|Typical fossils from the Potomac shores (click for larger view)|
Bone pieces, particularly Turtle Bone or Scutes (shell scales), are common to find, often standing out due to their dark material. Many show either a flat end from the shell, or imprints of the scales/bones. In fact, looking for darker material in areas where low tide has exposed mixed-sized pieces of rocks/shells is often one of the easiest methods of finding many fossils on beaches and shores.
Turritella shells and molds are another common find. These are in the form of the actual fossilized twisty shell, or sometimes just the twisted inner mold where material filled the shell and then became sedimentary rock. Finding complete pieces is not often easy.
Sharks' Teeth are what many people are primarily seeking when they go beach combing for fossils. The teeth of course were the hardest part of the cartilaginous sharks' bodies, and since sharks can shed hundreds of teeth throughout their lifetime, this offered more chances for them to be preserved as fossils. While there are many species, and the giant Megalodon is the prize find, the most common sharks found in my experience are Sand Tiger Sharks. These teeth are long, round, and very pointy with not much serration or edges to them.
Crocodile Teeth are much less common. These are typically conical, almost bullet-like, and have characteristic hollow or concave bases. These are my wife's favorite for she uses the hollow bases for earring posts and necklaces. She has formed the sight picture to find these and is much better at locating them than I am.
This is another point to make, forming a sight image of what you are looking for can greatly help in finding good pieces even in places already picked clean by other fossil hunters. Once you find the first piece, it gets easier each time to find the next of the same type. Finding that first piece is the primary issue.
Eagle Ray Dental Plates are often overlooked. These teeth are designed to crush mollusks and crustaceans, so are flat rather than sharp or edged. They form rows to functions as hammers, but complete rows are difficult to find. What you usually find is just one row or piece. Various species have either very wide or thin teeth.
Sting Ray Barbs are much more difficult to find. These typically are long and narrow, with serrated edges, and a groove along the side where the venom would travel. Finding a whole stinger is quite rare, something I have yet to do.
Fish Dental Plates are not that uncommon, but are difficult to recognize, especially since each species has a different look. They are typically flat but curved and have various impressions from the roof of the fish mouth present.
Coprolite is the term used for any fossilized feces. The most common are those of fish, but you can occasionally find those of other animals such as turtle or crocodile. They are not always easy to recognize either, but the often-dark coloration can offer a clue. Those of fish are also often either very smooth and round or oval, sometimes BB-like, offering yet another clue.
Petrified Wood does indeed look like wood, often showing the grain of the tree. It is usually also a much lighter color, often tan looking. These are uncommon, but it is a good idea to check any strange looking driftwood piece you find and try to snap it. If it does not, then that is a good clue that it is a fossil.
My family finds fossil hunting to be a fun activity, though we are often distracted or also doing something else like fishing simultaneously. The last two times for example, my son did not collect much, more interested in catching frogs, fish, and insects for me to photograph. But he still gets excited when he finds a particularly nice find. And who wouldn't, considering how old and the odds against that a piece of the past was preserved for us to find. It makes each find a treasurer regardless of what it is.