|Name Meaning||Middle Lizard||Height||N/A|
|Pronunciation||Mes-oh-sore-us||Length||1 meter (3.3 feet)|
|Era||Paleozoic – Permian Period||Weight||10–20 pounds|
|Classification||Reptilia, Mesosauria, Mesosauridae||Location||South America and South Africa|
Mesosaurus is a genus of lizard-like reptiles that lived during the Permian Period, about 290 million years ago.
Fossils of this reptile have been discovered in South America and South Africa, suggesting that the animal lived during a period when both continents were part of the supercontinent known as Pangea.
Mesosaurus had many features that confirm that it probably lived an aquatic lifestyle.
Based on these features, this reptile is considered one of the first reptiles to return to an aquatic ecosystem.
Mesosaurus lived in the freshwater lakes and rivers of the Permian Period, but it may have lived in hypersaline environments, analogous to full-marine ecosystems.
Mesosaurus was discovered in South Africa in the 1830s but wasn’t fully identified or described until the 1860s.
Since then, several more fossils of this reptile have been discovered across the two continents, providing scientists with numerous information about how it lived.
In this article, we’ll discuss some of the facts that we know about this fascinating animal.
Mesosaurus had a typical lizard-like body.
It looked a lot like a small, prehistoric crocodile, complete with a slender body and a long tail.
The Mesosaurus’ tail probably had a fin similar to that of many tetrapod amphibians that evolved earlier in the Paleozoic.
This reptile had four well-developed limbs, with each one having five digits.
The digits were webbed and paddle-shaped, which would have made the Mesosaurus an excellent swimmer.
Mesosaurus was a relatively small reptile, with a typical adult measuring about one meter (3.3 feet) in length.
The average weight of this reptile has been estimated to be about 10 to 20 pounds.
Mesosaurus had a small jaw with an elongated skull.
The jaws feature sharp filter-like teeth positioned at the tips and angled outwards in the reptile’s mouth.
The postcranial bones of this reptile were thick and dense.
The skull of the Mesosaurus was covered by a dermal membrane known as the cleithrum, a feature only seen in primitive bony fish and the early tetrapods.
The Mesosaurus’ ribs were large and banana-shaped, which helped to reinforce the ribcage and protect its internal organ while diving.
Habitat and Distribution
Mesosaurus lived during the Early Permian Period.
The existence of this aquatic reptile probably spanned between 299 and 270 million years ago.
Fossils of Mesosaurus have been found in parts of present-day South America and southern Africa.
More specifically, remains of this reptile have been recovered from rocks of the Paraná Basin in Brazil and the Karoo Basin in South Africa.
These regions were once part of the supercontinent known as Pangea, which was yet to split 300 million years ago.
One of the most notable attributes of this reptile is that it lived in an aquatic environment.
It is often described as a marine reptile.
This is inaccurate because the Mesosaurus lived in freshwater environments like rivers, lakes, and lagoons.
Some habitats where it lived (particularly in Uruguay) may have been hypersaline, but they were not your typical marine environment.
The streamlined body and well-developed paddle-like limbs of this reptile suggest it was most likely a skilled swimmer.
It lived in shallow waters and may have been capable of venturing onto dry land occasionally.
During the Early Permian Period, much of the landmasses on Earth were still connected as a single supercontinent known as Pangea.
This configuration influenced global climate patterns and ocean circulation.
The climate during the Early Permian was generally warm and humid, characterized by extensive equatorial swamps and tropical forests.
The Mesosaurus probably lived in some of these swamps, venturing on land occasionally.
Mesosaurus is popular as one of the first reptiles to return to a fully-aquatic lifestyle.
Earlier reptiles evolved in the swamps of the Carboniferous Period about 320 million years ago and soon colonized the terrestrial landscape.
However, a few genera, such as the Mesosaurus, returned to the aquatic environment about 20 million years later, living mostly as semi-aquatic or fully aquatic animals.
Behavior and Diet
As one of the earliest aquatic reptiles, Mesosaurus exhibited a range of behavior suited to its environment.
In terms of locomotion, this reptile was a good swimmer, but it was probably not very fast.
The top speed for this reptile has been estimated to be between 0.15 and 0.86 meters per second.
It had a streamlined body with well-developed limbs and paddle-shaped feet.
This body structure and the fact that it had a powerful tail all indicate an adaptation for efficient swimming.
It likely moved with an undulating motion of its body, similar to the aquatic reptiles that evolved later.
Mesosaurus also used its limbs and tail in a coordinated manner to propel itself through the water.
The position of this reptile’s nostrils above its head would have made it possible to breathe in the water without lifting its head out of the water.
But arguably the most significant adaptation that made terrestrial locomotion possible for this reptile was pachyostosis observed in its bones.
The Mesosaurus’ bones were dense and heavy.
The additional weight would have helped to stabilize this reptile at the water’s surface or in the topmost few meters of the water column where it lived.
Pachyostosis also gave the Mesosaurus greater momentum when diving below the water surface.
But while the Mesosaurus is often described as a fully aquatic animal, it could probably move onto land occasionally for short periods.
The elbows and ankles of this reptile were restricted, which means it was incapable of moving freely on land like present-day terrestrial lizards.
Locomotion on land would have involved pushing its entire body forward, similar to sea turtles.
The social behavior of Mesosaurus is not well-known due to a lack of direct evidence in the fossil record.
However, researchers believe that this reptile was most likely a solitary animal.
Due to their relatively small size and evolutionary stage, it is unlikely that this prehistoric reptile exhibited complex social structures.
