I want you to imagine just for a second a dynasty
of creatures that absolutely dominated the earth.
Okay. We are talking about a reign that lasted
for... Well over 190 million years. That is a
staggering amount of time. Right. I mean, they
watched the supercontinent Pangea fracture and
drift apart. They survived the catastrophic KPG
boundary. The literal asteroid impact. Exactly.
The impact that famously wiped out the non -avian
dinosaurs. They were survivors in the truest,
most grueling sense of the word. They really
were. And yet, despite that staggering record
of resilience across multiple geological eras,
massive group of animals was ultimately wiped
out by a single word. Just one word. Obsolete.
It is quite the dramatic twist, honestly. It
really is. To survive the literal end of a world,
you know, wading through the ash and ecological
collapse that doomed the dinosaurs, only to be
erased by the stroke of a taxonomist's pen millions
of years later. And that is exactly what we are
focusing our deep dive on today. We are looking
at the history of Mesosuchia. This is an obsolete
term for a truly massive ancient group of crocodile
morph reptiles. And we're pulling a lot of this
from a really comprehensive Wikipedia article
on the topic. It's a great source for this. Yeah.
So our mission today is to explore the history
of these heavily armored sort of in -between
crocodiles. We are going to unpack their incredibly
bizarre anatomy. We'll look at how they conquered
the globe and ultimately understand exactly why
modern science had to completely rewrite their
entire family tree. It serves as a perfect example
of how our understanding of the natural world
is, well, it's never truly static. We love to
put things in neat little boxes. But the story
of the Mesozoic shows us what happens when our
rigid categories collide with the messy, sprawling
reality of evolution. Nature doesn't care about
our filing systems. OK, let's unpack this, because
to really appreciate the Mesozoic, you have to
understand the sheer scale of their timeline.
You really do. These creatures existed from 199
million years ago all the way down to roughly
4 .57 million years ago. Which is just, it's
hard to wrap your head around. It spans from
the early Jurassic period right up to the late
Miocene or even the early Pliocene. It is an
almost incomprehensible amount of time to be
successful on this planet. It truly is. I mean,
when they first appeared, the world was a radically
different place. The climate, the flora, the
available prey. Everything was in flux. Everything.
And yet. this group persisted through the entirety
of the jurassic the entirety of the cretaceous
crossed that apocalyptic threshold 66 million
years ago and kept right on going. Through the
Paleocene, the Eocene. Through the Volococene
and into the Miocene. They were a constant dominant
fixture across multiple major climatic upheavals.
And their historical name actually reflects their
place in that massive timeline, doesn't it? It
does. Because the early scientific consensus
seemed to view their evolution as a very simple
three -act play. Right, a very linear progression.
You had the quote -unquote first early crocodile.
placed in a suborder called Protosuchia. Then
you jump to the modern extant species, the ones
you might see in a swamp today, which we call
Eusachia. Which translates to true crocodiles.
Exactly. True crocodiles. And right there in
the middle, serving as this massive evolutionary
bridge, was the Mesosuchia. The crocodiles in
between. Precisely. It was a very neat, linear
way of looking at their evolution, beginning,
middle, and end. But nature is rarely that neat.
No, never. What makes the Mesozoicians so compelling
is that they were far more than just a transitional
stepping stone waiting around to turn into modern
crocodiles. Right. They were a wildly successful...
dynamic, and incredibly diverse group of animals
in their own right. Diverse might actually be
underselling it a bit. Yeah. Because when you
say the word crocodile to someone today, you
immediately picture a very specific lifestyle.
You picture a swamp. Yeah, a swamp -dwelling,
semi -aquatic ambush predator lurking at the
water's edge. Waiting for a zebra to drink. Exactly.
Yeah. But these ancient creatures were not boxed
into that one ecological niche. We're talking
about... Terrestrial Mesozoans sprinting around
on land. Running down prey. Yeah. We have the
semi -aquatic ones, sure. But then there were
fully aquatic, essentially marine crocodile morphs
perfectly adapted for swimming in the open oceans.
That diversity of lifestyle is exactly why they
thrived for nearly 200 million years. They were
everywhere. They didn't just survive. They adapted
to exploit almost every available ecological
niche left open by other predators. Wow. But
here's the challenge for early paleontologists.
A terrestrial crocodile... Right, longer legs
versus... A paddle -like tail. Exactly. And yet
beneath all those specialized adaptations, they
shared a core set of defining anatomical features
that allowed early scientists to group them together
under that mezzosuchia banner. I really want
to dig into that biological blueprint because
looking at their anatomy, it becomes clear they
were operating very differently from the crocodiles
we know today. Very differently, yes. Let's start
with the skull. One major differentiator is that
mezzosuchians had a shorter secondary palate
compared to modern usuchians. Functionally speaking,
how did that change the way these animals operated?
