Oldest-Known Aquatic Reptiles Might Have Spent Some Time on Land

September 25, 2018

The study of fossils of Older Mesosaurs reflects that the adult reptiles didn’t spend most of their time in the sea.

The reptiles who secondarily adapt to an aquatic or semi-aquatic life in a marine environment are called Aquatic Reptiles. A lot of reptiles adapted to the sea-life during the Mesozoic era including Mesosaurs, the oldest aquatic reptile known to humanity. Recently, a study was published in ‘Frontiers in Ecology and Evolution’ which suggested that this reptile might have spent some time on land, as well.

The researchers performed a detailed examination of Mesosaurs fossils and found that bones from adults share significant similarities with the land-dwelling animals. In addition to that, the relative scarcity of land-weathered fossils of large specimens supported the idea that older Mesosaurs were semi-aquatic. However, the researching team did acknowledge that the juveniles spent most of their time in the water. Professor Graciela Piñeiro, the Lead Author of this study who works at the Facultad de Ciencias, Universidad de la República, talked about this in the following words:

“Despite being considered the oldest-known fully aquatic reptile, mesosaurs share several anatomical features with terrestrial species. Our comprehensive analysis of the vertebrae and limbs of these ancient reptiles suggests they lived in the water during the earliest stages of their development, whereas mature adults spent more time on land.”

This research carries a lot of significance as it showed the world that thorough analysis of fossilized remains from all stages of a reptile’s life is necessary to completely understand the behavior and lifestyle of the creature. The discovery of unusually large bones of Mesosaurs in the Mangrullo Formation of Uruguay urged the international team of researchers to figure out the theory behind this finding. The general length of discovered skeletons was around 90 centimeters but they found some large specimens which were about 2 meters in length. They wanted to know why these mature adults were not in abundance and this curiosity led them to an amazing discovery. Piñeiro referred to that and said,

“The larger specimens, at least twice the length of the more commonly reported Mesosaurus fossils, could just be exceptionally big individuals. However, the environmental conditions of the Mangrullo lagoon of where they lived were harsh, making it difficult for the occasional mesosaur to reach such a relatively large size and age. We then realized that in comparison to the smaller, better-preserved specimens, larger Mesosaurus fossils were almost always disarticulated, very weathered and badly preserved. This suggested these larger specimens had extended exposure to the air when they died.”

Different life stages of this ancient reptile were catered in this research to ensure all aspects are covered, thoroughly. The researchers tried to look for signs of terrestrial existence during the reconstruction and analysis of Mesosaurs skeletons. The skeletal structure is extremely important in determining the habitat of the animal because there is a marked difference in bone profiles of terrestrial, semi-aquatic, and aquatic creatures.

Consequently, the team of Piñeiro took help from morphometrics to examine the shape of the fossilized bones. A total of 40 Mesosaurs specimens were selected for examination, ranging from juveniles to adults, and all of them were compared to similar aquatic or semi-aquatic reptiles. Pablo Núñez, a member of the team at Universidad de la República, explained the results by saying,

“The adult mesosaur tarsus (a cluster of bones in the ankle region) suggests a more terrestrial or amphibious locomotion rather than a fully aquatic behavior as widely suggested before. Their caudal vertebrae, the tail bones, also showed similarities to semi-aquatic and terrestrial animals. This supports the hypothesis that the oldest and largest mesosaurs spent more time on land, where fossil preservation is not as good as in the subaquatic domain.”

According to Piñeiro, this research is of immense importance not only for the future research but also for understanding the reptile evolution. She mentioned that their work highlights that we must work with an entire population of a species before establishing any paleobiological interpretations of their behavior. She also talked about the implications of this research on the species that are closely related to the Mesosaurs. In particular, she talked about the evolution of the amniotic egg and said,

“For instance, thanks to our previous discovery of a mesosaur egg and embryos inside the mother’s body, our new findings can give support to earlier hypotheses suggesting that the amniotic egg might have appeared in aquatic or semiaquatic animals as a strategy to leave the water to avoid predation.”