Plesiosauria: A Comprehensive Guide to Mesozoic Marine Giants

Leave a comment / / Updated on: 24th September 2023


The Earth is home to millions of unique species, each with various features that make them stand out.

Because of how vast our planet is, it is almost impossible for experts to discover all these species.

However, the range of species currently existing does not compare to that of prehistoric Earth before the evolution of humans.

Before humans lived and occupied most of the Earth, the continents, as we know them, were all joined.

These joint areas were called supercontinents and were home to many ancient species that ruled our planet in those days.

Supercontinent | Ianm35 via Getty Images

Because of how vast these past eras were, experts split them, each focusing on different species and events that spurred evolution as we now know it.

One of the most popular eras is the Mesozoic Era, called the “Age of Reptiles.”

As this name implies, this era is famous for its role in the evolution of many reptile groups, especially dinosaurs.

Contrary to popular opinion, the Mesozoic Era does not focus solely on dinosaurs. This era is also famous for the evolution of other terrestrial and aerial creatures.

The Mesozoic Era is also known for its role in the development of marine reptiles.

Gage Beasley's Prehistoric Shirt Collection
Gage Beasley’s Prehistoric Shirt Collection
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Gage Beasley’s Prehistoric Plush Collection

The growth of marine reptiles is attributed to a combination of factors, including changes in sea levels, climate, availability of ecological niches, and the rise of aquatic prey.

The earliest marine reptiles emerged during the Triassic Period, the first period of the Mesozoic Era. One of the most fascinating groups of marine reptiles was the Plesiosauria, first appearing in the latest Triassic Period about 203 million years ago.

The discovery of this prehistoric group dates as far back as the early 17th century. During this time, initial findings were described by Richard Verstegen, and this was the first of many descriptions of this group throughout the 17th century.

Discoveries and depictions of the Plesiosauria group continued throughout the 17th and 18th centuries, and by the 19th century, specifically in 1835, Henri Marie Ducrotay de Blainville named the order Plesiosauria itself.

Like many other marine reptiles, the defining feature of the Plesiosauria group is its aquatic adaptations.

Hawkins’ demonic plesiosaurs battling other sea-monsters in primordial darkness | John Martin via Wikipedia

Their unique adaptations to oceanic life allowed them to flourish for millions of years, with some reaching enormous sizes and playing vital roles in prehistoric marine ecosystems.

The study of plesiosaurs has been an ongoing quest for paleontologists, who have dedicated countless hours to unearthing fossils, studying anatomical features, and reconstructing their biology and behavior.

Researchers have uncovered innumerable tales about their biology, behavior, and evolutionary connections via the diligent study of their remains and detailed intricacies.

This article focuses on the features of this prehistoric group, their distribution in ancient habitats, their evolutionary history, and other facts that help us understand what their life was like in the Mesozoic Era.

Characteristics that Define the Plesiosauria Group

1. Body structure

Plesiosaur Body Structure | ilbusca via Getty Images

Generally, the Plesiosauria group had a torso characterized by a broad and barrel-shaped structure, providing stability and buoyancy in the water.

This design allowed for efficient swimming and facilitated the creature’s predatory lifestyle.

These creatures had wide ribcages, with ribs tightly interlocked, providing structural support and rigidity to the torso.

The presence of gastralia, or abdominal ribs, further enhanced stability during swimming and contributed to the overall hydrodynamic profile.

One part of their body that experts found intriguing was the neck. Generally, initial discoveries caused these experts to believe these creatures had elongated necks.

However, further research revealed the group was divided into Plesiosauromorphs, which had elongated necks and small heads, and Pliosauromorphs, with short necks and elongated heads.

Rhomaleosaurus cramptoni (Pliosauromorph), Natural History Museum. | Niki Odolphie via Flickr (CC BY 2.0)

The vertebrae in the neck of Plesiosaurs were highly flexible, allowing for a wide range of movement and precise control.

This adaptation facilitated efficient hunting and enabled the Plesiosauria to snatch prey with remarkable agility.

The neck of Plesiosaurs also had powerful muscles, allowing them to manipulate and extend their necks swiftly. These muscles helped capture prey, maneuver in the water, and maintain stability.

