9.7: Fossils in Thin Section

Diagrams Showing Common Fossils

The following diagrams show schematic representations of some of the most common fossils encountered in thin sections; they are not organized taxonomically, but rather based on gross morphology.  

Figure \(\PageIndex{1}\): Fossils that are, or appear to be, made out of a walled shell or exoskeleton include brachiopods, bivalves, barnacles, and trilobites (Page Quinton via Wikimedia Commons; CC BY-SA 4.0)

Figure \(\PageIndex{2}\): Fossils that have, or appear to have, internal chambers include cephalopods, sponges, gastropods, and foraminifera (Page Quinton via Wikimedia Commons; CC BY-SA 4.0)

 

Figure \(\PageIndex{3}\): Fossils that are, or appear to be, colonial include tabulate corals, rugose corals, bryozoans, and sponges (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).

 

Figure \(\PageIndex{4}\): Other distinctive or noteworthy fossils encountered in thin section include crinoid plates, crinoid columnals, brachiopod spines, calcispheres, sponge spicules, and conodonts (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).

 

Porifera (Sponges)

Figure \(\PageIndex{5}\): Examples of sponges in thin section.  A and B) Sponge spicules are the most commonly type of sponge fossils in most settings; they vary from simple to complex rays and commonly resemble toy jacks.  C) Entire sponges are much less commonly preserved and, as is the case with these encrusting examples, can be difficult to distinguish from stromatolites (A-C from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

 

Cnidaria (Corals)

Figure \(\PageIndex{6}\): Examples of rugose (A &B) and tabulate (C&D) corals in thin section.  Rugose corals can be solitary or colonial and have corallites with prominent vertical subdivisions (septa).  Tabulate corals are colonial and have corallites with pronounced horizontal subdivisions that makes colonies look like storeys in a building (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Lophoporhata

Brachiopoda

Figure \(\PageIndex{7}\): Brachiopod fossils in thin section.  A-D) Brachiopod valves often appear crenulated and have a complex or ornamented appearance.  E & F) show brachiopod spines which have hollow interiors and finely laminated walls (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Bryozoa

 

Figure \(\PageIndex{8}\): Bryozoans can show a variety of morphologies (stony, branching, lacy, etc.) and show an equally wide range of morphologies in thin section.  Although they superficially resemble corals, they are not closely related and generally have much smaller openings (typically < 1 mm) than corals.  A) shows a cross section through  branching bryozoan, B and C) show sections through lacy forms (note that the bryozoan is the chambered feature in the NW quadrant of C; much of the rest of the image is filled with crinoid plates).  E & F) show variable cuts through probably lacy bryozoans (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Mollusca

Gastropods

Figure \(\PageIndex{9}\): Examples of gastropods in thin section.  A and B) show longitudinal cuts through gastropods with medium height and squat spirals.  C) Shows a transverse cut with the spirals extending into and out of the plane of the photo.  D) The left half of the image shows a transverse cut through a gastropod contained in the outermost spiral of  a larger gastropod that is cut in longitudinal section (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Cephalopods

Figure \(\PageIndex{10}\): Axial cross section through a possible coiled cephalopod showing progressive decrease in the diameter of the chamber.  Without preservation of a siphuncle or internal septum, it is also possible that this is a gastropod that does not coil out of plane.  Note the geopetal fill with sediment at the bottom and spar at the top (Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Arthropoda

Ostracods

Figure \(\PageIndex{11}\): Ostracods are small arthropods with thin valves.  The valves halves are held together with soft tissue, partially overlap, and have a hook-shaped edge opposite the hinge.  Note the large bivalve in C and numerous calcispheres in both A and D (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Trilobites

Figure \(\PageIndex{12}\): Trilobites were segmented arthropods that molted as they grew which makes their fossils particularly abundant in some beds.  In thin section they are preserved as curved fossils that can terminate with a distinctive shepard's hook that is morphologically similar to, but larger than that of ostracods (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Barnacles

Figure \(\PageIndex{13}\): Barnacles are small to medium-sized crustaceans that, in their adult form, are attached to a hard substrate.  They surround their bodies with a series of complexly-shaped plates that have a ribbed architecture that can appear folded and tooth-like in cross section (Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Echinodermata

Crinoids

Figure \(\PageIndex{14}\): Crinoid columnals are donut-shaped segments that make up the stalk of crinoids, they have a central canal and small arms (cirri) can attach to them.  Two dimensional cuts through them can result in wide variety of morphologies.  A) shows a transverse cut through a columnal with results in a circular shape with a clearly visible central canal. B and C) show transverse cuts through columnals that intersect the central canal.  D) shows a small intraclast surrounded by several transverse cuts through columnals that don't intersect the canal (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

Figure \(\PageIndex{15}\): A) Slightly oblique longitudinal cut through several stacked, articulated crinoid columnals.  B) Arm plate of a crinoid.  C) Numerous crinoid plates and a columnal with five-fold symmetry in the central canal.  D) A variety of crinoid skeletal elements, many of which form the cores of ooids (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Forminifera

Figure \(\PageIndex{15}\): Foraminifera a single-celled organisms, many of which build a calcareous test with internal chambers (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

Miscellanea and Algae

Figure \(\PageIndex{15}\): A selection of other distinctive fossils include A and B) the Permian marine gren algae Mizzia sp., C) Mississippian calcispheres which are likely of algal origin, D) Permian Archaeolithoporella - an encrusting organism of uncertain affinity, E) Tubiphytes, an encrusting organism of uncertain affinity, and F) conodonts are tooth-like structures (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

 

Figure \(\PageIndex{16}\): A and C) Stromatolites from as photosynthetic microbes grow and trap sediment; although local conditions may result in distinctive overall morphologies, they lack an internal structure aside from the laminae.  B and D) Stromatoporoids might be related to sponges; although they superficially appear similar to stromatolites, well-spreserved specimens show a more complex internal architecture with a lattice-like network of horizontal laminae and vertical pillars (all images from Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

Additional Readings and Resources

• Microfacies of Carbonate Rocks by Erik Flugel

• A Color Guide to the Petrography of Carbonate Rocks by Peter Scholle and Dana Ulmer-Scholle

• Carbonate World website