Have you ever wondered if there were dinosaurs that could live both on land and in water like modern amphibians? This strange concept might sound fictional, but some remarkable fossil discoveries over the past few decades have revealed a fascinating group of semi-aquatic reptiles that lived alongside the dinosaurs during the Mesozoic Era.
If you’re short on time, here’s a quick answer to your question: While technically not dinosaurs themselves, several prehistoric reptile groups like plesiosaurs and ichthyosaurs evolved amphibious lifestyles and could be considered ‘amphibian dinosaurs’.
In this nearly 3000 word article, we’ll take an in-depth look at the anatomy, habitats, and lifestyles of these aquatic reptiles of the Mesozoic Era and see just how dinosaur-like some of them became.
Defining What Makes a True Dinosaur
Taxonomic Classification of Dinosaurs
Dinosaurs belong to a group known as archosaurs, which also includes modern crocodilians and pterosaurs. To be classified as a dinosaur, an animal must fall under the clade Dinosauria, which is defined as “the last common ancestor of Triceratops and Neornithes, and all its descendants.”
So birds are actually considered a type of dinosaur under modern taxonomy!
There are two main groups of dinosaurs: Saurischia, which includes carnivores like Tyrannosaurus rex and long-necked herbivores like Brachiosaurus, and Ornithischia, which includes armored dinosaurs like Stegosaurus and horned dinosaurs like Triceratops.
Dinosaurs are further divided into orders, families, genera, and species based on anatomical similarities.
Unique Physical Traits of Dinosaurs
Unlike other reptiles, dinosaurs stood upright with legs positioned directly under their bodies. They also had unique skeletal adaptations like an S-shaped neck in sauropods and stiffened tails that aided balance in theropods.
Additionally, many species evolved bony armor, horns for display, and other anatomical structures.
However, the most unique physical trait of dinosaurs is that they were endothermic, meaning they generated their own internal body heat to regulate temperature. This allowed them to achieve massive body sizes. For example, the sauropod Argentinosaurus may have weighed over 70 tons!
| Trait | Dinosaurs | Other Reptiles |
|---|---|---|
| Stance | Upright legs under body | Sprawling legs |
| Temperature regulation | Endothermic | Ectothermic |
| Size | Often massive | Typically smaller |
Major Mesozoic Aquatic Reptile Groups
Plesiosaurs – The Long-Necked Marine Reptiles
Plesiosaurs were a diverse group of marine reptiles that thrived during the Mesozoic era. They had long necks, small heads, and paddle-like limbs to propel themselves through the ancient seas. The most iconic plesiosaurs had long, serpentine necks that made up about half the length of their entire body!
This unique body plan allowed them to stealthily ambush fish and other marine prey.
There were two main plesiosaur groups – the long-necked elasmosaurs and the shorter-necked pliosaurs. Elasmosaurs had necks containing over 70 vertebrae, longer than any other known animal! In contrast, pliosaurs had more robust bodies and shorter necks.
Both groups were adept ocean predators, but they likely filled slightly different ecological niches to avoid too much competition.
According to the Natural History Museum, over 100 valid species of plesiosaurs have been discovered so far. They reached their peak diversity during the Jurassic period, but managed to survive until the end-Cretaceous mass extinction 66 million years ago.
Ichthyosaurs – The “Fish Lizards” of the Ancient Seas
Ichthyosaurs were another successful group of predatory marine reptiles. As the name suggests (ichthys = “fish” in Greek), they had very fish-like anatomy, with a tuna-shaped body, crescent-shaped tail fin, and four flippers.
This allowed them to slice through the water at high speeds to catch fast-moving prey like squid and fish.
One ichthyosaur species, Temnodontosaurus, likely had the largest eyes of any known animal – up to 25 cm (10 inches) in diameter! This suggests they hunted in very deep, dark waters where good eyesight was at a premium.
Advanced eyes, combined with their speed and maneuverability, made ichthyosaurs formidable hunters.
According to recent estimates, there may have been over 100 ichthyosaur species throughout the Mesoizoic era before their extinction 90 million years ago. They were abundant for over 150 million years, playing key roles as mid-tier predators in marine ecosystems around the globe.