2. Limb adaptations 

Seeleyosaurus with a tail fin | Dmitry Bogdanov via Wikipedia (CC BY 3.0)

This group possessed remarkable limb adaptations that allowed them to navigate the ancient seas with agility and efficiency.

Their limb structure followed the basic tetrapod blueprint while being modified for an aquatic lifestyle.

The most notable adaptation was their specialized flippers, which were powerful propellers.

These flippers were modified forelimbs, elongated into paddle-like structures, which aided in maneuvering and propelling through the water.

Nothosaurs still had functional legs. | Dmitry Bogdanov via Wikipedia (CC BY 3.0)

The flippers were composed of several rows of bones that supported a web of skin, similar to the wings of a bat.

The arrangement of bones in the flippers varied across different genera, resulting in subtle differences in flipper shape and function.

The limb adaptations of the Plesiosauria also contributed to their exceptional maneuverability.

With their modified limbs, they could execute tight turns, pursue prey, and evade predators in the underwater realm.

Their unique limb structures, specialized flippers, powerful musculature, and skeletal reinforcements were all essential in their successful adaptation to life in prehistoric oceans.

3. Skull and dentition 

Cast of the “Puntledge River elasmosaur”, Canadian Museum of Nature | D. Gordon E. Robertson via Wikipedia (CC BY-SA 3.0)

The variation in the skulls of the Plesiosaurs suggests that they adapted to different eating strategies and ecological niches.

The skull’s structure and form varied significantly but were generally modest in proportion to the creature’s physical size.

While some species had more extensive, robust heads, others had elongated, narrow skulls.

Their skull morphology was further varied by cranial crests, ridges, or other bone ornaments.

The jaw mechanism of Plesiosaurs played a crucial role in capturing and manipulating their prey.

The lower jaws were composed of two halves, hinged at the back, which allowed for a wide gape and increased prey capture efficiency.

Cast of “Plesiosaurus” macrocephalus | FunkMonk via Wikipedia (CC BY-SA 3.0)

The jaw joint was flexible, enabling the reptile to open its mouth widely and engulf relatively large prey items.

The number and shape of the teeth varied greatly.

Some types had many teeth that resembled needles.

Enormous conical teeth with a round or oval cross-section were seen in most Plesiosaurs.

The teeth had vertically wrinkled enamel, were seated in dental sockets, and lacked a genuine carina or cutting edge.

For catching food, certain species’ front teeth were noticeably longer.

Major Organism Groups of the Plesiosauria Order

1. Pliosauroidea

Macroplata, part of the Pliosauroidea | Nobu Tamura via Spinops (CC BY 2.5)

The Pliosauroidea, a primary group within the taxonomic order Plesiosauria, were a diverse group of marine reptiles characterized by their large size, powerful forelimbs, and short necks compared to other plesiosaurs.

They belonged to the larger Sauropterygia clade and existed throughout the Mesozoic Era, from the Early Jurassic to the Late Cretaceous period.

Pliosauroids possessed a streamlined body with powerful flippers, enabling them to swim swiftly through the water.

Attenborosaurus | Nobu Tamura via Spinops (CC BY 2.5)

Their heads were elongated, housing rows of sharp teeth that helped capture and devour prey.

Their strong jaws and muscular necks allowed for efficient biting and tearing of flesh, making them formidable hunters in the ancient seas.

The structure of their teeth suggested adaptations for grasping and impaling slippery prey, while their powerful jaws allowed them to deliver devastating bites.

Some evidence indicates that pliosauroids may have engaged in pursuit predation, employing bursts of speed to chase down their prey.

2. Plesiosauroidea

Plesiosaurus from prehistoric times | Warpaintcobra via Getty Images

Plesiosauroidea, another group within the order Plesiosauria, comprised a diverse array of marine reptiles that ruled the ancient seas during the Mesozoic Era.

These remarkable creatures thrived from the Jurassic to the Cretaceous period, roughly over 100 million years, and possessed unique features, including elongated necks, streamlined bodies, and specialized flippers.

The Plesiosauroidea were distinguished from other marine reptiles by their distinctive body plans.

They had a long neck with several vertebrae and a small body containing their internal organs.

