Quick Guide to Common Fossils

Sponges

Sponges (Phylum Porifera) are the simplest major group of animals; their earliest fossils appear in the late Precambrian. Most modern sponges live in the ocean and usually have basket-shaped bodies. They live by filtering food and oxygen out of water pumped in through openings in their body walls and out through a larger opening at the top. The familiar bath sponge has no mineralized skeleton, but many other kinds of sponges have skeletons composed of tiny structures called spicules, which are made of calcium carbonate (CaCO₃) or silica (SiO₂). It is these skeletonized sponges that have the greatest likelihood of becoming fossils. Over their long history, such sponges have frequently been important contributors to reefs and reef-like mounds.

Archaeocyathans

Archaeocyathans were the first important animal reef builders, originating in the early Cambrian. These vase-shaped organisms had carbonate skeletons and are generally believed to be sponges. They went extinct in the late Cambrian, but were very diverse. Archeocyathids are often easiest to recognize in limestones by their distinctive cross-sections. Most paleontologists now recognize archaeocyathans as a subgrouping of sponges.

Rugose and Tabulate Corals

Corals are sessile relatives of jellyfish and sea anemones. They possess stinging tentacles, which they use to feed on small planktonic prey. Each group of coral possesses distinctly shaped "cups" that hold individual animals, or polyps. Colonial corals live in colonies of hundreds or even thousands of individuals that are attached to one another. Solitary coral lives independently, as a single isolated polyp.

Rugose corals were both colonial and solitary. (Solitary forms are often called "horn corals.") Tabulate corals were exclusively colonial and produced a variety of shapes, including sheetlike and chainlike forms. These corals receive their name from the table-like horizontal partitions within their chambers. Both rugose and tabulate corals went extinct at the end of the Permian. Modern corals―scleractinians―first appeared in the Triassic, and include both solitary and colonial species. Many scleractinian corals have photosynthetic symbiotic algae in their tissues, called zooxanthellae. This algae provides nutrition to the coral polyps, helping them to grow more rapidly.

Trilobites

Trilobites are iconic Paleozoic fossils; they were more common in the Cambrian and Ordovician than in later periods, and became extinct at the end of the Permian. They were marine arthropods, and had well-defined head, tail, and thoracic (leg-bearing) segments. Most had large compound eyes, often with lenses that are visible to the naked eye. In life, they had antennae like many other arthropods, but since these were not mineralized, they only fossilize under exceptional circumstances. Many could roll up for protection, and several species also had large spines.

Brachiopods

Brachiopods are filter-feeding animals that have two shells and are superficially similar to bivalves (such as clams).  Instead of being mirror images between shells (symmetrical like your hands), brachiopod shells are mirror images across each shell (symmetrical like your face). There are two major types of brachiopod shells, distinguished by how the two valves connect to each other: articulate brachiopods have tooth-and-socket hinges that tightly interlock, whereas inarticulate brachiopod shells lack hinge structures entirely. Internally, brachiopods are substantially different from bivalves, with a lophophore (filter-feeding organ made of thousands of tiny tentacles), and a small and simple gut and other organs. Bivalves, in contrast, have a fleshier body and collect their food with large gills.

Bryozoans

Bryozoans are colonial invertebrates, many of which build elaborate skeletons of calcium carbonate. Bryozoans are common in today’s oceans, where they are frequently found encrusting rocks or shells. During the Paleozoic era, however, bryozoans commonly grew off of the sea floor as erect structures. After they died, their skeletons accumulated into thick beds of limestone. Although they do not appear to be, bryozoans are actually closely related to  brachiopods—both groups have the same distinctive feeding and respiratory structure, the lophophore.