Eons

In 2003, a fossil was discovered in an ancient lakebed in whats now China.

It was an almost complete skeleton about the size of a platypus, with teeth that resembled a seals.

And in the rock surrounding it, there was a shadow.

That shadow was the preserved impression of skin and other soft tissue including a furry pelt, webbed feet, and a paddle-shaped tail.

It looked like a typical aquatic mammal, the kind you might see near a river or lake today.

But this creature lived in the Jurassic period, 164 million years ago, and it wasnt technically a mammal.

It was a mammaliaform, part of the group that includes mammals and our ancient relatives.

And for a long time, scientists had thought of the mammaliaformes that lived during the Mesozoic Era as small, shrew-like insectivore generalists scurrying in the shadows of dinosaurs.

But this discovery complicated the history of mammaliaforms...

It painted a picture of their explosive diversification, their mysterious disappearance, and how our own ancestors might have survived thanks to a leg up from some leafy allies Now, we know that placental mammals like us humans belong to a group called Eutheria, while marsupials like kangaroos and koalas belong to a group called Metatheria.

Eutherians and metatherians share a more recent common ancestor than other mammal groups, so together were referred to as therians. And monotremes, like the platypus and echidna, are our next closest living relatives.

Together, therians, monotremes and all the animals back to our most recent common ancestor form whats called the crown-mammal group.

The mammaliaformes are basically one more step removed from there.

Its the group that includes crown-mammals and their most recent common ancestor plus all the mammal-like animals that are more closely related to mammals than they are to anything else.

The common ancestor of the mammaliaformes would have looked something like Morganucodon a small, insect-eating generalist that scurried through the undergrowth of the Late Triassic, around 200 million years ago.

And the descendants of mammaliaformes like Morganucodon would eventually give rise to our mammal ancestors, which were also small insectivores.

For a long time, it was thought that all Mesozoic mammaliaformes were built like this because of the imposing ecological presence of the dinosaurs.

Many thought it wasnt until the Cretaceous-Paleogene extinction, that space was freed up for the mammals to adapt to new niches.

But mammaliaformes were rare in the fossil record.

And paleontologists were lucky to find more than a few teeth or parts of a jaw.

So this hypothesis wasnt exactly based on a ton of material.

Occasionally, more substantial fossils were uncovered, but even they were mostly tiny, like the genus from China known as Zhangheotherium.

And while its true that most mammaliaformes did fit this description, a site in Northern China has recently flipped the script.

There, layers of Jurassic lake sediment and volcanic ash deposits have preserved the fossils of a huge variety of organisms.

And thats where our mysterious, paddle-tailed mammaliaform was discovered.

It belonged to an ancient group called the docodonts, and was named Castorocauda from castor meaning beaver and cauda meaning tail, after its, well, beaver-shaped tail.

Measuring at least 42.5 centimeters long and weighing up to 800 grams, it may not have been huge by our standards, but its the biggest Jurassic mammaliaform ever found.

Castorocauda also sported the oldest evidence of a furry pelt, which indicates it was warm-blooded.

This suggests that some of the characteristics we associate with crown-mammals are actually ancestral traits that existed long before we emerged.

But its size and its fur are only part of what made this fossil so special.

Castorocauda was a far cry from a tiny, scurrying generalist.

With its webbed feet, paddle tail and teeth adapted for catching fish, the scientists who described it recognized it as a semi-aquatic specialist.

And this pushed the emergence of the first aquatic mammaliaformes back by over 100 million years.

Which meant that mammaliaformes were developing specialized traits much earlier than we thought, despite the supposedly restrictive presence of the dinosaurs.

And it soon became clear that Castorocauda wasnt the only fuzzy innovator in this fossil deposit.

Over the next few years, scientists began uncovering more evidence that theyd been kinda wrong about these ancient critters.

Mammaliaformes had actually experienced a period of major ecological radiation.

And not only that, this radiation revealed fast rates of evolutionary change that peaked in the mid-Jurassic.

But why?

What caused it?

Well, when separate lineages start to radiate at the same time, it suggests that something is going on in the environment that benefits multiple groups.

During this time, Pangea was in the process of breaking up, which transformed ecosystems.

And scientists soon realized that this breakup gave docodonts and other groups the chance to radiate into new niches and develop brand new adaptations... Like the tunneling Docofossor, with its sprawling limbs and paws like a moles for shoveling earth.

Or the squirrel-like, sap-eating Agilodocodon, with its curved claws and flexible elbow, wrist, and ankle joints for climbing.

Members of another mammaliaform group, Haramiyida, were also getting in on the action.

The fossilized imprint of skin membranes stretching between their limbs indicates that these animals could glide from tree to tree.

In addition to the environmental change, this mammaliaform radiation might have also been a result of good timing.

Because, ever since the Triassic, mammaliaform groups had been gaining adaptations like fur, warm-bloodedness, lactation, and modifications to the jaw and inner ear sometimes convergently.

Developing this critical mass of advantageous traits could have made it easier for them to explore new environments and adapt to specialized roles.

