Dominance of the Mesozoic period – dinosaurs, pterosaurs, and giant marine reptiles
Mesozoic world
Spanning about 186 million years (from ~252 to 66 million years ago), the Mesozoic era consists of the Triassic, Jurassic, and Cretaceous periods. During this time, reptiles (especially dinosaurs) were the most prominent large vertebrates, occupying land, sea, and air realms:
- Dinosaurs thrived in various terrestrial ecosystems.
- Pterosaurs (flying archosaurs) conquered the skies.
- Marine reptiles, such as ichthyosaurs, plesiosaurs, and mosasaurs, dominated the oceans.
This era followed the Permian–Triassic mass extinction, the deadliest extinction in Earth's history, and ended with another massive upheaval – the Cretaceous–Paleogene (K–Pg) extinction (~66 million years ago), during which not only non-avian dinosaurs but also many marine reptiles perished, freeing evolutionary niches for mammals and birds. The “Age of Reptiles” shows the final forms of archosaur evolution, revealing how they developed, spread, and eventually disappeared.
2. Beginning of the Triassic: after the greatest extinction
2.1 Recovery after the Permian and the rise of early archosaurs
Permian–Triassic (P–Tr) extinction (~252 million years ago) wiped out ~70% of terrestrial and ~90% of marine species, radically changing Earth's biosphere. Species that survived the early Triassic period, especially early archosaurs, rapidly diversified, occupying vacant ecological niches:
- Archosauromorphs: A broad group including the ancestors of crocodiles, pterosaurs, and dinosaurs.
- Synapsids (dominant in the late Paleozoic) had greatly declined, so archosaurs began to dominate predator and large herbivore roles in many ecosystems.
2.2 The first dinosaurs appear
In the Late Triassic (~230–220 million years ago), the first true dinosaurs appeared. Fossils in Argentina (e.g., Eoraptor, Herrerasaurus) and Brazil, and slightly later in North America (Coelophysis), show they were small, bipedal, and lightly built. Key dinosaur features include an upright posture (limbs under the body) and specialized hip, ankle, and shoulder joints, giving them agility and more efficient movement than "sprawling" reptiles. Over several million years, these early dinosaurs branched into two major groups:
- Saurischia ("lizard-hipped"): includes theropods (bipedal predators) and sauropodomorphs (herbivores leading to gigantic sauropods).
- Ornithischia ("bird-hipped"): various herbivores (ornithopods, thyreophorans like stegosaurs, ankylosauroids, and later ceratopsians) [1], [2].
2.3 Triassic marine reptiles
In the seas, new marine reptile lineages replaced Paleozoic forms:
- Ichthyosaurs: dolphin-like forms adapted to open ocean predator niches.
- Notosaurs, from which pachypleurosaurs and later plesiosaurs evolved: fin-limbed reptiles ranging from coastal areas to open seas.
These groups demonstrate rapid, repeated adaptive radiation after the P–Tr extinction, occupying marine niches from shallow coasts to deep waters.
3. Jurassic: dinosaurs thrive, pterosaurs expand
3.1 Dinosaur dominance on land
During the Jurassic period (201–145 million years ago), dinosaurs evolved into many iconic forms, such as:
- Sauropods (e.g., Apatosaurus, Brachiosaurus): gigantic, long-necked herbivores reaching 20–30+ m in length, considered some of the largest land animals ever.
- Theropods (e.g., Allosaurus, Megalosaurus): large bipedal predators, but also including smaller, more gracile lineages.
- Ornithischia: Stegosaurs with back plates, early ankylosauroids, and smaller, bipedal ornithopods.
Warm Jurassic climates, extensive flooding of continents by seas, and vast coniferous forests provided abundant resources. As Pangaea broke apart, dinosaurs could spread across wide connected territories. They dominated terrestrial ecosystems, overshadowing other reptiles and synapsids.
3.2 Pterosaurs: Masters of the Skies
At that time, pterosaurs perfected active flight:
- Rhamphorhynchoids: primitive, long-tailed forms common in the Early–Middle Jurassic, often smaller-bodied.
- Pterodactyloids: more advanced forms with shorter tails and often large head crests, appearing in the Late Jurassic. Later, gigantic forms appeared, like Quetzalcoatlus (Cretaceous), with wingspans over 10 m.
They exploited flying niches from insectivory to piscivory and were the main flying vertebrates until the rise of birds from some theropod dinosaurs at the end of the Mesozoic. [3].
3.3 Marine Diversity: Ichthyosaurs, Plesiosaurs, and Others
In Jurassic oceans:
- Ichthyosaurs reached their highest diversity but began to decline during the Cretaceous period. They had streamlined bodies, large eyes, adapted for deep-water hunting.
