What is Mesosaurus?
Mesosaurus is an ancient reptile genus that is now extinct. As per fossil records, this extinct genus lived in the Early Permian period, which is between 299 and 271 mya or million years ago. They are thought to be the early aquatic relatives of the present-day reptiles and dwelled in South Africa and southern Africa. The name Mesosaurus implies ‘middle lizard’. They are grouped under the family Mesosauridae of the order Mesosauria along with other genera such as the Stereosternum and Brazilosaurus.
On this page, we will learn about the Mesosaurus to great lengths discussing their characteristics, their evolutionary biology and more.
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Classification of the Mesosaurus
As per the classification given by Gervais, the extinct Mesosaurus genus can be classified as under:
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: †Mesosauria
Family: †Mesosauridae
Genus: †Mesosaurus
Type Species: †Mesosaurus tenuidens
Description of Mesosaurus
The skull of the Mesosaurus was elongated and larger as compared to that of Stereosternum and their teeth were longer. Their teeth were also angled, particularly the teeth present at the tips of the jaws. Their postcranial skeleton had thick bones as a result of having gone through pachyostosis. One characteristic that distinguished Mesosaurus from other reptiles is the presence of a cleithrum. This is a type of dermal bone overlying the scapula. This type of bone is generally observed in tetrapods and the more primitive bony fish. They had their interclavicle head in the shape of a triangle which is distinct as compared to the head of other early reptiles which were shaped like a diamond. They were approximately 3.3 feet or 1 m long given their long and narrow skull and tail. They had large, banana-shaped ribs. As in the case of other reptiles, the Mesosaurus also had lungs instead of gills and breathed oxygen in from the atmosphere rather than acquiring it from underwater. Their nostrils were positioned high up which allowed them to breathe in air through them while the remainder of their bodies remained below the surface of the water.
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Habitat and Diet of Mesosaurus
For a long time, Mesosaurus were thought to be the first-ever marine reptiles. As newer data emerges, it has been suggested that the Mesosaurus that were found in Uruguay mostly inhabited water bodies of the hypersaline kind instead of a marine environment where these reptiles are typically found. The Mesosaurus, however, had many adaptations that allowed it to live a lifestyle that was fully aquatic. They are rarely likely to have ventured onto land and unlikely to have traversed great distances of the open ocean. Their diet primarily consisted of small crustaceans among other prey.
Distribution of Mesosaurus
The remains or the fossils of the Mesosaurus have been found in the eastern part of South America (Brazil, Irati Formation, Melo Formation and Uruguay) and in Whitehill Formation, southern Africa.
Reproduction in Mesosaurus
In the fossil remains discovered in Brazil and Uruguay, Mesosaurus were found to have amniote-type fossil embryos in an advanced developmental (foetus) stage. Ever since the amniotes first appeared in the Late Carboniferous period, they have been inferred to have this typical reproductive strategy but these fossil records of Mesosaurus are the earliest amniote foetus recorded. Before the description of the Mesosaurus’, the oldest amniote foetuses known were from the Triassic period.
A particularly coiled foetus isolated from Mesosaurus was found lacking in calcareous eggshells. This suggested that the oviduct glands in the reptile and by extension all Paleozoic amniotes were unlikely to be able to secrete CaCO3. Another Mesosaurus specimen is indicative that the genus, akin to the other marine reptiles, gave live birth to their young ones. This is thought because the specimen was of a medium-size adult that contained a small individual within its rib cage. This condition is referred to as foetus ‘in utero’. This interpretation, if considered correct, is representative of the most primitive record of viviparity among fossilised remains. A different foetus specimen isolated, however, indicates that the reproduction strategy adopted by Mesosaurus was oviparous in nature.
Evolutionary Biology of Mesosaurus
After the early tetrapods migrated to land in the Late Devonian period or even later during the Paleozoic era, Mesosaurus were the first of the reptiles that were known to have returned to the water. It is likely that their streamlined body with webbed feet and a long tail supported a fin. Their flexible tail and long hind legs are thought to have provided them with the ability to propel themselves through the water. Their sideways movement is also likely to have been facilitated by their flexible bodies. However, as their ribs were heavily thickened, twisting movement of their bodies are likely to have been prohibited.
