Many people’s first memories of learning about meteorites were probably prefaced by “the one that killed the dinosaurs”. This most famous and yet now non-existent meteorite crashed into our planet in Yucatan Pininsula, in what is now modern-day Mexico. In science textbooks there is an ever so familiar artists impression of dinosaurs gazing in awe at a night sky now filled with the blaze of an incoming massive space rock.

Dinosaurs – witnesses to their own extinction! Credit: Time Scavengers

But what exactly is a dinosaur? Apart from cool ancient monsters of course. Well, the definition of dinosaur is actually quite narrow, and a little more complicated than you might think!

Montage of six dinosaur fossils: (clockwise from top left) Microraptor gui, Apatosaurus louisae, Edmontosaurus regalis, Triceratops horridus, Stegosaurus stenops, Pinacosaurus grangeri.

First of all, dinosaurs are still alive today! Not just reptiles related to dinosaurs, like crocodilians, but actual animals that descended from theropods (that’s big bipedal carnivores like T-rex) and were the only dinosaur lineage to survive the extinction that killed most dinos 65 million years ago. And don’t worry, this isn’t some obscure taxon that you’ve never heard of before, but animals you see every day; they’re our flying feathered friends the birds (yes that does mean birds are technically reptiles…biology gets things wrong sometimes too).

Row 1Sinosauropteryx primaCryolophosaurus elliotiVultur gryphus Row 2Gypaetus barbatusConcavenator corcovatusCarnotaurus sastrei Row 3Deinocheirus mirificus & Tarbosaurus bataarBalaeniceps rexArchaeopteryx lithographica & Compsognathus longipes Row 4Anchiornis huxleyiDakotaraptor steini & OrnithomimusNanuqsaurus hoglundiPasser domesticus Row 5Spinosaurus aegyptiacusBubo scandiacus & Anas rubripes, Unnamed genus kayentakatae

So we have dinosaurs living among us, (and if you look at some birds the dinosaur jumps out at you. Looking at you, ostriches) but what about what we think of as dinosaurs, the big lumbering reptiles? There were way more dinosaur lineages than just theropods, only a tiny fraction of which survived.

So we can sort all dinosaurs into avian dinosaurs (the birds) and non-avian dinosaurs (all the rest; the extinct dinos). In biology (in the modern day, anyway), we classify things based on their relationship to other organisms, so dinosaurs end up being something like: ‘the group consisting of the most recent common ancestor of triceratops (or Megalosaurus, or Iguanodon) and modern birds, and all its descendants’. Not necessarily the easiest definition to grasp. So, we can define dinosaurs a bit more simply by saying they are made up of two groups. These groups are the ornithischians and the saurischians, and both existed from the Triassic Period, through the Jurassic and Cretaceous periods, and all except the avian dinosaurs became extinct during the K-Pg (Cretaceous-Paleogene extinction).

The Ornithischian dinosaurs (literally meaning the ‘bird-hipped’ dinosaurs because their hip bones looked like the picture here) are made up of a number of smaller groups. Ankylosaurians, Ceratopsians, Stegosaurians, Pachycephalosaurians, and Ornithopods all belong to Ornithischia. Now, those names may sound intimidating, but you would probably recognise some of the dinosaurs in each of those groups from a picture.

Euoplocephalus, a typical Akylosaur with a short, armoured head, and club on its tail.

Our first group of dinosaurs from the Ornithischia are the Ankylosaurs. Ankylosaurids were squat, quadrupedal armoured dinosaurs with short necks and short limbs. They were mostly herbivores, and despite their short stature they were not to be messed with. For starters they were not small, simply low to the ground. They likely weighed between 2 and 3 tonnes, and could almost always take on a T-rex in a fight. Some Ankylosaurids, such as Euoplocephalus, had heavy clubs on the end of their tails to fend off attackers, and their heavily armoured skin could take a few bites. So much so that these tough critters survived quite a long time, from the early Jurassic until the end of the Cretaceous period, when the K-Pg extinction event finally managed to do it in.

Edmontonia rugosidens. This type of Ankylosaur had a longer snout and lacked the club at the end of its tail; instead it had exaggerated shoulder spikes.
Skull of Pachycephalosaurus at the Oxford University Museum of Natural History.

Pachycephalosaurs were pretty interesting guys. They were adapted to do one thing and one thing only; to headbutt. Their name literally means ‘thick headed lizard’, and their heads are definitely thicc. The dome on the top of this dinosaur’s head was made out of thickened bone, with spikes and nodes surrounding it and along the nose.

The dome on their skulls was used as a battering ram in combat, when fighting others of its kind and those who would try to eat it. The pachycephalosaur was a herbivore , which likely ate low lying vegetation during the late Cretaceous of what is now North America and Asia.

Reconstruction of an unnamed pachycephalosaur.

