The Elements of Evolution – Dinosaurs


The Mesozoic era was the time of dinosaurs. They arose 240 MYA, during the Triassic, with considerable speciation toward the end of that period. Reptiles had diversified into numerous families and hundreds of species by the close of the Triassic. The dramatic calamity that ended the Triassic wrought changes in the mix.

Why dinosaurs survived the end-Triassic extinction event is still poorly understood, but this was their opportunity for ascent, which transpired gradually. As they evolved, dinosaurs lived alongside their reptilian cousins for millions of years.

The ascendancy of dinosaurs was as accidental and opportunistic as their demise and replacement by therian mammals at the end of the Cretaceous. ~ American paleontologist Paul Sereno

Continental Shifts

Dinosaurs evolved when much of the Earth’s landmass was concentrated into the supercontinent Pangea. They roamed throughout Pangea, which began to break apart during the early Triassic, caused by a seafloor-spreading rift. Pangea’s partitioning resulted in isolated populations, engendering speciation as climates diverged into different biomes, with distinct plant and animal life.

Dinosaurs rapidly radiated to fill available ecological niches. Their bodies burgeoned in size 220 MYA, as a way to dominate competitors. Upsizing slowed by 200 MYA, and even went into reverse in some lineages, most impressively toward the eventuation of birds.

Dinosaurs proliferated during the early Jurassic, becoming the dominate land animal, fitting into most every ecological niche: as scavengers, hunters, and herbivores. 1,850 dinosaur genera are estimated to have existed. Most genera had only 1 to a very few species, so there may have been just ~2,000 different types of dinosaurs.

The Jurassic (201–145 MYA) was the great age of dinosaurs, though the biggest and best-known dinosaurs did not emerge until the Cretaceous (145–66 MYA). By that time there were thousands of species. Some supersized to become the largest terrestrial animals of all time.

The supercontinent Pangea was breaking up throughout the Jurassic and Cretaceous periods. The process became especially marked during the Cretaceous. Sea level rose so much that there were shallow seas on the continents. About 1/3rd of the land was covered with water.

In the initial stages of Pangea becoming piecemeal the large landmasses had extreme continental climates, with marked seasons and little rain. These arid conditions returned at the end of the Cretaceous, when sea level fell.

The tectonic exertions enlivened volcanic activity. As such, these periods were punctuated with extinction events. At the end of the Cretaceous, there was a great outpouring of basaltic lava in India, known as the Deccan Traps.

Global temperature was high in the early- to mid-Cretaceous. Swamp forests flourished along the borders of the Tethys Ocean.

As the leaves and trees of the forest fell, peat accumulated. Upon geological pressure and heat, the land deposits eventuated into coal, while organic marine decay produced petroleum: the fossil fuels.

Body Heat

Reptiles are ectothermic: maintaining body temperature through environmental means. Conversely, birds and mammal independently evolved endothermy: internally maintaining a metabolically favorable body temperature.

Normal body temperature for endotherms varies by group but ranges 38–41 ºC in birds and 25–38 ºC in mammals. Feathers, fur, and body fat help conserve heat. Bodily organs produce heat, as do active muscles, either voluntarily or by shivering.

Heat is shed via evaporation – either sweating or panting – and by convection and radiation from the body’s surface. A small animal has a larger surface-to-volume ratio than a larger one. Hence, body size and configuration are life-history variables correlated with an animal’s natural biome.

Adapting to habitats which have a different ambient temperature range often involves bodily changes to better maintain internal temperature. Seabirds, for instance, are generally stouter than those that live over land, as are birds that migrate long distances compared to those that stay close to home.

Birds and mammals are the only animals to universally practice endothermy, and even some of them save energy and lower their temperature by going into a state of torpor. Bats may do this during the day. When disturbed, they shiver briefly to restore body heat before flying off.

Some hummingbirds become torpid at night. Their body temperature falls with air temperature until 20 ºC. If the external temperature falls further, hummingbirds maintain themselves at 20 ºC.

Body temperature may fall to as low as 6 ºC in mammals that hibernate, such as bears and hedgehogs. The body temperature of the Arctic ground squirrel may drop as low as minus 2.9 ºC. Its blood stays liquid and its tissues are protected by the biological equivalent of antifreeze.

The brains of hibernating animals are kept warmer than their bodies, even as the cold takes its toll on nerve cells near the surface. 80–90% of the energy used during hibernation goes to keeping the brain alive.

Endothermy necessitates a metabolism that runs several times that of ectotherms. The trade-off involves what an animal is relying upon the environment for: heat (ectothermy) or abundant food (endothermy).

In being economical in every way, from digestion to movement, sloths have a well-earned name. 3-toed sloths do not even maintain a constant body temperature: allowing a nearly 5 °C swing in their everyday lives.

Sloths are on the reptile end of being a mammal. ~ English zoologist Rebecca Cliffe

At the expense of greater energy consumption, endotherms can grow and develop much faster than ectotherms. Shorter adolescence allows the development of parental care without taking too much of the parent’s life span and so curtailing multiple broods.