Now, the ancestors of crown-mammals were also a part of this mid-Jurassic radiation.

But while they saw a major increase in species diversity, they stayed shrew-like For example, the late Jurassic Juramaia, another mammal from that lakebed formation and the earliest known therian.

If Juramaia wasnt our direct ancestor, it was at least a close relative, and would have looked a lot like our Jurassic ancestors.

So if other mammaliaformes werent nearly as constrained by dinosaurs as we thought, why did our ancestors stay so small?

Well, a recent study found that other mammaliaformes actually had a greater role in suppressing the diversification of the therians than the dinosaurs did, because of their good timing.

Those mammaliaformes had shown up on the scene earlier than the crown-mammals, and occupied the available specialist niches before therians had a chance to compete.

Now, that doesnt mean that the dinosaurs had no influence.

As far as we know, they did prevent ancient mammaliaformes from ever getting bigger than a hefty housecat.

But it goes both ways the presence of mammaliaformes in these smaller-bodied roles would have also limited the ability of dinosaurs to exploit those same niches.

And our ancestors were perfectly suited to the insectivore niche they occupied, which may have been their saving grace for the changes that were to come.

After all, theres a reason that were here now.

So why did our ancestors survive while other mammaliaformes didnt?

Trying to figure out how, when and why most of the others disappeared is challenging.

Because, we cant identify a line in the fossil record with diverse mammaliaform remains on one side, and only therians and monotremes on the other.

Even with all the new discoveries, mammaliaform fossils are still relatively rare.

And when they do turn up, its usually only teeth.

But those teeth allow us to identify which group these critters belonged to and draw conclusions about their lifestyle, distribution, and diversity at different times.

For example, from the mid to late Jurassic, about 175 to 145 million years ago, there are teeth belonging to all kinds of mammaliaformes in the fossil record.

But around this time, many mammaliaformes were also starting to experience a decline.

This decline was not marked by a destructive cataclysm, but instead by a gradual, two-phase process.

A study analyzing mammaliaform fossil distribution throughout the Mesozoic found that they first experienced a restriction in range.

And then during the Cretaceous, around 125 million years ago, the record reveals a decrease in species richness.

During this time, in most areas, two types of teeth replaced all the others in the fossil record.

The first type were cimolodont molars, belonging to the multituberculates, a rat-like group of crown-mammals whose teeth mark them as plant-eaters.

The second type were tribosphenic molars.

These are the kind of molars you have, and theyre the kind of molars ancestral placentals and marsupials had.

Studies indicate that this kind of tooth has a greater capacity to evolve into varied forms than some of the other tooth types.

And in the late Cretaceous, they took on a form that could cut, pierce, and chew exoskeletons -- these were the teeth of a small, insectivorous generalist.

This change coincided with the Cretaceous Terrestrial Revolution, a time of ecological upheaval that lasted from 125 to 80 million years ago.

It was an event that would alter ecosystems forever: the spread of flowering plants.

Flowering plants, or angiosperms, made up less than 5% of terrestrial plant species at the start of the period.

But by the end of the Cretaceous, they dominated the land, comprising over 80% of terrestrial plant species.

As flowering plants spread, small insecty mammals were in the perfect position to take advantage of the insect pollinators that diversified in response to this expanding niche.

The ability of angiosperms to outcompete other plants in many regions even helped make the climate more humid, expanding the worlds wet, tropical habitats, which began to spread into whats now North America and Eurasia.

And mammals ability to climb trees and travel across uneven terrain likely helped them to navigate the spreading tropical forests.

At the same time, the growing abundance of fleshy fruit and seeds provided new nutritional opportunities for the herbivorous multituberculates.

And their high energy needs may have driven them to compete more fiercely for resources.

Such a dramatic change in the environment would have been a stressor for highly specialized animals that relied on specific resources, leaving many mammaliaformes struggling to adapt.

But small, generalist therians that were less reliant on stable conditions persisted.

In this way, evidence suggests that the spread of angiosperms helped our ancestors succeed, by tipping the scales against their ancient mammaliaform relatives.

Some recently-published fossil finds suggest that during this time, monotremes had radiated into diverse forms in Australia.

But therians werent free to take over quite yet other crown-mammal groups were still around and had started to specialize.

The most successful of these was the multituberculates, a group that had taken advantage of the new abundance of vegetation brought on by the spread of angiosperms.

They and several other mammal groups would go on to survive the K-Pg mass extinction, diversifying and competing with placentals, marsupials, and monotremes.

But we now know that this era of evolutionary rivalry was nothing new for the mammals.

Instead, it was the next chapter of a story that had played out over millions of years of mammaliaform history, from the diverse docodonts of the Jurassic to the angiosperms that helped our ancestors get a foot in the evolutionary door.

Eventually, competition and a changing climate caused the last of those other mammal groups to disappear, leaving the monotremes and therians to diversify.

The diversification resulted in the return of countless forms, from the shovel paws, to the gliding membranes, and the paddle tails that our ancient relatives pioneered.

And that shift left our own group, the placentals, to become the most successful, diverse mammaliaformes to ever live.