Mosasaurs – The Apex Marine Predators
Mosasaurs were the last major group of aquatic reptiles, dominating the Late Cretaceous oceans. They breathed air, but were well-adapted to a marine lifestyle with paddle-shaped limbs and long, muscular tails.
Some species grew to over 15 meters (50 feet) long, making mosasaurs larger than most marine predators today!
Mosasaurs were at the very top of the Cretaceous marine food chains. Analysis of fossil stomach contents and tooth marks on bones reveals that they preyed on other marine reptiles like plesiosaurs and turtles as well as large bony fish and ammonites.
Powerful jaws filled with conical teeth made short work of such challenging prey.
The fossil record indicates that mosasaur diversity increased steadily throughout the Late Cretaceous. According to UC Berkeley, there may have been up to 20 genera by the end of the period. Unfortunately for them, mosasaurs perished in the end-Cretaceous mass extinction along with their dinosaur contemporaries.
Amphibious Lifestyles and Habitats
Coastal and Estuary Dwelling Species
Many ancient reptiles were well-adapted to living in coastal regions and estuaries. These habitats provided ample food sources like fish, crustaceans, and aquatic plants. Species like Dakosaurus and Metriorhynchus had hydrodynamic bodies and powerful tails that enabled them to swiftly swim and hunt in shallow waters (1).
Fossil evidence indicates some even had webbed feet for extra propulsion. These creatures were the crocodiles of their time, stealthily ambushing prey in murky waters.
Other creatures like the peculiar-looking Tanystropheus inhabited tidal flats and coastlines. At over 16 feet long, these reptiles had extremely elongated necks comprising over half their body length – an adaptation for fishing (2). Their necks allowed them to strike at fish with lightning speed.
Remains show they swallowed fish whole, just like modern herons and cormorants. With access to abundant food sources, coastal species flourished in the Triassic and Jurassic periods.
Deep Diving Adaptations
While most reptiles stayed near the surface, a few remarkable species evolved to dive to ocean depths. Plesiosaurs were equipped with stiff torsos and paddle-like limbs that enabled marine mobility. Analysis of their bones and lungs shows they could plunge over 3,000 feet deep and withstand tremendous pressure (3).
Even more incredibly, fossil embryos indicate some plesiosaurs gave live birth far below the surface – a feat no modern reptile can achieve.
The immense Mosasaurus, reaching over 50 feet long, represents the pinnacle of reptilian marine adaptations. With a streamlined body, powerful flippers, and massive tail, Mosasaurus was built for speed and agility in open waters.
Its jaws bristled with sharp teeth, and it could dislocate its lower jaw to swallow large prey. Clearly, these predators dominated the Cretaceous seas (4).
Giving Birth in the Water
Live birth in water posed major challenges for air-breathing reptiles. Nevertheless, plesiosaurs and mosasaurs evolved innovative solutions. Analysis of fossils indicates pregnant females had widened pelvises to ease birthing (5).
Newborns likely had underdeveloped lungs but were buoyant, letting them reach the surface to breathe. Mothers may have assisted babies or kept them safely tucked under their flippers. Live birth meant young were mobile from day one – a huge advantage over reptiles constrained to laying eggs on land.
For ancient reptiles to undergo such a profound transition, live birth must have conferred considerable evolutionary benefits. Giving birth in the water enabled offspring to inhabit their natural aquatic environment immediately, avoiding the need to migrate from land.
It also allowed reptiles to exploit aquatic niches requiring deep, sustained diving – thereby spurring the evolution of magnificent creatures we are still unraveling today.
Evolutionary Relationships to Dinosaurs
Archosaurs – The Ancestral Connection
Modern amphibians like frogs share an ancestral link with dinosaurs through archosaurs, which were ancient reptiles that lived around 250 million years ago during the Triassic period. Archosaurs descended from amphibian-like tetrapod ancestors and eventually diverged into two major branches: crocodilians and avemetatarsalians.
The avemetatarsalians later split into pterosaurs and dinosaurs.
