An Ultimate Guide to Shonisaurus: The Shoshone Lizard

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NameShonisaurusDietCarnivorous or Piscivorous
Name MeaningThe Shoshone LizardHeightN/A
PronunciationSHOH-ni-SAW-rusLength13.5–15 meters (44–49 feet)
EraMesozoicTriassic PeriodWeight29.7 metric tons (65,000 pounds)
ClassificationIchthyopterygia, Ichthyosauria & ShastasauridaeLocationUnited States (North America)

Shonisaurus Pictures

3D illustration of Shonisaurus isolated in a white background
3D illustration of Shonisaurus isolated in a white background | Elenarts108 via iStock

The Shonisaurus

Gage Beasley Prehistoric's Shonisaurus Concept
Gage Beasley Prehistoric’s Shonisaurus Concept

Shonisaurus was a massive marine reptile that lived during the Triassic Period about 237 million years ago. 

It was an ichthyosaur, a relative of the more famous Ichthyosaurus

Members of this family are known for their fish-shaped bodies even though they were air-breathing reptiles.

Shonisaurus is particularly notable as one of the largest members of this family, with a length of about 50 feet, measured from snout to tail. 

It is also one of the largest prehistoric animals ever discovered. 

Shonisaurus lived in North America, with most of its fossils recovered from present-day Nevada, USA. 

Pencil drawing of shonisaurus
Pencil drawing of shonisaurus | ArthurWeasley via Wikimedia Commons CCA 2.5

The name of this prehistoric reptile translates as “Lizard of the Shoshone Mountains,” a reference to the location where the first fossil of this reptile was recovered from.  

The first fossil site had an assemblage of up to 40 Shonisaurus individuals preserved together in the same fossil bed. 

Since its initial discovery, many aspects of the Shonisaurus’ life have been reviewed as new details emerge. 

In this article, we present an overview of some of the fascinating details about this prehistoric reptile, including what it looked like, where it lived, and how it lived back in the Triassic Period. 

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Gage Beasley’s Prehistoric Plush Collection

Physical Characteristics

Gage Beasley Prehistoric's Shonisaurus Size Comparison Chart
Gage Beasley Prehistoric’s Shonisaurus Size Comparison Chart

Like other ichthyosaurs, Shonisaurus was a large reptile with an elongated, fusiform body.

Despite being a reptile, the fish-like creature looked a lot like modern-day sharks or dolphins. 

This streamlined body shape was an adaptation for efficient locomotion through the water. 

Shonisaurus was one of the largest ichthyosaurs ever discovered. 

This massive prehistoric reptile reached an average length of about 13.5 to 15 meters (44–49 feet) and weighed as much as 29.7 tons.

One possible species of this reptile was even bigger. 

Shonisaurus sikanniensis was up to 21 meters (69 feet) in length and weighed about 81.5 metric tons. 

This makes it the largest marine reptile of all time. 

Illustration of shastasaurus
Illustration of shastasaurus | DiBgd via Wikipedia CC BY-SA 4.0

However, the actual identity of this Triassic creature is still being debated as some scientists think it was a Shastasaurus (another Triassic ichthyosaur) and not a Shonisarus species. 

Shonisaurus had a large rounded torso, with forelimbs and hindlimbs modified into flippers.

These flippers were short compared to the rest of the reptile’s body but were still longer and narrower compared to that of other ichthyosaurs. 

Most old depictions of this Shonisaurus show that it had a dorsal fin. 

However, this has since been discounted since no skeletal evidence has been found for this. 

If it had dorsal fins at all, it may have been formed by connective tissue and not bones, which would explain its absence in the fossil record. 

Shonisaurus had a long snout and robust, conical teeth
Shonisaurus had a long snout and robust, conical teeth | MR1805 via iStock

Shonisaurus had a very long snout and is often reported to have possessed numerous sharp teeth. 

However, more recent findings suggest that the teeth were only present in the smallest juvenile specimens but not in adult forms. 

Habitat and Distribution

Shonisaurus lived during the Late Triassic Period, approximately 230 million years ago. 

Its fossil remains have been primarily found in what is now North America. 

The Shonisaurus lived in deep oceanic environments. 

The reptile’s eyes were exceptionally large, even for its immense body size. 

This adaptation suggests that the Shonisaurus spent considerable time in very deep waters. 

The massive size of this reptile also supports the idea that it was a deep-sea dweller. 

Shonisaurus would have inhabited ancient oceans, preying on marine life
Shonisaurus would have inhabited ancient oceans, preying on marine life | MR1805 via iStock

Also, given its size, this reptile would not have survived in shallow water. 

