Coelurosauria

Coelurosauria, meaning "hollow-tailed lizards", is a group of theropod dinosaurs. These dinosaurs are more closely related to birds than to carnosaurs. This group includes interesting dinosaurs like compsognathids, tyrannosauroids, ornithomimosaurs, and maniraptorans. Some scientists think megaraptorans might also be part of this group, but we are still learning more about them.

One amazing fact about coelurosaurs is that most of the dinosaurs we know had feathers are in this group. Scientists like Philip J. Currie thought that all coelurosaurs might have had feathers. But we also found some coelurosaurs that had pebbly, scaly skin, showing that they did not all look the same.

Maniraptora, a part of Coelurosauria, includes the birds we see today. This means birds are the only dinosaurs still alive. In the past, people used the name Coelurosauria for all small theropods, but now we know this group is much more specific and exciting.

Anatomy

Bodyplan

Studying coelurosaurs shows that their last common ancestor could eat plants, which may have helped them. Some later groups kept this ability, while others ate insects (Alvarezsauridae), plants (Therizinosauridae), or meat (Tyrannosauroidea and Dromaeosauridae). Coelurosaurs include some of the largest (Tyrannosaurus) and smallest (Microraptor, Parvicursor) meat-eating dinosaurs. Special traits of coelurosaurs include:

• a sacrum (a row of bones attached to the hips) longer than in other dinosaurs
• a tail that stiffens at the end
• a bent ulna (a lower arm bone)
• a tibia (lower leg bone) longer than the femur (upper leg bone)

Integument

Main article: Feathered dinosaurs

Fossils show that early coelurosaurs had skin covered mostly in feathers. Feather traces are rare but found in most major groups. Most coelurosaurs also had scales or bony plates on parts of their bodies, especially their feet. Some of these animals had feathers elsewhere.

Feathers or feather-like structures were once thought to be only in coelurosaurs, but they have also been found in some ornithischian dinosaurs (like Tianyulong and Kulindadromeus_) and pterosaurs.

Nervous system and senses

Rarely, complete brain shapes of theropods are known from fossils. Scientists can create models of theropod brains using special scans. These discoveries are important because they show how modern bird brains evolved from earlier reptiles. The part of the brain called the cerebrum grew larger in coelurosaurs.

Fossil evidence and age

Some early fossils that might be from Coelurosauria go back to the late Triassic, but these fossils are not very clear. The first certain Coelurosauria fossils come from the late Middle Jurassic, like Proceratosaurus and Kileskus. Many almost complete fossils from Coelurosauria are known from the Late Jurassic, such as Archaeopteryx from around 155–150 million years ago.

In the early Cretaceous, many well-preserved fossils of Coelurosauria, including early birds, were found in the Yixian Formation in Liaoning. Most groups of Coelurosauria went extinct around 66 million years ago during the Cretaceous–Paleogene extinction event, but birds, which are part of the Coelurosauria group, survived and evolved into the many types we see today.

Scientists agree that birds are descendants of Coelurosauria. Today, birds are considered the only living members of this group. In 2015, a small piece of a tail from a young Coelurosaur was found preserved in amber.

Classification

The way scientists group coelurosaurs has changed a lot over time. In the past, many small meat-eating dinosaurs were put together in a group called Coelurosauria. But in the 1960s, scientists saw that some of these dinosaurs looked very different and began to sort them into new groups like Ornithomimosauria, Deinonychosauria, and Oviraptorosauria.

Today, scientists define Coelurosauria as all dinosaurs that are more closely related to birds than to another big meat-eater called Allosaurus. With this new definition, some big dinosaurs like the tyrannosaurids are now considered part of Coelurosauria too. Even some plant-eating dinosaurs, like the segnosaurs, are now known to be part of this group and are related to Therizinosaurus.

Scientists keep studying and naming new groups within Coelurosauria. Some recent groups include Tyrannoraptora, which links tyrannosaurids and birds. Another group, Maniraptoromorpha, includes dinosaurs more closely related to birds than to tyrannosaurids. The most recent group, Maniraptoriformes, may be special because the dinosaurs in this group might have had bird-like feathers and wings. This group includes ornithomimosaurs and maniraptorans, which are closely related to birds.