Here we come to a few names we might recognise more readily. Stegosaurians and ceratopsians are two groups that were alive during the cretaceous period and were both herbivores with some unique features. Early ceratopsians were bipedal, such as the Psittacosaurus.

Psittacosaurus mongoliensis

 Psittacosaurus is one of the dinosaurs we know most about, appearance wise – we have data which even allows us to predict its colouration! And although it has the usual ceratopsian beak and some unusual wide cheekbones, it is not what you first imagine when you hear ‘ceratops’.

Triceratops horridus

But Triceratops probably is! It is the best known ceratopsian, and was distinctive due to the three horns on its head, made of bone and covered with keratin – the same thing that makes up deer antlers and your hair and nails. It also had the characteristic bony frill of most ceratopsians, and the function of these decorations on its head is still unknown, but we can say that it is more likely to have been a display adaptation similar to the peacock’s tail, rather than one used for fighting.

Now, moving from dinosaurs that had huge skulls for their body, we come to the opposite. Stegosaurus was another quadrupedal armoured dinosaur (related to the ankylosaurs) which had a beak, but this group had large plates situated vertically along their necks, backs and tails, and had a tiny head! They also had large spikes right at the end of their tail called the thagomizer, which they could use as a weapon.

Stegosaurus stenops

Finally for the Ornithischia, we have the ornithopods (meaning bird feet), yet more herbivores which started out in the Triassic period as bipedal runners, and grew in size until the K-Pg to become either facultative quadrupeds or facultative bipeds – where they could choose to walk on two legs or four depending on the situation. This group included the duck-bill dinosaurs or hadrosaurs, which includes Parasaurolphus, below. The main adaptation of these animals to their environment, and what allowed them to survive and dominate for so long was a novel chewing mechanism, the most efficient amongst non-avian dinosaurs.

Parasaurolphus walkeri – we have skin impressions for this dinosaur as well.

 This allowed them to break up their food before swallowing it and made their digestion much more efficient, allowing them to become bigger over time.

This is what Saurischian hips looked like.

You may have noticed however, that neither the bird-hipped dinosaurs, nor the bird feet dinosaurs include the theropods. This means that, ironically, birds did not evolve from the Ornithischians, but from the Saurischians, the ‘lizard-hipped’ dinosaurs – which is just confusing, really.

But the Saurischians were a simpler group than Ornithischians. The members of this group followed one of two major body plans.

First of all we have the Sauropodomorphs. As seems to be the way with dinosaurs, the prosauropods started off as bipedal animals during the mid-Triassic and the Sauropods had evolved to become quadrupedal by the late Cretaceous. They were the dominant herbivores on land, and only the ornithopods rivalled them by the end of the Cretaceous (again due to their advanced chewing method – sauropodomorphs simply had gizzard stones in their stomach to grind up what they’d eaten, as some modern birds – again, ostriches – do).  But, you’ll recognise many of the names of sauropods and certainly know what they look like. Brachiosaurus, Brontosaurus, Diplodocus and Apatosaurus were Sauropods, which includes the largest animals to have ever lived on land (there is debate over which sauropod takes the title – some aren’t complete fossils and as such are just estimations). These giants had long necks and tails and stood on four columnar legs (a bit like an elephant’s). They also had hollow bones, an adaptation that reduces their weight to allow them to live on land.

And last but certainly not least, we come to the only other Saurischian group, the one, the only, the theropod. And this is probably the most well-known group of dinosaurs, containing both the Tyrannosaurus rex, and the Velociraptor, and of course, modern birds. This group is characterised by hollow bones and three toed limbs. They are mostly carnivorous, although some groups converted back to being vegetarian, such as the Therizinosaurs. This meant that these were the only large land carnivores at the time when most were alive, from the late Triassic to the late Cretaceous (birds appeared in the Jurassic and are the only dinosaurs to have survived the K-Pg). A lot of these dinosaurs show fossil evidence of feathers and protofeathers, as well as evidence of carnivory – lizards found in stomachs, etc. And as you probably know, these dinosaurs were bipedal, fast runners with small forearms.

Compsognathids are a group of small theropods, this is Sinosauropteryx
Maniraptoriformes was another group of small feathered hunters to which Velociraptor belonged. This is a cousin, Utahraptor.
Tyrannosaurids were a group of large carnivores with feather like coverings on some of their body, as shown here by Tyrannosaurus rex.

So, this has been a whirlwind tour through the clade Dinosauria, and you may have spotted that some of your favourites are missing. Completely aquatic reptiles such as ichthyosaurs, plesiosaurs, mosasaurs and pliosaurs, as well as flying reptiles or pterosaurs, such as Pteranodon and Pterodactyl, while they were alive at the time of the dinosaurs, do not belong to the dinosauria. Dinosaurs were exclusively terrestrial animals, living at least most of their lives on land (although there are theropods such as Spinosaurus that hunted in shallow water and could probably swim well).


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