Classification of the Dinosaurs and Related Beasts:
The first vertebrates to live on land were the amphibians. There was a time in Earth’s history (about 370 million years ago) when amphibians were the largest animals living on land. These early amphibians still had very fish-like skeletons, and since most modern amphibians are highly dependent on water, these critters surely were too. (By the way, the scenario most people imagine is saltwater fishes crawling out of the sea and becoming amphibians, but amphibians appear to have evolved from
freshwater fish. Probably, they evolved from fishes living in ancient swamps.) What these early amphibians clearly had were four legs adapted to supporting their weight on land. So, these animals and all their descendants are known as the
tetrapods.
Amphibians have thin, water-permeable skins, and the eggs they lay are not waterproof either. In addition, the young of most amphibians are very fish-like, living in water and breathing with gills rather than lungs. These facts make them highly dependent on water, because they will quickly die of dehydration in a dry environment, and their eggs must be deposited in wet environments.
Somewhat over 300 million years ago, a series of rather important evolutionary innovations occurred. From the amphibians arose a group of animals with skeletons that were better adapted to supporting their weight out of water. They also evolved skin that was more or less waterproof and was covered with protective
scales. Perhaps most importantly, they laid eggs that had waterproof outer shells and internal protective membranes including the
amnion that kept the developing embryo wet. These are known as
amniotic eggs and these animals and all their descendents are therefore known as the
amniotes. Because amniotes have waterproof skins and because they can lay their eggs on land (or in many species, retain them within the mother’s body as the embryos develop), they are far more independent of water than are amphibians.
The earliest amniotes, with their dry, scaly skins and their egg-laying ways would be called (*sigh*) “reptiles.” From these early reptiles evolved four distinct lineages, which are classified according to their skull anatomies. These were the
anapsids,
synapsids,
euryapsids, and
diapsids. The drawing below should help to clarify things. (In the drawing, the “orbit” is the eye socket, “Sq” indicates the
squamosal bone, one of the bones that makes up the skull, and “Po” indicates the
postorbital bone.)
In the
anapsids, the skull is solid in the temporal region — that is, the region behind and above the eye socket. Early anapsids included the
pareiasaurs, which were up to about the size of a modern hippopotamus, but the only surviving anapsids are the turtles.
In the
synapsids, there is a single opening in the temporal region of the skull. Early synapsids included an interesting group of animals known as the
pelycosaurs, with distinctive dorsal sails. These included the well-known
Dimetrodon, a predator that lived about 250 million years ago and reached 10 feet (3 meters) or so in length.
What the heck was the function of that sail? Well, pelycosaurs were clearly reptiles — notice how the legs were splayed out to the side like a lizard’s, for instance. It seems likely that they had slow, reptile-style metabolisms and were not warm-blooded. There are real advantages to being warm, however, because the warmer your body (up to a point), the faster are the chemical reactions that drive your metabolism. So, warmer animals have more energy available and can be more active; they also tend to have faster reflexes.
The spines on a pelycosaur’s back were quite thin, and it’s likely that a thin layer of skin covered those spines to form a sail. If a pelycosaur were to orient itself perpendicular to the sun, the sail would make an effective absorber of solar energy. Blood pumped into the sail would be warmed and would in turn warm the body. So, pelycosaurs likely used those sails to absorb solar energy. This would give them a great advantage over animals that lacked such sails, because pelycosaurs could quickly and efficiently warm their blood on sunny days and so maintain a more active and energetic lifestyle — very handy for a predator that had to chase down prey.
If the sail could be used to
absorb solar energy, it could also be used as a radiator to prevent overheating by
shedding excess heat. In hot weather, a pelycosaur could move into shade or orient itself so that the sail was parallel to incoming sunlight, and blood pumped into the sail would radiate away excess heat. In short, it’s likely that the pelycosaurs’ sails allowed them to regulate their body temperatures much more effectively than could animals that lacked them.
