How Are Dinosaurs Classified?

The Classification Systems Used for Dinosaurs, Pterosaurs and Marine Reptiles

brachiosaurus
Brachiosaurus, the prototypical example of a saurischian dinosaur (Nobu Tamura).

In a sense, it's much easier to name a new dinosaur than it is to classify it--and the same goes for new species of pterosaurs and marine reptiles. In this article, we'll discuss how paleontologists classify their new discoveries, assigning a given prehistoric animal to its proper order, suborder, genus and species. (See also a Complete, A to Z List of Dinosaurs and The 15 Main Dinosaur Types)

The key concept in the classification of life is the order, the broadest description of a distinctive class of organisms (for example, all primates, including monkeys and human beings, belong to the same order).

Under this order you'll find various suborders and infraorders, as scientists use anatomical traits to dintinguish between the members of the same order. For example, the order of primates is divided into two suborders, prosimii (prosimians) and anthropoidea (anthropoids), which are themselves divided into various infraorders (platyrhinii, for example, which comprises all the "new world" monkeys). There's also such a thing as superorders, which are invoked when a regular order is found to be too narrow in scope.

The last two levels of description, genus and species, are the most common designations used when discussing prehistoric animals. Most individual animals are referred to by genus (for example, Diplodocus), but a paleontologist may prefer to invoke a particular species, say, Diplodocus carnegii, often abbreviated to D. carnegii. (For more on genus and species, see How Do Paleontologists Name Dinosaurs?

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Below is a list of the orders of dinosaurs, pterosaurs and marine reptiles; just click on the appropriate links (or see the following pages) for more information.

Saurischian, or "lizard-hipped," dinosaurs include all the theropods (two-legged predators like Tyrannosaurus Rex) and sauropods (bulky, four-legged plant eaters like Brachiosaurus).

Ornithischian, or "bird-hipped," dinosaurs include a wide range of plant eaters, including ceratopsians like Triceratops and hadrosaurs like Shantungosaurus.

Marine reptiles are divided into a baffling array of superorders, orders and suborders, which comprise such familiar families as pliosaurs, plesiosaurs, ichthyosaurs and mosasaurs.

Pterosaurs are comprised of two basic suborders, which can roughly be divided into early, long-tailed rhamphorhynchoids and later, short-tailed (and much bigger) pterodactyloids.

Next Page: The Classification of Saurischian Dinosaurs

The order of saurischian dinosaurs comprises two seemingly very different suborders: theropods, the two-legged, mostly meat-eating dinosaurs, and sauropods, prosauropods and titanosaurs, about which more below.

Order: Saurischia The name of this order means "lizard-hipped," and refers to dinosaurs with a characteristic, lizard-like pelvic structure. Saurischian dinosaurs are also distinguished by their long necks and asymmetrical fingers.

Suborder: Theropoda Theropods, the "beast-footed" dinosaurs, include some of the most familiar predators that roamed the landscapes of the Jurassic and Cretaceous periods. Technically, theropod dinosaurs never went extinct; today they're represented by the vertebrate class "aves"--that is, birds.

  • Family: Herrerasauridae The herrerasaurs comprise only five dinosaurs, the most well-known of which are Staurikosaurus and Herrerasaurus. Among the first dinosaurs, herrerasaurs are characterized by weird anatomical traits, such as only two sacral vertebrae and a more primitive hand structure than later theropods (some paleontologists even dispute whether herrerasaurs were dinosaurs at all!). Herrerasaurs went extinct at the end of the Triassic period, well before the better-known dinosaurs of the Jurassic and Cretaceous.
     
  • Family: Ceratosauridae Unlike the case with the more primitive herrerasaurs, it's generally accepted that ceratosaurs were true dinosaurs. They were characterized by their hollow bones, S-shaped necks, and unique jaw structures, and are the first dinosaurs to show any sort of resemblance to birds (which evolved tens of millions of years later). The most well-known ceratosaurs are Ceratosaurus, Dilophosaurus and Coelophysis.
     
  • Clade: Coelurosauria Technically, what sets coelurosaurians apart from other theropods is that they're more closely related to birds than to their sister family, the carnosauria (described below). One problem with this "clade"--the membership of which is far from set in stone--is that it includes such an enormous amount of members, ranging all the way from Velociraptor to Ornithomimus to Tyrannosaurus Rex. Coelurosaurs are distinguished by the structure of their sacrums, tibias and ulnas, among other skeletal features.
     
  • Clade: Carnosauria You might expect a clade named carnosauria to include such terrifying meat-eaters as Tyrannosaurus Rex, but that isn't the case. Besides their carnivorous diets, carnosaurs were distinguished by the comparative lengths of their femurs and tibias, the size of their eye sockets and the shapes of their skulls, among other anatomical features. They also had fairly large front arms, which is why T. Rex didn't make the cut. Famous examples of carnosaurs include Allosaurus and Spinosaurus.
     
