Mesosaurus

About Mesosaurus

Mesosaurus: Adaptation to Water and Gigantism

Mesosaurus was an early aquatic amniote (reptile) with an elongated skull that lived in an inland sea during the Early Permian period (about 280 million years ago).

It swam through the water using a long, powerful tail and large hind legs that are thought to have been webbed. Its bones were highly dense and heavy (pachyostosis), which likely functioned as "weights" to suppress buoyancy and maintain stability in the water.

Mesosaurus was once thought to be a "creature that spent its entire life completely in water" with a total length of about 1 meter. However, recent research has revealed the existence of giant individuals reaching up to 2.2 meters in total length. When they were young, their bone joints were flexible, and they lived completely in water. However, as they grew larger, their skeletons became stronger, and they could also spend time on land (coastal areas). In this way, it is known that they practiced "ontogenetic niche partitioning," changing their habitat as they grew.

What Did They Eat? (Diet and Cannibalism)

Mesosaurus's elongated jaws were lined with countless long, slender teeth, much like a comb. It was once thought that it used these fine teeth like a "sieve," sucking in water along with plankton and filtering it out to eat, a process known as "filter feeding."
However, examinations of fossilized stomach contents and feces (coprolites) have shown this to be incorrect. Mesosaurus mainly ate "pygocephalomorphs," small crustaceans resembling shrimp about 2 cm in length. The long, slender teeth were suited for entangling and catching fast-moving prey in the water like a cage.

Additionally, the inland sea where they lived was a very harsh environment. Therefore, evidence has been found that they sometimes scavenged on carcasses or engaged in "cannibalism," eating juveniles of their own species.

Settlement of the "Autotomy" Controversy Over Mesosaurus's Tail

For many years, the "tail" of Mesosaurus has been a subject of debate among paleontologists. The tail bones (caudal vertebrae) of Mesosaurus sometimes have crack-like lines. Because of this, there was a theory that it performed "autotomy," deliberately detaching its own tail when attacked by an enemy, just like modern lizards.

However, recent research using X-rays has revealed that these lines were not for autotomy. This was a primitive skeletal structure called a "composite centrum," made up of multiple bones joined together. It has been concluded that Mesosaurus used its tail as a powerful propulsion for swimming and did not detach it itself.

Classification and the Secret of the Skull

In older encyclopedias, other genus names such as "Stereosternum" and "Brazilosaurus" are sometimes introduced as relatives of Mesosaurus. However, after re-examining hundreds of specimens, it was found that these were all either young individuals of the same "Mesosaurus (Mesosaurus tenuidens)" or just had different fossil preservation states.

Furthermore, in recent years, a major discovery was made when an extremely well-preserved skull was found. A hole called the "lower temporal fenestra" was open behind the eye. For a long time, Mesosaurus had been considered a representative of the "anapsids," animals with no holes in their skull bones, but this discovery led to a major rewriting of the evolutionary history of early reptiles.

Ovoviviparity and the World's Oldest Embryo Fossil

In Uruguay and Brazil, fossils of Mesosaurus babies still inside the mother's body and fetus fossils have been found. This is the discovery of the world's oldest amniote embryo (a growing baby) in the fossil record.

These embryos lacked hard eggshells, proving that Mesosaurus was "ovoviviparous" (or viviparous), hatching eggs inside their bodies and giving birth to live young. It is thought that they grew their young to a certain size before giving birth in order to survive in the harsh aquatic environment.

"Crucial Evidence" for Continental Drift and Mass Mortality

Mesosaurus fossils are found only in the southern parts of South America and southern Africa, across the present-day Atlantic Ocean. Since it would be impossible for Mesosaurus to swim across a vast ocean, this is strong evidence for the "continental drift theory," which states that these two continents were once joined together (Pangea).

The Irati-Whitehill inland sea where they lived was a unique environment where the surface was calm, but toxic hydrogen sulfide accumulated on the seabed. The reason Mesosaurus are found in large numbers as beautiful, articulated fossils is thought to be because massive storms churned up the toxic water from the seabed, causing them to die en masse (from suffocation or poisoning) all at once.