Since the origins of life on Earth (2,700 million years ago), evolution has left the footprint of countless living organisms on the planet. Some evolved into new species, while others, due to different environmental factors, have disappeared. As is the case with the dinosaurs.

When did the dinosaurs live?

The Mesozoic Era (from 251 to 65 million years ago) in the time scale of the Earth is divided in three periods of time: Triassic, Jurassic and Cretaceous. During these periods, dinosaurs were the dominant terrestrial vertebrates in the world. However, this highly diverse clade of reptiles was practically wiped out during the Cretaceous–Paleogene mass extinction, marking the end of the Mesozoic Era.

 

Digital illustration of the spiral of ecological time. By Nicolas Primola | Shutterstock.com
Digital illustration of the spiral of ecological time. By Nicolas Primola | Shutterstock.com

Paleontology, dinosaurs and birds

Evolutionary studies, as well as studies of fossil remains, have so far identified more than 500 dinosaur genera and 1,000 distinct species. Despite differences in size (from the 35 cm tall Anchiornis huxleyi to the 25 m long Amphicoelias fragillimus), dinosaur species had a lot in common. For example, they all had horns or crests, four straight legs, and they laid eggs in nests. Other groups of animals, like pterosaurios or pelicosaurios, are popularly considered dinosaurs, but they are not – they didn’t have straight legs. Nevertheless, there is evidence of aquatic dinosaurs, as well as dinosaurs that were able to fly or, at least, glide.

The first dinosaurs walked on their two hind legs, similar to birds. Later came quadruped species or species that could walk two or four legs. Most paleontologist that work with dinosaurs state that birds can be considered dinosaurs. Birds are classified under the Maniraptora subgroup, which are Coelurosauria, which belong to order Saurischia, which are also dinosaurs. Contrary to popular belief, dinosaurs cohabited with other animal groups, such as mammals. These were small mammals that lived in reduced ecological niches in a world dominated by big reptiles. With the mass extinction, dinosaurs practically disappeared, but the surviving species evolved into the current birds and other groups of smaller animals.

Paleontologists recovering dinosaur skeleton in an excavation. By natural paleontologist | Shutterstock.com
Paleontologists recovering dinosaur skeleton in an excavation. By natural paleontologist | Shutterstock.com

The mass extinction

There are several theories about the non-avian dinosaur extinction

During the Mesozoic Era the temperature on Earth was warmer, less changeable and its atmosphere had higher levels of carbon dioxide and oxygen. When the volcanic activity in the Cretaceous-Paleogene came to an end, all of these atmospheric factors were affected. Almost all dinosaur species were large and, according to what many scientist say, due to their high demand for oxygen they weren’t able to survive this process.

Moreover, the presence of larger tetrapods had a significant impact on dinosaurs, as they occupied their ecological niches. Insects were not affected by the environmental changes, therefore providing smaller species with a source of food and increasing their ability to survive. Finally, the emergence of flowers and the reduction of the coniferous plants that dinosaurs depended on added to their inability to adapt and contributed to their eventual disappearance.

Another common theory states that a meteorite with a diameter of 5-15 km collided close to the peninsula of Yucatan, creating the Chicxulub crater. According to this theory, the drastic changes in the planet’s temperature and atmosphere brought about by the impact caused the death of most species. Some scientists believe that this mass extinction could have taken place in only a matter of hours.

Deccan Traps, found in India, is one of the largest volcano formations on Earth. It is assumed that their formation led to the expulsion of dust and gases, affecting the sunlight’s penetration, limiting the photosynthesis of plants and creating a greenhouse effect. The sudden increase in temperatures would have led to the extinction of many species. Although this theory has been discarded by most scientists as the main cause of the disappearance of dinosaurs, many suggest it contributed to their extinction.

