by Elsa Panciroli1
The study of the earliest mammals is an exciting part of palaeontology, telling us not only about strange animals that once lived on Earth, but also about how our own ancestors evolved alongside the dinosaurs. Early mammal fossils are very rare and often we only find a few teeth and bones, but we can tell a lot about the animals’ ecology and evolution from these remains. Discoveries of more-complete skeletons, particularly in China, are now revealing that early mammals were more successful and diverse than anyone had suspected. They specialized to exploit new habitats, diets and ways of living that would lead to their ultimate success.
I want to give you an overview of the earliest mammals: mammals from the time of the dinosaurs. We will look at what defines an animal as a mammal; their anatomy; some of the mammal groups alive at the time of the dinosaurs; and, finally, what the latest scientific research is revealing about these extinct cousins of ours from the Mesozoic era.
The ‘mammal-like reptiles’:
Mammals belong to a larger group of animals called cynodonts. The early cynodonts are sometimes called ‘mammal-like reptiles’, because they evolved from reptiles and still had many reptilian features in their skeletons, but were already noticeably like mammals. Early cynodonts probably had fur and whiskers, and almost certainly laid eggs. They had wide zygomatic arches (cheekbones); this is important because it shows that they had an increased amount of muscle around the jaws, giving them more control over chewing and biting than their more reptilian ancestors.
Early cynodont teeth were complex in shape, not just simple pegs or spikes like reptile teeth. Cynodonts also have a secondary palate — the bone that forms the roof of the mouth and allows these animals to breathe through their nose while chewing food. These changes in the way they ate are important for the later evolution of mammals, because they allowed the cynodonts to process food more efficiently – to chew it into smaller pieces – getting more energy from what they ate. This probably helped to fuel them to be more warm-blooded and active, with higher metabolisms. However, the first cynodonts did not have a dentary–squamosal jaw joint (see below), and so they were not true mammals.
Cynodonts first appeared in the late Permian period, long before the first dinosaurs. They survived a mass extinction that occurred 252 million years ago, marking the end of the Permian and the start of the Triassic period. They were successful throughout the Mesozoic, but their success is often overlooked because we tend to concentrate on the dinosaurs and the marine and flying reptiles that populated the world during this period. The first mammals appeared in the Late Triassic, becoming more diverse over time. The last group of non-mammalian cynodonts, the tritylodonts, survived alongside the mammals until the early Cretaceous period, before going extinct. This makes them one of the most successful cynodont groups (Figure 1).
What makes a mammal?
Before going any further, we have to ask what exactly a mammal is. If someone asked you what makes a mammal today, you would probably list characteristics such as fur or hair, the production of milk and being warm blooded. However, for a Mesozoic mammal palaeontologist — studying the very first mammals and their ancestors from the Triassic to the end of the Cretaceous — it is very difficult to tell whether these long-extinct animals had any of these features. Palaeontologists must rely on the fossil record for evidence, and characteristics such as fur and warm blood are not usually directly preserved in the rock record, so we must instead look at what does preserve: the bones and teeth.
A feature or trait that palaeontologists observe in the skeleton or teeth is called a character. These are used to describe the differences between different groups of animals. There are four important skeletal and dental characters that define the first mammals:
• Changes in the jaw — a dentary–squamosal joint
• Changes in the ear — the formation of the middle ear
• Changes in the teeth — diphyodont dental replacement, specialization of tooth types and precise dental occlusion
• Changes in the size of the skull and brain — modifications to the skull, especially the frontal and parietal regions, due to increased brain size
These changes in skeletal structure are all linked, with one often evolving alongside another, or making the next change possible. Let’s look at these defining skeletal characters and find out what they are and why they were vital for the evolution of mammals.
Changes in the jaw:
To understand this change, we must first look at the reptilian jaw, which is made up of several bones, including the dentary, the angular and the articular (Figure 2). The reptilian jaw joint is between the articular and the bone in the skull called the quadrate.
By contrast, in the ancestors of mammals, the dentary got bigger until it made up most of the jaw, while the other bones of the jaw became much smaller. The different parts of the skull were also rearranged. Eventually this meant that the jaw joint changed. The mammals evolved a joint between the dentary bone in the jaw and the squamosal in the skull (Figure 2). This is known as the dentary–squamosal joint, and is still found in modern mammals, including humans.
Although they had the defining mammalian dentary–squamosal joint, some of the very first mammals also kept their quadrate–articular jaw joint; they had both at the same time. The quadrate–articular jaw joint was eventually lost entirely in mammals.
Changes in the ear:
So what happened to the other bones in the jaw, the angular and articular (Figure 2)? In mammals, these bones became progressively smaller until they detached from the jaw and were incorporated into the ear.
