The Big Hill Formation is a geologic formation in Michigan. It preserves fossils dating back to the Ordovician period. A fossiliferous site on the Stonington Peninsula (in Delta County) includes a dolomite bed of the Big Hill Formation which has abundant and well-preserved fossils. The most common fossils are soft-bodied medusae (jellyfish), followed by linguloid brachiopods, algae, and arthropods (namely chasmataspidids, leperditiid ostracods, and eurypterids). This site is considered a Konservat-Lagerstätte, and is commonly referred to as the Big Hill Lagerstätte or Big Hill Biota.[1][2]
Depositional environment
The Big Hill formation likely represents a shallow, sheltered basin within the photic zone, suggested by the completeness and lack of transport of the algal specimens. However, the lack of dessication cracks and adhesion structures suggests that there was no subaerial exposure and the absence of ripple marks suggests that the basin was at least several meters deep.
The basin was likely dysoxic, as suggested by the limited diversity and lack of bioturbation. In addition, the lack of rhynchonellate brachiopods and echinoderms as well as the abundance of lingulids suggests that the basin had a different salinity than normal marine environments; however, direct evidence of hypersalinity like evaporite molds is absent. Microbial mats provide evidence of ecological stress.
While the basin was largely sheltered, it was likely connected by an extent to open marine ecosystems, as shown by the presence of chasmataspidids, eurypterids, and scyphozoan medusae. The medusae represent different stages of decomposition, suggesting that they were not all deposited in a single event but rather accumulated over time on the seabed via waves or currents. In addition, the orthocones may have been empty shells that drifted into the basin.
Fossil content
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Color key
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Notes
Uncertain or tentative taxa are in small text; crossed out taxa are discredited.
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Chelicerates
Trilobites
Crustaceans
Lophophorates
Mollusks
Cnidarians
Algae
See also
References
- ^ a b c d e f g h i j k l Lamsdell, James C.; LoDuca, Steven T.; Gunderson, Gerald O.; Meyer, Ronald C.; Briggs, Derek E. G. (2017). "A new Lagerstätte from the Late Ordovician Big Hill Formation, Upper Peninsula, Michigan". Journal of the Geological Society. 174 (1): 18–22. doi:10.1144/jgs2016-059. ISSN 0016-7649. S2CID 85551499.
- ^ a b Lamsdell, James C.; Gunderson, Gerald O.; Meyer, Ronald C. (2019-01-08). "A common arthropod from the Late Ordovician Big Hill Lagerstätte (Michigan) reveals an unexpected ecological diversity within Chasmataspidida". BMC Evolutionary Biology. 19 (1): 8. doi:10.1186/s12862-018-1329-4. ISSN 1471-2148. PMC 6325806. PMID 30621579.
- ^ a b Bingham, Joseph; Lamsdell, James C.; Meyer, Ron; Gunderson, Gerald (2024-09-22), EARLY EVOLUTION OF SEA SCORPIONS: EURYPTERIDS (CHELICERATA: EURYPTERIDA) OF THE BIG HILL LAGERSTÄTTE, Geological Society of America Abstracts with Programs, doi:10.1130/abs/2024AM-401218
- ^ Lamsdell, James C.; Falk, Amanda R.; Hegna, Thomas A.; Meyer, Ronald C. (29 May 2025). "Exceptionally preserved ovaries in an ancient horseshoe crab". Geology. doi:10.1130/G53317.1.
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Michigan
United States
