Theropods include the most terrifying dinosaurs that ever walked the earth, from T. rex to Velociraptor to Spinosaurus. They are also one of the most remarkable groups for their close relationship to birds. The name Theropod means ‘beast foot’ in Greek, hinting at the large, clawed feet of these agile hunters.

 Even though chickens and songbirds may not seem as interesting as their dinosaur ancestors, they help us uncover the amazingly rich and diverse history of one of the most fascinating groups of animals of all time. Here, we’ll go over the basic traits of Theropods as well as their evolutionary history and their living cousins.

If you are curious about a particular dinosaur, see this full list of all the dinosaurs we have covered.

The Fossil Record of Theropods

Theropods are some of the oldest dinosaurs that we know of. The first relatives of Theropods are Herrerosaurids, which are about 231 million years old in the Late Triassic. Whether or not Herrerosaurids are actually Theropods or just their primitive relatives is still being researched. However, the first true Theropods are Coelophysoids from South America from the Late Triassic and Early Jurassic. Theropods are the only dinosaur clade to still be alive today! Even though most modern birds have four toes, they are the only living descendants of Theropods—meaning dinosaurs technically never went completely extinct. 

Just like bird fossils, small Theropod fossils are hard to find because their bones are hollow and fragile. This is why so much of the fossil record for Theropods are large carnivores like Allosaurus and T. rex. The first Theropod ever found was Megalosaurus, although it was not thought to be a dinosaur when it was first found. 

Part of a femur was discovered in the 1600s in the United Kingdom which was described first as a Roman war elephant. Later, this was changed to a biblical human giant. In 1824, it was finally described as a giant lizard and given the name Megalosaurus. In 1842 Richard Owen used Megalosaurus and two other animals, Iguanodon and Hylaeosaurus, to create the group Dinosauria. 

When did the First Theropods Live?

This makes Megalosaurus the first Theropod fossil ever found, but the first Theropod that ever lived is harder to determine. Eodromaeus is a small, agile dinosaur from approximately 230 million years ago in Argentina. Before this discovery, Eoraptor was thought to be the earliest known Theropod dinosaur, but the team of paleontologists that described Eodromaeus found that Eoraptor was most likely an early Sauropodomorph and Eodromaeus was the true earliest Theropod. Only five years later, it was suggested that Eodromaeus was a primitive Saurischian dinosaur just like Eoraptor and therefore not a Theropod at all.

Another contender for the title of earliest Theropod is Tawa. Larger than Eodromaeus but still relatively small, Tawa came from New Mexico around 215 million years ago. For many years, several different studies described Tawa as a basal or primitive Theropod. In recent years, it has been suggested that it may be a primitive Saurischian or a Herrerasaur—another type of dinosaur that was once thought to be a Theropod. 

The Herrerasaurids from Late Triassic Argentina were the first to be considered early Theropods. However, in 1988 a nearly complete skeleton of Herrerasaurus was discovered that revealed it was not a Theropod at all, and the group was given their own family as basal Saurischians.

No matter which of these Late Triassic dinosaurs was the first Theropod, the fact remains that Theropods evolved in the Late Triassic in North America or Gondwana, a paleo continent that included modern South America, Africa, and Australia. These three continents were still connected at the time as Pangaea continued to break apart. These dinosaurs quickly spread across the globe, where their descendants can still be found today.

Theropods Anatomy in Detail

Most Theropods, even as far back in the Late Triassic, had the same general body shape and characteristics. Most species are bipedal with three-toed feet and are built for running in order to chase down their prey. They come in a wide range of sizes, from huge, intimidating predators to tiny, agile hunters. 

Spinosaurus is one of the largest Theropods, growing up to 49 feet long and weighing up to 8 tons. On the other end of the scale, the petite Anchiornis only grew about 1 foot long and weighed around 4 ounces, or one quarter of a pound. Some Theropods have names that hint at their magnitude, like Giganotosaurus or Megalosaurus, and others hint at their compact size, like Microraptor. 

Because so many Theropods are carnivorous hunters, their teeth are usually long, conical, and sharp with serrated edges. The numbers of fingers on their hands and presence of claws vary across the group. For example, T. rex had shortened limbs that ended in two clawed fingers but Therizinosaurus had long arms ending in three fingers with huge sickle-like claws.

Not All Theropods are Carnivore

One of the defining characteristics of Theropods is their carnivorous nature. The clade was first created within Dinosauria in order to put all of the carnivorous dinosaurs into one group. Early finds included a Compsognathus fossil with a lizard in its stomach as well as some Theropods fossilized in the middle of hunting for their prey, such as a Velociraptor with Protoceratops or a T. rex with Triceratops. The discovery of herbivorous Theropods like Therizinosaurus, Oviraptor, Troodon, or Gallimimus came at the end of the 1900s. These groups are some of the dinosaurs most closely related to modern birds, which also can be anything from carnivorous to herbivorous.

Theropods are not the only dinosaurs to evolve feather-like structures, but they are the group where these structures are the most common. Even so, many Theropods still kept scaly skin, especially in adulthood. Non-avian dinosaurs were typically not capable of flight, so their feathers were most likely used for warmth and coloration. The 160 million year old Epidexipteryx even had four long feathers on its tail for some sort of display: the first example of ornamental feathers.

