Stereosternum and the giant stromatolites

Stereosternum tumidum is one of the three species of Mesosauridae. These guys were the first tetrapods to become secondarily aquatic, at the beginning of the Permian period. And yet, there are very few depictions of them. This species, in particular, inhabited the shallow waters of Whitehill-Irati Sea (or Mesosaur Sea, which is a much cooler name), where giant stromatolites, up to three meters wide, existed.