A reptile has a stapes bone in the ear, which transmits sound from the eardrum to the inner ear (Figure 2). In mammals, the quadrate and articular got smaller and moved alongside the stapes, and these three tiny bones were used to transmit sound from the eardrum to the inner ear. These bones form the middle ear, and the two bones that used to be in the jaw are the malleus (articular) and incus (quadrate).
The mammalian ear arrangement is much more sensitive to sound than the reptilian one. That would have had big implications for hunting or foraging for food, escaping predators and communication. Having better hearing at different frequencies probably contributed to the ecological diversity of mammals (see below).
Changes in the teeth:
There are three important characteristics of mammal teeth that contributed to the group’s success. The first is diphyodont dentition. This means that the first set of ‘baby’ teeth is replaced by a permanent adult set (as in humans). Sometimes the baby teeth are called milk teeth, because when they are replaced is linked with how long the young feed on milk from their mothers. Producing milk is another unique mammalian characteristic. Mammals grow very rapidly when young, especially their skulls, so that they have space to develop an adult set of teeth as soon as possible. Once they have these teeth, they are no longer reliant on milk from their mothers and can feed on adult food. Many other animals, including reptiles, are polyphyodont, meaning that they replace their teeth continuously throughout their life, with new ones coming in every few months.
The second characteristic is that mammal teeth tend to be specialized into different types (Figure 2). In reptiles, teeth are usually all similarly shaped wherever they are in the mouth. This makes them good for one purpose: grabbing food. In mammals, teeth became specialized so that there were teeth for grasping food at the front of the mouth (incisors), teeth for biting (canines) and teeth for cutting and grinding at the back of the mouth (pre-molars and molars).
This leads us to the third characteristic of mammal teeth: precise occlusion. This means the teeth fit together closely, and allows mammals to process food in sophisticated ways, because food is ground up or sliced as the teeth move against each other.
All three of these characteristics go together: if teeth are replaced only once, they can grow to occlude precisely, which means that they can become specialized to nip, grind, slice and chew in different parts of the mouth. All of this allows mammals to process their food efficiently, enabling them to become specialist feeders and get the maximum energy out of what they eat.
Changes in the skull:
Mammal brains are noticeably larger than those of other animals when compared to body size. On average, a modern mammal brain is around ten times the size of the brain of a reptile with a similar body mass. The earliest mammals had brains around one and half times as big as the brains of their closest relatives, the non-mammalian cynodonts (see above). Differences are especially noticeable in the front of the brain, which is not only larger, but also more complex and folded than in other animals. One of the reasons for this is that mammal senses became more sophisticated, particularly their sense of smell. The olfactory bulb, the area in the brain that processes smell, was much larger in the earliest mammals than in their ancestors. It is probable that many of the first mammals were nocturnal, and so may have needed a good sense of smell to hunt, avoid predators and find one another in the dark.
The skull had to change shape to accommodate the changes in brain size, especially in parietal and frontal regions (Figure 3). Meanwhile, the changes in mammal teeth and jaws meant that the chewing and biting muscles also moved into different positions and expanded, so the skull also altered shape to allow that. The result was a uniquely mammalian skull, different from that of any animal that had lived before.
Early mammal groups:
There are a lot of terms used to talk about the first mammals: mammaliamorphs, mammaliaforms, stem mammals, early mammals, Mammalia. It can be very confusing! The important thing to remember is that these are all animals in the huge and branching tree that we also belong to, with its roots in the Triassic.
The earliest mammals probably had fur, and almost certainly laid eggs like their ancestors. They were much smaller on average than their immediate ancestors, the non-mammalian cynodonts. Most mammals remained mouse- to badger-sized until the extinction of the dinosaurs 66 million years ago. They were probably mostly nocturnal and ate insects to begin with, but in the Middle Jurassic and Cretaceous periods (between 174 million and 66 million years ago) they split into many different groups with different lifestyles, or ecologies.
Early mammal groups include the morganucodontans, docodontans, and multituberculates, as well as the groups that still have living members today, the australosphenidans (the ancestors of monotremes, like the platypus) metatherians (marsupials) and eutherians (placental mammals) (Figure 4).
The morganucodontans include many of the very first mammal species. They probably evolved somewhere in the northern landmasses in the late Triassic, and by the early Jurassic period they were more or less globally widespread. There are very few complete skeletons preserved, but those that are tell us that morganucodontans were mouse- to rat-sized (weighing up to 200 grams), and were accomplished insect eaters. They are best known from their distinctive molar teeth, which had three cusps (the pointed protrusions on the surface of the teeth) in a row (Figure 4).
The docodonts are an especially interesting mammal group. They lived from the middle Jurassic to the early Cretaceous, and most were rat- to ferret-sized. They had more-complex molar teeth than most other early mammals (Figure 4), with shearing and grinding surfaces that evolved again in later mammal groups. They are an offshoot of mammals that did not leave any surviving descendants, but what makes them so fascinating is their wide range of lifestyles. Finds from China reveal that some docodontans evolved to be aquatic, like otters, and may have eaten small fish. Others were specialist diggers, like the modern mole, whereas yet others were tree-dwellers and may have eaten sap (Figure 5). This proves that mammals were flourishing to exploit different opportunities in their ecosystems (see below), adding to our understanding of the diversity of early mammal groups.