Their flippers, which served as the primary means of locomotion, were adapted for maneuverability and efficient swimming.

They could travel the ancient oceans with incredible grace and agility because of their changes.

3. Cryptoclidia

Life restoration of Muraenosaurus | Dmitry Bogdanov via Wikipedia (CC BY 3.0)

Cryptoclidia, a suborder of plesiosaurs that thrived during the Mesozoic Era, presents a fascinating group of marine reptiles with unique characteristics.

These creatures, which lived roughly 180 to 70 million years ago, exhibited a distinct combination of fish-like and reptilian features.

Unlike other plesiosaur groups, Cryptoclidians had relatively short necks, typically composed of 19 to 25 vertebrae.

Their bodies were robust and streamlined, equipped with paddle-like flippers that facilitated agile swimming.

Their unique blend of fish-like and reptilian traits and their role as transitional forms within the Plesiosauria group makes them a subject of ongoing research and fascination.

4. Polycotylidae

Trinacromerum bentonianum from the Late Cretaceous of Kansas | Nobu Tamura via Spinops (CC BY 2.5)

The Polycotylidae were a fascinating family of marine reptiles that lived during the Late Cretaceous period, around 100 to 85 million years ago.

Belonging to the order Plesiosauria, these creatures were well-adapted to an aquatic lifestyle.

Unlike their long-necked relatives, they had relatively shorter necks.

Their limbs had evolved into paddle-shaped flippers, allowing them to navigate the water with agility and speed.

One of the most distinctive features of the Polycotylidae was their large, powerful jaws lined with sharp teeth.

Their teeth were conical or peg-like, ideal for seizing and gripping prey.

Life reconstruction of Eopolycotylus rankini | Connor Ashbridge via Wikipedia (CC BY-SA 4.0)

This characteristic dental arrangement suggests a carnivorous diet, consisting mainly of fish, cephalopods, and other marine organisms.

Their powerful jaws, coupled with sharp teeth, were ideal for snatching and holding onto prey.

Fossil evidence indicates that they may have engaged in opportunistic feeding, exploiting available food sources in their habitat.

Notable Examples of Organisms within the Plesiosauria Order

1. Plesiosaurus

Gage Beasley Prehistoric's Plesiosaurus Concept
Gage Beasley Prehistoric’s Plesiosaurus Concept

Plesiosaurus is a genus within the Plesiosauria order, which lived from the Early Jurassic to the Late Cretaceous period, approximately 195 to 66 million years ago.

Plesiosaurus had a distinct body structure that set it apart from other marine reptiles of its time.

It had a relatively small head with sharp teeth for catching prey such as fish and cephalopods.

The creature’s head was narrow, with its eye sockets placed halfway along the length of the skull.

One of the most fascinating features of this creature was its elongated neck which had approximately 40 cervical vertebrae, 42 in some.

Fossil evidence suggests that Plesiosaurus inhabited different marine environments, including open oceans, coastal waters, and lagoons.

2. Elasmosaurus

Gage Beasley Prehistoric’s Elasmosaurus Concept

With an estimated length of 43-45 feet, the Elasmosaurus is a marine creature within the Plesiosauria order with one of the longest necks on record, estimated at around 23 feet, more than half its entire length.

This exceptional adaptation allowed the reptile to explore a wide range of depths and reach prey that may have been inaccessible to other marine predators.

Elasmosaurus was a carnivorous predator, feeding primarily on fish, cephalopods, and other small aquatic creatures.

It would have used its sharp teeth to catch and hold onto its prey before swallowing it whole.

Its elongated neck provided a distinct advantage when hunting, allowing it to launch surprise attacks on unsuspecting prey from a greater distance than other predators.

3. Cryptoclidus

Life reconstruction of C. eurymerus | Nobu Tamura via Spinops (CC BY-SA 4.0)

Cryptoclidus is a genus of plesiosaur reptiles from the Middle Jurassic period.

This creature was initially discovered in 19th-century England, and more fossils have been found across the country, France, Northern Russia, and Cuba.

It was named Cryptoclidus, which means “hidden clavicle,” referring to the internal bone structure that supported its shoulders.

This creature’s body was medium-sized, averaging 13 feet.