  • Family: Therizinosauridae This family was once known as segnosauria, and it's hopped back and forth all over the evolutionary map: the latest trend is to consider therizinosaurs as closely related to birds, hence their classification as theropods. These herbivorous and omnivorous dinosaurs were characterized by their extremely long claws, backward-facing pubic bones (similar to birds), four-toed feet, and (mostly) large sizes. Not many dinosaurs belong to this family; the most prominent examples are Therizinosaurus and Segnosaurus.

Suborder: Sauropodomorpha The none-too-bright herbivorous dinosaurs known as sauropods and prosauropods often reached astonishing sizes; they're believed to have split off from a primitive ancestor shortly before dinosaurs evolved in South America.

  • Infraorder: Prosauropoda As you might guess from their name, the prosauropods ("before the sauropods")--small- to medium-sized, occasionally bipedal herbivorous dinosaurs with long necks and small heads--were once thought to be ancestral to big, lumbering sauropods like Brachiosaurus and Apatosaurus. However, paleontologists now believe that these late Triassic and early Jurassic dinosaurs weren't the direct ancestors of sauropods, but more like their great, great, etc. uncles. A classic example of a prosauropod is Plateosaurus.
     
  • Infraorder: Sauropoda Sauropods and titanosaurs were the true giants of the dinosaur age, including such lumbering beasts as Diplodocus, Argentinosaurus and Apatosaurus. These four-legged, long-necked herbivores were characterized by their erect limbs (similar to those of modern elephants), long necks and tails, and relatively small heads with tiny brains. They were especially numerous toward the end of the Jurassic period, though lightly armored titanosaurs prospered right up to the K/T Extinction.

    Next page: The classification of ornithischian dinosaurs

    The order of ornithischians includes the vast majority of plant-eating dinosaurs of the Mesozoic Era, including ceratopsians, ornithopods, and duckbills, described in more detail below.

    Order: Ornithischia The name of this order means "bird-hipped," and refers to the pelvic structure of its assigned genera. Oddly enough, modern birds are descended from saurischian ("lizard-hipped"), rather than ornithischian, dinosaurs!

    Suborder: Ornithopoda As you can guess from this suborder's name (which means "bird-footed"), most ornithopods had birdlike, three-toed feet, as well as the birdlike hips typical of ornithischians in general. Ornithopods--which came into their own during the Cretaceous period--were quick, bipedal herbivores equipped with stiff tails and (often) primitive beaks. Examples of this populous suborder include Iguanodon, Edmontosaurus, and Heterodontosaurus. Hadrosaurs, or duck-billed dinosaurs, were a particularly widespread ornithopod family that dominated the later Cretaceous period; famous genera include Parasaurolophus, Maisaura and the huge Shantungosaurus.

    Suborder: Marginocephalia The dinosaurs in this suborder--which include Pachycephalosaurus and Triceratops--were distinguished by their ornate, oversized skulls.

    • Infraorder: Pachycephalosauria The name of this infraorder means "thick-headed," and that's not an exaggeration: pachycephalosaurs were characterized by their extremely thick, bony heads, which they presumably used to duel one another for the right to mate. These Cretaceous dinosaurs were mostly herbivores, though some isolated species may have been omnivorous. Well-known pachycephalosaurs include Pachycephalosaurus, Stygimoloch, and Stegoceras.
       
    • Infraorder: Ceratopsia As pachycephalosaurs were distinguished by their skulls, ceratopsians were set apart by their horns and frills--some of which grew to gargantuan proportions, as in Triceratops and Styracosaurus. Ceratopsians often had thick hides as well, a means of defense against the tyrannosaurs and raptors of the late Cretaceous period. Overall, these large herbivores were behaviorally very similar to modern elephants and rhinoceroses.

      Suborder: Thyreophora This small suborder of ornithischian dinosaurs includes some large members, including Stegosaurus and Ankylosaurus. Thyreophorans (the name is Greek for "shield bearers"), which include both stegosaurs and ankylosaurs, were characterized by their elaborate spikes and plates, as well as the bludgeoning tails evolved by some genera. Despite their fearsome armament--which they most likely evolved for defensive purposes--they were herbivores rather than predators.

      Previous page: the classification of saurischian dinosaurs

      Next page: the classification of Marine Reptiles

      The marine reptiles of the Mesozoic Era are especially difficult for paleontologists to classify, because, in the course of evolution, creatures living in marine environments tend to take on a limited variety of body forms--which is why, for example, the average ichthyosaur looks so much like a large bluefin tuna. This trend toward convergent evolution can make it difficult to distinguish between the various orders and suborders of marine reptiles, much less individual species within the same genus, as detailed below.

