Name: Archaeocidaris brownwoodensis
Location: Texas, USA, Winchell Formation
Age: 303-306 million years ago, Carboniferous Period
Nine animals are crowded into this photograph. A single sea urchin, now crushed, was the home for a neighborhood of life.
The other members of the community, besides the sea urchin, are bryozoans and brachiopods. Bryozoans are microscopic animals that live together in colonies and build lacy skeletons (marked in blue in the map on the upper right). Brachiopods are soft animals that live inside a pair of shells (marked in green).
These fossils are the oldest evidence of community-building between sea urchins and other animals.
When these bryozoans and brachiopods were young, they floated in the ocean. When it was time to change into an adult, they settled on the spines of a sea urchin. They stayed there for the rest of their lives, filtering food out of the water around them.
Is there a benefit to these communities? The tiny hitchhikers probably didn’t benefit the sea urchin at all, but they didn’t hurt the sea urchin, either.
In contrast, sea urchin spines were an ideal home for the smaller animals. The hitchhikers, unable to move on their own, could find new food sources on the back of a sea urchin. The spines provided protection that they could not provide for themselves. Finally, hard spines made a better home than the alternative - soft mud that could bury a small, immobile animal alive.
Specimen Number:TMM 1967TX61
Schneider, Chris L. “Hitchhiking on Pennsylvanian echinoids: Epibionts on Archaeocidaris.” Palaios 18(2003):435-444.