Dinosaurs are among the most iconic creatures to have roamed the Earth, and today they hold a special place in the popular imagination. But what were the earliest ones to appear on the planet?

Dinosaurs belong to a larger group of animals known as archosaurs, which first appeared around 251 million years ago. This group includes modern birds, modern crocodiles and their ancestors, as well as pterosaurs—the now-extinct flying reptiles.

When Were Dinosaurs First Discovered

The question of when dinosaurs were first discovered is a complex one. First, it is important to note that some dinosaurs have actually survived to the present day—scientists consider modern birds to be dinosaurs. So the response to the question would be that humans discovered dinosaurs when they first came across birds.

But to simplify things, let’s focus on non-avian dinosaurs (in other words, all dinosaurs other than birds), which are now extinct.

For centuries, humans came across fossils of these creatures without the finders knowing their true origin.

It wasn’t until 1842 that English biologist, anatomist and paleontologist Richard Owen first officially recognized dinosaurs, or Dinosauria, as a distinctive group of animals, according to London’s Natural History Museum.

Owen’s classification was based on a trio of fossil discoveries of extinct reptiles made in southern England in the early 1800s. The first of these was Megalosaurus, which was described in 1824 by William Buckland—technically, the first non-avian dinosaur to be validly named. The other two—Iguanodon and Hylaeosaurus—were described by Gideon Mantell in 1825 and 1833, respectively.

Even though these fossils were not considered to be the remains of dinosaurs at the time of their discovery, Owen found that they shared certain characteristics, which required the creation of a new group.

Stock image: A computer-generated 3D illustration of the dinosaur Coelophysis. This creature is among the most well-known early dinosaurs. iStock

Potential scientific evidence of the existence of Megalosaurus may go back even further, though. In 1677, English naturalist Robert Plot published a description and illustration of a huge bone found in a quarry in Oxfordshire, England, that he initially suspected belonged to an ancient Roman war elephant.

While the bone has since been lost, scientists later suspected it may have come from a Megalosaurus, based on the illustration, although this has not been confirmed.

When Did Dinosaurs First Appear?

The first known “true” dinosaurs appeared around 230 million years ago, during the Triassic period (252 to 201 million years ago), the fossil record indicates. But several aspects of their early history are still not well understood.

“Just a few million years later, we find them in nearly all major ecosystems across the planet,” Sterling Nesbitt, a paleontologist and associate professor of geosciences at Virginia Tech, told Newsweek.

Theropods (the group of dinosaurs that includes birds) and sauropodomorphs (the group that includes the gigantic, long-necked dinosaurs) appeared at the same time. The third major lineage, ornithischians, appeared a few million years later, although the early fossil record for this group is severely lacking.

Over the past two decades, scientists have significantly improved their understanding of the closest relatives of the dinosaurs, according to Nesbitt. Most of these relatives were smaller than the early dinosaurs, but there was a huge diversity of forms and species. These animals included silesaurids, pterosaurs and lagerpetids (now thought to be flightless pterosaurs).

“These close relatives have many dinosaur features, but not all of the features shared by theropods, sauropodomorphs and ornithischians,” Nesbitt said. “Further down the family tree of reptiles, you have crocodylians and their relatives.”

Silesaurids, which were first described in 2003, are leggy, quadrupedal, lizard-like creatures that are among the closest relatives to the dinosaurs.

“But there is this 10-15 million year gap that separates the common ancestor of silesaurids and dinosaurs from the first true dinosaurs,” Paul Barrett, a dinosaur researcher at the NHM, said in an article published on the museum’s website.

Nearly all of the oldest known dinosaurs are from Argentina and Brazil. Among the key examples are Saturnalia (Brazil) and Eoraptor (Argentina), which were small, bipedal sauropodomorphs that were transitioning to a herbivorous diet.

“About 230 million years ago in Argentina we have the first sauropodomorphs,” Ashley Poust, a paleontologist at the San Diego Natural History Museum, told Newsweek. “These weren’t yet the ponderous giants that their descendants would become, but they had evolved some distinct differences from other dinosaurs at the time.”

Poust continued, “They differed by changing their gait to sometimes walking on all fours. And they may have already begun to become omnivores, including plants in their diet, something other known Triassic dinosaurs were unlikely to do. One South American sauropodomorph was even named Panphagia, mainly because it was adapted to eat anything.”

A replica display of Herrerasaurus, exhibited at the Art Science Museum in Singapore on January 23, 2014. Herrerasaurus was the first large carnivorous dinosaur. ROSLAN RAHMAN/AFP via Getty Images

Another important early dinosaur was Herrerasaurus from Argentina. This creature was the first large carnivorous dinosaur, although scientists don’t understand its relationships with other dinosaurs very well, according to Nesbitt. Early dinosaurs tended to be small in comparison to their later relatives.

One of the most famous Triassic dinosaurs, Coelophysis, was not discovered in Argentina or Brazil. This theropod was first found in Ghost Ranch, New Mexico, where scientists famously dug up a vast death assemblage consisting of numerous individuals in the 1940s and ’50s.

“These dinosaurs are skinny, toothy little meat eaters and are among the best-known Triassic dinosaurs,” Poust said.

The oldest ornithischian dinosaurs are highly debated, but clear evidence of them first appears in the fossil record around 200 million years ago, with small plant-eaters like Scutellosaurus, according to Nesbitt.

When Dinosaurs Ruled the Earth

Ghost Ranch continues to be an important site for researchers studying the early history of dinosaurs.

“One of the coolest lessons about the earliest dinosaurs has been brought home by new excavations there in the last decade by some of our most accomplished paleontologists is that dinosaurs didn’t rule right away,” Poust said. “They shared the scene with many other creatures and only later in the Jurassic Period, 201 to 145 million years ago, do they seem to outlast the competition.”

He went on, “Even as late as the late Triassic when Coelophysis was around there were tons of other crazy animals in the ecosystems that were bigger and scarier than the dinosaurs. They included huge crocodile-like phytosaurs and plant-eating armored aetosaurs that had enormous shoulder horns. The Triassic also had the cow-sized Placerias and the elephant-sized Lisowicia, which were more closely related to mammals than to dinosaurs. Other new things, like pterosaurs, were spreading along with the early dinosaurs.”

By the Jurassic Period, however, dinosaurs had truly taken center stage, occupying many of the conspicuous roles in ecosystems on land, and many had grown to huge sizes, according to Poust.

While theropods and sauropodomorphs did well in the Triassic Period, all three major dinosaur lineages dominated in the Jurassic, Nesbitt said.

A mass extinction event at the end of the Triassic enabled dinosaurs to become dominant on land, wiping out virtually all other archosaurs that they were in competition with.

What Killed the Dinosaurs?

Eventually, the dinosaurs’ reign came to an end as a mass extinction event changed the course of evolutionary history again. About 66 million years ago, a giant asteroid smashed into the Earth. Three-quarters of the plant and animal species on our planet are thought to have been wiped out in the ensuing event.

Among the animals that disappeared during the extinction were non-avian dinosaurs, pterosaurs and ammonites, as well as many bird and mammal species. Some birds, however, managed to survive. This group of animals continues to thrive today. Because scientists consider birds to be “avian dinosaurs,” technically the dinosaurs were never completely wiped out.