I approached the table, holding my breath. In front of me were two palm sized, flat rocks. They held two halves of one animal, pressed between their solid mustard pages.
The skeleton was surrounded by a dark smudge that appeared to be the remnants of a fuzzy body. It was small enough to pop in your pocket. The head and shoulders were in profile, and it looked for all the world like a tiny Nosferatu tip-toeing across the stone with claws outstretched.
This was the Jurassic ‘mole’, Docofossor brachydactylus. Over 100 million years before today’s denizens of the underground, this little creature was the first mammal to fully adapt to life beneath the surface.
Spectacular early mammal fossils like these reveal how animals interacted with their ever-shifting environments – with tell-tale consequences for their anatomy. They tell us how mammal bodies have been assembled and shaped through deep geological time, and importantly, their repeated convergence on the same adaptations to meet the challenges of survival.
Docofossor had long turned to stone, but it felt light in my hand as I gingerly lifted it to peer closer. Its skull was snubby, with teeth similar to those of modern mammals that forage for worms. The limb bones were proportionally short, but it had long sharp elbows; these provided lots of attachment area for powerful upper arm muscles to dig through soil.
There was one further attribute that set it apart from all that went before: its most impressive front feet. These were nature’s first ‘mole’ paws.
Moles in the modern world include animals from completely separate families. Although not closely related, they have several things in common, including their shovel-shaped paws.
When most people talk about moles, they mean members of Talpidae, which are closely related to shrews and hedgehogs. These scourges of the perfect lawn include the European mole (Talpa europaea), the short-faced mole in Asia (Scaptochirus moschatus), and the eastern mole (Scalopus aquaticus) in North America.
They have liquorice-black to chocolate-brown velvety fur, chubby round bodies with short limbs, and pointed noses. This family is made up of specialist diggers with body plans modified for their subterranean lifestyle.
Mole front feet are wide, with long, thick claws ideal for shifting dirt to make tunnels and find food. They lack external ears, and have tiny eyes – sight, after all, is not very important to them. Touch, on the other hand, is vital: many moles have sensitive Eimer’s organs in their noses, modified skin cells packed with nerves that help them find food and thrive in the complete darkness of the earth.
But being mole-like is not confined to talpids. Two other groups of modern mammal include members that have convergently developed the same adaptations. The golden moles, Chrysochloridae, are common in southern Africa and related to other animals of that continent such as elephants and aardvarks. Meanwhile in Australia there is a marsupial mole, Notoryctes.
Like its above-ground kin, Notoryctes has a pouch in which to raise its young, which opens to the back to prevent it from filling with sand. Everything else about these animals screams of the underground. They have become mole-like in similar ways, despite sitting at entirely different tips of the mammal tree. Natural selection has acted on them to solve the same ecological problem, and in each case has come up with the same results.
This sheds light on two fundamental rules of evolution: convergence, and the relationship between form and function. For palaeontologists studying extinct life, these are crucial to understanding the fossil record. One of the big surprises of the last two decades of research on ancient mammals, is that many of the lifestyles we see in modern mammal groups appeared much further back in time than we previously expected.
Docofossor was living the mole-life in the time of dinosaurs, long-before the first member of Talpidae appeared.
Turning our attention even further back in time, we discover more than 252 million years ago the first animals to specialise in plant-eating and hyper-carnivory. Many of them also belong to our mammal-lineage, called the synapsids.
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To understand why synapsids (including mammals) look and behave the way they do, we need to step-back over 350 million years. The common ancestor of all four-limbed, backboned animals was a early tetrapod. Their fossils are found in rocks that once sat at the equator, but are now part of Greenland and the Canadian Arctic. We don’t know which was our direct ancestor – this is not something we can say about any fossil – but our ancestors were probably similar to animals like Acanthostega.
It looked like a salamander the length of your arm, with a flat head. Its eyes were on the top of its head, looking skyward in permanent exasperation. Four limbs stuck out around it like a skydiver’s, with digits wide to form paddles, and a rudder-like tail. early tetrapods didn’t evolve their adaptations for life on land, but the adaptations they already had for pushing aside foliage in the water turned out to be useful platforms from which to launch themselves out there.
Hold up your hand, palm facing away from you. Anatomists have allocated your fingers ascending numbers: your thumb is number one, your pinkie is number five. This numbering system is universal in all tetrapods because we share a five-fingered body plan dating back to our common ancestor. As animals have specialised for different environments and lifestyles, this basic plan has been altered and modified in a kind of evolutionary recycling. But all limbs and digits are homologous, a fancy way of saying that they share the same evolutionary origin, even if their function changes. So whatever happens, your middle finger will always be number three, even if you lose some fingers through natural selection, mutation or accident.
