Dinosaurs Were Evolutionary Copycats of These Long-Lost

Postosuchus and Desmatosuchus

Many of the reptiles that thrived during the Triassic were crocodile relatives that dinosaurs would later mimic through evolutionary happenstance, such as Postosuchus (back) and Desmatosuchus (front).
Evolutionnumber9 CC BY-SA 4.0

Had a volcano-driven mass extinction not occurred at the end of the Triassic 201 million years ago, we likely would have had something closer to an Age of Crocodiles than the Age of Dinosaurs that actually followed. Ancient relatives of today’s swamp-dwelling reptiles were more diverse than dinosaurs and came in an even greater array of shapes and sizes, with dinosaurs largely on the ecological sidelines. When intense volcanic outpourings caused global climates to rapidly swing between hot and cold, however, fuzzy and warm-blooded dinosaurs were better able to cope. Crocodiles and other forms of ancient reptiles were hit much harder.

The Triassic began 252 million years ago as life was recovering from the worst mass extinction of all time, also caused by the aftereffects of incredible volcanic eruptions. Protomammals were decimated by the catastrophe, but reptiles of all sorts quickly evolved to open new niches in the water, on land and in the air. Dinosaurs were certainly among this saurian surfeit, but they weren’t necessarily trendsetters.

The earliest dinosaurs date back to around 243 million years ago, but the animals were pretty small and not especially diverse. Only after the end-Triassic mass extinction, in the Jurassic, did dinosaurs start to truly flourish—before that, the world saw a very different cast of ruling reptiles. Here’s a short list of Triassic animals that evolved forms and behaviors we now associate with dinosaurs, long before the “terrible lizards” would do the same.



While smaller, Postosuchus was the Tyrannosaurus rex of its time.

Jeff Martz

Tyrannosaurus rex wasn’t the first toothy terror to stalk the Earth. More than 220 million years ago, in what’s now Texas, the most fearsome animal on land was Postosuchus. Described in 1985, this crocodile relative could grow to about 20 feet long and had a deep skull full of curved, blade-like teeth. Rather than walking on all fours, however, Postosuchus probably moved around on its hind legs and kept its comparatively tiny forelimbs off the ground. Sound familiar? The paleontologist who described Postosuchus thought the crocodile relative looked so much like T. rex that he suggested Postosuchus might have been a T. rex ancestor—a claim later refuted despite the broad-brush resemblance. Experts now know that Postosuchus belonged to a group of reptiles called pseudosuchians, the same one that includes alligators and crocodiles today.



Desmatosuchus was a crocodile relative that evolved elaborate armor long before ankylosaurs.

Jeff Martz

Armored dinosaurs have always stood out to paleontologists for their unusual anatomy. Species like Ankylosaurus were covered from snout to tail in armor. But dinosaurs were far from the first animals to evolve such ornamentation. Aetosaurs were crocodile relatives that lived during the Triassic and were covered in bony plates and spikes. Some species were so thoroughly coated in armor that even the openings for their reproductive and excretory tracts had spikes around them.

Desmatosuchus wore one of the most elaborate arrays of armor. This omnivorous reptile walked on all fours and could grow to be about 20 feet long, and it was immediately recognizable by the huge, backward-curving spikes growing from its shoulders. How effective these plates were against predators is questionable. Some Desmatosuchus bones show signs of being munched on by Postosuchus. Instead of only a defensive function, then, the unique arrangement of scutes on Desmatosuchus may have played a social role and allowed these animals to recognize each other and size each other up, just as paleontologists expect the much later ankylosaurs did.



Caelestiventus had a mouthful of teeth that helped this Triassic flier catch insects.

Nobu Tamura CC BY-SA 4.0

Pterosaurs were the first vertebrates to take to the air. Their origins are still murky. The current evidence indicates that these airborne reptiles diverged from a common ancestor with dinosaurs more than 243 million years ago. And even though bones of the earliest flying pterosaurs have yet to be discovered, these reptiles were flapping through the air by the end of the Triassic. Paleontologists have found multiple pterosaurs such as Caelestiventus, Preondactylus and Caviramus that flew over the Earth during that time frame.

Pterosaurs are easily recognized by their aerodynamic anatomy. Their wings were made of a membrane that stretched between the body and a ludicrously elongated fourth finger, and their bodies, not including the wings, were covered in protofeathers. Dinosaurs would later take to the air, too, primarily relying on feathery wings to get them aloft—but some evolved what pterosaurs had already perfected. A small dinosaur named Yi had both feathers and membranes between its fingers, resembling the combination of fuzz and leathery membranes that pterosaurs had been using to fly for tens of millions of years.



Effigia walked like a dinosaur but was actually more closely related to crocodiles.

Nobu Tamura CC BY 2.5

Evolution keeps spinning off animals that look like emus. Tens of millions of years before the origin of ostriches and their relatives there were the ornithomimosaurs—beaked, flightless dinosaurs covered in feathers. And tens of millions of years before ornithomimosaurs lived Effigia. The Triassic crocodile relative caused a stir when it was described in 2006, because it clearly walked on two legs with each one tucked under the body, a trait that was thought to be unique to dinosaurs among reptiles. More than that, Effigia had a toothless beak and roughly resembled the ornithomimosaurs that would come much later. Paleontologists expect that Effigia was an omnivore with a preference for plants, although precisely what this Triassic reptile ate is as yet unknown.


Dome-headed animals only evolve every so often. The pachycephalosaurs of the Late Cretaceous are the most famous—they’re well known for their thick, spiky headgear that continues to confound paleontologists. But those dinosaurs were not the first to wear the fashion. A very different animal, Triopticus, evolved a similar dome-headed profile back in the Late Triassic.

Thus far, paleontologists have only uncovered part of the animal’s skull. The few clues gleaned from this specimen indicate Triopticus was an archosauriform, or part of the broader group that included dinosaurs, crocodiles and their evolutionary relatives. Despite such a distant relationship to the pachycephalosaurs, however, Triopticus independently evolved a very similar noggin, all the way down to spike-like protuberances around the dome. With any luck, researchers will someday find the rest of the animal.



Asilisaurus and related silesaurs pioneered ecological roles that would later be filled by small, plant-eating dinosaurs.

Zissoudisctrucker CC BY-SA 4.0

The biggest, most ornate and fiercest-looking dinosaurs often get the most attention. The small, lithe herbivorous dinosaurs of the Jurassic and Cretaceous—like Lesothosaurus and Dryosaurus —are either overlooked or cast as fodder for the carnivores of their time. But little plant- and insect-eating dinosaurs were a part of the saurian story for tens of millions of years, and they had a Triassic counterpart in the silesaurs. These reptiles were relatively small, lanky and had leaf-shaped teeth for shearing leaves and busting beetle shells. Paleontologists have also recognized them as dinosauriforms, or reptiles very close to the first dinosaurs and possibly the group from which dinosaurs arose. But the details haven’t yet been settled. Silesaurs might have been protodinosaurs that lived alongside the earliest dinosaurs, or, as some experts argue, they are a diverse collection of early dinosaurs that were mislabeled. Either way, these animals started filling a niche that dinosaurs would fill throughout their heyday, thriving at small sizes even as other reptiles grew larger.