Mesosaurus

Mesosaurus, which means "middle lizard," was an extinct genus of aquatic reptile.

These animals lived during the late Early Permian period, roughly 275 million years ago. They were members of the family Mesosauridae and the order Mesosauria. Mesosaurus represents one of the earliest lineages of reptiles to adapt to a fully aquatic lifestyle. They lived along the shorelines of the Irati–Whitehill sea. This was an epicontinental sea that covered parts of southern Pangaea.

Today, scientists study these reptiles to understand early vertebrate evolution and the history of our continents.

The physical body of Mesosaurus was highly specialized for life in the water. They had slender bodies and long, narrow tails. Their limbs were paddle-like, and soft tissue evidence shows they had webbed feet.

A unique feature was pachyosteosclerosis, which means their bones were both thick and dense. This heavy bone structure likely helped them achieve neutral buoyancy in the upper water column. It may have also stabilized them at the surface or provided momentum while gliding. Their nostrils were located on the top of the head. This allowed them to breathe while keeping most of their head underwater, much like a modern crocodile.

Mesosaurus had specific biological features that changed as they grew. Their skulls became proportionally shorter as they aged, even though the snout elongated. Their teeth also became more elongate over time.

During growth, the hands, or manus, and the hindlimbs became proportionally shorter. Scientists have also noted that their teeth underwent replacement. Their jaws contained numerous slender, conical teeth, as well as palatal teeth on the roof of the mouth. These dental features were once thought to be for filter feeding, but new research suggests a different diet.

Recent studies have clarified the diet and movement of these reptiles. Mesosaurus used its teeth to catch small, swimming prey known as nektonic organisms. They specifically targeted pygocephalomorph crustaceans, which were abundant in their environment.

Evidence from coprolites, or fossilized feces, confirms they ate these crustaceans. Some fossils even suggest they may have engaged in cannibalism by eating smaller juveniles. Regarding movement, they were likely relatively slow swimmers. Biomechanical analysis suggests optimal speeds between 0.15 and 0.41 meters per second. While they were adapted for water, they might have moved on land by pushing themselves forward like nesting sea turtles.

The history of Mesosaurus discovery is quite unusual. The holotype specimen, nicknamed the "Griqua Mesosaurus," was found in South Africa around 1830.

Interestingly, it was being used as a pot lid in a hut. The French paleontologist Paul Gervais eventually received the specimen and named it Mesosaurus tenuidens in 1865. Other species like Brazilosaurus and Stereosternum were once considered separate genera. However, recent studies show these were likely just juvenile or fragmentary specimens of Mesosaurus. Now, M. tenuidens is recognized as the only valid species.

Mesosaurus provides vital evidence for the theory of continental drift. Their fossils are found in southern Africa and eastern South America.

These regions are now separated by a vast ocean. The presence of the same genus in both places proves the landmasses were once connected. They inhabited the Irati–Whitehill sea for up to 4 million years. Eventually, this sea dried up, which led to the extinction of the Mesosaurus. This event marked the end of their specific lineage in that environment.

Reproduction in Mesosaurus is also a subject of great scientific interest. Fossils found in Uruguay and Brazil represent the earliest known records of amniote fetuses.

One specimen, MCN-PV 2214, shows an adult with a fetus inside its rib cage. This suggests that Mesosaurus may have practiced viviparity, or giving live birth. Other fossils, such as specimen FC-DPV 2504, might point toward ovoviviparity instead. These findings help scientists understand how early land-laying vertebrates transitioned their reproductive strategies over millions of years.