      Superorder: Ichthyopterygia "Fish flippers," as this superorder translates from the Greek, comprises ichthyosaurs--the streamlined, tuna- and dolphin-shaped predators of the Triassic and Jurassic periods. This abundant family of marine reptiles--which includes such famous genera as Ichthyosaurus and Ophthalmosaurus--largely went extinct at the end of the Jurassic period, supplanted by pliosaurs, plesiosaurs and mosasaurs.

      Superorder: Sauropterygia The name of this superorder means "lizard flippers," and it's a good description of the diverse family of marine reptiles that swam the seas of the Mesozoic Era, starting from about 250 million years ago to 65 million years ago--when sauropterygians (and the other families of marine reptiles) went extinct along with dinosaurs.

      Order: Placodontia The most ancient marine reptiles, placodonts flourished in the oceans of the Triassic period, between 250 and 210 million years ago.

      These creatures tended to have squat, bulky bodies with short legs, reminiscent of turtles or overgrown newts, and probably swam along shallow coastlines rather than in the deep oceans. Typical placodonts included Placodus and Psephoderma.

      Order: Nothosauroidea Paleontologists believe these Triassic reptiles were like small seals, scouring shallow waters for food but coming ashore periodically on beaches and rocky outcroppings.

      Nothosaurs were about six feet long, with streamlined bodies, long necks and webbed feet, and they probably fed exclusively on fish. You won't be surprised to learn that the prototypical nothosaur was Nothosaurus.

      Order: Pachypleurosauria One of the more obscure orders of extinct reptiles, pachypleurosaurs were slender, smallish (about one and one-half to three feet long), small-headed creatures that likely led an exclusively aquatic existence and fed on fish. The exact classification of these marine reptiles--the most commonly preserved of which is Keichousaurus--is still a matter of ongoing debate.

      Superfamily: Mosasauroidea Mosasaurs, the sleek, fierce, and often giant marine reptiles of the later Cretaceous period, represented the pinnacle of marine reptile evolution; oddly enough, their only living descendants (at least according to some analyses) are snakes. Among the most fearsome mosasaurs were Tylosaurus, Prognathodon and (of course) Mosasaurus.

      Order: Plesiosauria This order accounts for the most familiar marine reptiles of the Jurassic and Cretaceous periods, and its members often attained dinosaur-like sizes. Plesiosaurs are divided by paleontologists into two main suborders, as follows:

      • Suborder: Plesiosauroidea The prototypical plesiosaur was a large, streamlined, long-necked predator possessing big flippers and sharp teeth. Plesiosaurs weren't as accomplished swimmers as their close cousins, the pliosaurs (described below); they cruised slowly along the surface of rivers, lakes and oceans, extending their long necks to snap up unwary prey. Among the most famous plesiosaurs were Elasmosaurus and Cryptoclidus.
         

        Suborder: Pliosauroidea Compared to plesiosaurs, pliosaurs had much more fearsome body plans, with long, toothy heads, short necks, and barrel-shaped bodies; many genera resembled modern sharks or crocodiles. Pliosaurs were more agile swimmers than plesiosaurs, and may have been more common in deeper waters, where they fed on other marine reptiles as well as fish. Among the scariest pliosaurs were the gigantic Kronosaurus and Liopleurodon

        Compared to saurischian and ornithischian dinosaurs, not to mention marine reptiles, the classification of pterosaurs ("winged lizards") is a relatively straightforward affair. These Mesozoic reptiles all belong to a single order, which is itself divided into two suborders (only one of which is a "true" suborder in evolutionary terms).

        Order: Pterosauria Pterosaurs--almost certainly the first large animals on earth ever to evolve flight--were characterized by their hollow bones, relatively large brains and eyes, and, of course, the flaps of skin extending along their arms, which were attached to the digits on their front hands.

        Suborder: Rhamphorhynchidae In legalistic terms, this suborder has a shaky status, since it's believed that the pterodactyloidea (described below) evolved from members this group, rather than both groups having evolved from a last common ancestor. Whatever the case, paleontologists often assign smaller, more primitive pterosaurs--such as Rhamphorhynchus and Anurognathus--to this family. Rhamphorhynchoids are characterized by their teeth, long tails, and (in most cases) lack of skull crests, and lived during the Triassic period.

        Suborder: Pterodactyloidea This is the only "true" suborder of pterosauria; it includes all the large, familiar flying reptiles of the Jurassic and Cretaceous periods, including Pteranodon, Pterodactylus, and the enormous Quetzalcoatlus. Pterodactyloids were characterized by their relatively large size, short tails and long hand bones, as well as (in some species) elaborate, bony head crests and lack of teeth.

        These pterosaurs survived up until the K/T Extinction 65 million years ago, when they were wiped out along with their dinosaur and marine reptile cousins.