Rock Boring Marine Invertebrates Can Make Holes in Fossils

One of the Fossil Forum moderaters recently let me have a bag of fossils.  Among the specimens was a partial dugong rib.  The dugong of the North Atlantic ocean was a sea going manatee that probably went extinct about 3 million years ago when South America drifted close enough to North America to form a land bridge.  This geological transformation disrupted ocean currents and probably is the cause of a major marine extinction event.

Upon examining the dugong specimen, I noticed what I thought for sure were gnaw marks made by some small carnivore.  I happen to possess a house cat skull.  The teeth and jaw span exactly match what looked like canine puncture marks on the specimen.  I couldn’t find much information about small carnivores of the Miocene and Pliocene, so I contacted Dr. Richard Hulbert of the Florida Museum of Natural History and described the specimen to him in order to come up with a list of possible culprits.  Dr. Hulbert is the author of Fossil Vertebrates of Florida and dozens of articles in scientific journals.  He informed me that a small carnivore couldn’t bite hard enough to make canine puncture marks in a dugong bone.  Instead, the holes and marks were probably made by marine invertebrates that use enzymes to bore through rock in much the same way shipworms (Teredo navalis) do to wood.

This looks like gnaw marks but were probably made by marine invertebrates.  Note the canine-like puncture holes.

House cat skull is a perfect fit for burrow marks probably made by a clam or worm.

Canine tooth is a perfect fit for both holes that were probably made by worms or clams.

I was aware of bioturbation.  Bioturbation occurs when animals burrow up and down sediment.  Some times this activity mixes sediment from different ages and can cause problems for scientists when they try to date strata.  But I didn’t realize these invertebrates also burrow through rock (and fossils).

I researched more about rock borers and discovered that there are an astounding 8 superfamilies of rock boring clams, including pholadidae, the piddock clams.  Some species prefer boring through soft rock, others hard rock.  They make popular aquarium pets.  Some aquarium owners complain that the clams move around too much when placed on a sandy substrate.  These people don’t realize the clams are searching for a rock to bore into and won’t feel safe until they find one.

There are also rock boring worms and sea urchins.

Worms, as well as wind and water, are the elements that can erode fossils into nothingness.  This makes the survival of any recognizable fossil all the more remarkable.