Pelycosaurs and their relatives dominated the land for millions of years, but let me point out that they were
not dinosaurs. They lived millions of years before the dinosaurs arose. For whatever reason, the synapsids became much less prominent when the dinosaurs arose, but they did indeed survive. They gave rise to the
mammals. So, when you see a drawing of a
Dimetrodon, keep in mind that this was not a dinosaur at all — it was a distant relative (though not a direct ancestor) of
you and me.
In the
euryapsids, there was only a single temporal opening in the skull, but it was positioned higher than the opening in the skull of a synapsid. The euryapsids appeared a little later and seem to have been descended from diapsids, so they’re often classified simply as diapsids. The euryapsids included marine reptiles that dominated the world’s oceans at the same the dinosaurs dominated the land. These marine reptiles were the
plesiosaurs, in which the four limbs were modified into flippers, and the fishlike
ichthyosaurs, which were the Mesozoic equivalent of whales and dolphins.
Finally, there were the
diapsids, with two temporal openings in the skull. This was and is a very diverse taxon that is generally subdivided into two subtaxa, the
Lepidosauria and the
Archosauria. The lepidosaurs include the familiar snakes and lizards, as well as the extinct mosasaurs. Mosasaurs were essentially large sea-going lizards that shared the Mesozoic seas with ichthyosaurs and plesiosaurs. (Mosasaurs were quite distinct from plesiosaurs, but like the plesiosaurs, their limbs were modified into swimming paddles.)
Archosaurs include the
crocodilians (crocodiles, alligators, and their kin),
avians (birds),
pterosaurs (“flying reptiles”) and
dinosaurs. The dinosaurs were distinguished from mere “reptiles” by the fact that their legs were not splayed out to the side like those of a reptile, but oriented directly under the body.
In the top of the drawing, you can see how a typical reptile, such as a lizard, has its legs splayed out to the sides. By contrast, the bottom of the drawing shows how a dinosaur’s legs (like those of modern mammals) were oriented directly under the body. This positioning of the limbs was much more efficient at supporting body weight, which allowed dinosaurs to grow to much larger sizes than had any of their predecessors. The upright limb posture also allowed for much more efficient locomotion and so a more active lifestyle.
The dinosaurs are generally divided into two major groups,
ornithischians and
saurischians, according to the arrangement of bones in their hips. Three partially fused bones made up the hip of a dinosaur, the
ilium (which I’ve colored red in the drawing), the
ischium (blue), and the
pubis (yellow). (In this drawing, the animals’ heads would be to the left, and the tails to the right. A typical ornithischian’s pelvis is shown on the left, and a typical saurischian’s on the right.)
In ornithischians, the pubis was turned back toward the tail and often partially fused with the ischium. The name “ornithischian” means “bird-hipped” and refers to the superficial similarity between the hips of ornithischian dinosaurs and modern birds. Birds are not descended from ornithischian dinosaurs, however. Interestingly, there are no ornithischians that appear to have been carnivores. All known species had the sorts of teeth you’d expect to find in herbivores.
The ornithischians included the “armored dinosaurs” that had bony structures embedded in their skins which might have provided some protection against predators. Perhaps the best-known of these were
Stegosaurus and its kin. These were the four-legged dinosaurs with distinctive plates on their backs and spikes on their tails that were presumably used to defend against predators.
Another group of armored ornithischians were the
ankylosaurs, which were built like Mesozoic tanks. They had heavy bony armor covering their backs and long spikes extending out from their flanks. Many species had bony “clubs” at the end of their tails which would have made dangerous defensive weapons. Ankylosaurs included such genera as
Ankylosaurus,
Nodosaurus, and
Edmontonia.