If ectothermic crocodiles were to care for their young until adulthood, they would be able to reproduce only every 9 years or so. Most birds and mammals have 1 or more broods per year.

Not only can endotherms offer better parental care, they can provide some degree of warmth within their nests. In temperate latitudes this extends the breeding season.

There are exceptional ectotherms who achieve this by special adaptations. The diamond python keeps its eggs suitably warm by coiling around them.

There are some ectothermic animals that provide parental care, but only short-term. Ectothermic parental care does not extend until offspring are well on their way to being full grown, as is common with birds and mammals.

Endothermy affords cognitive stability in the wildly fluctuating temperatures of terrestrial habitats. This improves response time to danger, or for hunting, independent of the weather.

Overall, endothermy improves survival prospects and affords shorter generation time. Together these constitute an intrinsic potential for faster population growth, which offers an evolutionary edge.

Mesotherms rely on metabolic heat to raise their body temperatures above ambient temperature, but do not defend a thermal set point as do endotherms. ~ American zoologist Robert Eagle et al

As mesotherms, dinosaurs split the difference: able to internally generate body heat, but not to the degree of maintaining a thermal homeostasis like endotherms. Great white sharks are one the few living animals that are mesothermic.

Relative to reptile ectotherms, dinosaur mesothermy afforded increased speed and agility. Dinosaurs were faster predators, or faster to flee danger, than the reptiles that dominated the early Mesozoic. This performance advantage was instrumental in the rise of dinosaurs.

Dinosaur thermophysiology was not uniform. Different lineages had distinct levels of bodily warmth.

During the Triassic there were numerous large predators that preyed upon smaller reptiles and early mammals. One way to reduce predatory risk was to become nocturnal, or at least crepuscular. Although there are now specialized nocturnal predators (e.g., owls), most minute mammals retain their nocturnality, or at least a strong preference for being in the shadows.

Because large ectothermic predators can retain body heat longer when the ambient temperature drops, endothermy is a great advantage for small animals; hence the evolution of endothermy in mammals, which started their ascent when they were modestly sized.

 Tegu Lizards

Black-and-white tegu lizards inhabit the plains east of the Andes Mountains. During the autumn and winter, tegus hibernate in their burrows.

Come spring they breed. A female lays a clutch of eggs in a nest insulated with moist grass, twigs, and other cozy litter. The mother remains with her eggs in the nest, keeping them warm.

During incubation, the tegu lizard endothermically raises her body temperature 10 °C above ambient. She does not eat while tending her nest, so metabolism does not generate the warmth.

The tegu is an exception to squamates being entirely ectothermic. Diamond python females also construct insulated nests which they keep warm. How these reptiles generate retentive body heat is not known.

Many bird and mammal females have upgraded thermogenesis during reproduction. These instances of improved endothermy for enhancing offspring survival independently evolved.


Opah, also known as moonfish, are comically rotund, but not to those who live in fear of them. Opah are deadly deep predators: plying cold, dark waters as far down as 300 meters. Most predatory fish are slowed by the cold at the depths opah hunt, waiting for meals to come to them. By contrast, moonfish are nimble pursuers.

The adaptation that makes the moonfish lifestyle possible is endothermy. Opah are the only fish known to maintain their body temperature in the chilly waters where they live. Other top sea predators, such as tuna and sharks, must reheat their bodies in warmer waters near the ocean surface after diving deep for food.

 Cold Tolerance

They combine warm- and cold-blooded animals in one. ~ Russian American physiologist Elena Gracheva

Leopard ground squirrels (shown) and golden hamsters are comfortable in the cold. This makes hibernation easy: they don’t have to fatten themselves up like bears for an extended slumber during the winter.

These rodents manage by having a specialized cold-sensing protein that does its job of signaling bodily functions but allows an imperviousness to the cold: aware but unperturbed. Other, related adaptations in metabolism, heart rate, and breathing afford a cold body hibernating in comfort.

The 1st Dinosaurs

Reptilian diversity prior to dinosaurs was considerable. Unsurprisingly, the different families of dinosaurs emerged from various reptilian lineages. (Only an estimated 1/4th of the dinosaur species that lived are known.)

The earliest dinosaurs were carnivores or omnivores that walked on 2 hind limbs that were positioned directly under the body to support it from below. Long tails provided balance.

Euparkeria was a reptile genus that lived 245–230 MYA. This sharp-clawed biped with numerous needle-like teeth began in the early Triassic the size of a housecat.

Euparkeria was an agile, nimble predator of insects, grubs, and other small animals in the undergrowth. It could rear up and sprint on its powerful rear legs, using its long tail for balance.

Euparkeria was not the daddy of dinosaurs, but some of the earliest dinos were much like it. Euparkeria was at least close to the ancestry of archosaurs.