- Plesiosaurs became more specialized, branching into long-necked elasmosaurids with very long necks and short-necked pliosaurs (e.g., Liopleurodon), possibly reaching impressive sizes.
Many fish groups, ammonites, and other marine invertebrate communities also thrived in warm, shallow oceans. After the marine period ended, the ecological vacuum left by Triassic marine reptiles was fully filled by these new top marine predators.
4. Cretaceous: New Evolutionary Ideas and the Last Flourishing
4.1 Continental Breakup and Climate
During the Cretaceous period (145–66 million years ago), Pangaea continued to split into Laurasia (north) and Gondwana (south), resulting in more distinct faunal regions. Warm "greenhouse" climates, high sea levels, and the expansion of epicontinental seas shaped diverse dinosaur faunas on different continents. This was the "peak of prosperity" for more advanced dinosaur groups:
- Ornithischia: Ceratopsians (Triceratops and others), hadrosauroids ("duck-billed" dinosaurs), ankylosauroids, pachycephalosaurs.
- Theropods: Tyrannosaurs in the north (T. rex), abelisaurs in the south, as well as smaller raptor-like dromaeosaur forms.
- Sauropods: Titanosaurs in Gondwana, including especially large species (Argentinosaurus, Patagotitan) [4], [5].
4.2 Origin of Birds and Feathered Dinosaurs
Some theropods, especially coelurosaurs (e.g., raptor-type maniraptorans), developed feathers for warmth or signaling. True birds (dinosaurian origin) had already appeared in the Late Jurassic or Early Cretaceous, e.g., Archaeopteryx, which is an intermediate form. Cretaceous fossil finds in China (Jehol fauna) show how feathered dinosaur lineages expanded, filling the morphological gap between "raptor" dinosaurs and modern birds, clarifying how flight evolved from small, feathered theropods.
4.3 Changes in marine reptiles: mosasaur dominance
While ichthyosaurs went extinct around the middle of the Cretaceous and plesiosaurs survived, a new group—mosasaurs (large marine lizard-related forms)—became the dominant marine predators. Some mosasaurs reached lengths over 15 m, feeding on fish, ammonites, and other marine reptiles. Their global distribution in Late Cretaceous seas indicates a steady shift in marine predator dominance.
5. Ecosystem complexity: high productivity levels and niche diversity
5.1 Breakthrough of angiosperm (flowering) plants
During the Cretaceous, there was also the rise of flowering plants (angiosperms), along with new pollination strategies, fruits, and seeds. Dinosaurs adapted to this plant community; hadrosauroids, ceratopsians, and other herbivores could have played roles in seed dispersal or indirectly aided pollination. Together with intensive insect pollination, terrestrial environments became even more complex.
5.2 Interaction of insects and reptiles
A great diversity of plants stimulated an evolutionary burst in insects. At the same time, pterosaurs (some specialized in insectivory) and small feathered theropods (partly insectivorous) indicate a close food web. Larger dinosaurs or reptiles altered the landscape through grazing and trampling effects, similar to today's megafauna.
5.3 Emergence of mammals
Although overshadowed, mammals existed during the Mesozoic—mostly small, nocturnal, or highly specialized insectivores or frugivores. Some more advanced forms (e.g., multituberculates, early therians) occupied their niches. However, their time to dominate larger bodies had not yet come until the K–Pg extinction, when dinosaurs disappeared.
6. Pterosaur evolution and extinction
6.1 Late Cretaceous giants
Pterosaurs flourished most from the Early to the Middle Cretaceous but gradually declined with the advancing evolution of birds. However, some pterosaurs (azhdarchids) reached enormous wingspans (~10–12 m) in the Late Cretaceous, such as Quetzalcoatlus. They may have fed on carrion or preyed on food with long legs like storks. By the end of the Cretaceous, pterosaurs nearly vanished, except for a few lineages that, like non-avian dinosaurs, perished during the K–Pg extinction [6].
6.2 Possible competition with birds
With the improvement of bird flight efficiency, an ecological conflict may have arisen with small or medium-sized pterosaurs, contributing to the decline of pterosaurs. However, what truly caused their extinction—whether direct competition, climate change, or that final disappearance—remains debated. In any case, pterosaurs are the only group of reptiles to have developed active flight, demonstrating their exceptional evolutionary success.
7. K–Pg extinction: the end of the “Age of Reptiles”
7.1 Catastrophic event
About 66 million years ago, a large celestial body (~10–15 km in diameter) struck near the present-day Yucatán Peninsula (Chicxulub crater). This impact, combined with intense volcanic activity (Deccan Traps in India), rapidly altered global climate, ocean chemistry, and light penetration. Within a few millennia (or possibly faster), ecosystems collapsed:
- Non-avian dinosaurs became extinct.