 

Casowary. The bird close to non-avian dinosaurs. By Sanit Fuangnakhon and Matt Cornish | Shutterstock.com
Casowary. The bird close to non-avian dinosaurs. By Sanit Fuangnakhon and Matt Cornish | Shutterstock.com

Feathers

The common image of dinosaurs is that of large, scaly reptiles. For decades it has been known that their appearance was quite different, however the popular imaginary has always been the “impressive giant lizard” rather than the “oversized ostrich”. Nothing could be further from the truth. Although there were dinosaurs that didn’t possess feathers, the vast majority of them did. With the discovery of Archaeopteryx in 1864, a new idea was considered, that this species could be the missing link between dinosaurs and birds. Today there are many fossils that support this theory. So now, it isn’t hard to think of a bird like the cassowary (see image) as an ancestor of the dinosaurs. Suddenly, with this in mind, dinosaurs don’t seem so distant and distinct.

 

Dinosaurs

There are many scientific and artistic representations of how we think dinosaurs may have looked, but cinema was the most powerful tool in bringing these extinct animals back to life. Films such as “A Million Years BC” or “King Kong” included these giant reptiles in their footage, but it was with Jurassic Park and its subsequent renditions (the latest Jurassic World: The Fallen Kingdom by J.A. Bayona) that realism became fiction. Even though many of the dinosaurs in these films are quite unrealistic, their special effects and designs give viewers the opportunity to imagine what they might have been like in a totally believable way.

From the extensive list of species that formed these prehistoric reptiles, I present below a selection of the most characteristic representatives of each taxonomic group, with some of their distinctive qualities.

Dilophosaurus (Coelophysoidea)

You may remember this species from Jurassic Park. In the film it was depicted as quite a small reptile with an extendable cowl (like the lizard Chlamydosaurus kingii) and the ability to spit poison. All three of these features were cinematic inventions. In fact, it was 7 meters long and weighed about 350 kg. It is one of the first theropods of the early Jurassic epoch. Its main characteristic was the two crests on its skull that they used possibly as a display (sexual dimorphism). They had weak front teeth which suggests that they were scavengers (because they wouldn’t have been able to hold large prey).

Dilophosaurus (Coelophysoidea). By Herschel Hoffmeyer | Shutterstock.com
Dilophosaurus (Coelophysoidea). By Herschel Hoffmeyer | Shutterstock.com

Coelophysis (Coelophysoidea)

This group of dinosaurs consisted of three species, one of which was the best-known primitive dinosaur on account of the number of complete fossils unearthed. It measured 2.5 to 3 meters in length and weighed about 28 kilograms. They are believed to have been fast running carnivores, probably feeding on smaller lizards. They had an elongated snout with large openings that reduced the weight of their skull. It’s possible that they lived in large herds and were able to hunt larger prey. The curvature of their necks allowed for a quick biting movement.

Coelophysis (Coelophysoidea). By Kostyantyn Ivanyshen | Shutterstock.com
Coelophysis (Coelophysoidea). By Kostyantyn Ivanyshen | Shutterstock.com

Ceratosaurus (Ceratosauria)

This dinosaur lived at the end of the Jurassic era. The main characteristic of these animals was the horn-shaped structure on their skull, behind their nostrils, and two more horn-shaped ridges in front of their eyes. This was one of the few theropods that had armoured skin, in the the form of small bony plates. Its tail measured half its total length and its back was lined with vertebral spines. They lived in modern-day United States, Tanzania and Europe and it was one of the first dinosaurs to be discovered.

Ceratosaurus (Ceratosauria). By Catmando | Shutterstock.com
Ceratosaurus (Ceratosauria). By Catmando | Shutterstock.com

Monolophosaurus (Megalosauroidea)

The name Monophosaurus refers to the ridge on top of its skull. It ran from between the nostrils up to the eyes. It was hollow inside and may have been used to attract females during the mating season. It is a species of Carnosaurus that lived in Asia at the end of the Jurassic era. It was up to 6 meters long and 2 meters high and had very sharp teeth. Remains of this species have always been found close to water, suggesting that they may have inhabited areas near lakes or oceans.