Archosaur forms the clade of reptiles that includes dinosaurs, pterosaurs (winged lizards), today’s crocodiles, and birds. When archosaurs first appeared, and their exact descent, remains in the realm of debate.

There were other, similar reptiles to Euparkeria, in what was a clear adaptive trend. 2–3 meters long, Nyasasaurus had several skeletal features characteristic of dinosaurs, including a bipedal stance. The genus emerged 243 MYA.

Adaptive pressures and convergent evolution played important roles in the emergence of dinosaurs. Various packages of life-history variables played out. The sizes, shapes, lifestyles, and behaviors of dinosaurs evolved as a biotic gyre. This was typical of evolutionary trends in macrobes throughout life’s history.

Dinosaur Classification

Dinosaurs were first recognized as distinct from reptiles in 1842 by English comparative anatomist Richard Owen. Owen coined Dinosauria, meaning “terrible reptile.”

In 1887, English paleontologist Harry Seeley divided dinosaurs into 2 orders: ornithischians (Ornithischia) and saurischians (Saurischia). The noted difference was in their pelvises: whether their pubic bone pointed forward (saurischians) or backward (ornithischians). Saurischians were “lizard-hipped,” while ornithischians were “bird-hipped.”

Seeley’s basic division and nomenclature held despite birds evolving from saurischians, not ornithischians. The shape of bird pelvises came as a convergent evolution to the similar structure of unrelated ornithischians.

In 2017, English paleontologist Matthew Baron proposed a revised dinosaur classification based upon extensive new findings since Seeley’s day and an unbiased analysis. Baron retained Saurischia, but found that theropods belonged with ornithischians, labeling this new clade Ornithoscelida.

Baron’s choice of Ornithoscelida was a revival of an old name, albeit not using the same taxonomic rationale. English biologist Thomas Henry Huxley had defined Ornithoscelida in 1869 by how heavyset dinosaurs were. Huxley’s classification was overshadowed by Seeley’s 1888 grouping.

Baron’s classification better identifies the 2 major clades of dinosaurs. Baron recognized theropods and ornithischians as sister taxa. Another telling distinction is that Baron’s new categorization incorporates the earliest dinosaurs – herrerasaurids – as saurischians; something which Seeley’s classification could not accommodate.


In numerous ways dinosaurs resembled earlier reptiles. Some, such as tyrannosaurs, had protective scales on their bodies. Others sported plumage, although more fuzzy proto-feather than bird feather. Fluffy coloration arose for mating display. This evolutionary impetus would culminate in a functional form that allowed dinosaur descendants to take wing.

The earliest dinosaurs were herrerasaurids, emerging 245 MYA, in the Late Triassic. Eoraptor is exemplary: a 1-meter omnivore, weighing 10 kg.

A million years later, Eodromaeus appeared as a basal theropod: a 1.2-meter carnivore weighing 5 kg. Most theropods were carnivores, though various theropod groups were insectivores, omnivores, or even herbivores. Almost all theropods were bipedal.

Sauropods showed up shortly after theropods. Sauropods were thick-legged herbivores that walked on all fours, with small heads, elongated necks, and long tails.


Carnivorous tyrannosaurs were one of numerous theropod families. Tyrannosaurs arose 170 MYA. The earliest were the size of a modern human.

Tyrannosaurs diversified, but their rise to the top of the food chain was long checked by the dominant allosaurs that arose in the mid-Jurassic and lasted until 93 MYA, in the mid-Cretaceous.

In their prime, allosaurs were almost as large as tyrannosaurs eventually grew to. A mass extinction event, with global warming and sea level fluctuations, laid allosaurs low and allowed tyrannosaurs to take their place.

Tyrannosaurs apparently developed giant body size rapidly, late in the Cretaceous, and their success enabled by their early-evolving keen senses. ~ American paleontologist Stephen Brusatte et al

Tyrannosaurs were fierce predators. They hunted and scavenged and were not above fighting and eating another tyrannosaur.

T. rex

The most famous dinosaur, Tyrannosaurus rex emerged in the Late Cretaceous; near the end of the tyrannosaur family line. T. rex was one of the largest land carnivores of all time: up to 13 meters long and weighing over 6 tonnes. T. rex had a massive skull, and an enormous jaw that delivered a bone-crushing bite. The size of a chicken as a hatchling, T. rex took 15–18 years to reach full size. Making it to maturity was tough. Only ~40% survived their first year.

T. rex‘s girth slowed it down. Its top speed was 19 kph.

The available herbivorous dinosaurs in its environment were all much slower than an adult T. rex. It was slaughter in the slow lane. ~ Canadian paleontologist Thomas Carr

For all its ferocity, T. rex has been the butt of jokes for its puny arms. While not even long enough to reach its mouth, T. rex used its muscular arms for slashing at close quarters: an offensive defense.

