@futurebird the shell was on its side, and the valves sat lengthwise like a pair of scissors. The modern heart cockles rest on the side like this, but not as elongated of a shell. Some heart cockles have windows to allow in light to symbionts, but Shikamaia's shell was too thick for that. The researchers propose that if it was photosymbiotic, it extended its mantle over the edges of the shell, like giant clams do. However, we will need better preserved shells to look for geochemical evidence!
futurebird