Life is hard is you’re a sessile animal. Admittedly, there’s a certain couch-potato attraction to sitting around all day, waiting for food to blunder into your mouth. But there are lots of challenges that come with being anchored to a single spot for your entire life, starting with where you should attach your anchor. The problem is that soft (sandy or muddy) seafloors don’t provide good anchor points, unless you have specialized structures to allow this (and some animals do). But if you lack such a structure, what’s your best option? Attaching to the broken, shattered body of some other animal, of course!
We’ve been collecting a large number of fossils for VMNH’s upcoming exhibit on the Ordovician Period. Many Ordovician deposits are dominated by sessile filter feeders, many of which form colonies that encrust the shells of other animals. Encrusting colonies often start small, such as with the patches of bryozoans seen on the brachiopod shell above. But if conditions are favorable the colonies will spread to cover all the available space.
In the Ordovician a favorite target for attachment was the solitary rugose coral Grewingkia canadensis, seen here partially covered by another coral (possibly the tabulate coral Protaraea richmondensis):
An encrusting organism doesn’t necessarily have to wait on its anchor point to die, either. As long as the target doesn’t have any means of removing the encrustation, it’s a perfectly good possibility for colonization. The brachiopod below may well have still been alive when an edrioasteroid (an encrusting echinoderm, distantly related to starfish) began to grow on its shell (specimen courtesy of the Earlham College Geology Department):
Colonization by encrusting organisms isn’t just an event from the distant past. In Cenozoic deposits and in modern environments we still see lots of evidence of encrusting organisms, including bryozoans and the more recently evolved barnacles.