See the following examples of fossil specimens - plants, dinosaur teeth and eggs - along with clues to identifying them.
Which Are the Fossil Plants?
The large black slab shows late-Middle to Late Pennsylvanian (~308 to 300 million years old) plants from the famous St. Clair fossil beds of Pennsylvania. The leaf fronds are from the genus Alethopteris, and although they look like ferns, they are actually seed ferns. Seed ferns are not closely related to ferns and went extinct long ago.The small rectangular slab is from the famous limestone fossil quarries of Solnhofen, Germany where Archaeopteryx comes from. But these marks are not fossils. They are what are called manganese-oxide dendrites. These are naturally-forming features that occur when minerals in solution percolate into tiny fissures in rocks and are deposited in a tree-like form. This beautiful plant-mimic grows along fractures in the bedding planes of Solnhofen rocks, and since this is also where the fossils are found, dendrites are often found radiating out from a genuine fossil making it look like it’s covered in moss.
Which Are the Dinosaur Teeth?
Clockwise from top: Tyrannosaurus rex tooth (Late Cretaceous, western US), tyrannosaur tooth (Late Cretaceous, Judith River, MT), rugose coral (Middle Devonian, Falls of the Ohio, Clarksville, Indiana), rugose coral (Paleozoic, locality unknown), ornithomimid toe claw (Late Cretaceous, western US), rugose coral (Paleozoic, locality unknown), tyrannosaur tooth (Late Cretaceous, Judith River, MT), rugose coral (Paleozoic, locality unknown), theropod tooth (Late Cretaceous, Ojo Alamo, NM), tyrannosaur toe claw (Late Cretaceous, western US).
All are fossils except, technically, the T. rex tooth, which is actually a cast of a real tooth. The rugose corals (also known as solitary or horn corals) are very often mistaken for dinosaur teeth but are older than the oldest dinosaurs, having gone extinct around 250 million years ago (the oldest known dinosaurs are around 230 million years old).
Which Is the Dinosaur Egg?
The specimen on the lower right is an approximately 80 million-year-old fossil dinosaur egg from the Late Cretaceous Djadochta Formation of Shahbarakh Usu, Mongolia. It was collected by A. F. Johnson on 17 July 1923 as one of a group of 3 weathered oviraptorid eggs.
The object on the upper left is a water-worn rock, most likely from a river. Its resemblance to an egg is merely accidental.
River-rounded rocks are commonly mistaken for fossil eggs. This is just one method by which nature produces rocks that resemble eggs. Sedimentary concretions are another common imposter. Concretions often form when some object acts as a “seed” for the deposition and cementation of sequential layers of sediment. On occasion, the matter that initiates the concretion, the “seed,” can be a fossil. To find this out requires cracking or cutting open the concretion.
Genuine fossil eggs usually have an easily identifiable shell which differs significantly from the enclosed sediments either by having a fine surface ornamentation (the smoother the "shell," the less likely it is to be a non-bird dinosaur egg) or a specific type of crystalline structure in cross-section. Also, because eggshell tends to be brittle, the shell is almost always heavily cracked with clear shifting of the eggshell bits. Ironically, one strong indication against a dinosaur egg identification is a very egg-shaped specimen: most fossil eggs are not "egg-shaped" because most fossil eggs come from non-avian dinosaurs and are everything from spherical to torpedo-shaped.
Additionally, the thickness of the “shell” can rule out an egg ID. Embryos in hard-shelled terrestrial eggs need shells through which they can conduct gas exchange—basically so they can breathe. Past a certain thickness, this becomes impossible.
And don’t forget, if a chicken egg were included in the photo, it, like the oviraptorid egg would be properly identified as a dinosaur egg.