The multituberculates are named after the multiple ‘tubercles’ (bumps) on their molar teeth (Figure 4). They had very rodent-like skulls, and their teeth were especially effective at grinding food. Most were mouse- to rat-sized, with a few larger species the size of a domestic cat. Multituberculates are some of the longest-lasting mammals in Earth’s history; appearing in the middle Jurassic, they survived the extinction of the dinosaurs and other giant reptiles (the K–Pg extinction, see below), and persisted until around 40 million years ago.
Humans belong to the group Eutheria, and our origins stretch back into the time of dinosaurs (Figure 4). Eutherian mammals are placental, meaning that they feed their developing baby in the womb through a placenta, and give birth to live young rather than laying eggs. They have unique characters in their skeleton that distinguish them from other mammal groups. One of the earliest known eutherian fossils is Eomaia scansoria, which means ‘Dawn mother who climbs’ (Figure 5).
The latest fossil evidence — ecological diversity:
Until recently, most early mammal fossils were just individual teeth and jaws, with very few whole skeletons preserved. As a result, people generally thought that all early mammals were very similar: small, more or less mouse-like in appearance and mainly insectivorous.
However, in the past 20 years there have been many new fossil discoveries, particularly from China (Figure 5). Some of these are complete skeletons, even showing impressions of fur. These fossils tell us that there was a lot more to early mammals than palaeontologists previously suspected. By the Cretaceous, some mammals were up to the size of a badger; some ate fish, others fruit, and some even ate baby dinosaurs!
We now know there were species that could glide, such as Volaticotherium, using a flap of skin between the arm and leg like a flying squirrel does today. There were swimmers such as Castorocauda, which had a flattened tail like an otter, and mole-like diggers such as Docofossor. These first mammals were exploiting different ecological niches in the environment. The changes that took place in their skeletons (see above) allowed them to adapt to unique ecological niches. This proves that although reptiles such as dinosaurs were the biggest animals on Earth in the Triassic, Jurassic and Cretaceous, mammals were very successful too, living alongside the dinosaurs for millions of years and eventually outlasting them.
The end of the dinosaurs — the rise of modern mammals:
At the end of the Cretaceous, 66 million years ago, there was a mass extinction called the K–Pg extinction event (sometimes still called by its older name, the K–T extinction). It occurred when an asteroid hit Earth just off the coast of Mexico and caused massive global climate change. The dinosaurs died out (apart from the birds), as did the flying reptiles, marine reptiles and a great many other animal groups (including a lot of mammals). However, the mammals that survived recovered very quickly. With the other animal groups gone, there was a lot of empty ecological space, and mammals quickly adapted to fill it. They became larger and began to specialize as carnivores, herbivores, runners and climbers. Within 10 million to 20 million years, many of the modern mammal groups we know today had appeared (Figure 6), including the first whales.
Some people argue that mammals could have evolved into these ecological niches even if the dinosaurs had not died out. This is probably not true, because dinosaurs and marine reptiles successfully filled most of these niches, especially the larger plant and meat-eating spaces in the ecosystem. It is unlikely that mammals would replace them without a driving cause, such as the extinction event. But it is important to remember that mammals were very successful during the time of the dinosaurs.
Mammals appeared at around the same time as the dinosaurs, survived alongside them for 150 million years, and then made it through a mass extinction that killed so many of the other animal groups on Earth. Their unique anatomy meant they could exploit roles in the ecosystem that had never been filled before, and they have become one of the longest-lived and most successful vertebrate animal groups on Earth. They have diversified and occupied ecosystems on every continent, even returning to the sea and taking to the air.
So the next time someone tells you that small mammals are boring, remember the tiny pioneer mammals that outlived even the mightiest giant reptiles — and took over the world!
Suggestions for further reading:
Hu, Y., Meng, J., Wang, Y. & Li, C. Large Mesozoic mammals fed on young dinosaurs. Nature 433, 149–152 (2005) DOI: 10.1038/nature03102
Kielan-Jaworowska, Z., Cifelli, R. L. & Luo, Z.-X. Mammals from the Age of Dinosaurs (Columbia University Press, 2004).
Luo, Z.-X. Transformation and diversification in early mammal evolution. Nature 450, 1011–1019 (2007). DOI: 10.1038/nature06277
Pond, C. M. The significance of lactation in the evolution of mammals. Evolution 31, 177–199 (1977). DOI: 10.2307/2407556
1University of Edinburgh, School of Geosciences, Kings Buildings, Edinburgh, Scotland, U.K. Email: firstname.lastname@example.org, Twitter handle: @gsciencelady