1. The Fidelity of Fossil Assemblages
An Exercise at Recognizing Good vs. Poor Spatial and Temporal Fidelity
The Fidelity of Fossil Assemblages
The Fidelity of Fossil Assemblages: An Exercise at Recognizing Good vs. Poor Spatial and Temporal Fidelity

2. Assemblage Types
Biocoenosis: All members of the community are represented.
Thanatocoenosis: All fossils present were members of the community, but some members are missing.
Taphocoenosis: Some fossils present were not members of the community – time averaged (poor temporal fidelity) or transported (poor spatial fidelity) assemblage
Assemblage Types:
1. Biocoenosis: All members of the community are represented.
2. Thanatocoenosis: All fossils present were members of the community, but some members are missing.
3. Taphocoenosis: Some fossils present were not members of the community – time averaged (poor temporal fidelity) or transported (poor spatial fidelity) assemblage

3. How Do You Distinguish Among Assemblage Types?
Each should have some predictable set of biostratinomic features.
Each should have different bioclast fabrics.
Each should have different sedimentologic characteristics.
How Do You Distinguish Among Assemblage Types?
Each should have some predictable set of biostratinomic features.
Each should have different bioclast fabrics.
Each should have different sedimentologic characteristics.

4. Problem: Case of Unusual Preservation
Find a strangely preserved fossil assemblage in Hamilton Group rocks (Middle Devonian, 350 Ma) from western New York State.
Found in thin limestone beds.
A large interior seaway sat on the interior of North America at this time.
Your task: Determine which type of assemblage is represented.
The following slides and associated notes will guide you through the problem.
Problem: Case of Unusual Preservation
Find a strangely preserved fossil assemblage in Hamilton Group rocks (Middle Devonian, 350 Ma) from western New York State.
Found in thin limestone beds.
A large interior seaway sat on the interior of North America at this time.
Your task: Determine which type of assemblage is represented.
The following slides and associated notes will guide you through the problem.

5. Shown is an outcrop of the Centerfield Member of the Middle Devonian Hamilton Group at Brown’s Creek in Genesee County, western New York State. The Centerfield is composed of shales, mudstones, and limestones. The red arrow points out the mud-rich rocks (shales and mudstones), and the blue arrows denote the thin limestones. It is within the thin limestones that the unusually preserved fossil assemblages occur. The mud rocks exhibit more typical fossil preservation (shown in a subsequent slide).

6. This is a stratigraphic section showing the stratigraphy of the Centerfield Member. The Brown’s Creek outcrop exposes the lower half of the section, including sampling intervals (or horizons) The thin limestones containing the unusual fossil assemblage are represented by horizons 11 and 13.

7. a b c
Upper surface of a mudstone, rather than a limestone, that shows the typical preservation of fossil assemblages within the Centerfield Member. Fragments of crinoids (belonging the Phylum Echinodermata; labeled “a”) and bryozoan colonies (“b”) are common; brachiopods (bi-valved organisms) are typically disarticulated; molluscs composed of aragonite are preserved as molds; and trilobites (Phylum Arthropoda) are found as isolated, disarticulated segments (“c”).

8. This and all subsequent slides show various styles of preservation within the thin-bedded limestones. Shown is a proetid trilobite (Phylum Arthropoda) preserved in the orientation seen here. The cephalon or head is at a right angle to the thorax and pygidium (the main body and tail). The cephalon rests on the upper surface of the limestone with the remainder of the body positioned vertically down into the limestone. A trilobite thorax is composed of numerous articulated segments.

9. This bivalve mollusc’s shell (belonging the Class Bivalvia, Phylum Mollusca) was originally composed of aragonite, but is preserved as recrystallized calcite. Both valves are articulated and the shells have not been laterally compressed. The specimen is positioned in the same orientation it was found within the limestone; the umbo (beak) is oriented up in the limestone. The matrix of the limestone is seen adjacent to the fossil.

10. A B C
Shown is a polished cross section of one of the thin-bedded limestones. The slab is oriented as found; up in the limestone was up in the outcrop. Within the limestone are fossil brachiopods (Phylum Brachiopoda). Brachiopods secrete calcitic shells; these fossils are still composed of calcite. The two valves composing the brachiopods are found still articulated. Most articulated specimens have coarse crystalline calcite infillings (seen at “A”), while a small number are filled with sediment (labeled “B”). One specimen still shows preservation of the lophophore support structure (labeled “C”).

11. Shown is a Greenops trilobite (Phylum Arthropoda) preserved as a molt
Shown is a Greenops trilobite (Phylum Arthropoda) preserved as a molt. Trilobites, like all arthropods, grew by molting; this species shed its cephalon and then crawled out of its thorax and pygidium. The surface upon which the trilobite is preserved is a bedding plane of the limestone.

12. This is another limestone bedding plane surface with trilobites
This is another limestone bedding plane surface with trilobites. This is a grouping of at least 3 individuals of the genus Otarion, but these specimens are not molts. Some paleontologists have suggested that trilobites gathered for copulatory events (a phenomenon known as a coterie), similar to behaviors exhibited by chelicerate arthropods like modern horseshoe crabs.

13. A B C a b c d
Four other unusual fossil occurrences. (1) Shown are 2 specimens of fossil crinoids (Phylum Echinodermata); these animals are composed of numerous articulated plates that make up the stem (a), calyx (b), arms (c), and pinnules (d). All structures are seen here intact. (2) A number of different fossils are shown on this upper surface of a limestone bedding plane. “A” is a spiriferid brachiopod with its two valves still articulated, resting with its commissure (the valve opening) up and its broad hinge area down. “B” is a gum-drop-shaped colonial tabulate coral (Phylum Cnidaria) resting with its flat lower surface down. Lastly, at “C” is an encrusting tabulate coral that appears to rest directly upon the upper surface of the limestone. (3) This is a possible coterie of the trilobite Phacops; these specimens are not molts. (4) A collection of articulated fossil blastoids (Phylum Echinodermata), which, like crinoids, are composed of numerous skeletal plates. Only calyces are shown; stems and arms are not preserved.

14. Fossil rugose corals (Phylum Cnidaria) are common within the limestone, but can have variable preservational qualities. Some corals have a well-preserved epitheca (the exterior wall of the coral), like the one preserved on the left; others have had their epitheca corroded away, like the specimen seen on the right.

15. A
Yet other rugose corals can have one half, in this case the lower, well preserved with the epitheca of the other half stripped away. This specimen was oriented like this in the outcrop. This specimen has encrusting tabulate corals (labeled “A”) on the upper surface; no encrusters are found on the lower half.