Monolophosaurus (Megalosauroidea). By DM7 | Shutterstock.com
Monolophosaurus (Megalosauroidea). By DM7 | Shutterstock.com

Spinosaurus (Megalosauroidea)

This species lived in North Africa during the Cretaceous period. The Spinosaurus was the largest of all the carnivorous dinosaurs. It could measure up to 18 meters in length and weighed in excess of 20 tons. They had a skull similar to that of a crocodile and spikes measuring up to 1.65 meters protruded from their backs. Covered and joined together by skin, they would have been used for both thermoregulatory and display purposes. They walked on all fours most of the time and it is believed they spent a lot of time in the water, feeding on fish.

Spinosaurus (Megalosauroidea). By Herschel Hoffmeyer | Shutterstock.com
Spinosaurus (Megalosauroidea). By Herschel Hoffmeyer | Shutterstock.com

Allosaurus (Carnosauria)

The Allosaurus is a theropod that lived at the end of the Jurassic period. They were large (up to 9 meters long), bipedal and had strong claws and sturdy legs. They used their long, heavy tail to balance as they moved. Their skull was relatively large and featured two ridges in front of and above the eyes. Despite their large size, these predators were relatively light. Their teeth were serrated, but not very big, and it is believed that they hunted by stalking their prey and striking them hard with their jaws.

Allosaurus (Carnosauria). By Herschel Hoffmeyer | Shutterstock.com
Allosaurus (Carnosauria). By Herschel Hoffmeyer | Shutterstock.com

Compsognathus (Compsognathidae)

These small reptiles lived during the end of the Jurassic era. It’s one of the smallest species, measuring just 1 meter in height and about 3 kilograms in weight. They were carnivores that fed on small lizards. It’s the closest known relative to the prehistoric bird Archaeopteryx, the missing link between feathered dinosaurs and present-day birds. They had long hind legs, a long tail to maintain their balance, an elongated skull with a pointed nose, small serrated teeth and relatively large eyes.

Compsognathus (Compsognathidae). By Linda Bucklin | Shutterstock.com
Compsognathus (Compsognathidae). By Linda Bucklin | Shutterstock.com

Tyrannosaurus rex (Tyrannosauridae)

The T-Rex lived at the end of the Cretaceous period. It was one of the last dinosaurs around before the great extinction, as well as being one of the largest predators to have set foot on our planet. It was bipedal, with strong hind legs, an enormous skull, a long and thick tail for balance. It possessed small but strong front legs, equipped with two clawed toes. Experts are still debating whether they were hunters or scavengers. According to the latest studies, the T-Rex appears to have been covered with feathers like many other dinosaurs.

Tyrannosaurus rex (Tyrannosauridae). By Herschel Hoffmeyer | Shutterstock.com
Tyrannosaurus rex (Tyrannosauridae). By Herschel Hoffmeyer | Shutterstock.com

Struthiomimus (Ornithomimosauria)

The name of these animals comes from Latin and means ‘imitator of ostriches’, due to their similarity. They were more than 3 meters tall and inhabited part of the current United States during the end of the Cretaceous. Their bones were hollow which made them lighter, possibly allowing them to run faster. There have been plenty of remains found of this species, which would suggest that they were not carnivores, and the hypothesis is that they lived in herds. They had no teeth and their snout was more like a beak. They may have been covered with feathers.

Struthiomimus (Ornithomimosauria). By Herschel Hoffmeyer | Shutterstock.com
Struthiomimus (Ornithomimosauria). By Herschel Hoffmeyer | Shutterstock.com

Mononykus (Alvarezsauroidea)

These dinosaurs inhabited the Gobi Desert at the end of the Cretaceous. They measured 1 meter and weighed about 3 kilograms. They had strong hind legs and were fast, highly agile runners. Their front legs were very short and had just one finger, ending in a long claw. Supposedly, these animals were covered with feathers. Their small skull, contained little, sharp teeth. They are believed to have eaten insects and small animals, and that they hunted at night, like the Velociraptor, thanks to their specially adapted eyes.