During the Triassic Period, the continents had a completely different configuration which affected the distribution of the world’s oceans. 

All the continents were connected to form a single landmass known as Pangea. 

Pangea began to break up about 200 million years ago, but the ocean spreading that led to the continent’s break up began earlier in the Triassic. 

This created suitable habitats for various marine reptiles, including the Shonisaurus

Earth’s oceans were generally warmer during the Triassic than today’s. 

There were no ice caps at the poles, and the sea level was higher. 

Behavior and Diet

A Shonisaurus Ichthyosaur stabs a prehistoric fish trying to get away from these predators in a Triassic ocean
A Shonisaurus Ichthyosaur stabs a prehistoric fish trying to get away from these predators in a Triassic ocean | CoreyFord via iStock

Shonisaurus had a bulkier build, with a more streamlined body compared to the tuna-like appearance of the ichthyosaurs that evolved after it. 

Shonisaurus also lacked the vertical, crescent-like tail fluke present in other ichthyosaurs, suggesting that its method of locomotion in the water differed from theirs. 

Instead, its tail flipper was thick and two-pronged.

The difference in the tail anatomy of this reptile also suggests that it swam with a sideways motion of its body instead of swimming like a dolphin. 

However, it was capable of swimming up and down as well. 

One key piece of evidence for this is the size of the limbs of this marine reptile.

All four limbs were of the same size. 

This extinct ichthyosaurs primarily fed on cephalopods and other marine animals
This extinct ichthyosaurs primarily fed on cephalopods and other marine animals | Vac1 via iStock

This is different from what is observed in other marine creatures whose front limbs tend to be larger to counter the downward effect of their tail while swimming. 

All four limbs of the Shonisaurus being the same size meant this reptile could control its motion in the vertical plane more effectively. 

This means it could pitch its body to swim up or down quickly, which makes sense when you consider the reptile’s diet. 

Shonisaurus was a carnivorous predator that preyed on squid and other soft-bodied prey. 

These invertebrates tend to swim up and down in the water column instead of side-to-side. 

An ability to pitch its body to swim upward or downward would have been effective for catching this type of prey. 

Shonisaurus hunting for food in the Triassic seas
Shonisaurus hunting for food in the Triassic seas | MR1805 via iStock

Evidence from the coprolites (fossilized poop) of the Shonisaurus also seems to support this theory. 

The coprolites contained fossilized squids, belemnites, and soft fish that were a part of the Shonisaurus’ diet. 

Given the relatively small size of these prey items, Shonisaurus would have had to eat large quantities of them to survive.  

Adult Shonisaurus individuals didn’t have teeth in their long snouts. 

Their feeding habit was probably similar to that of modern basking sharks or whale sharks.

This would involve opening their mouth to swallow small prey swimming in the water column. 

Shonisaurus scouring the Triassic ocean
Shonisaurus scouring the Triassic ocean | MR1805 via iStock

Since it was a reptile, Shonisaurus was probably also capable of opening its jaws wide to gulp in large amounts of prey. 

The social behavior of Shonisaurus is not well-documented, but it’s plausible that these marine reptiles exhibited some level of social interaction, especially during mating and nesting seasons.

The discovery of a large fossil bed with at least 40 individuals buried together supports the idea that Shonisaurus congregated like this at least during certain times.  

Congregating like this facilitated mating opportunities and also protected against potential threats while giving birth in the water. 

Life Cycle

Although it was a reptile, Shonisaurus was fully adapted to an aquatic lifestyle. 

This means their entire life cycle, from mating to birth, took place in the water. 

Model of ichthyosaur Shonisaurus
Model of ichthyosaur Shonisaurus | image via Wikimedia Commons CC A-SA 4.0

After mating, Shonisaurus females would have carried their young in their bodies. 

They were either viviparous or ovoviviparous. 

Either way, the Shonisaurus juveniles were born young by the mother instead of hatching from eggs. 

The juveniles were born relatively well-developed and would have been capable of swimming and living in the marine environment where they were born right away. 

Scientists have interpreted the fossil bed discovered in Nevada as a possible birthing ground for Shonisaurus females. 

Like some species of modern whales, Shonisaurus females probably congregated near the Triassic coastline to give birth to their young. 

A reconstruction of Shonisaurus popularis about to breach
A reconstruction of Shonisaurus popularis about to breach | PaleoEquii via Wikimedia Commons CC A-SA 4.0

This area would have been deep enough for the Shonisaurus to survive but also shallow enough to keep large prey away and protect the young. 

