Modern sea pens bear a remarkable resemblance to fossils found in Australia.
Running up and down the stem of a modern sea pen are polyps that are specialised for feeding and reproduction.
The skeletons of sea pens are flexible and horny, and they belong to a group called the soft corals along with another organism called dead man's fingers.
Soft corals can grow in water up to 6000 metres deep, but their close relatives the stony corals - the ones which produce reefs - can live no deeper than forty metres.
Coral polyps live only on the surface of their structures, each in a tiny limestone cell. But they are connected to each other by thin strands so that the whole skin is a living network.
As new polyps sprout from the connecting branches, they secrete cells that grow over the early ones and stifle them.