Major Groups of Dinosaurs

Dinosaurs are most commonly defined as the most recent common ancestor of Triceratops and modern birds, and all its descendants. They are generally divided into two main groups based on how their hip bones are arranged - the Ornithischians ("bird-hipped") and the Saurischians ("lizard-hipped"). Rather interestingly (or confusingly) we now know that birds actually evolved from the "lizard-hipped" saurischian group and then later evolved a hip bone arrangement similar to that of Ornithischian dinosaurs. This is an example of convergent evolution - where similar features evolve independently in different species or at different times.

The hip contains an upper bone called the ilium, and beneath this are the ischium and the pubis. In Ornithischian dinosaurs, the pubis points backwards - towards the tail of the animal. In some cases there is a forward-facing projection from the pubis, but this is generally small. In saurischian dinosaurs, the pubis points forwards towards the front of the animal. Another feature of Ornithischian dinosaurs is the presence of a "beak-forming" bone called the predentary in their lower jaw.

Sauropodomorphs include the sauropods (long-necked herbivores) such as Brachiosaurus and Diplodocus, and the prosauropods - earlier relatives of sauropods such as Massospondylus. Sauropods reached truly massive sizes - some sauropod necks have been found to reach 15 m in length (6 times the longest recorded giraffe neck!), and with weights reaching 60 - 80 tonnes (60,000 - 80,000 kg) and higher.
The heads of sauropods were generally quite small relative to their size, often with weak peg-like teeth - suggesting they did not orally process their food. This helped to reduce the weight of the head, allowing the neck to get longer and helping them reach taller vegetation. Essentially, the teeth were just used to strip leaves off plants - and they relied on their gut to process the food.

Some sauropods have also been found to have body armor, such as Shunosaurus - a sauropod from the late Jurassic which had a tail club with 2 spikes, and Saltasaurus - a sauropod from the late Cretaceous which had bony osteoderms along its back. Like modern birds, sauropods had hollow bones.

Theropods were bipedal dinosaurs including well-known carnivores like Allosaurus, Tyrannosaurus Rex and Giganotosaurus. The name theropod means "beast-footed". Theropods generally had short arms that ended with 2 or 3 claws, and also had hollow bones like modern birds. Most theropods had very sharp, serrated teeth.

A number of theropod species have been discovered with evidence of feathers - ranging from simple hair-like filamentous "protofeathers" to much more complex feathers like the long tail feathers of Epidexipteryx. While most theropod species were carnivores, some theropod species have been found to have been herbivores, omnivores, insectivores or piscivores (fish-eaters).

Theropods ranged in size from small animals weighing only a few hundred grams to enormous carnivores like Tyrannosaurus Rex, and some are likely flying above your head as you read this - this is the group that modern birds are descended from.

Stegosaurs were quadripedal herbivores, with the most well-known member of the group being Stegosaurus. Their heads were small relative to their size, and their front legs were much shorter than their rear legs. While they were mostly quadripedal, some paleontologists have suggested that they may have been able to rear up on their hind legs when threatened or to reach taller vegetation. Stegosaurs had rows of large, bony plates along their back called "scutes", and large bony spikes on their tails.

The distinctive arrangement of the 4 tail spikes in Stegosaurs is semi-formally referred to as a "Thagomizer". This term was initially coined by a "Far Side" cartoon in which a caveman is seen pointing to a slide with an image of a stegosaurus tail and saying "This end is called the thagomizer... after the late Thag Simmons" (presumably another caveman in the group who had an unfortunate encounter with those spikes). The term has since been adopted by a number of paleontologists and institutions.

The exact function(s) of the plates on the backs of Stegosaurian dinosaurs has been a subject of much debate. Suggested functions have included display, thermoregulation and defense. While the thermoregulation theory is often stated in popular books etc., probably the most endorsed theory by paleontologists today is that they were primarily used for display. However, this is not certain - and being large structures on their body it is certainly possible that they had some thermoregulatory effect. The plates varied significantly between different stegosaur species - with some species having smaller, narrower plates and others such as Kentrurosaurus having plates along only part of the back. These variations meant that some species had plates which were not optimized for thermoregulation, and would seem to support the theory that they were for display and species recognition.

Ankylosaurs (meaning "fused lizard") are another group of Ornithischian dinosaurs often referred to as the "armored dinosaurs". These were quadripedal herbivores with armor in the form of osteoderms (bones which develop in the skin) over much of their bodies including their back and skull. The most well-known member of this group is Ankylosaurus which was also one of the largest - being around 6 - 8 m long and with estimated weights of up to 8 ton (8,000kg). It was heavily armored, even having a bony plate on its eyelids!

Like stegosaurs, the front legs of ankylosaurs were shorter than their rear legs - though this difference is generally less pronounced in ankylosaurs. Some ankylosaurs, like the famous Ankylosaurus, also had a large bony club at the end of their tail. Ankylosaurs and Stegosaurs both belong to a larger group of armored dinosaurs called Thyreophora. Ankylosaurs likely fed on low-lying plants such as cycads, ferns, and angiosperms (flowering plants).

The next group is the Ceratopsians - these dinosaurs had parrot-like beaks and often had large neck frills (though this feature is absent in some members). Some earlier ceratopsians were bipedal, with the later, larger members being quadrupeds. The most well-known member of this group is Triceratops - this was one of the largest ceratopsians, and lived until the mass extinction at the end of the Cretaceous period. Large horns were common among ceratopsians, and many had dental batteries for grinding vegetation.

While many people might only be familiar with Triceratops, the frills and horns of ceratopsians were spectacularly diverse - as shown in the images here. Many people assume that the frills on ceratopsians were mainly intended for protection, but it is thought that they were actually primarily display structures. Triceratops is actually unusual in having a solid, thick frill - many other ceratopsian frills were thinner with holes (fenestrae) in them, and they were ornamented with bony extensions called epossifications.

Many species of ceratopsian dinosaurs have names that end in "ceratops" but this is not always the case - for example Psittacosaurus and Centrosaurus are both ceratopsian dinosaurs.

Ornithopods were generally bipedal - the name means "bird-footed" (although their feet were not really bird-like). These were one of the most successful groups - they flourished and diversified until the end of the Cretaceous period. Some later, larger members likely walked on all fours much of the time, but are thought to have been bipedal when running (and to have been able to run quite fast). Early ornithopods were quite small, around 1m long, but later members were much bigger - such as Shantungosaurus (approx. 15m in length), or the more well-known Iguanodon (approx. 10m long).

The final group of Ornithischians is the pachycephalosaurs ("thick-headed lizards"), also known as the "dome-headed dinosaurs". These were bipedal dinosaurs with short front arms and thickened skulls which, in some species, forms a thick raised dome. The function of this dome has been debated, with many paleontologists suggesting it was used for head-butting and some have suggested it may have been used for species recognition. Research has suggested that the domes of pachycephalosaurs could have withstood combat stresses - however this does not prove that they actually did butt heads. Pachycephalosaurs lived exclusively in Laurasia, a large continent which included North America, Europe, and most of Asia (except India).