Solitary Rugose Corals
Solitary rugose corals are extinct animals. They grew separately from one another and they are also called horn corals because that’s what they look like!
The coral skeleton was made of calcite and so the fossils we see are white. The animal inside was a tiny sea anemone like creature called a polyp that had a mouth and probably stinging tentacles. They were technically carnivores but are often called microcarnivores because their prey was so small.
Rugose means wrinkled, and the corals had wrinkled walls. You can see their daily growth by tiny lines in the wrinkles the same way you can count years in tree trunks. Bescause of this, we can measure how the Earth’s rotation has been slowed down over time by the moon's gravitational influence. Rugose corals from just after the time that Tyndall Stone was created show that there were 420 days in a year back then!
If the fossil is cut horizontally across the coral, we see a circular shape with lines called septa radiating from the centre. If the fossil is cut vertically, we can see these same septa elongated in a longer shape.