Receptaculites
Receptaculites are a big palaeontological mystery. They were originally believed to be sponges and they are also called sunflower corals but they are likely neither. They are most commonly accepted to be calcareous green algae but there is still debate about this.
Receptaculites are now extinct. They grew in a large circular colonial structure that was flat or gently domed, and made of aragonite pillars with square or diamond-shaped heads. They have a double-spiral structure of intersecting clockwise and counter-clockwise patterns, similar to the spiral structure we see in pinecones and sunflowers.