Based on fossil evidence, Cryptoclidus is believed to have been a piscivorous predator, primarily feeding on fish.

The head’s and teeth’s delicate structure forbids any kind of fighting with prey and points to a diet high in tiny, soft-bodied creatures like squid and shoaling fish.

4. Muraenosaurus

Side view of M. leedsi | Nobu Tamura via Spinops (CC BY 2.5)

Muraenosaurus is a genus of marine reptiles that thrived in the Late Cretaceous period, approximately 70 to 65 million years ago.

Muraenosaurus was a medium-sized plesiosaur, with the largest specimen measuring up to 17 feet long.

Its limbs had evolved into paddle-like structures, facilitating agile movement through the water.

The jaws of Muraenosaurus contained sharp teeth well-suited for capturing and consuming its prey.

Fossil evidence suggests that Muraenosaurus inhabited oceans that covered present-day Europe, particularly regions including England, France, and Germany.

These areas were part of the ancient Tethys Sea, which provided an ideal habitat for numerous marine species during the Late Cretaceous period.

There have also been discoveries in the Wyoming area of the USA.

Feeding Strategies and Behavior of the Plesiosauria Order

Feeding Strategies

Reconstructed Elasmosaurus skull and neck, North American Museum of Ancient Life | Zach Tirrell via Flickr (CC BY-SA 2.0)

This genus displayed a wide range of dentition adaptations, reflecting their diverse dietary preferences.

Some species, like Elasmosaurus, possessed numerous small, needle-like teeth ideal for catching and holding slippery prey, such as fish and cephalopods.

Others had long, conical teeth designed for grasping larger prey.

The presence of heterodont dentition in some species suggests niche partitioning, with different Plesiosauria species occupying various trophic levels within the marine ecosystem.

According to experts, many Plesiosauria likely employed suction feeding as a primary feeding strategy.

By rapidly expanding their pharyngeal cavity, these reptiles created a powerful vacuum, which aided in capturing prey.

This technique was particularly effective for targeting small and agile organisms, like fish and squid.

The long necks of these creatures allowed them to strike at a considerable distance, enabling them to surprise their prey with lightning-fast lunges.

A school of fish try to get away from a Liopleurodon Plesiosaur near a tropical reef. | CoreyFord via Getty Images

On the other hand, some species in this order used their long necks to adopt a filter-feeding lifestyle similar to today’s modern-day filter-feeding marine animals like baleen whales.

By opening their mouths and swimming through schools of planktonic organisms, these creatures could strain out tiny prey using an array of slender, comb-like teeth or keratinous rakers lining their jaws.

While many Plesiosauria targeted smaller fish, some larger species were apex predators and practiced macrophagy.

These gigantic marine reptiles possessed robust jaws lined with massive, interlocking teeth, suggesting they fed on large marine vertebrates.


Restored skeleton of Plesiosaurus | Kim Alaniz via Flickr (CC BY 2.0)

The Plesiosauria group encompasses a wide range of species, varying in size, body shape, and ecological adaptations.

However, based on fossil evidence, it appears that most Plesiosauria species led solitary lives. Fossil discoveries typically reveal isolated individuals rather than groups.

This solitary lifestyle is commonly observed in marine predators, as competition for resources, such as food and mates, often leads to individuals establishing and defending their territories.

Fossilized embryos found within adult individuals suggest that these reptiles gave birth to live young, akin to modern-day marine mammals.

This reproductive mode implies the existence of mating rituals and social interactions related to courtship.

Polycotylus with fetus. | Dawn Pedersen via Flickr (CC BY 2.0)

While the specifics of Plesiosauria mating rituals remain speculative due to the scarcity of direct fossil evidence, some hypotheses suggest that male Plesiosauria may have engaged in territorial displays or combat to establish dominance and gain access to females during breeding seasons.

This behavior might have involved neck and limb movements to impress potential mates and deter rivals.

Although solitary lifestyles were prevalent among Plesiosauria, some experts propose the possibility of occasional group behavior, particularly during cooperative hunting.

Modern marine predators, such as orcas and dolphins, employ group hunting strategies to capture elusive prey.

Similarly, it is plausible that certain Plesiosauria species might have engaged in cooperative hunting to increase their hunting success.