Mononykus (Alvarezsauroidea). By Linda Bucklin | Shutterstock.com
Mononykus (Alvarezsauroidea). By Linda Bucklin | Shutterstock.com

Therizinosaurus (Therizinosauria)

This animal, also known as “scythe lizard” on account of its claws, lived in Asia at the end of the Cretaceous period. Among the individual bones unearthed of this dinosaur were its enormous, 70 cm long claws. It was estimated to be 8 to 12 meters long and weighed up to 6 tons. Unlike other theropods, the Therizinosaurus stood on two hind legs, with four toes holding its body, instead of three.

Therizinosaurus (Therizinosauria). By Catmando | Shutterstock.com
Therizinosaurus (Therizinosauria). By Catmando | Shutterstock.com

Oviraptor (Oviraptorosauria)

This non-avian dinosaur was very similar to a bird. Originally, they were thought to have fed on eggs because the first specimen was found near a nest. Later studies showed that it was in fact their own nest. They were about two meters long and weighed about 11 kilograms. Besides having a rib cage similar to that of a present-day bird, they also had feathers on their bodies, tails and front legs. According to studies of relatives of this species, we can assume that they had a casque similar to that of the Australian cassowary.

Oviraptor (Oviraptorosauria). By Linda Bucklin
Oviraptor (Oviraptorosauria). By Linda Bucklin

Microraptor (Deinonychosauria)

The discovery of the Microraptor was an important source of information for evolutionary studies between birds and dinosaurs. They lived at the beginning of the Cretaceous in Asia and had long feathers that formed supporting surfaces on all four legs and tail. Similar to today’s birds, They are thought to have been able to fly or at least glide with their four wings and tail (used as a rudder). It measured between 42 and 83 centimetres and weighed no more than 1 kilogram (one of the smallest known dinosaurs).

Microraptor (Deinonychosauria). By Warpaint | Shutterstock.com
Microraptor (Deinonychosauria). By Warpaint | Shutterstock.com

Velociraptor (Deinonychosauria)

In contrast to their oversized depiction in Jurassic Park, Velociraptors were a relatively small animal, around 1.8 meters long, including the tail. They weighed about 15 kilograms and lived in late Cretaceous Asia. On their hind legs (they were bipeds, with long legs indicating that they were especially fast) they had three claws, one of which was larger, elongated and curved, with which they killed their prey. It is assumed, according to studies of previous species, that they were also covered with feathers.

Velociraptor (Deinonychosauria). By Linda Bucklin | Shutterstock.com
Velociraptor (Deinonychosauria). By Linda Bucklin | Shutterstock.com

Plateosaurus (Plateosauridae)

This dinosaur genus lived during the Triassic period in what is now Europe and Greenland. They were bipedal animals with a very long and mobile neck made up of 10 vertebrae, which fed on plants with their sharp yet stubby teeth. They had strong hind legs and short arms with prehensile fingers and claws that they used to defend themselves and grab hold of food. They measured from 5 to 10 meters and could weigh up to 4,000 kilograms. They had a long, strong tail with which they kept their balance when on the move.

Plateosaurus (Plateosauridae). By Linda Bucklin | Shutterstock.com
Plateosaurus (Plateosauridae). By Linda Bucklin | Shutterstock.com

Apatosaurus (Diplodocoidea)

These giant animals were 22.8 meters long and weighed about 22 tons. Together with the Diplodocus they are the two main representatives of this family, although the Apatosaurus had bigger and stronger bones. To lighten the load on their vertebrae, they were perforated with air sacks. They were herbivorous animals that fed on the least accessible vegetation. Their tails were very long and the hypothesis is that they would use them almost like a whip in order to generate loud sounds.

Apatosaurus (Diplodocoidea). By Catmando | Shutterstock.com
Apatosaurus (Diplodocoidea). By Catmando | Shutterstock.com

Alamosaurus (Titanosauria)

This is one of the largest known dinosaurs to have existed. Reaching up to 30 meters and 74 tons in weight, it was the largest in North America. They lived at the end of the Cretaceous unlike all other Titanosaurus. They had a small head, supported by a long and thick neck, a wide and robust torso, cylindrical legs and a tail, which they helped them to balance. This was a herbivorous animal and we can assume it took advantage of its size and long neck to reach the highest and most inaccessible vegetation.

