
Not all dinosaurs are created equal – there are many different types that can be organized into groups around common features that they share. So, what exactly are the different groups and what do those groups tell us about evolution, extinction and the animals that share our world today?
What are the different types of dinosaurs?
Dinosaurs are classified into two main groups or clades:
Saurischia
Saurischia is a major group of dinosaurs known as a clade. Traditionally, dinosaurs were divided into two orders—Saurischia (lizard-hipped) and Ornithischia (bird-hipped). Today, many scientists see Saurischia more as a clade, which means it’s a group of species that share a common ancestor, rather than a strict order or genus. This clade includes both the meat-eating theropods and the long-necked sauropodomorphs.
What does Saurischia mean?
The term Saurischia is used to describe a group of dinosaurs whose hip structure is similar to that of modern lizards. The name comes from the Greek words “sauros,” meaning lizard, and “ischion,” meaning hip. This group includes both meat-eating theropods and the long-necked sauropodomorphs. The lizard-like hip bones are a key characteristic that sets them apart from the other major dinosaur group, Ornithischia, or “bird-hipped” dinosaurs.
What types of dinosaurs belong to the Saurischia clade?
The Saurischia group is divided into two sub-categories of dinosaurs:
Theropods:
These were mostly two-legged, meat-eating dinosaurs. Famous examples include Tyrannosaurus rex and Velociraptor. They often had sharp teeth and claws, and some later evolved into birds starting about 150 million years ago.
Sauropodomorphs:
This group includes the long-necked, four-legged herbivores such as Diplodocus and Brachiosaurus. They were among the largest land animals, known for their massive bodies, long tails, and relatively small heads.

What are some living examples of Saurischia?
Somewhat confusingly, modern birds are ancestors of ancient theropods rather than members of the Ornithischia clade. Apparently, the reason is, it’s all in the hips! Modern birds descend from dinosaurs like T. rex which had lizard-like hip bones. Whereas dinosaurs like Triceratops, although they had ‘bird-like hip structures’, did not evolve into today’s avian species.
If anyone can make more sense out of that classification kerfuffle, please help us out in the comments below.
How did Theropods evolve into modern birds?
This is one of the great stories of evolution. One that is still being written. It’s such a massive leap to see the prehistoric characteristics of a colossal Tyrannosaur in the tiny features of the pigeon hopping around your local town square.
It all started about 150 million years ago, give or take a few millennia. And it took the accretion of many small changes over that incredibly long time period for the predatory dinos of the Late Jurassic age to evolve away from early, relatively small tyrannosauroids into today’s French fry stealing seagull.
Here’s how it might have gone:
Feathers First:
Early theropods started growing feathers. At first, these feathers might have helped with keeping warm or displaying to mates, rather than flying.
From Insulation to Flight:
As feathers became more complex, they helped some theropods glide or move through the air. Over time, these gliding abilities improved into true powered flight.
Skeleton Changes:
The bones of these dinosaurs gradually became lighter and more adapted for flight. For example, some bones fused together to create a strong yet lightweight structure, similar to what we see in modern birds.
Other Adaptations:
Over time, other features evolved, like the development of a beak (replacing teeth) and changes in muscle structure, which made flight more efficient.
Fossils like Archaeopteryx show a mix of dinosaur and bird features, giving us a glimpse into this long evolutionary journey.
Ornithischia
The other major clade of dinosaurs is Ornithischia which covers ornithopods, thyreophorans, and ceratopsians.
What does Ornithischia mean?
Ornithischia comes from the Greek words “ornithos” (bird) and “ischion” (hip). This name refers to the hip structure of these dinosaurs, which some thought was similar to that of modern birds. However, as we’ve already seen, despite the name, ornithischians are not directly related to modern birds.
What types of dinosaurs belong to the Ornithischia clade?
This clade breaks out into three main kinds of Ornithischia.
Ornithopods (Ornithischia):
These herbivores ranged from small, fast runners to larger, duck-billed dinosaurs like Hadrosaurs and Iguanodon. Many were bipedal, which means they could walk on two legs, especially when moving quickly.
Thyreophorans (Ornithischia):
Known as armored dinosaurs, this group includes creatures like Stegosaurus and Ankylosaurus. They were covered in bony plates, spikes, or armor, which helped protect them from predators.
Ceratopsians (Ornithischia):
These dinosaurs, such as Triceratops, had distinctive beaks, frills, and horns. Their features likely helped in defense and possibly in display or social interactions.

Are there living examples of Ornithischia?
Unfortunately, we said goodbye to the herbivorous Ornithischia dinosaurs during the Cretaceous period 66 million years ago. Unlike theropods, which gave rise to modern birds, there are no living descendants of ornithischian dinosaurs.
Why did the Ornithischian dinosaurs go extinct?
Ornithischian dinosaurs went extinct as part of the mass extinction event at the end of the Cretaceous period, which was likely caused by a large asteroid impact near what is now the Yucatán Peninsula. This impact likely triggered wildfires, tsunamis, and a “nuclear winter” effect that disrupted food chains.
At the same time, the situation wasn’t helped by widespread, significant volcanic activity, such as the eruptions that created the Deccan Traps in present-day India, which may have worsened the environmental stress on land-based dinosaurs.
All members of this clade, such as hadrosaurs, ceratopsians, stegosaurs, and ankylosaurs, disappeared during this mass extinction event—the fifth mass extinction event in Earth’s history, also known as the Cretaceous-Paleogene (K-Pg) extinction event.
What are the “big five” mass extinctions?
The “big five” mass extinctions include:
- End-Ordovician (around 444 million years ago):
- What went extinct: This event mostly hit marine life. Many types of brachiopods, trilobites, graptolites, and early reef-building organisms disappeared. Since most life at the time was in the ocean, the extinction mainly impacted sea creatures.
- Late Devonian (around 372–359 million years ago):
- What went extinct: This extinction affected marine communities over an extended period. Major losses occurred among armored fish (placoderms), many reef-building organisms, and other marine invertebrates. The decline in these groups reshaped marine ecosystems and affected early terrestrial life as well.
- Permian-Triassic (about 252 million years ago):
- What went extinct: Known as the “Great Dying,” this was the most severe mass extinction. It wiped out a vast number of species—up to 90% of marine species and a large proportion of terrestrial species. Many marine invertebrates, such as trilobites and brachiopods, vanished. On land, many amphibians, early reptiles (including many synapsids that would eventually lead to mammals), and various insects were hit hard.
- Triassic-Jurassic (around 201 million years ago):
- What went extinct: This event mainly impacted many groups of reptiles and amphibians. Some large amphibians and non-dinosaurian archosaurs were lost, which opened ecological niches that allowed dinosaurs to become the dominant land vertebrates in the following Jurassic period.
- Cretaceous-Paleogene (around 66 million years ago):
- What went extinct: This extinction event is famous for the loss of non-avian dinosaurs. In addition, many marine reptiles, ammonites (spiral-shelled cephalopods), and a variety of other marine invertebrates were wiped out. While some bird lineages survived, the overall impact reshaped both marine and terrestrial ecosystems.
The story of dinosaurs and their modern descendants in a nutshell
So, there you have it. Birds came from Theropods, starting about 150 million years ago. They evolved enough to have gained the ability to fly by the K-Pg asteroid strike. And, when all the land-based dinosaurs went extinct the birds were able to carry on—surviving to frequent the feeder hanging in your backyard right now.
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