(T. rex was not the only theropod with puny arms. The trait evolved independently, and similarly, in Gualicho shinyae, a large-jawed theropod that lived in modern-day Argentina. Gualicho stood 2 meters and weighed nearly 0.5 tonnes.)

Juveniles had proportionately longer arms. Young dinos likely had more use for their arms.

Tyrannosaurs didn’t need big arms to hunt, because their powerful bites and hyper-bulldog necks did the job. ~ American evolutionary biologist Eric Snively

T. rex had eyes that faced forward, giving it stereo-scopic vision with depth of field for pinpointing prey.

T. rex may well have ears that were forward focused, like that of modern-day birds of prey. With these acute senses,

T. rex may have been a nocturnal predator.

T. rex had a snout as sensitive as human fingertips. The tactile nose was employed to explore surroundings and build nests. It also allowed the dinosaur to pick up fragile eggs and offspring. A touchy nose may also have been important in courtship, with T. rex couples rubbing their faces together in foreplay.

The neck of T. rex was like that of modern birds. Like many birds, T. rex likely raised its head and fixed its vision on its prey before lowering its head to attack.

Taking a bite involved thrusting its head up, biting, then shaking its head and pulling back with its legs to rip a hunk off: the approach of birds combined with the shake-feed of crocodiles.


T. rex had several close relations. One was a diminutive cousin: Nanuqsaurus, which was 2.4 times smaller, and lived in the chilly high latitudes of North America. Nanuqsaurus‘ relatively small size was a cold-weather adaptation.


Dakotaraptor was a North American native. Unlike T. Rex, Dakotaraptor had long arms, with wings that stretched 1 meter.

Dakotaraptor was far too big to fly. The wings improved balance when running, providing lift that gave speed.

Dakotaraptor was a fierce predator. An agile runner with long legs, Dakotaraptor had sickle claws the size of a man’s hand on both front and rear limbs that acted as grappling hooks.

These claws could grab on to anything and just slice them to bits. It was utterly lethal. ~ American vertebrate paleontologist Robert DePalma

At 5–6 meters long, Dakotaraptor was one the largest dromaeosaurids (feathered theropods). Only Utahraptor, at 7 meters, was larger. Utahraptor was an earlier, close cousin to Dakotaraptor, with similar wings and claws. Both evolved late in the dinosaur reign.

Dakotaraptor probably practiced pack hunting, with ad hoc stratagems to bring down prey. Able to run down animals that T. Rex could only watch, the speed of Dakotaraptor was fearsome. Feathery wings got off to an impressive start.


Dinosaurs were social animals. Herbivorous dinosaurs traveled in herds.

Tyrannosaurs lived in family groups, as is common for large predators. They hunted in packs. Tyrannosaurs likely had social dominance hierarchies. The usual constraints – food, mates, and territory – were likely points of contention.


The first time I examined the specimen, I even questioned whether it was a genuine fossil. ~ Italian paleontologist Andrea Cau

About the size of a mallard, Halszkaraptor was something of a cross between a duck and a penguin: it waddled on 2 legs but had flipper-like forelimbs to aquatically maneuver like penguins. Unable to fly, it was a skillful swimmer.

Halszkaraptor’s goose-like neck made it a fine ambush hunter. Its bill housed sharp teeth.

Like all dinosaurs (and later birds), Halszkaraptor had to come onto land to lay its eggs. This novel maniraptoran arose in the late Cretaceous and belongs in the clade from which birds descended.

The unexpected mix of traits makes it difficult to place Halszkaraptor within traditional classifications. ~ Andrea Cau


Various enormous carnivores emerged from several dinosaur subgroups as a culmination of earlier developments.

Similar body plans illustrate convergent evolution. Some of these predators, especially the smaller ones, were pack hunters; others, opportunistic scavengers.


Dinosaurs were landlubbers. Until this one. ~ Paul Sereno

Giant fish lived in the rivers of North Africa during the Cretaceous. A theropod arose to take advantage of the aquatic feast.

Spinosaurus grew to be the largest known carnivorous dinosaur: 12.6–18 meters long, weighing 7–21 tonnes. Spinosaurs had large, strong forelimbs with scythe-like claws. Its hind legs were short, with splayed toes. Spinosaurs may have paddled in the water like a duck. Its long, flexible tail assisted propulsion.

Conical teeth in a crocodilian snout overlapped, acting as a snare for trapping fish. Spinosaurs had nostrils halfway up its skull, so it could stick its snout into the water and still breathe.

Spinosaurs’ center of mass was too far forward for it to stand on its hind legs on land, like other theropod predators. Instead, spinosaurs ambled on all 4 legs.

Similar to the synapsids of old, spinosaurs had spines growing out of its vertebrate – up to 1.65 m long – that provided for a sail which likely served for sexual display, and perhaps aided thermoregulation.

The skull and lifestyle of spinosaurs were suggestive of crocodiles, which later convergently evolved to take advantage of freshwater aquatic fare. Like crocodiles, spinosaurs spent much of their day in the water, while resting and mating on the riverbank.

