The oldest known aquatic reptiles, the mesosaurs, probably spent part of their life on land, reveals a new study published in Frontiers in Ecology and Evolution. The fossilized bones of adult Mesosaurus share similarities with land-dwelling animals, which—coupled with the relative scarcity of land-weathered fossilized remains of large specimens—suggests that older mesosaurs were semi-aquatic, whereas juveniles spent most of their time in the water. This new research emphasizes the importance of thoroughly analyzing fossilized remains from across all stages of a reptile’s life to get a full appreciation of its lifestyle and behavior.