So while frogs seem very different from dinosaurs today, if we trace back the evolutionary tree far enough, they both descended from fish-like creatures with four limbs adapted for walking on land. This common ancestral connection means amphibians and dinosaurs share some basic skeletal features and body plans deep in their genetics, even if their modern forms have specialized and diversified dramatically.
Convergent Evolution of Fish-Like Forms
Beyond inheriting a tetrapod body baseline from archosaur ancestors, some amphibians and later dinosaurs both evolved fish-like aquatic forms independently through convergent evolution. For example, in the Triassic period, there were odd amphibian creatures like Eocyclotosaurus and Metoposaurus with flat heads and long bodies ideal for living in water.
Similarly, many dinosaur groups developed semi-aquatic forms with paddle-like limbs and streamlined profiles that were well-adapted to rivers and shorelines. The ostrich-like Spinosaurus had a crocodile-esque snout and fins to aid swimming.
And duck-billed dinosaurs like Parasaurolophus may have used their distinctive head crests as snorkels!
So while amphibians and later dinosaurs took different evolutionary routes, they both faced selective pressures that led some lineages to adapt a primarily water-based lifestyle by reverting to the fish-like forms of their primeval ancestors.
The Extinction of Aquatic Reptiles
Climate Changes and Habitat Loss
The extinction of aquatic reptiles like ichthyosaurs, plesiosaurs, and mosasaurs at the end of the Cretaceous period around 66 million years ago was likely caused by a combination of climate changes and habitat loss (McLachlan, 2019).
The greenhouse conditions that allowed these marine reptiles to thrive started to decline as atmospheric carbon dioxide levels dropped. This cooling trend reduced shallow ocean temperatures and impacted coral reefs and coastlines that were important ecosystems for many species (Fischer et al., 2016).
In addition, the breakup of continents and associated marine regression saw substantial reductions in the extent of shallow epicontinental seas that had high biodiversity and were breeding grounds for many aquatic reptiles.
The lowering sea levels shrank habitats and food sources, making survival more difficult, especially for larger marine reptiles that required abundant prey. Smaller populations were more vulnerable to extinction when faced with sudden events like asteroid impacts or volcanic eruptions.
Some adaptable species survived by moving to deeper waters, but many iconic aquatic reptiles disappeared entirely (Foffa et al., 2019).
Competition with Early Whales
Increased competition from early whales (cetaceans) in the late Cretaceous also contributed to the demise of marine reptiles. As mammal species diversified and occupied similar ecological niches, they may have outcompeted reptiles for shared food sources like ammonites and belemnites (Kelley & Pyenson, 2015).
Whales evolved quickly and had advantages like warm-blooded metabolisms, live births, and lactation that allowed them to adapt better to changing marine environments (Lindgren et al., 2011). Their high intelligence and predatory skills made them formidable rivals.
While there was likely some direct competition leading to displacement or predation, the greater threat came from whales being better suited to the cooling oceans. They were able to conserve heat, migrate to remain in optimal habitats, and give birth to live young that needed less parental care.
Marine reptiles laid eggs on land that were vulnerable, especially with lower sea levels. Their slow metabolism and cold-blood made it hard to thrive as temperatures declined. Ultimately, cetaceans assumed the ecological roles of aquatic reptiles, though it took millions of years for whale and dolphin biodiversity to reach Cretaceous reptile levels.
Conclusion
While not technically dinosaurs themselves, the aquatic reptiles of the Mesozoic Era like plesiosaurs and ichthyosaurs evolved remarkably amphibious lifestyles and occupied many of the same ecological niches that we associate with dinosaurs.
With their extraordinary adaptations to life in the water, it’s easy to see how they could be considered ‘honorary dinosaurs’ that ruled the prehistoric seas while their land-dwelling cousins dominated terrestrial habitats.
Even after the extinction of the dinosaurs, the age of reptiles lasted for many millions more years due to these successful aquatic species. The fossils of these magnificent swimming archosaurs provide a glimpse into an alien world long gone, but one that continues to inspire awe and wonder.
