A few weeks ago, I gave you a polished slab of coral for Fossil Friday, thinking that the answer would be quite simple! I was told that it was obviously a Permian coral, specifically from the order Rugosa (aka horn coral).
But then the answers started coming in, and people were certain it was from an extinct group of sponges called archaeocyaths. Well, they certainly look very similar, but archaeocyaths date back to the Lower Cambrian 530 million years ago and were almost gone by the middle Cambrian. Definitely two different creatures.
So I went back to my sources at the museum, and asked them what they thought.
Discussions ensued, followed by disagreements, followed by arguments and eventually by fossilized tibia throwing. It was gruesome. To make matters worse, the original specimen mysteriously disappeared from where I had originally found it.
But I did hunt it down. And sure enough, it was a Permian coral, Rugosa to be specific. How can you tell the difference? Well, it is tricky to tell in the picture, but an archaeocyath will have a distinctive internal and external wall that is not present in this specimen.
And if that doesn't convince you...well, I photographed the tag as extra confirmation!