These ancient reptiles with their long jaws, small skull and nostril located atop breathed solely the upper sides of their heads. This breathing by breaking the surface is in a manner akin to what is found in the present-day crocodiles. Evolutionary biologists originally thought that the teeth of the Mesosaurus acted as a straining device that allowed them to filter feed on planktonic organisms. This assumption was based on the idea that the Mesosaurus had numerous teeth that were closely spaced together within their jaws. The fossils of the extinct reptile examined newly, however, revealed that the Mesosaurus had fewer teeth and their dentition seemed more favourable in the capturing of nektonic prey like the crustaceans.
It is suggested that the Mesosaurus was able to attain a natural buoyancy in a few upper metres while inside the water column. This hypothesis is supported by the pachyostosis observed in the bones of the extinct animal. This additional weight is also thought to have provided stability when the animal was at the surface of the water. An alternate hypothesis also suggested that the pachyostosis provided a greater momentum when the Mesosaurus was gliding underwater.
A number of features observed in the Mesosaurus indicate that the animal had a wholly aquatic lifestyle. However, it has been suggested that they likely moved onto the land for brief periods of time. It also appears that they were unable to walk on account of the movement of their ankles and elbows being restricted. So, in the case they moved onto land, they were more likely to have pushed themselves forward in a similar fashion the living sea do when they nest on the beaches. A further study on the Mesosaurus’ vertebral column is indicative that young ones of the reptile were fully aquatic while the adult animals spent a certain amount of time as land-dwelling reptiles. This hypothesis has been supported by the fact adult animal remains are relatively rare in aquatic settings along with the evidence of drying fractures processed in the coprolite. Palaeontologists are, however, unable to determine the exact time period of the terrestrial dwelling of these animals as their aquatic lifestyle adaptations and pachyostosis would have made it difficult for them to forage on land.
So, we have learnt a great deal about the extinct reptile Mesosaurus in this article. This reptile is important in the study of the paleobiology of reptiles as well as palaeogeography of the Earth.
FAQs on Mesosaurus
1. What is the Significance of Mesosaurus?
Answer: Mesosaurus is significant for it provides evidence in support of the theory of continental drift. The fossilised remains of the Mesosaurus have been found in part of southern Africa and South America, two regions greatly separated in the present day. Mesosaurus was a coastal animal and thus, unlikely to have crossed the vast saline stretches of the Atlantic ocean. Their distribution is, therefore, indicative of the paleogeographic evidence that the two continents present in the Southern Hemisphere were at one point of time joined together and provides evidence of the continental drift.
2. Why is the Mesosaurus Called the Middle Lizard?
Answer: Mesosaurus is thought to be one of the earliest aquatic reptiles, relatives of the present-day class. The name of the genus has been derived from the Latin word, ‘meso’ meaning middle and is a befitting name for the ancient reptile. Evolutionary biologists who first described the Mesosaurus fossils considered the genera to be in between reptiles and amphibians as they displayed features characteristic of both the phyla.
A number of features of the Mesosaurus - their webbed feet and hands, laterally compressed tail and nostrils closely placed to the eyes at the top that allowed them to breathe in a manner similar to crocodiles, i.e., without lifting their head out of the water surface, all these features indicated that the lifestyle of the Mesauraus was predominantly aquatic. Additionally, their thick, dense bones similar to that of plesiosaurs and manatees are likely to have provided them with the ability to dive in the water effortlessly. These features allowed the scientists to predict that Mesosaurus were almost wholly aquatic and it was only on rare occasions that they came out of water.
The Mesosaurus, however, also displayed features that were more similar to the land-dwelling animals. These included their hip and heel structure. Added to it is the fact that, the native habitats and environment where the Mesosaurus dwelled are believed to have had shallow waters that were shrinking and drying.
All these features contribute to the reason why the Mesosaurus is known as the middle lizard.