Surprisingly, Acanthostega had eight fingers and toes on each limb, while other early tetrapods had seven. It wasn’t until later that five became the magic digital number, probably for practical reasons during weight-bearing, because too many fingers may have been cumbersome, reducing the flexibility of the wrist or ankle and making harder to move on land.
As vertebrates left the water, they took advantage of the fact that air has up to 30 times more available oxygen than water. Changes in their skeletons helped them survive out of water: stronger vertebrae, larger limbs, the restructuring of the ankle to support their body weight and optimise foot movements for walking.
As they started breathing air using their chest muscles (instead of pumping it into their lungs using their jaws) they were free to re-purpose their skull and jaw muscles for new types of feeding. For the first time they snaffled vegetation, a diet that requires skilled gripping and pulling. Some of these first tetrapods, called amniotes, also developed the shelled egg. These are the ancestors of all mammals (and reptiles). With a chesty sigh, they strode from the water’s edge, laid their eggs, and took on the world.
It is a common misconception that mammals evolved from reptiles. We now know this is not remotely true. Mammals and reptiles share a common ancestor, but this amniote was neither mammal nor reptile, because those groups had not evolved yet. The split between them runs deep, cleaved in the Carboniferous world.
Mammals belong to the bigger group Synapsida, which means ‘one arch’. There is a single hole behind the eye in synapsids, creating an arch in the skull bones. You can feel it by placing your fingers in the hollow behind your eye, above your cheekbone. If you were a reptile, you would have two such holes in each side of your skull (or none if you were a turtle). These skull holes are one of the fundamental differences between the reptile-line and mammal-line. The opening provides space for muscles that open and close the mouth, so it could be that the different arrangements were linked to different ways of biting and feeding in our ancient ancestors.
With the basic body-plan in place, synapsids achieved shocking diversity long before the dinosaurs appeared. In the Permian, the mammal-line accomplished such a range of sizes, shapes and ecologies that this period could be called the first Age of ‘Mammals’. These are not the cold-blooded ‘mammal-like reptiles’ of old parlance, but the core of the first complex ecosystems on land. They included swift, sabre-toothed hunters and dangerous armoured browsers, climbers in the highest branches and diggers of deep soil. These trend-setting critters streamlined vegetarianism and gave apex predation its test drive.
You may already know one of the most famous mammal-relatives from the Permian: the sail-backed Dimetrodon (above). Many people’s earliest encounters with it are from cartoons; in Fantasia, it lolls by a pool enjoying Stravinsky. Its sprawling posture and long body were superficially ‘reptile-like’, but it was in fact our distant relative.
The iconic sail-back probably served to attract a mate, like a peacock’s tail. Its big head was full of pointed teeth, sharpened for meat-eating. Later meat-eating synapsids elongated their canines into the first sabre-teeth, and their posture changed, providing speed for the chase.
It was in the Permian that herbivory also took off, a landmark moment for tetrapods. Plant-eating poses unique challenges, one of the biggest being that plants are made of tough cellulose, which can only be broken down with certain enzymes that vertebrates don’t possess. This meant that despite foliage flourishing across our planet, animals couldn’t get enough nutrition from eating it to give it a taste. It was like walking through a well-stocked supermarket with an empty wallet.
To get around the problem, herbivores harnessed the power of microbes. Plant-eaters today, like cows, sheep, wildebeest, or llamas, have big bodies filled with fermentation chambers where friendly bacteria work to disassemble cellulose for their host. They have lots of intestine too, squeezing every last nutrient from their leafy chow. Those that don’t chew their food manage meal-times with proportionally smaller mouths than meat-eaters – it is, after all, just a hole to put the plants into. Big neck and shoulder muscles are helpful to power repetitive browsing and grazing motions, and blunt, spatula-shaped teeth clamp down on leaves and shear them off from their stalks. Among modern mammals, ungulates are a massive and wildly successful group that have made eating veg their trademark move.
The rules have changed little in 280 million years. Herbivore body plans are evident in the ancient mammal-line: many are massive, bulky creatures, built like nightclub bouncers. They were probably the first to employ micro-organisms in digestion, maybe initially ingesting them when they ate decomposing plant matter, or plant-feeding insects. Some of the bacteria survived in the gut, and a symbiotic relationship developed as animals with more micro-organisms extracted more nutrients, improving their survival. Although herbivory evolved independently in multiple animal groups, it is in synapsids that we find some of the first dedicated adaptations. Their lineage were at the forefront of this niche for the next 50 million years.