It’s like a cross between an aquatic bird and a crocodile. ~ Paul Sereno


As egg layers, all dinosaurs were brooders, providing parental care. With a tepid climate, protection rather than warmth was what eggs needed to hatch. Like modern reptiles, dinosaurs were largely precocial.

Dinosaurs lived on Earth 180 million years; as a dominant group for ~140 million years. While fierce carnivores are the most celebrated, most dinosaurs were herbivores.

Dinosaur Dominion

Dominion is a dramatic word. Certainly, dinosaurs excluded other vertebrates from proliferating as large animals.

Mammals are the most obvious example of this, but many other animals came along before the 1st dinosaur took a breath, and those other animals are still around.

Consider the cockroach, which debuted 360 MYA, during the Carboniferous (the earliest dinosaurs: 245 MYA). The cockroach thrives today, largely unchanged, except for a somewhat deteriorated diet owing to the evolution of fast-food restaurants.

As to dominion, microbes have ruled the world since they made the scene at the beginning of life, back before dirt was young. The power of the smallest life can never be overstated.

Viruses have the ability to manipulate the life histories and evolution of their hosts. ~ American virologists Forest Rohwer & Rebecca Vega Thurber


The history of animal life on Earth repeatedly showed a correlation between atmospheric oxygen and animal diversity as well as body size: times of low oxygen saw, on average, lower diversity and smaller body sizes than times with higher oxygen. These same relationships held for dinosaurs. ~ Peter Ward & Joe Kirschvink

Dinosaur diversity was roughly constant from the time of the first Triassic dinosaurs into the Jurassic. Atmospheric oxygen was low during this period. From the mid-Jurassic, global atmospheric O2 shot up. Dinosaurs gained girth and speciation rose – a trend that continued until 140 MYA, in the mid-Cretaceous.

Various explanations have been made as to why dinosaurs succeeded so well. For one, the climate of the times suited them. The world was warm and moist during the Jurassic. For the last 2/3rds of the period, there was no polar ice. Sea levels were high. Vast areas of land were flooded. Temperate and subtropical forests were pervasive. The extensive water moderated seasonal swings.

The earliest sauropods were small and ran on 2 legs, relying on speed to evade predators. Then, in evolutionary time, they grew until girth became their edge against predation.

Though most long-necked sauropods lumbered on 4 legs to support their bulk, a few up to the size of rhinoceroses went from walking on 4 legs to 2 as they matured. During development, their center of mass moved back toward their hips as their tail muscles became bulkier. This allowed these herbivores to reach higher in the trees to feed.

Earth was so warm during the Jurassic that dinosaurs lounged in Antarctica, which was a much larger continent at the time. Giant sauropods weighing up to 77 tonnes seasonally migrated to maintain their ravenous diet of greens: requiring over 450 kilograms each day, twice that of modern elephants.

Sauropods started life as an egg the size of a grapefruit. Baby sauropods were less than a half meter long and weighed under 10 kilos.

To reach their gargantuan size, sauropods had a tremendous growth rate between 5–25 years old. They reached their adult size by the time they were 30.

Sauropods became a disparate group, with sizes and body plans varying as plate tectonics drove species radiation. Separate sauropods lived side by side. A diversity of dining habits on various vegetation afforded each sauropod species its own niche.

The climate engendered prolific vegetation. As always, the success of animals ultimately rides upon greenery.

Some of the same sociality exhibited by birds and mammals was shared by dinosaurs. Predators often hunted in packs. Gregarious sauropod herbivores foraged in herds, though juveniles and adults may have done so separately, as their diets differed, as did their bulk.

Sauropods arose in the Late Triassic. By the end of their spectacular 150-million-year run, when all large dinosaurs became extinct, sauropods had lived on every continent.

The dinosaur’s eloquent lesson is that if some bigness is good, an overabundance of bigness is not necessarily better. ~ American businessman Eric Johnston


Dinosaurs and lizards evolved as separate reptile groups. Dinosaurs developed a distinct upright posture that lizards lack, yet they share a common ancestor.

Modern lizards that incubate at warmer temperatures are mentally sharper than those that come from a cold nest. Perhaps the warm climate provided the conditions needed to optimize dinosaur adaptability.


Ornithischians arose in the Jurassic. All were beaked herbivores; typically, low browsers. The surfeit of herbivorous dinosaurs owed to niche differentiation: specialization to favor different plants. The vast diversity of vegetation provided the platform upon which these picky plant lovers evolved.

Like sauropods, ornithischians were typically herd animals, though some appear to have led largely solitary lives. Although descendant from bipeds, almost all ornithischians had the ability to walk on all fours, if not natural quadrupeds.

Though less predominant than sauropods, ornithischians were among the most numerous, diverse, and longest-lasting lineages of dinosaurs. They were typically smaller than sauropods, and often prey to theropods.

