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Mystery fossil #63 is...

a tabulate coral

Mystery fossil #63

Pulling back a bit, it's easier to see that this is a chunk of fossil coral. This is a colonial tabulate coral found in the Silurian rocks of Siskiyou County, California. In this specimen, you can see that the colony is made up of closely packed polygonal tubes. If we cut a vertical section through this fossil, we'd see numerous horizontal partitions, or tabulae, within each tube.

Tabulate corals (see Mystery Fossil #40), once a major component of Paleozoic reef systems, were victims of the Permian-Triassic mass extinction about 245 million years ago. In the mid-Triassic, a new group of corals, not closely related to the tabulate corals, began to appear. Filling the niche vacated by the tabulates, some of these new corals evolved into colonial forms that built massive and extensive reefs. These corals, the scleractinians, have continued to dominate most reef habitats ever since. Read more about anthozoans (the corals, as well as sea anemones, sea pens, and sea fans) in the fossil record here.
 

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