Ceratopians or “horned dinosaurs” were up to about the size of modern rhinos. They had parrot-like beaks which they presumably used for cropping plants, but they’re best known for the large frills that extended from their skulls and covered their necks and for having horns extending from their faces. They probably used these horns like modern antelopes use theirs; that is, the males probably interlocked their horns and wrestled each other for access to females. In emergencies, the horns would also have made good defensive weapons. Well-known ceratopians included
Triceratops,
Torosaurus and
Styracosaurus.
Ceratopians were quite common during the latter portion of the Mesozoic, but perhaps the most common ornithischians were the
hadrosaurs or “duck-billed dinosaurs.” They had broad, duck-like beaks and teeth that were very well adapted for grinding plant matter. Most of them seem to have been bipedal and well-adapted for fast running, but they could probably go on all fours as well. Hadrosaurs didn’t have armored bodies, nor clubs or spikes on their tails, nor dangerous horns, so they apparently depended on raw speed to escape predators. Many of them had elaborate “helmets” on their heads into which the nasal cavities extended. These helmets appear to have served as resonating chambers, and the different species would have had very distinctive-sounding calls, depending on the shapes of their helmets. Well-known hadrosaurs included
Corythosaurus,
Lambeosaurus and
Parasaurolophus.
A rather interesting group of ornithischians were the
pachycephalosaurs or “thick-headed dinosaurs.” Like the hadrosaurs, they were apparently bipedal, but their heads were covered by quite thick growths of bone. It seems likely that male pachycephalosaurs got into “head-butting” contests in much the same way that modern Bighorn Sheep do. Well-known pachycephalosaurs included
Pachycephalosaurus and
Stygimoloch.
In saurischians, the pubis and ischium were distinctly separate, and the pubis usually pointed forward, toward the animal’s head. The saurischians were divided into two major groups, the
sauropods and the
theropods.
Sauropods were quadrupedal plant-eaters that often grew to enormous sizes. Unlike most ornithischian plant-eaters, most sauropods did not have teeth that were well-adapted for grinding up plant matter. Instead, most of them appear to have swallowed small stones (called
gastroliths or “gizzard stones”). If these dinosaurs had muscular gizzards like many modern birds, gastroliths would have been quite effective at grinding up plant matter.
Sauropods included the
brachiosaurs, such as
Brachiosaurus and
Ultrasaurus, which were heavily-built animals that grew to enormous sizes. Some may have weighed 100 tons or more, making them as heavy as the largest whales. Brachiosaurs had longer forelimbs than hindlimbs and appeared to hold their necks upright. They were apparently the Mesozoic equivalents of giraffes. The head of a
Brachiosaurus towered something like 50 feet above the ground, which would have made it well-suited for grazing on trees. Other sauropods included the
diplodocids or
brontosaurs, with their very long necks that were apparently held more horizontally and their long, whip-like tails. Well-known brontosaurs included
Apatosaurus,
Barosaurus,
Diplodocus, and
Seismosaurus.
Seismosaurus, at perhaps 120 feet (36 meters) or more in length, may have been the longest animal that ever lived.
Theropods were bipeds, and virtually all of them were clearly adapted for eating meat. It’s generally believed that birds evolved from small theropods, and many of the theropods were indeed strikingly bird-like in their anatomy. (Strictly speaking, according to many paleontologists, birds
are theropod dinosaurs.) Early theropods included the
allosaurs, which had teeth that appeared to be suited toward delivering slashing wounds. Allosaurs were probably pack hunters that went after large sauropods, much the same way that modern wolves go after moose and other such large prey. Later theropods included the
dromaeosaurs, which were clearly adapted for running and jumping, and had large sickle-shaped “killing claws” on their hindlimbs. They probably used those killing claws to disembowel prey. Dromaeosaurs included such famous critters as
Deinonychus and
Velociraptor. Perhaps the most famous theropods were the
tyrannosaurs, including
Daspletosaurus and
Tyrannosaurus rex. Tyrannosaurs were perhaps the largest theropods and had powerful jaws with teeth that would have been well-suited to delivering deep, gouging bites.
Dinosaur Classification
Hybrid Mode