By the mid-Jurassic, ornithischians had a wide variety of body plans. This diverse group included thyreophorans, which were armored herbivores, such as the spiky plated stegosaurs (8–9 meters; 2–3 tonnes). The spiked tail of stegosaurs could deliver a painful piercing blow to a potential predator.

Owing to their relatively small brains, stegosaurs earned renown as the “dumbest” dinosaurs. As brain size is no indicator of intelligence, stegosaurs are not the ones deserving such labeling.

While armor may have been helpful for defense, it may also have evolved via sexual preference, or served for thermoregulation. Juvenile stegosaurs lacked the large back plates.

In contrast were the relatively small and agile bipedal ornithopods known as hypsilophodonts (1.5–2.3 m; 20–40 kg), which lived lives like those of the antelopes and gazelles today: herds foraging on ground vegetation while keeping a keen lookout for predators.

The differences between herding hypsilophodonts and heavy thyreophorans point out 2 viable strategies for avoiding predation: either be armored or raise the odds against being eaten by being small, nimble, and one in a crowd.

By the end of the Cretaceous, more ornithischians had arrived: among them duck-billed hadrosaurids, such as Parasaurolophus (12 m; 3 tonnes), and horned ceratopsians, of which Triceratops (9 m; 5–8 t) is the best known. Both were herbivores. The duck-billed dinosaurs had sophisticated dentation that let them grind tough plant matter.

Triceratops compensated for dental ingenuity with overwhelming numbers: over 400 teeth that replaced themselves when they wore down.

Triceratops at first lacked the magnificent horns that they are so well-known for. Early on they sported a longer beak and shorter horns.

Triceratops became a favorite food of tyrannosaurs. Triceratops‘ horns grew to even the odds against becoming an easy meal.

Beyond defense, ornaments that various ornithischians had may well have developed from sexual selection. The neck frills of triceratops are exemplary. Female choice of mate that evolves into exaggerated looks has repeatedly occurred.

Dinosaur head ornaments were for a select few. To sport standout head gear, a dinosaur had to have a big body to viably support it.


 Dinosaur Fleas

These were insects much larger than modern fleas, and from the size of their proboscis, we can tell they would have been mean. ~ American zoologist George Poinar, Jr.

Dinosaurs had fleas. Big ones: 10 times the size of the modern flea.

It really appears as though they were specialized for working their way into some heavy hides, such as those on dinosaurs. ~ American palaeoentomologist Michael Engel

In subsequent evolution, fleas downsized. This required acquiring the strong spring-legged jump that lets modern fleas leap 100 times their height.

Without the incredible jump, dinosaur-era fleas had to scurry and hop on their prey. Alternately, ambush-style, they may have dropped from vegetation.

The flea also adapted to suck blood without causing so much pain, thereby allowing getting a full meal without unduly agitating its host. This involved going from a jaw with saw-like projections to smooth mouth parts.

 Water Fleas

While humongous fleas were dining on dinosaurs, wee water fleas were abundant denizens of freshwater habitats. Water fleas arose during the Permian.

Water fleas are tiny crustaceans in the order Cladocera, so-called for their bouncing locomotion. They continue today to play a key role in most terrestrial ecosystems.

Like sharks, water fleas emerged with a near-ideal set of traits, including agile adaptability to survive significant ecological shifts. Their basic biomechanics have scarcely evolved since their arrival ~260 MYA, while diversifying into over 700 species that occupy almost all freshwater environments.

When conditions are favorable, female cladocerans reproduce asexually. Conversely, when environmental conditions deteriorate and variety becomes critical, males are produced and sex becomes the norm. This conditional reproductive system is known as cyclical parthenogenesis.

Though water fleas have various defense mechanisms to escape predators, some decided on a tamer domain: in groundwater. No longer needing sight, these subterranean crustaceans economically went blind and lost their eyes.

 Ostrich Dinosaurs

Several dinosaurs were beaked, including the herbivorous Triceratops and Stegosaurus. Ornithomimosaurs were the only meat eaters to evolve beaks.

One of the best known ornithomimosaurs was Struthiomimus, an omnivore that enjoyed a rounded diet of vegetation and small animal fare, including eggs and insects. This nimble creature was 3–4 meters long, stood 2 meters tall, and weighed 140 kg.

The whole group is commonly called ostrich dinosaurs, though the similarity is only an example of parallel evolution, as these theropods were not related to emergent birds. Convergence included beaks without teeth: an avian feature. Early ornithomimosaurs had tiny teeth, as did nascent birds. This trait wore away as adaptation progressed in later species.

Ornithomimosaurs were ground-bound, with long tails. This tail distinguishes these dinosaurs from all birds, ostriches included.

The lifestyle of ornithomimosaurs was quite like ostriches, which are an omnivorous flightless bird. Like ostriches, when in danger, their response was to run. Ornithomimosaurs were likely the fleetest of all dinosaurs. Some were faster than ostriches, which can run 80 km/hr.

