The black squiggly lines you see on the Fossil Megalodon shark tooth above are created by a serpulid marine worm. The serpulid tube worm attaches itself to the surface of a megalodon tooth. All serpulids build hard tubes of Crystalline calcium carbonate and mucopolysaccharide matrix using glands located on the collar. These tube worms will die off when covered with sediment leaving only the tube-like structure attached to the tooth. When the tube worm on the shark tooth is covered by sediment it will die. The sediment creates a low-oxygenated environment for Anaerobic bacteria to thrive. They will start digesting the mucopolysaccharide matrix. The anaerobic bacteria creates a chemical reaction that produces hydrogen sulfide which leaves a black stain. The black stain on the shark tooth is exactly where the tube worm was. This same black stain can be seen with oysters and encrusted bryzoan that attach themselves to Megalodon teeth. Usually, it's more of a black spot.