There is evidence from their bones that the metabolisms of ancient mammal relatives were already elevating – the first hints of warmer blood. Controversial fossils suggest some may have possessed a few hairs, although solid evidence for fur is not known until the time of dinosaurs. The Permian synapsids built an astonishing ecosystem, laying the foundations for predator-prey relationships ubiquitous among tetrapods ever since. This astonishing time period was brought to a crushing halt by a mass extinction that nearly wiped out life on Earth, shuffling the deck of life completely.
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In the time of the dinosaurs, reptiles took over the large-bodied niches in Earth’s ecosystems. The mammal-line set off in a different direction: they shrank. This was a not a relegation, but an innovation. Indeed, of the 5,500 plus mammal species on Earth today – mostly rodents – 90 per cent are small, many less than 1kg (2.2lb). They seized the Mesozoic moment with endothermic gusto, combining increasingly warm-blood with care for their young through milk-feeding. Their complex teeth could process a smorgasbord of foods, and we know from the lack of colour-processing structures and genes in modern mammals that these ancestral critters were mostly nocturnal, a lifestyle still dominant in living groups.
This brings us back to Docofossor and its Jurassic friends, with their incredible plethora of adaptations. There are some snags when analysing animals this far back in time. The first is the paucity of complete fossils. Scientists long assumed mammals only radiated and explored new niches after the disappearance of the dinosaurs.
Their once-paltry fossil record comprised mostly teeth and jaws, giving no hint at the incredible ecological diversity they achieved so early in their evolution. Aalthough there are now spectacular skeletons from places like China, among hundreds of early mammal species named, most are still only known from teeth and jaws. For fossils that include limb bones, those bones are often damaged or squashed. This makes them difficult to study, compressing the features researchers need for their analyses.
But perhaps the most challenging issue of all is that these animals are separated by hundreds of millions of years of evolutionary change. Superficially, we might think mammals in the time of dinosaurs resemble mice or shrews.
Their bones however, tell a more complicated story. Many are a mixture of shapes inherited from their predecessors, alongside new features. For small-bodied animals, there are additional problems: tiny creatures don’t need to adapt their skeletons much to move in new ways, because they are light enough to climb or scoot using existing musculature. This means the signals in their bones that would reveal how they lived, are far harder to detect.
Researchers have developed new techniques to understand how long-lost animals moved and lived. Did they run, hop or clamber across the face of the Earth, and what was their role in their environment, their ecology? These old bones have long hinted at their lifestyle through their shape, a relationship known as form and function. It’s an old concept, but the way we explore it has changed a great deal. We can now compute enormous datasets of bone shape to test hypotheses about form and function mathematically, to see if they are statistically significant.
One of the most common ways to do this is through geometric morphometrics. This method uses points on the bones that together capture its overall shape, creating a constellation of landmarks. It’s a bit like motion-capture filming used in special effects: by placing landmarks in the appropriate points you track the parts of the bone important for movement, often the sites of muscular attachment. By identifying the same landmarks on large datasets of different species, researchers can statistically compare their distribution across the entire dataset, rather than simply eyeballing each specimen.
The results are plotted on a chart, and the position an individual animal’s datapoint should tell us about the shape of their bones compared to other animals. Hopefully, all of the specialist diggings cluster together on the chart, whereas those that habitually climb and live in trees will cluster elsewhere. You can test these results statistically, and by carrying out this analysis using a dataset of modern animals – for which locomotion and ecology are known – you can then add a fossil to see where it plots.
With luck this will tell you something about how they lived in their environment.
Analyses of bone shape are now being deployed on mammals from the time of dinosaurs, testing their true ecological diversity. Palaeontologists have also nicked a few tricks from engineering, using methods for testing the properties of building materials and applying them to fossils to understand their capabilities, such as the biting strength in jaws. All of this reveals a far richer world that previously thought, one in which most of the major tricks animals pull today were test-driven and manufactured long ago by natural selection as solutions to survival.
Using fossils, we are tugging back the curtain to see the mechanisms of evolution in deep time. Although direct comparisons between a living animals and their long-lost cousins may be hard to interpret because their bodies aren’t arranged in quite the same way, the results of these transformative analyses have repercussions for our understanding of ecology and biology, as well as applications for human and veterinary medicine.
By understanding how extinct animals lived and adapted to their environment – and more crucially, to environmental changes – we can learn lessons that apply to conservation efforts and climate change mitigation. With each new piece of evidence, the tantalising path of life is further revealed.
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