Ornithomimosaurs emerged in the Late Cretaceous: one of the last dinosaur groups to appear. Their continuing evolution was apparent as the reign of dinosaurs ended.


Anzu was another bird-like dinosaur of the Cretaceous; dubbed the “chicken from hell” by its discoverers. Anzu was an oviraptorosaur, characterized by a beaked, parrot-like skull, with or without a bony crest atop its head.

Oviraptorosaurs were a group in the maniraptoran clade, which included both bird-like and non-avian dinosaurs. Maniraptorans had long arms and 3-fingered hands. They are the only dinosaurs with breast bones: an avian trait. The only dinosaurs to ever fly were maniraptorans, though how far back they first took flight remains unknown. Birds are maniraptoran descendants.

Like other theropods, oviraptorosaurs were feathered. Some had feather-adorned tails, which males shimmied as a courtship display.

Anzu stood 1.5 meters high and reached more than 3 meters from beak to tail. An adult weighed up to 300 kg.

Aznu resembled a beefy emu. The creature had a long neck; its head topped with a tall, thin crest. At the end of its forelimbs were long, sharp claws.

Anzu lived on ancient floodplains, omnivorously feeding on plants, small animals, and possibly eggs. It had a huge beak with sharp edges, and a strangle sliding jaw joint used to cut vegetation and meat.

Like birds, Anzu had hollow bones. As with Struthiomimus, Anzu‘s meter long tail precluded flight.

In the Skies

Besides behemoth land lubbers and their nimbler carnivore cousins, reptiles also took to the air. Pterosaurs were the 1st flying vertebrates. They reigned over the skies 228–66 MYA.

Pterosaurs were neither dinosaurs nor the progenitor of birds. Some were scaly, others furry.

Pterosaurs were able fliers. Their wings were quite unlike those of birds: more like bat wings, in being made of thin membranes stretched between the arms and hind limbs.

Pterosaurs’ hollow bones, respiratory airs sacs, and enlarged brains were reminiscent of avian traits. In these regards, birds were an instance of convergent evolution.

Early pterosaurs had long, trailing tails, and teeth in their beak-like mouths. Later ones, the pterodactyls, were short-tailed, and had no teeth. This was, once again, convergent evolution with only distantly related birds.

Remarkably, pterodactyls were highly precocial and could take wing just after they hatched. Lacking parental care, flaplings were on their own, and so were well stocked with precocious knowledge as well as the ability to fly.

Some of the smaller pterosaurs were covered with fur. That and their doubtless flights in cool air over great distances suggest that pterosaurs were endothermic.


Anurognathus was a small pterosaur, the size of a swift. Its legs were stronger than any bird or bat alive today.

Some birds, such as swifts, chase insects through the sky: a technique termed hawking; not Anurognathus. Instead, it sat in wait for its prey to fly by, then Anurog-nathus instantly launched itself on an intercept course, which is called sallying. Anurognathus‘ powerful legs let it leap into the air, losing no time, nor wasting energy, on hawking about.

At Sea

After spending millions of years attuning themselves to living on land, some reptiles returned to the sea, as well as inhabiting freshwater bodies. Some stayed close to the shoreline, feeding off the bounty in seashore sand.

The fact that marine reptiles and sharks were so large during the Jurassic and Cretaceous suggest that the marine food supply was ample. Over half of the oil now being exploited was laid down during the Cretaceous.

The seas were more densely populated by phytoplankton than at any other period in geological history. Diatoms emerged in the Late Jurassic, greatly radiating and becoming abundant during the Cretaceous. As today, tiny autotrophs were then the fodder that fed the oceanic food chain.

 Stony Coral

One of the most significant marine keystone species made its debut in the Triassic seas: stony corals, the supreme builders of reefs. They did not start that way. The earliest stony corals were solitary and not as architecturally inclined.

These polyps did not radiate until the Jurassic, becoming the dominant reef-builders in all the world’s oceans from then. By this time these corals had become industriously colonial. The spur to their success appears to have been symbiotically teaming up with algae.

Stony coral were not the only contributors to oceanic reefs. Calcified seaweeds and sponges also did their part.

 Seaside Sifter

During the Middle Triassic, 245 MYA, estuarine mud and seabed sand were home to a surfeit of small animals. A dinosaur arose to take advantage of this potential feast.

Atopodentatus grew to 3 meters long. Not as well-adapted to life at sea as aquatic icthyosaurs, this quadruped had paddle feet but could also walk on land.

Atopodentatus‘ dentition was unlike any other animal ever. Its head was shovel-shaped, armed with an arc of over 175 tiny, needle teeth that fused to the sides of the jaw rather than sitting in sockets. Inwardly, the teeth were bladelike, arranged like a comb. Most of the teeth in the upper jaw faced each other, in a split running between the 2 halves of the upper jaw.

Atopodentatus was a filter feeder: it stuck its snout in the mud, took a mouthful of sediment, and sifted for puny prey, such as worms. Squeezing the sand through its teeth let Atopodentatus comb through prey like a baleen whale traps krill.


