an omnivore that fed on insects, fruits and small vertebrates.

Figure 6: Pair of skeletons of the opossum Pucadelphys andinus from the early Paleocene of Bolivia, preserved in a life-like way just as resting in their nest. Skeleton on top of the figure faces to the right, skeleton at the bottom (without skull) to the left. From Rich & Rich (1993).

Abundant fossil Didelphimorphia are known from middle Paleocene fissure fillings in Brazil. Members of this order cover a large range of size and adaptation at that time. Minusculodelphis was close in size to the smallest known mammals. It is based on fossil jaws that contain teeth less than a millimeter in size! This tiny animal probably hunted for insects. Eobrasilia was much larger and had bulbous teeth similar to Late Cretaceous Didelphodon. It was previously considered as a relative of the carnivorous Sparassodonta discussed below, but is now regarded as an independent branch of opossum relatives that evolved towards a more carnivorous niche.

A second group of marsupials, the order Paucituberculata, is known in South America from the Paleocene to the present. During the Paleocene the most important members were the extinct polydolopids, rat to rabbit-sized animals that were somewhat similar to rodents in ecology. The middle Paleocene genus Epidolops is known from a nearly complete skull. All other polydolopids are only known from fossil jaws and teeth. Like rodents, polydolopids had enlarged front teeth. Still more distinctive was that some of their cheek teeth, especially the last premolars, formed large blades for cutting food. Similar blade-like cheek teeth have evolved independently in several groups of mammals, for instance the Mesozoic to early Tertiary multituberculates and todays rat-kangoroos. Such teeth could be useful for processing tough parts of plants, but an animal with this dentition could be an omnivore as well. Polydolopids declined in diversity during the Eocene and became extinct in the Oligocene, probably due to competition with other groups of Paucituberculata and with immigrating rodents and primates.

Figure 7: Lower jaw of the polydolopid Epidolops ameghinoi from the middle Paleocene of Brazil. Note the large front teeth and the blade-like last premolar. Later polydolopids like Polydolops developed strong serrations on this tooth. From Paula Couto (1952).

Finally, a group of marsupials started during the Paleocene to occupy the ecological role of carnivorous mammals in South America. These animals are classified in the extinct order Sparassodonta, which culminated in the dog- or bear-like borhyaenids and the saber-toothed cat-like thylacosmilids of the later Cenozoic. Paleocene members of this order demonstrate the transition from primitive opossum-like animals such as Pucadelphys to ferocious predators. Mayulestes from the early Paleocene of Bolivia was a small predator which occupied a ecological niche close to that of weasels or martens. Adaptations like a prehensile tail show that this agile animal lived partially in trees. On the ground Mayulestes was probably capable of relatively fast but short runs to catch its prey.

The first larger sparassodonts are known from the middle Paleocene of Brazil. They include the medium sized genus Patene (meaning 'fox' in a native South American language) and a poorly known wolf-sized predator. These marsupial carnivores developed basically the same tooth structure for cutting meat as the true (placental) carnivores of the northern hemisphere. Unlike true carnivores, however, sparassodonts did not develop particular adaptations for fast running. This may have been one reason for their decline and finally their extinction after the arrival of true carnivores in South America in late Cenozoic time.