Alamosaurus (Titanosauria). By Herschel Hoffmeyer | Shutterstock.com
Alamosaurus (Titanosauria). By Herschel Hoffmeyer | Shutterstock.com

Ankylosaurus (Ankylosauria)

This is the most remarkable species of armoured dinosaurs, which lived at the end of the Cretaceous period. They had strong, heavy bone armour, comprised of plates that were often fused together. It covered their entire bodies and shielded them from attack. They were 6.25 meters long and weighed about 6 tons. It was a quadruped animal, wide and robust, with a large skull and two horns pointing on back of the head. The mouth ended in the shape of a beak with small teeth. At the end of the tail they had a large club which they were thought to have used to defend themselves.

Ankylosaurus (Ankylosauria). By Warpaint | Shutterstock.com
Ankylosaurus (Ankylosauria). By Warpaint | Shutterstock.com

Stegosaurus (Stegosauria)

These animals lived at the end of the Jurassic period in what is now North America. It was a large quadruped herbivore with a rather unusual body shape. They had a small head which hung close to the ground, short front legs and longer back legs. This meant they had a very arched back and a rigid tail which was suspended in the air with the end covered with large spikes. Along its back were bony plates, but there’s still debate about what they would have been used for: thermoregulation, display or defence.

Stegosaurus (Stegosauria). By Herschel Hoffmeyer | Shutterstock.com
Stegosaurus (Stegosauria). By Herschel Hoffmeyer | Shutterstock.com

Iguanodon (Ornithopoda)

The iguanodon lived at the beginning of the Cretaceous period in what is now Europe. It was the second dinosaur to be named after the Megalosaurus, and it got its name because its teeth were similar to those of an iguana. They were large herbivores (about 12 meters long and weighed 3 tons) with strong, robust legs. One notable characteristic was a claw on their thumbs which they probably used to defend themselves against their predators. They are believed to have alternated between walking on two and four legs.

Iguanodon (Ornithopoda). By Warpaint | Shutterstock.com
Iguanodon (Ornithopoda). By Warpaint | Shutterstock.com

Parasaurolophus (Ornithopoda)

These dinosaurs were herbivores that also walked two and four legs. They inhabited North America at the end of the Cretaceous period. The main feature of these animals was the long curved tube projecting upwards and back from the skull, giving it an unusual appearance. It is assumed that the function of this was visual recognition of the species and sex, acoustic resonance and thermoregulation. They belong to the group of duck-billed dinosaurs, and are considered one of the rarest types of dinosaur.

Parasaurolophus (Ornithopoda). By Warpaint | Shutterstock.com
Parasaurolophus (Ornithopoda). By Warpaint | Shutterstock.com

Triceratops (Ceratopsia)

Triceratops lived at the end of the Cretaceous period, in what is now North America, and were one of the last genera to appear before the Cretaceous–Paleogene mass extinction. These animals had a large bony frill (posterior cranial platform) and three horns. Their robust body, strong legs were similar in many ways to a rhinoceros. There is an extensive debate about the function of their bony frill and horns, whether they were used for defence, in courtship or a display of dominance as other current species.

Triceratops (Ceratopsia). By Herschel Hoffmeyer | Shutterstock.com
Triceratops (Ceratopsia). By Herschel Hoffmeyer | Shutterstock.com

Pachycephalosaurus (Pachycephalosauria)

This species lived in late Cretaceous North America. The distinct characteristic of these animals was the possession of an extremely thick skull roof. Experts initially believed that this thick skull was used in combat, but this theory was later ruled out. They were bipeds, with long hind legs and short front legs. They were about 5 meters long and weighed up to 2 tons. Their brain was small and their semi-frontal eyes gave them binocular vision. Their teeth were small and serrated.

Pachycephalosaurus (Pachycephalosauria). By Herschel Hoffmeyer | Shutterstock.com
Pachycephalosaurus (Pachycephalosauria). By Herschel Hoffmeyer | Shutterstock.com

Thanks to Pablo Jaramillo (Teacher at the National Museum of Natural History of Chile) for his notes on some errors in the text.

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