To understand the impact, let's visualize the
secondary palate. It's essentially the bony roof
of the mouth, which separates the nasal passage
from the oral cavity. In Mesozoans, this palate
is formed by three specific bones. The premaxillary,
the maxillary, and the palatine bones. Spot on.
Because it is physically shorter than what we
see in modern crocodiles, it drastically shifts
where the inner nostrils, the koani, are located.
So where do they end up? In these ancient creatures,
those inner nostrils are usually situated right
between the palatine and pterygoid bones. And
crucially, the text notes that Chorana is not
surrounded by the medial pterygoid plate. Exactly.
That is a major defining characteristic that
separates them from modern forms. How does that
compare to a modern crocodile? In modern Yusushians,
that bony palate extends much further back. It
completely surrounds the nasal opening. Which
lets them do what? It allows a modern crocodile
to breathe while its mouth is submerged or full
of water, as long as the tip of its snout is
above the surface. Oh, wow. So they can drown
prey without drowning themselves. Precisely.
The shorter palate of the Mississutians means
their respiratory and feeding mechanics weren't
quite as separated, which heavily influenced
how they hunted and processed their prey, particularly
in aquatic environments. That makes a lot of
sense. And moving down from the skull, there
is a fascinating detail about their spine. They
possessed what's called an amphicoelum of the
vertebrae. Yes, amphicoeolus vertebrae. How does
that compare to the skeletal structure of a modern
crocodile? An amphicoeolus vertebrae refers to
the specific shape of the individual's spinal
bones. They are concave on both ends. Okay. So
if you were to isolate a single bone from a mesosucian
spine, the main body of it would look a bit like
an hourglass. Dented in on both the front and
the back. Right. As opposed to the modern crocodile
spine. Right. Modern crocodiles generally have
percolous vertebrae, which are concave in the
front and convex in the back. Kind of like a
ball and socket joint. Exactly. More of a ball
and socket arrangement. The amphicolelous structure
in Mesozoans provided a very specific kind of
flexibility and strength. It's an older design.
It's an older Argosaurian design, but clearly
it was incredibly effective given that it supported
them from the early Jurassic all the way to the
Miocene. Let's jump back to the skull architecture
for a second. Sure. Because it sounds like they
had incredibly robust, tightly packed heads.
Their jawbones sometimes joined dorsally, which
actually... push the nasal bones away from the
premaxillary bones and their outer nostrils usually
merge together. It paints a picture of a very
specialized rigid snout. It does. And that rigidity
is further supported by another key feature.
They almost always lost the preorbital foramen.
Remind me what that is. The preorbital foramen
is a small opening or hole in the skull located
just in front of the eye sockets. It's a common
feature in many other archosaurs, including early
dinosaurs. So losing that hole essentially solidifies
the facial skeleton. Precisely. When you close
up those fenestrae or windows in the skull, you
are trading lightness for structural integrity.
Making it much heavier. Heavier but much stronger.
A solidifying of the facial skeleton means the
skull can withstand immense stress and impact.
Which ties perfectly into the next feature. Their
superior temporal fossa was massive, and the
frontal bone actually reached its edge. Yes.
And functionally, the superior temporal fossa,
those openings at the top of the skull behind
the eyes, they accommodate the jaw muscles. So
the larger hole means larger muscles. Exactly.
The larger the fossa, the more space there is
for massive muscle attachment. Wow. So a huge
temporal fossa combined with a heavily solidified
snout strongly suggests these animals had absolutely
devastating bite forces. They were just built
to clamp down on struggling prey and not let
go. Absolute powerhouses. And just to add to
that tough, rugged build, their postterminal
arch was often located directly under the skin.
Giving them a very distinct look. Right, so you'd
have these pronounced bony ridges sitting right
below the surface behind their eyes. Plus, there
is a quirk in their hips. The pubis bone. Yeah,
the pubis bone does not enter the acetabulum,
which is the hip joint socket. That pelvic structure
is a subtle but crucial differentiator. The way
the femur articulates with the hip dictates an
animal's gait and locomotion. So it changes how
they walk? Very much so. The fact that the pubis
is excluded from the acetabulum in these creatures
is a distinct evolutionary signature. It influenced
how efficiently those terrestrial mesosuchines
could move on land compared to other reptile
groups. They were just incredibly adapted machines.
But my absolute favorite detail from this anatomical
breakdown has to be the armor. The carapace.
Yes. Their carapace was usually extremely well
-developed. I really want you to picture this.
