One of these things is not like the other. One of these things just isn't the same . . . .
One of the more frequently asked questions at any paleontological institution is, “How do you know what’s a fossil, and what’s not?” Though it makes sense to question something so fundamentally important to a study as recognizing its subject, some people ask it in an almost embarrassed way, as if the answer should therefore be obvious. Well, it's a good question, and it's always good to examine the fundamentals, so let's take a look.
Before delving too deeply—literally or figuratively—it bears repeating that fossil hunting has some legal considerations you should always know before you go. Of course, trespassing on private land is never a good idea. Make sure you get a land owner's permission before prospecting. Anyone can hunt for fossils on public lands IF the rules of the agency in charge of the land permit it. In the US, the Bureau of Land Management will have different rules than any given national forest. Wilderness areas prohibit disturbing minerals or the plants that grow around them. ALWAYS research an area before engaging in any fossil recovery, and make sure you understand and abide by the rules of the land. If you have questions, local geological surveys are hubs of information that can save you a lot of trouble, like stiff fines or even jail time. Finally, if you do come across something unusual or if you're not certain about a situation, err on the side of leaving the fossil in place. Moving fossils can sometimes destroy vital information about them. Take photos, bury it if you have to, make sure you can find the location again, and then GET HELP. If you take the time to learn proper techniques beforehand, you can help out paleontologists in some incredible ways. If you don't, you could end up destroying the only means of solving a mystery millions of years in the making.
KNOW BEFORE YOU GO.
That said, the key to recognizing fossils in the rock lies with the difference between minerals and organisms. While both of these things grow, minerals do so only according to their molecular structures. Salt molecules stack in cubes. Water molecules follow a six-sided symmetry thanks to the 60˚ angle formed by their hydrogen atoms. It’s all about fitting into the smallest possible space allowed by the bonded atoms and their electrochemical properties. If you ever examine a large pile of coins, you’ll notice some of them naturally form cylindrical stacks in places for no other reason than the pressure of gravity acting on them and the surrounding coins cause them to pack into the paths of least resistance. Since they already have a cylindrical shape, their “crystals” tend to look cylindrical, though longer and skinnier.
Larger crystals like, say, iron pyrite usually exhibit a clear geometric structure. Exceptions to this rule usually involve the interference of another force like the edge of a preexisting mineral deposit, erosion, or the intrusion of crystals composed of another substance. Even then, the path of least resistance pattern of growth still applies. Close examination will show that even complex crystal forms organize according to simple geometry at smaller levels. As a result, they tend to look ironically artificial in recognizable ways.
Organisms, however, grow structures designed to interact with and change their surrounding environment—these structures are the organs the name “organism” refers to. A plant, for example, has stems that function as structural support for the leaves as well as material transporters for growth. Their shapes allow them to defy surrounding forces like gravity and suck up water like straws. If we can truly say that crystals grow to fit the environment, organisms grow to fight the environment. Even when surrounded by or suffused with rock, wood will still have a wood pattern designed for bearing weight and sucking water. In short, organisms' shapes have a job to do. If you infer the job, that gives you an idea or hypothesis to test against other parts of the fossil. If the hypothesis consistently predicts structures elsewhere on the specimen, science supports it. If not, back to the ol' drawing board.
Though most people don’t dive into these differences in the pedantically detailed way science sometimes requires, nearly all of us can still tell the difference between rock and wood if we just settle down and take the time to do it. Our brains recognize patterns better than any other tool out there, and once people realize this, they pick up on fossil hunting pretty quickly. It does still take some practice and some experience for trickier situations, but fortunately the internet abounds with examples you can use to train your eyes. Seriously, jigsaw puzzles or eye-spy books make great training as well.
Of course, the Stewart Museum at the Eccles Dinosaur Park makes an excellent resource for training them peepers, as well as fossil exhibits at other museums. The picture above features a specimen from the Stewart collection called a dendrite. It looks like a black plant fossil in tan shale at first, but if you take a closer look, you’ll see that it lack the organs plants have: leaves, stems, and so on. You’ll also notice that the black crystals follow the path of least resistance, like cracks in the rock. Water slowly seeping between rock layers redistributes minerals to form dendrites, so you may recognize the same sorts of lichen-like patterns on water-damaged posters or cardboard.
A more entertaining but weirder way for you to practice fossil hunting at the Dinosaur Park involves the footprint tiles across from the bathrooms. Ignore the footprints—they’re just sculptures—but make a close examination of patterns in the marble. Some of those patterns follow organic structures: coiled shells with air chambers, odd cigar-shaped shells with growth rings, and even the seemingly random squiggles of tunnels that somehow retain the uniform width of the worm that made them. Some of these shells intersect with the sculpted footprints, demonstrating that they go below the surface. If that weren't amazing enough, consider that these fossils occur in a metamorphic rock even though grade school curricula insist only sedimentary rocks contain fossils! (Sedimentary rocks just contain the best-preserved fossils.)
Again, stay safe out there, don't play pirate, and keep your eye on fossil preservation when you find them. And if you can't make it outside for whatever reason (winter, weather, pandemics, whatever), remember to train those eyes in pattern recognition. Good luck!
https://geology.utah.gov/ --if you're not from Utah, this still makes a good example of what a geological survey can do. Meanwhile, you can Google the name of your state, province, or country + "geological survey" and that should get you started. Good luck! Again!