Young Shonisaurus individuals had a jaw lined with several sharp teeth, suggesting that they had a diet completely different from the adult forms. 

Their teeth probably helped to protect them from predators too. 

As they grew, their diet would gradually change to soft-bodied prey in their ecosystem. 

Evolution and History

Shonisaurus is a member of the ichthyosaur family, a diverse and successful group of marine reptiles that evolved during the Mesozoic Era.

Ichthyosaurs are not directly related to modern reptiles like crocodiles or lizards. 

The name Shonisaurus is derived from the Shoshone Mountains in Nevada, where its remains were first discovered
The name Shonisaurus is derived from the Shoshone Mountains in Nevada, where its remains were first discovered | MR1805 via iStock

They’re also not related to the other groups of marine reptiles, such as the plesiosaurs and pliosaurs that also lived during the Mesozoic. 

Instead, they represent a separate reptile lineage that adapted to an aquatic lifestyle sometime during the Triassic about 250 million years ago. 

Shonisaurus was a primitive ichthyosaur, so it did not have any of the features seen in later ichthyosaurs. 

For instance, no evidence of a dorsal flipper was found for this reptile, even though it was present in younger ichthyosaurs. 

The flipper was either completely absent or was made up of soft cartilages that became ossified in later species. 

A fully-developed tail fluke was also absent in this reptile. 

3D rendering of a shonisaurus isolated on white background
3D rendering of a shonisaurus isolated on white background | Vac1 via iStock

Consequently, it lacked the distinct tuna-like profile seen in younger ichthyosaurs and would have moved differently. 

The lower flippers of the Shonisaurus, which evolved from the walking limbs of their ancestors, were relatively narrower and longer than those of its relatives. 

These are some of the features that changed as the ichthyosaurs continued to evolve and develop more efficient swimming abilities to become better suited to a fully aquatic lifestyle. 

Interactions With Other Species

Shonisaurus was one of the largest predators in the aquatic ecosystem of the Triassic Period. 

However, considering how it fed, this reptile was not an apex predator. 

Its diet mainly included smaller fish, squid, and other small marine invertebrates. 

Shonisaurus was attacked by nothosaurus
Shonisaurus was attacked by nothosaurus | CoreyFord via iStock

Shonisarus was an efficient swimmer, well adapted to hunting these small prey. 

However, the absence of teeth in its jaws meant this reptile could not have preyed on the larger animals in the marine food web of the Triassic. 

Even though it was not an apex predator, being such a large animal did come with some advantages. 

The Shonisaurus had only a few natural predators that could have preyed on it within its ecosystem.

Only large marine reptiles, such as other ichthyosaurs or some types of marine crocodile-like reptiles, could have posed a threat to the Shonisaurus

Juveniles and smaller individuals were the most at risk from these threats, but larger individuals were probably unaffected. 

Cultural Significance

Illustration of a Shonisaurus ichthyosaur
Illustration of a Shonisaurus ichthyosaur | SEBASTIAN KAULITZKI via GettyImages

Shonisaurus is one of the best-known prehistoric reptiles. 

The popularity of this creature is partly due to its size and also due to the abundance of fossil remains. 

Although it is not well-known to the general public due to little representation in popular media, this reptile is very popular in the scientific community for various reasons. 

The well-preserved fossils of this reptile have provided valuable insights into the adaptations, behaviors, and ecological roles of an ancient marine reptile that lived so long ago in the Triassic Period. 

They also contain clues of an ancient ecosystem considerably different from present-day conditions. 

The Shonisaurus’ status as a basal ichthyosaur also makes it an important genus in the family. 

By studying Shonisaurus, we can learn a lot about its relatives and trace their evolutionary journey from primitive reptiles to the more advanced forms that dominated the aquatic ecosystem during the Late Triassic. 


Shonisaurus was one of the largest marine reptiles ever discovered. 

It was a massive primitive ichthyosaur that lived in the deep oceanic environment of the Middle Triassic Period. 

Being a basal ichthyosaur, Shonisaurus was different from many of the later members of the genus in its general appearance and adaptations. 

The difference in the tail structure and the absence of dorsal fins were among the most notable adaptations observed in this reptile. 

For such a massive reptile, Shonisaurus didn’t have a very remarkable diet. 

Adult forms of this reptile did not have teeth in their mouth, suggesting that they preyed on small invertebrates and small vertebrates in their ecosystem. 

But this isn’t unusual since some of the largest marine creatures today aren’t voracious predators either. 


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