🦎 BEFORE DINOSAURS: THE ARCHOSAURS
PAGE 1 OF 5 — REPTILES BEFORE THE DINOSAURS
250 MA
THE RULING REPTILES
Long before T. rex, a group called archosaurs, meaning "ruling reptiles", dominated land and water. They were not dinosaurs yet, but they carried the blueprint. Archosaurs first appeared in the Early Triassic, recognisable by two clever features: an extra opening in the skull in front of the eye, and a hole in the lower jaw. From this single group, life split into two great branches: one leading to crocodiles, the other to dinosaurs and, eventually, birds.
⚡ WHAT MAKES AN ARCHOSAUR?
A skull opening in front of the eye (the antorbital fenestra), teeth set in sockets, and a hole in the lower jaw. These tiny clues let scientists trace the family tree of crocodiles, dinosaurs and birds back to one ancestor.
"The ruling reptiles before the first dinosaur."
ROOTS!
CROC LINE
🐊 Low-slung armoured predators
🦴 Ancestors of modern crocodiles
🛡️ Bony back plates called osteoderms
🦴 Ancestors of modern crocodiles
🛡️ Bony back plates called osteoderms
DINO LINE
🦖 Slender, fast bipedal runners
🐦 Ancestors of dinosaurs and birds
🦵 Legs tucked straight under the body
🐦 Ancestors of dinosaurs and birds
🦵 Legs tucked straight under the body
PAGE 2 OF 5 — TWO BRANCHES, ONE FAMILY
THE SPLIT
ONE ANCESTOR, TWO DESTINIES
Around 250 million years ago, the archosaurs split into two great lineages. The Pseudosuchia kept a sprawling gait and heavy, armoured skulls, the crocodile path. The Avemetatarsalia stood taller, balanced on their toes, and ran faster, the path that produced dinosaurs, pterosaurs and birds. For most of the Triassic the crocodile-line cousins were actually the bigger, more successful group. Dinosaurs were the underdogs, waiting for their moment.
🌳 TWO BRANCHES OF THE FAMILY
🐊 Pseudosuchia → crocodiles and alligators
🦖 Avemetatarsalia → dinosaurs, pterosaurs and birds
⏳ Both branches still have living members today
🦖 Avemetatarsalia → dinosaurs, pterosaurs and birds
⏳ Both branches still have living members today
SPLIT!
SKULL SHAPE
🦷 Different jaw hinges
👃 Different nostril placement
👃 Different nostril placement
ANKLE BONE
🦴 Crucial ankle difference
🏃 Allowed upright running
🏃 Allowed upright running
HABITAT
🌊 Rivers and floodplains
🏜️ Open Triassic plains
🏜️ Open Triassic plains
PAGE 3 OF 5 — SURVIVING THE GREAT DYING
252 MA
🌋 Volcanoes and climate collapse
💀 96% of species vanished
💀 96% of species vanished
LUCKY FEW
🦎 Small generalist bodies
🍖 Varied diets helped
🍖 Varied diets helped
TRIASSIC REBOUND
EMPTY WORLD, NEW RULERS
After the Permian "Great Dying" wiped out 96% of species, the world was nearly empty. With competitors gone, archosaurs exploded into every empty role, fearsome predators, plant-eaters, and the first flying vertebrates, the pterosaurs. Giant crocodile-line hunters like Postosuchus ruled the land. Dinosaurs were still small, rare players in this drama, quietly perfecting the upright legs and light bones that would later make them unstoppable.
🌋 RECOVERY TOOK MILLIONS OF YEARS
Life needed around 10 million years to bounce back from the Permian extinction. Archosaurs were among the biggest winners of that recovery, and the empty world gave dinosaurs the opening they needed.
RISE!
PAGE 4 OF 5 — FROM ARCHOSAUR TO DINOSAUR
FIRST DINOS
DINOSAURIA IS BORN
Around 230 million years ago, in what is now Argentina, the first true dinosaurs appeared. Animals like Herrerasaurus and Eoraptor were small, bipedal and fast, archosaurs in body, dinosaurs in name. What set dinosaurs apart was a special hip socket with a hole in it, letting their legs lock straight underneath for efficient, upright walking. For 30 million years they stayed in the shadow of their crocodile-line cousins, until the end-Triassic extinction cleared the way and the dinosaur age truly began.
🦴 THE DINOSAUR CHECKLIST
🦵 Open hip socket (a hole where the leg meets the hip)
🏃 Legs held straight under the body
🦎 Specific ankle, neck and skull features that crocodiles lack
🏃 Legs held straight under the body
🦎 Specific ankle, neck and skull features that crocodiles lack
BORN!
BIRDS LATER
🐦 Birds are living dinosaurs
🐊 Crocs are living cousins
🐊 Crocs are living cousins
PTEROSAURS
🦇 Flying archosaurs
🚫 Not dinosaurs
🚫 Not dinosaurs
KEY TRAIT
🦴 Upright stance
⚡ Efficient movement
⚡ Efficient movement
PAGE 5 OF 5 — THE LEGACY LIVES ON
TODAY
ARCHOSAURS NEVER LEFT
Every crocodile basking on a riverbank and every sparrow on a fence is living proof: archosaurs won the long game. Both branches of the family survived to today, around 26 species of crocodilians on the croc line, and over 10,000 species of birds on the dinosaur line. Dinosaurs held the spotlight for 165 million years, but the archosaur story never ended. It simply continued, in feathers and armour, all around us.
LIVE!
RECAP
🦎 Archosaurs = crocodile + bird line
🦖 Dinosaurs = bird-line branch
🦖 Dinosaurs = bird-line branch
REMEMBER
🦕 KEY FACTS
🌡️ Mesozoic climate next
🦕 How hot was the dino world?
🦕 How hot was the dino world?
🧠 QUIZ TIME!
BEFORE DINOSAURS: THE ARCHOSAURS · 5 QUESTIONS
QUESTION 01
What group of reptiles gave rise to both dinosaurs and crocodiles?
QUESTION 02
Which archosaur branch includes crocodiles?
QUESTION 03
Which branch includes dinosaurs and birds?
QUESTION 04
What mass extinction cleared the way for archosaur dominance?
QUESTION 05
Are birds considered dinosaurs today?
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