A large marine reptile, ichthyosaurs had a porpoise-like head, a long toothy snout, and a tuna-like body. They swam with side-to-side movement like catfish. Of the seafaring reptiles, ichthyosaurs were the most meticulously adapted to their watery world.

Averaging 2 meters long and 80–100 kg, ichthyosaurs were a parallel development to the lineages that led to the modern-day dolphin and whale. One ichthyosaur species could dive down 610 meters. This was another instance of convergent evolution, in that all 3 families were air breathers, fast swimmers, capable of deep dives, and bore live young.

The varied diet of the ichthyosaurs, capable of hunting at night, gave them a long run: from 250–90 MYA. The first ichthyosaur evolved only a few million years after the Great Dying.

Ichthyosaurs died out as environmental volatility in the oceans overwhelmed their ability to adapt. Global changes profoundly reorganized marine ecosystems during the Cenomanian age (100.5–94 MYA).

Most reptiles are egg layers, but adaptation to living in the sea demanded viviparity. This was true of ichthyosaurs, nothosaurs, plesiosaurs, and mosasaurs. Having few large offspring meant that these creatures were likely to having been caring mothers.


Nothosaurs were another early aquatic reptile (245–200 MYA). They were slender, long necked piscivores with sharp, interlocking teeth. Nothosaur bodies were somewhat like crocodiles: streamlined for aquatic predation, including webbed feet. Their lifestyle was like that of modern seals, in being largely aquatic, while coming ashore from time to time. Various nothosaur species ranged in size from several centimeters to 4 meters.

Nothosaurs propelled themselves over the seafloor by rowing their forelimbs in unison. Paddle feet scooped the soft mud, disturbing fish and shrimp there, which a nothosaur snapped up with its needle-sharp teeth.

Nothosaur limbs suggest that they could move on land, albeit with the sprawling gait of seals, which may have later adopted the nothosaur lifestyle: fishing at sea and relaxing on land.

Nothosaurs were supplanted by pleiosaurs. Both were sauropterygians: aquatic reptiles that evolved from terrestrial tetrapods soon after the end-Permian extinction. While nothosaurs and plesiosaurs are closely related, that nothosaurs begat plesiosaurs is uncertain.


Plesiosaurs (203–66 MYA) became common during the Jurassic and lasted to the end of the Cretaceous. This diverse group of marine reptiles replaced ichthyosaurs as the top aquatic predator during the Late Jurassic. While starting at 2 meters, some plesiosaurs were massive: up to 20 meters, the size of a sperm whale.

Compared to nothosaurs, plesiosaurs had further adaptations to an aquatic life: 4 paddle-shaped flippers instead of webbed feet, and elongated necks. Like penguins and sea turtles, plesiosaurs swam largely with their forelimbs, but with a difference: the watery vortices created by the front flippers were woven by the back flippers to move more efficiently.

Plesiosaurs were forelimb-dominated swimmers that used their hind limbs mainly for maneuverability and stability. ~ Chinese paleontologist Shiqiu Liu et al

Elasmosaurus (13–14 m; 2 tonnes) was one of the last plesiosaurs. Its neck ran 6–7 meters; the longest of the plesiosaurs. Elasmosaurus had 72 vertebrates in its neck; more than any known animal. With its tiny head on small sea creatures Elasmosaurus fed. How Elasmosaurus managed to get enough to eat is an enduring mystery.


Mosasaurs were air-breathing marine reptiles that appeared in the Early Cretaceous, descended from aquatic lizards. They radiated into considerable diversity, with a peak of 38 genera.

During the last 20 million years of the period, with the extinction of the ichthyosaurs and pleiosaurs, mosasaurs became the dominant seafaring predator. Powerful swimmers, they coursed through the water using their strong tails, like sharks and ichthyosaurs. Their 4 flipper limbs provided fine movement control.

The smallest mosasaur was less than 1 meter long. Larger mosasaurs were more typical, with many species over 4 meters. The largest mosasaur reached 17 meters.

Mosasaurs had large, sharp teeth in double-hinged jaws, long snouts, and flexible skulls, much like sharks (an instance of convergent evolution), affording a fearsome bite and the ability to swallow large chunks. Sharks were mosasaurs’ main competitors: an evolutionary pressure which may have refined sharks toward their modern forms. Both sharks and mosasaurs had a worldwide presence.

Mosasaurs may have been endothermic, as were ichthyosaurs – an unusual adaptation for reptiles.

Today’s monitor lizards are a close relative to mosasaurs, which went extinct 66 MYA.


Dinosaurs were not the only ones living large. Some trees too were towering. Gymnosperm conifers had benefited from the Permian–Triassic extinction event. They were the skyscrapers of the time.

Meanwhile, plant innovation was happening on a much smaller scale, and at a lovelier level. Flowering plants were coevolving with their pollinators, and with their pests.