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 remembering that our success as crown mammals
started much, much earlier. Alright...so now we know about our mammal
relatives from the Mesozoic, but what happened after during the Cenozoic? Check out our episode, “From the Fall of
Dinos to the Rise of Humans” to find out! And big thanks to this month’s crown group
Eontologists: Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick
Seifert! By becoming an Eonite at patreon.com/eons,
you can get fun perks like submitting a joke for us to read, like this one from Jules Why do land animals hate ichthyosaurs? They’re all ICK-Y! That's a fun play on words And as always thank you for joining me in
the Konstantin Haase studio. Subscribe at youtube.com/eons for more adventures
in deep time.
PBS Eons, Moth Light Media, Henry The Paleo Guy, Stefan Milo, Ben G Thomas.
All fantastic youtube channels with great content that's usually related to biology and evolution. But boy do I get sucked in and fall down the rabbit hole when I decide to watch just one video.
getting reliable notifications for this channel i think is the best part of being in the subreddit. Youtube is so bad at letting me know
Great video thanks!