Receptaculitids are problematic fossils of the Paleozoic, usually found in rocks indicating a shallow calm marine environment. There is no real agreement on where they belong phylogenetically; they have been compared both to poriferans and to cnidarians. More recently, they have been placed among the calcareous green algae, since the structure of their skeletons is similar to certain living algae (Dasycladales).
Receptaculid skeltons were calcareous and had a very distinctive shape and structure. In the picture of the genus Ischadites shown above, you can see the distinctive spiral shape of the receptaculid skeleton. The sequence of skeletal structures creates a pattern that is the same as that seen in sunflowers and pine cones, the so-called Fibonacci spiral.