An article published in the journal “Frontiers in Ecology and Evolution” describes a research on mesosaurs, the most ancient reptiles whose adaptation to aquatic life is known. A team of researchers carried out an accurate analysis of a number of fossils concluding that the bones of adults have similarities with those of land-dwelling animals and this suggests that they were semi-aquatic while their juveniles were aquatic. It’s a result obtained by examining fossils of specimens of various ages, a type of study that can reveal changes that occur with the growth of an individual of a species.
Mesosaurs are a genus of reptiles that lived in the Permian period, between about 299 and 251 million years ago. A species called Mesosaurus tenuidens is known, whose fossils have been discovered in both Africa and South America and described for the first time over 150 years ago. However, for a long time paleontologists believed that adults were about a meter long but specimens about twice as long were discovered in the Mangrullo Formation in Uruguay.
Professor Graciela Piñeiro, one of the authors of this research, explained that the environmental conditions in the Mangrullo Formation were sharsh so it would have been difficult for a mesosaur to reach an age advanced enough to justify a relatively large size. She also pointed out that the fossils of these very large mesosaurs were in a much worse state than the smaller ones, suggesting that they suffered significant air exposure when they died.
The researchers examined a total of 40 mesosaur skeletons of varying lengths to assess their adaptations to aquatic life. They were compared to skeletons of other reptiles of the Permian period, 4 of Hovasaurus boulei, another aquatic species, 2 of Thadeosaurus colcanapi, a land-dwelling species, and 4 of Claudiosaurus germaini, a species that was perhaps semi-aquatic. Other archive data used in the research concerned extant reptile species such as spectacled caiman (Caiman crocodilius), Bengal monitor (Varanus bengalensis) and water monitor (Varanus salvator).
The result of the analysis of some anatomical characteristics such as that of the tarsus, the set of foot bones in the ankle area, some vertebrae and others show differences between adult and juvenile mesosaurs. The comparison with other reptile species suggests that adult mesosaurs spent more time on land while juveniles lived an aquatic life.
The harsh environmental conditions in which mesosaurs lived could explain the scarcity of elderly specimen and relatively large dimensions. The fact that they spent time on land would explain the lower quality of the fossils with a worse preservation caused by the exposure to air.
In conclusion, mesosaurs may have been partially amphibious and Professor Graciela Piñeiro pointed out the importance of studying fossils that represent the entire population of a species with a wide range of juveniles and adults to correctly establish their lifestyle.
This research was carried out by examining 40 skeletons of mesosaurs but in the field of paleontology sometimes they find only one specimen of a species and often it’s incomplete. This can make all the difference in understanding how a certain species lived and is one of the reasons why there may be controversy about certain studies. The interpretations concerning a species can be used in the study of similar species therefore they have ramifications that can be really important and this research stresses the importance of a complete vision in those interpretations.