We aren't just talking about thick, leathery
reptilian skin. Yeah, not at all. We are talking
about highly developed, heavy, bony armor plating.
covering these creatures. Whether they were sprinting
across the plains or swimming in the seas, they
were essentially organic prehistoric tanks. The
development of that carapace was undoubtedly
a primary factor in their long -term survival.
Think about the neighborhoods they lived in.
Rough neighborhoods. Very rough. When you were
sharing early ecosystems with massive theropod
dinosaurs or navigating oceans filled with apex
marine reptiles, having a well -developed suite
of heavy bony armor is a distinct evolutionary
advantage. It let them take a hit. It allowed
them to hold their ground in highly competitive
environments. Here's where it gets really interesting.
Because they didn't just survive alongside those
other massive predators. Right. In many cases,
they were the top of the food chain themselves.
There's an incredibly fascinating discovery that
really proves how dominant they were long after
the dinosaurs were gone. The Hispaniola find.
Yes. A study published in the Proceedings of
the Royal Society B details a creature called
Sabicus ichirohinus. Fantastic paper. This is
a South American sebicide discovered in the Miocene
deposits of Hispaniola. That is a phenomenal
piece of evidence, and it really changes the
narrative. How so? Well, the Miocene Epoch is
deep into the age of mammals. And finding Sabicus
echiorhynus in Hispaniola, which is part of the
Greater Antilles in the Caribbean, is incredibly
significant from an ecological standpoint. I
was struck by that location. How does a massive
terrestrial predator end up dominating a Caribbean
island? It speaks to their sheer adaptability
and dispersal capabilities. Island ecosystems
are fascinating because they are isolated. Right.
It's hard to get there. Exactly. The species
that manage to get there, often by rafting on
vegetation or crossing narrow seaways, have to
rapidly adapt to limited space and specific local
prey. And they thrived. They did. The paper specifically
documents these sebaceids as apex predators in
those early West Indies environments. It completely
shatters that old idea of them just... being
a transitional middle group. Completely. This
isn't a story of animals that barely scraped
by in the shadows of the dinosaurs and then quietly
faded away. No. This shows that millions of years
after the dinosaur extinction, these Mesozoician
descendants were still highly successful, dominant
predators, conquering isolated island ecosystems.
Exactly. They were the undisputed rulers of their
domains. And when you look at the sheer sprawling
diversity... of their broader classification,
the scale is genuinely mind blowing. It really
is. The list of subgroups within this classification
is massive. It's huge. You have the Chartegasuchoidea
group, which includes genera from Asia like Shantungasuchus
and Sequanasuchus. Those particular Asian genera
represent some of the fascinating, highly specialized
branches of this lineage. It shows just how globally
distributed the group was. They were everywhere.
They weren't confined to one continent. They
were a global phenomenon. Then you have the Metasuchia.
Right. Which flits off into the Notosuchia and
the Neosuchia. And the Neosuchians, which in
this older classification explicitly excludes
the modern true crocodiles, contains a staggering
number of families. It really does. You've got
the Oedipus sordae, the Goniophilitidae, the
Paraligotoridae. And some amazing names in there.
And, of course, the Thalatosuchia, those incredible
open ocean marine crocodile morphs we talked
about earlier. It's a wildly expansive family
tree. It is a vast phylogenetic tree. And listening
to that list of diverse families, you know, terrestrial
runners, pelagic marine swimmers, heavy ambush
predators, it highlights the incredible adaptive
radiation of these animals. Adaptive radiation,
meaning they spread out into different roles.
Exactly. They filled nearly every conceivable
ecological role available to a carnivore over
a span of nearly 200 million years. Which brings
us to the biggest plot twist of the entire deep
dive. Yes. If they were this successful, if they
lived for 190 million years, if they conquered
the land, the sea and Caribbean islands, and
if they were apex predators all the way into
the Miocene. How did they lose the name? Exactly.
Why on earth is the term Mesozoic now considered
completely obsolete? How does a dynasty like
that just get canceled by modern science? That
is the crux of the issue. And it all comes down
to the strict, unforgiving rules of modern taxonomy
and cladistics. Our filing system again. Right.
The term. Mesasuchia is no longer regarded as
genuine because biologists realized it belongs
to what we call a paraphyletic group. Okay, I
think this is where human categorization really
clashes with nature. Break down paraphyletic
for us. What makes a group paraphyletic, and
why is that invalid in modern evolutionary science?