Male and female Mesosaurus individuals probably came together occasionally to mate but didn’t live together during other periods.
The diet of this reptile is another attribute that has been a bit controversial.
Initially, it was interpreted as a filter-feeder that combed plankton and other small food particles from the water.
This is due to the comb-like nature of its teeth, which looked like a structure for filtering food out of the water.
However, the discovery of additional specimens shows that the Mesosaurus didn’t have enough teeth in its mouth to form an efficient filter-feeding apparatus.
Based on this, it has been suggested that the Mesosaurus was more likely an active carnivore that preyed on small fish and invertebrates in its freshwater ecosystem.
To obtain food, Mesosaurus probably pursued prey actively in the water.
Its streamlined body shape would have allowed it to move quickly and stealthily through the water, making it a formidable predator for the smaller organisms in its ecosystem.
Like other reptiles, Mesosaurus likely reproduced sexually.
It exhibited internal fertilization, where males deposit sperm into the female’s body directly, after which the fertilized eggs are released outside the body to hatch.
Mating probably took place in the water, after which the fertilized embryo would develop in amniote-type eggs (eggs with three layers; amnion, chorion, and allantois) within the female’s body.
This is evidenced by the discovery of amniote-type fossil embryos belonging to the Mesosaurus in Uruguay and Brazil.
These well-developed fetus fossils are the earliest record of amniote eggs in the fossil record.
Although this reproductive strategy has been recorded for reptiles that evolved earlier in the Carboniferous Period, no actual fossil has been found before the discovery of the Mesosaurus.
One specimen of a medium-sized Mesosaurus has been found with a small fetus in its ribcage.
Although some scientists often cite this as evidence of viviparity in this reptile, it is more likely to be evidence of ovoviviparous reproduction, in which the female would give birth to live young that hatched from eggs within its body.
Mesosaurus females gave birth to their young in the water.
Unlike adults that could spend short periods on land, juveniles were most likely fully-aquatic.
The Mesosaurus embryos show evidence of pachyostosis even before they were hatched.
This suggests that mesosaur juveniles could swim at birth or shortly after.
Young Mesosaurus likely faced various challenges in their aquatic environment, including finding food and avoiding potential predators.
As they grew, they would have developed the necessary adaptations to become more efficient swimmers and better hunters in their ecosystem.
Evolution and History
Mesosaurus is the type genus for a group of aquatic reptiles known as mesosaurs.
Members of this group are among the first reptiles to adapt to a fully aquatic lifestyle.
Their reptilian ancestors first emerged out of the water during the Devonian or Carboniferous Period, about 320 million years ago.
Roughly 20 million years later, the mesosaurs emerged and gradually returned to the aquatic environment.
Mesosaurs were characterized by their streamlined body shape and well-developed limbs for swimming effectively in the water.
Over time, Mesosaurus and other mesosaurs diversified into various species adapted to live in fully-aquatic environments.
While the mesosaurs as a group eventually went extinct about 270 million years ago, they paved the way for the subsequent evolution of more diverse and specialized marine reptiles.
Interactions With Other Species
An active carnivorous diet is now accepted for this reptile instead of the filter-feeding habit initially proposed.
Mesosaurus likely preyed on small aquatic organisms like fish, small amphibians, and various aquatic invertebrates present in its habitat.
The pygocephalomorph crustaceans have been identified as one of the main invertebrate prey of this reptile.
The sharp teeth and streamlined body of the Mesosaurus suggest that it was an efficient predator in its freshwater habitat.
But its small size also means it was likely prey for other animals in its ecosystem as well.
While there’s no direct evidence of Mesosaurus being preyed upon by other animals in the fossil record.
Larger aquatic predators, such as other marine reptiles or large fish, might have posed risks to juvenile or smaller Mesosaurus individuals.
Mesosaurus was an odd but very important prehistoric reptile.
The discovery of this aquatic reptile is notable for a lot of reasons.
This reptile is renowned as one of the first reptiles to return to a fully aquatic lifestyle after its forebears became land-faring animals.
Consequently, this reptile’s morphological and behavioral adaptations have been of considerable interest to scientists seeking to understand the transition of some marine reptiles from terrestrial to aquatic environments.
Understanding the Mesosaurus’ anatomy and lifestyle can prove pivotal in tracing the evolutionary history of several other marine reptiles that lived after it.
But perhaps even more interesting is the significance of the location where fossils of this reptile have been found.
Mesosaurus lived in eastern South America and southern Africa during the Permian Period.
Both locations are currently separated by the Southern Atlantic Ocean.
Since it is unlikely that the Mesosaurus swam across these two locations, the presence of this reptile on both continents is evidence that supports the theory of continental drift.
This theory proposes that South America and Africa were once joined together as part of a supercontinent known as Pangea about 300 million years ago before they began drifting apart.
Although this theory is widely-known today, it wasn’t as popular in the 1800s and 1900s when the first fossils of this reptile were discovered.
Mesosaurus was an aquatic reptile that lived during the Permian Period.
It was a relatively small reptile with a streamlined body and webbed feet that helped with locomotion in the water.
Mesosaurus is a member of a group of reptiles that made a return to the aquatic ecosystem several million years ago.
As a result, this reptile shows both aquatic and terrestrial adaptations.
It likely gave birth to live young in the water but was also capable of coming on land occasionally.
The Mesosaurus is an important fossil because it provides valuable insights into the transition of terrestrial reptiles to aquatic environments.
It also provides us with the oldest evidence of amniote eggs in the fossil record.
The distribution of the fossils of this reptile on both sides of the Atlantic Ocean is also considered strong evidence for the theory of continental drift.