Where did they live? 

Even in prehistoric times, Theropods could be found all over the world. Here’s a short list of some Theropods from each continent, including some we’ve already written about.

Interesting Points about Theropods

From Dinosaurs to Birds

Legendary paleontologist Othniel Charles Marsh created this group. Because Theropods include so many species of dinosaurs, the relationships between them can be difficult to determine—especially in the case of species that do not have a large amount of the skeleton fossilized. On the other hand, because there are so many Theropod species of dinosaurs, the fossil record contains thousands of well-preserved specimens that reveal a lot about the evolution of not only this group, but dinosaurs as a whole. 

Birds exist as an exciting link to some of the most awe-inspiring animals of the past. It can be difficult to look at a pigeon and try to imagine how it could possibly be related to something as intimidating as a T. rex, but they share several traits that reveal their close relationship. The primary features connecting Theropods and birds are feathers, wishbones, hollow bones, and nesting behavior. 

The evolution from dinosaur to bird was a long process and even the most primitive Theropods in the Late Triassic shared some of these traits with birds. When the feathered Archaeopteryx was first discovered in China, it seemed as though birds suddenly appeared in the fossil record with very few transitional dinosaurs in between. Further study from paleontologists slowly revealed that early dinosaurs were more like birds than we might think.

Size Mattered As Theropods Evolved Into Birds

One of the most important details of how dinosaurs became birds is their smaller stature. Birds today simply do not reach the humongous sizes of dinosaurs—very few animals do. But the rate at which dinosaurs became smaller and smaller by the end of the Late Cretaceous is surprisingly fast. And while some birds, such as the fearsome cassowary or the strange emu, still look like dinosaurs to us, the group has one big difference from their ancestors. 

Adult birds actually look more like baby dinosaurs than anything else because of their large eyes and short snouts that they had replaced with beaks. It is thought that this is how they shrank so rapidly, by changing the maturation process so that they retained their juvenile forms into adulthood. In fact, even juvenile dinosaurs are more advanced than modern birds. The one fossil that bird skulls most closely resemble are embryos, still inside of their eggs. 

The Discovery of Dinosaurs With Feathers

Feathers are, of course, the first thing that comes to mind when discussing the relationship between dinosaurs and birds. The discovery of feathered dinosaurs was a benchmark in the field of paleontology and the research of this controversial topic continues today. Paleontologists still cannot agree which species may have had feathers, how much of the body these feathers covered, and what they may have been used for. 

The oldest example of a dinosaur with protofeathers is not a Theropod, but Kulindadromeus from the Middle Jurassic. The discovery of this dinosaur was also the first time that feather-like structures had been found outside of Theropods. The oldest Theropod with feathers, however, is Anchiornis. This dinosaur was not just feathered, it was fully covered down to its feet and toes. 

Hollow Bones Saves Weight

Birds are known for their delicate, hollow bones but even some of the oldest dinosaurs had hollow bones too. The wishbone, or the furcula, is a forked bone in the chest of birds and their relatives. It connects to their shoulder joint and is something that helps birds fly today by allowing for extra muscle attachments in the wings and chest.

Compared to living animals, not much is known about the behavior of prehistoric animals. Dinosaurs are no exception, but sometimes spectacular fossils help expose hints of how they lived in the past. Nesting behavior can be seen from fossils of dinosaurs still on top of their eggs as well as the way nests themselves are built and arranged. Oviraptor and Citipati fossils have been found laying on top of eggs with their wings spread, just like birds do today.

Living Dinosaurs

Unlike pigeons, some birds are very easy to relate to dinosaurs. Perhaps the most notable of these is the flightless cassowary from Australia and nearby islands. These birds not only look primitive, but they can be dangerous when provoked. Their most dinosaur-like feature is their casque, the large skin-covered protrusion on the top of their head. Their skull is reminiscent of dinosaurs such as Corthyrosaurus, Pachycephalosaurus, and Dilophosaurus. Strong, scaly, three-toed legs and thin, hair-like feathers also draw up the picture of dinosaurs that had just begun their transition into birds. 

Nearby, the flightless emu of Australia has its own suite of dinosaur-like characteristics. Its feathers are much like the cassowary’s, and its wings have shrunken to the point of being vestigial. On their wings, however, is a very unbirdlike feature—a claw. This claw is just as vestigial as the rest of their wings, unfortunately, and these birds are almost entirely herbivorous.

Birds With Functioning Claw

One of the only modern birds with a functioning claw is the hoatzin from South America. Just like the cassowary, their large plumage and blue-skinned face make them look like living dinosaurs. Hoatzin chicks retain two claws on each wing that they lose in adulthood. They use these claws to climb around in the trees before they are able to fly. Despite the fact that this looks just like the three-clawed wings of Archaeopteryx, it is most likely that clawed wings were lost in early birds and reappeared later in certain species that needed them.

In Africa, the shoebill is a gentle giant with an intimidating appearance. Its large bill and forward-facing eyes give it the look of a capable and dangerous predator and they can grow up to nearly 5 feet tall. These birds are carnivorous and mostly they eat fish, but they are capable of eating even baby crocodiles. Even so, they are known for being docile around humans.

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Featured Image Credit: Xing Lida, CC BY-SA 4.0, via Wikimedia Commons