Let's use a family tree analogy. In modern taxonomy,
a valid, genuine group, what we call a monophyletic
group or a clade, All of its descendants. All
of them. You cannot arbitrarily chop off a branch
of the family tree just because those specific
descendants evolved to look a little different
over time. So if I start a defined family group,
starting with my great -grandmother, I have to
include all her kids, all her grandkids, and
all her great -grandkids to be scientifically
accurate. I can't just leave out my cousins because
they moved to another country and changed their
last name. Precisely. Now think back to that
old linear classification we discussed earlier.
Proto -Sushia is the first chapter. Right. Meso
-Sushia is the middle chapter. And Yu -Sushia
as the final chapter of modern crocodiles. Right.
Meso -Sushia was specifically designed to be
the middle group. The bridge. But as paleontologists
mapped out the genetics and morphology more accurately,
they realized a massive flaw in the filing system.
What was it? The modern true crocodiles, the
Usuchians, they didn't just replace the Mesosuchians.
They actually evolved from within one of the
Mesosuchian branches. Oh, so the modern crocodiles
in our swamps today. are technically the direct
evolutionary descendants of this middle group.
What's fascinating here is that by creating a
distinct box called Mesosuchia that deliberately
excluded modern crocodiles, early scientists
had created a textbook paraphyletic group. They
cut out the cousins? They did. They had drawn
a neat little circle around an ancient ancestor
and most of its descendants, but purposely left
the surviving modern descendants entirely out
of the circle just to keep their categories clean.
They were trying to force... nature into neat
historical chapters. Which is very human of us
to do. But nature doesn't work like that. It's
a continuous, branching, messy tree. Very messy.
You can't have a category that just abruptly
stops simply because the animals successfully
evolved into something that happens to still
be alive today. Exactly. In modern phylogenetics,
that makes the Mesozoic grouping artificial.
It doesn't reflect the true evolutionary reality.
So the scientific community had to fix the filing
system. So what did they replace it with? How
do you recategorize 190 million years of history?
The obsolete term Mesozoic was replaced by its
true phylogenetic equivalent. Mezucrocodilia.
Mezucrocodilia. Yeah. That is a bit of a mouthful.
It is, but it perfectly and accurately describes
what the group actually is. This new classification
contains all the original taxa that were housed
in the old Mezosuchia, plus the crocodilia, which
are the modern extant forms. Got it. And a variety
of other allied forms that have been more recently
discovered by paleontologists. So it brings everyone
back to the reunion. It corrects the historical
error by finally drawing the circle large enough
to include the ancestor and... all of its descendants,
whether they are fossilized in Miocene Rock or
currently sunning themselves on a riverbank in
Florida. That is brilliant. So the Mesozoans
didn't actually disappear at all. They just got
their modern grandkids legally added back into
the family well. That is a very apt way to look
at it. The animals themselves remain as magnificent,
terrifying, and historically significant as ever.
They didn't change. No. What changed wasn't the
fossil record. What changed was our human methodology
for understanding their relationships. That's
incredible. It highlights the fact that scientific
knowledge is a living, breathing process. As
we uncover new fossils and better understand
the intricate relationships of evolution, we
must be willing to tear down our old assumptions
and adapt our categories. It really is wild to
think about. We build these entire scientific
frameworks and then one realization about a family
tree. forces us to relabel centuries of museum
exhibits. If we connect this to the bigger picture,
we just explored a phenomenally successful group
of organisms. We are talking about heavily armored,
highly diverse crocodilomorph. The organic tanks.
Yes. They dominated terrestrial ecosystems with
specialized hips and deep skulls. They waded
into the open oceans and they survived global
mass extinctions, reigning for nearly 200 million
years. And yet. And yet their original scientific
grouping was entirely... dismantled and rewritten
simply because it was an artificial in -between
category that failed to recognize their modern
evolutionary descendants. It tells us as much
about the evolution of human science as it does
about the evolution of reptiles. So what does
this all mean? I think it means that... Trying
to understand the deep past requires a tremendous
amount of humility. Absolutely. It means we have
to be willing to completely change our labels
and update our worldview when the evidence demands
it. The Mesozoicians were ultimate survivors.
adapting to everything the planet threw at them.
But they couldn't survive a taxonomic update.
Though it was an update that ultimately honored
their true legacy by permanently connecting their
ancient history directly to the animals that
still swim in our rivers and swamps today. They
aren't just an extinct middle chapter anymore.
Exactly. They are part of an unbroken, ongoing
story. That is beautifully put. And that leaves
us with a really fascinating question to ponder.
I want you to think about this. If a dominant
190 million year dynasty of armored apex predators
had its entire scientific classification rewritten
just because it was a messy in -between category,
what labels, definitions, or categories are we
heavily relying on in our modern world right
now that future generations will look back on
and realize we're just an obsolete temporary
state of understanding?