Sometime in the Mesozoic Era, maybe as far back as 218 million years ago, the first true mammal appeared.

It was probably small, weighing less than half a kilogram.

It would’ve had fur and nursed its babies, like all living mammals do today.

And it would’ve lived alongside a bunch of its mammal-like relatives - things that, if you saw them today, you’d have a hard time telling them apart from true mammals.

Because, the dividing line between the true mammals and the not-quite-mammals is actually kind of blurry.

Lots of the traits we think of as defining us as mammals show up pretty early, during the time of the dinosaurs.

And, in some cases, they show up a LOT earlier….and in things that weren’t mammals at all.

Eventually though, all those defining traits came together in one of our ancient ancestors - that first true mammal, a mysterious creature that we may never find a fossil of.

But we can trace our mammal features through time as they appeared.

This is one of our earliest origin stories - one that starts in the Mesozoic or even before - and continues with us, now, in what’s sometimes called the “age of mammals.” So what even is a mammal?

And how do we figure out what counts as one in the fossil record?

Mammals today are all part of what’s called crown group Mammalia.

A crown group includes all of the living members of a group back to its most recent common ancestor.

It also includes all the extinct descendents of that common ancestor.

For crown group mammals, our most recent common ancestor is the one shared by placentals like us, marsupials like the kangaroo, and the monotremes like the platypus.

Now, just outside the crown group are its extinct relatives - the ones that branched off before the common ancestor of the crown, but are still more closely related to the crown group than to anything else.

If we trace these guys back to their most recent common ancestor with crown mammals, we end up with a group known as the mammaliaforms.

So that’s us, plus our extinct relatives outside crown mammals.

And those early mammaliaforms were like us in some ways...but weren’t quite there yet in others.

Today, it’s relatively easy to take a living animal and figure out if it’s a true mammal, because all crown mammals have these four things in common.

One: they have fur or hair.

Two: they have a jaw joint made up of only two bones - the dentary or mandible, and the squamosal, which, for us, is part of one of the bones of our skull.

Three: they have a middle ear made up of three bones - the incus, malleus, and stapes.

And four: they have the ability to produce milk.

While two of these traits can be seen easily in fossil mammals, the other two, fur and lactation, are tougher...but that doesn’t mean we can’t find them.

In fact, fur is probably the feature that originated first.

Its origins may actually go back to the Permian Period, based on evidence from fossilized poop!

Paleontologists have found coprolites from the Late Permian with what appear to be hair-like structures in them.

They think that these were made by animals called therapsids, the group that would later go on to include mammals -- but that poop came from members of this group that were around before any mammaliaforms existed.

After the evolution of fur, the first mammaliaforms appeared sometime between 215 and 205 million years ago - and their fossils show us that mammal-like jaws were the next features to arise.

One of the first of these was a tiny, shrew-like creature from the Late Triassic called Morganucodon.

Its fossils have been found mostly in Europe and China.

And something really interesting was happening with its jaw joint, which is actually a key moment in the evolution of mammals, as weird as that may sound.

The earliest ancestors of mammaliaforms had four bones in their jaws, and the jaw joint was formed by two bones called the articular and the quadrate.

And Morganucodon still had this old-style jaw joint.

But!

It also had a second jaw joint made up of the same two bones as the jaw joints of all living mammals - the dentary or mandible, and the squamosal.

And, in mammals today, the two bones of that old-style jaw joint have since become part of our middle ear.

Morganucodon’s jaw represents what’s sometimes called a “transitional mammalian middle ear.” This means that, while it has a mammal-like jaw joint, the bones that would become the middle ear were still attached to the jaw, but had gotten much smaller, indicating they were starting to help with hearing.

This is a trait that’s shared among other mammaliaforms, and it’s this feature that typically separates Morganucodon and mammaliaforms like it from the crown mammals.

In crown mammals, these bones are completely detached from the mandible and are part of the three-boned middle ear.

But it’s possible that crown mammals weren’t the first or only ones to evolve this set up.

A tiny mammaliaform called Hadrocodium may have already had a three-boned middle ear in the Early Jurassic period.

And one reason this trait might’ve been important for early mammaliaforms and then for the first crown mammals is that many of them were small insectivores who probably hunted at night.

Having more bones in the middle ear improved their hearing, so they were better able to hear prey and potential predators in the dark.

But what about that fourth classic crown mammal trait: lactation?

Did it actually evolve in mammaliaforms first, too?

Like fur, this one’s harder to see in the fossil record - we don’t have any fossilized mammary glands.

But we do have some indirect evidence from bones that might give us some clues.

In 2019, a new species of mammaliaform from the Middle Jurassic called Microdocodon was described - and it had a preserved hyoid bone.

The hyoid is a special bone that sits in your throat and is only attached to the rest of your skeleton by muscles and ligaments.

It supports the tongue and allows for better control over swallowing.

For mammals, the hyoid is part of what gives babies the ability to suckle milk.

This gives it a very distinct saddle shape with movable joints.

Reptiles just have a simple rod for their hyoid, which gives them a wide but not-very-muscularized throat.

And it looks like the hyoid of Microdocodon was very similar to the hyoid bones of modern mammals, so it’s very likely that Microdocodon could suckle, too.

And if it could suckle, then it probably lactated and fed its young with milk, just like all crown mammals do.

So, a lot of traits that we think of as being unique to crown mammals actually evolved earlier, in various different mammaliaforms.

But what does all of this mean for our lineage of mammaliaforms, the crown mammals?

Well, it shows that different branches of Mesozoic “mammals,” including ours, were converging on some similar traits, even if they weren’t closely related.

And the reason for that is probably because they were good strategies for survival in the niches that they occupied: being small to medium-sized insectivores living in a world full of large reptiles.

This worked for a while, but ultimately many of these early mammaliaform groups didn’t last.

Most of them came and went as the Mesozoic Era continued, and eventually only the ancestors of the crown mammals - and a few lingering mammaliaforms - were left by the end of the Cretaceous Period.

These would go on to survive the K-Pg extinction event 66 million years ago.

And the ancestors of the crown group would diversify into all the different mammals we know today, united by those four classic mammalian traits, while the last of the ancient mammaliaforms would die out.

So, while the Age of Mammals only really got going when the Mesozoic ended, it’s worth