Dolichorhynchops was a aquatic carnivorous Plesiosaur that lived in the seas of the Cretaceous Period. | CoreyFord via Getty Images

The group could have corralled schools of fish or surrounded larger prey, effectively increasing their chances of capturing prey that would be difficult to subdue individually.

While the solitary nature of Plesiosauria suggests limited social structures, there is a likelihood that social hierarchies existed within some species.

Dominance hierarchies are common among modern reptiles, where individuals establish a rank order based on factors like size, strength, or aggressive displays.

If Plesiosauria engaged in territorial behaviors or mating displays, it is plausible that dominant individuals occupied preferred territories or had priority access to mating opportunities.

However, further fossil discoveries and analyses are needed to provide more conclusive evidence on the existence and nature of social structures within Plesiosauria.

Paleobiogeography of the Plesiosauria Order

Plesiosaur skeleton of Meyerasaurus in the Museum am Löwentor, Stuttgart, seen from below | Ra’ike via Wikipedia (CC BY-SA 3.0)

Generally, the Plesiosauria group existed in the Mesozoic Era, approximately 252 to 66 million years ago, but their origins can be traced back to the Triassic Period (252-201 million years ago).

The early diversity of Plesiosauria was characterized by two distinct groups: the long-necked, small-headed Plesiosauromorpha and the short-necked, large-headed Pliosauromorphs.

Fossil evidence suggests that these early plesiosaurs had a widespread distribution across the ancient Tethys Sea and Panthalassa Ocean.

During the Jurassic Period (201-145 million years ago), Plesiosauria experienced significant diversification and occupied a variety of marine environments around the world.

The breakup of the supercontinent Pangaea into separate landmasses led to the opening of new oceanic basins, creating opportunities for plesiosaurs to disperse and colonize new regions.

First published plesiosaur skeleton, 1719 | William Stukely via Wikipedia

Plesiosauria fossils from the Jurassic have been discovered on nearly every continent, including Europe, North America, South America, Africa, Antarctica, and Asia.

The Cretaceous Period (145-66 million years ago) witnessed further changes in the paleobiogeography of Plesiosauria.

By this time, the continents had begun to drift apart, resulting in the formation of distinct oceanic regions.

Plesiosaurs of the Late Cretaceous exhibit a wide array of body forms and ecomorphological adaptations, indicating their occupation of various ecological niches.

During the Late Cretaceous, elasmosaurids, characterized by their extremely long necks and small heads, became the dominant plesiosaur group.

Fossils of elasmosaurids have been found primarily in regions that were once part of the Western Interior Seaway, a vast inland sea that stretched across North America.

Elasmosaurus | Daniel Eskridge via Getty Images

Elasmosaurids also inhabited other parts of the world, including South America, Europe, and Antarctica.

Generally, creatures in the Plesiosauria group were marine reptiles, but each species preferred specific habitats.

Some plesiosaur species, particularly the pliosaurids, favored coastal regions and shallow marine environments.

These areas provided abundant food sources, including fish and cephalopods, which plesiosaurs preyed upon.

Coastal shallows and lagoons also offered protection and breeding grounds, promoting the survival of the young.

Other plesiosaur species, such as the elasmosaurids, were well-suited for the open ocean.

With their long necks and streamlined bodies, they could effectively search for prey in deeper waters.

Elasmosaurids likely occupied niches similar to today’s deep-diving marine mammals, hunting for fish and squid in offshore habitats. 

Three Plesiosaurus dinosaurs migrate along with a school of fish to warmer Jurassic seas. | CoreyFord via Getty Images

The distribution of plesiosaur fossils provides crucial insights into their preferred habitats.

Fossils found in sedimentary rocks associated with marine deposits suggest their affinity for saltwater environments.

Conversely, fossils discovered in freshwater or brackish sediments indicate their ability to explore estuaries or river systems.

Paleontologists analyze sedimentary facies and depositional environments surrounding plesiosaur fossils to reconstruct their habitats.

By examining the presence of marine fauna, such as ammonites or marine reptiles, researchers can infer the marine nature of the plesiosaur’s environment.

Similarly, the identification of freshwater or brackish-water species indicates their occupation of estuarine or riverine settings.


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