- Pterosaurs died out.
- Marine reptiles, such as mosasaurs and plesiosaurs, became extinct.
- Ammonites and many other marine plankton groups died out or were severely reduced.
7.2 Survivors and consequences
Birds (avian dinosaurs), small mammals, crocodiles, turtles, and some lizards and snakes survived. With the disappearance of large dinosaurs, mammals rapidly began evolutionary radiation in the Paleogene, emerging as new large terrestrial vertebrates. Thus, the K–Pg boundary is a crucial break, ending the Mesozoic era and starting the Cenozoic (the “Age of Mammals”).
8. Paleontological insights and unresolved questions
8.1 Dinosaur physiology
Bone histology studies, growth rings, and isotopes suggest many dinosaurs may have had relatively high metabolic rates—some propose a “mesothermic” or partially warm-blooded model. Feathered theropods were likely even more bird-like in regulating temperature. Questions about how giant sauropods regulated body heat or how fast tyrannosauroids ran remain debated.
8.2 Behavior and social structure
Tracksite deposits indicate herd or pack behavior in some dinosaur species. Nesting sites (e.g., Maiasaura) demonstrate parental care—a sophisticated trait that may have contributed to dinosaur success. Ongoing evidence of possible communal nesting or protective behavior further enriches knowledge of dinosaur social complexity.
8.3 Paleobiology of marine reptiles
Marine reptiles like plesiosaurs still raise questions: exactly how did the long-necked elasmosaurs feed or maneuver? Were they warm-blooded like some marine mammals? Ichthyosaurs' fish-like adaptation shows convergent evolution with modern dolphins. Discoveries of new fossils—such as pregnant ichthyosaurs or unusual skull shapes—continue to detail marine reptile life strategies.
9. Why did reptiles dominate for so long?
- Post-permian opportunity: Archosaurs rapidly expanded as synapsid dominance declined, occupying key predator and large herbivore niches.
- Evolutionary innovations: Upright posture, efficient respiratory system, complex social/parental care traits in certain groups.
- Stable Mesozoic climate: Warm “greenhouse” regimes and extensive continental connectivity allowed dinosaurs to spread widely.
- Lack of competition: Other major herbivore or predator lineages (synapsids, amphibians) remained toppled or confined to small niches.
However, these success factors did not protect them from sudden catastrophe during the K–Pg event, highlighting the role of chance in Earth’s history.
10. Legacy and modern perspective
10.1 Birds: the living dinosaurs
With the survival of avian dinosaurs (birds), the Mesozoic legacy has carried into the present. Every bird—from hummingbird to ostrich—is the sole surviving dinosaur branch, continuing skeletal, respiratory, or perhaps behavioral traits formed in the Mesozoic.
10.2 Cultural and scientific significance
Dinosaurs, pterosaurs, and giant marine reptiles remain iconic symbols of paleontology and popular culture—reminding us of the ancient Earth world and the dynamism of life. Strong public interest drives new excavations, advanced analytical technologies, and scientific collaboration. The “Age of Reptiles” testifies to evolutionary potential when ecological opportunities open and the fragility of the largest animals in the face of global upheavals.
10.3 Future discoveries
As fossil searches continue in Asia, South America, Africa, and elsewhere, we likely still await discoveries of new dinosaur species or even entire groups. Advanced computer tomography, isotopic analysis, and 3D reconstructions can reveal behavior, colors, diet, or growth rates previously inaccessible. Also, reviewing museum collections with new technologies often yields new discoveries. Undoubtedly, the Mesozoic “Age of Reptiles” story continues and expands with each new find.
Links and further reading
- Benton, M. J. (2019). Dinosaurs Rediscovered: The Scientific Revolution in Paleontology. Thames & Hudson.
- Brusatte, S. L. (2018). The Rise and Fall of the Dinosaurs: A New History of a Lost World. William Morrow.
- Padian, K., & Chiappe, L. M. (1998). “The Origin and Early Evolution of Birds.” Biological Reviews, 73, 1–42.
- Upchurch, P., Barrett, P. M., & Dodson, P. (2004). “Sauropod Dinosaur Research: A Historical Review.” In The Sauropods: Evolution and Paleobiology, University of California Press, 1–28.
- Carrano, M. T., & Sampson, S. D. (2008). “The phylogeny of Tetanurae (Dinosauria: Theropoda).” Journal of Systematic Palaeontology, 6, 183–236.
- Witton, M. P. (2013). Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press.