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mbeyer747

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Between 2020-23, two collectors who scuba dive for fossils throughout Florida and Georgia have recovered 5 chesapecten (including two paired valves) with morphological characteristics that signal a Miocene age. These characteristics include an acute byssal notch and a byssal fasciole that is strongly differentiated from the shell’s auricle in terms of sculpture and elevation. The largest of the adult shells also displays an active ctenolium. Additionally,  one of the paired specimens displays significant gapes between valves when matched (the other pair was preserved as found by glue according to the collector and cannot be matched). These aforementioned traits are also emblatic of Miocene age for Chesapecten.  

 

These shells were recovered from the following areas in Georgia and Florida:

 

Savannah River, Effingham County, Georgia (Collector 1) Specimen 1 (W = 108.0 mm) 

R valve

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L valve

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R valve - close up of byssal notch and fasciole (most of fasciole has been degraded) 

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R valve - close up of ornamentation

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L valve - close up of ornamentation

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Profile

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Close up of matrix, gray sand

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Savannah River, Effingham County, Georgia (Collector 1) Specimen 2 (W = 101.6 mm) 

R valve

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R valve - interior

681774241_s2backr.jpg.f233f56a0d4604fed08e0f65736d240f.jpg

 

R valve - close up of byssal notch and fasciole

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L valve - note barnacles are modern species, not fossilized

828291118_s2back.jpg.f58d1043ef97040ed8cb121770c93ec1.jpg

 

L valve - interior

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L valve - close up of ornamentation on auricle

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Side profile of pair, showing gapes

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Front profile of pair, showing gapes

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Cumberland Island, Camden County Georgia (Collector 2) Specimen 3 (W = 114.3 mm)

R valve, note encrustation is recent not fossilized 

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R valve interior, thick shell apparent

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Close up of byssal notch and fasciole

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Close up of ctenolium, although modern encrustation makes it difficult to see what is going on in the ctenolium

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Close up of ornamentation

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St Mary’s River, Nassau County, Florida (Collector 2) Specimen 4 (W = 117.5 mm) 

R Valve

1623302825_sp4front.thumb.jpg.62c7d842afaafb2f2cb79803b8b4ef64.jpg

 

R valve interior, active ctenolium and thick shell apparent

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Byssal notch and fasciole

1721861078_s4notch.jpg.02ae008eb81d081c27e69d4dae6ac976.jpg

 

Close up of original sediment, note the olive and gray coloration

930170292_s4withdirt.thumb.jpg.2e18fbc736165aa8bc2b6251e19dc224.jpg

 

Profile 

85422641_s4profile.jpg.8f0124dc015448df6376dcef807f5344.jpg

 

 

Suwanee River, Hamilton County, Florida (Collector 2) Specimen 5 (W = 69.9 mm) 

R valve, subadult specimen

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R valve interior, shell is thick for a subadult

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Unfortunately, stratigraphic data were not collected for these shells. However, among the Miocene strata from Coastal Georgia and NE Florida currently described in the literature, the Ebenezer Formation of Weems and Edwards (2001), of Upper Miocene (Tortonian age), appears to be the most suitable match based on the age of the Ebenezer and the characteristics of the shells found. The shells collected resemble Chesapecten middlesexensis of the Upper Miocene of Virginia and North Carolina.

 

The Ebenezer was originally defined by Huddleston (1988) as a member of the Coosawhatchie Formation (Middle Miocene). Weems and Edwards later elevated it to formational rank based on differences in lithological and dinoflagellate composition compared to the rest of the Coosawhatchie. The Ebenezer formation consists of gray to olive-gray, fine- to medium-grained micaceous sand and stretches from South Carolina to NE Florida. Five mappable members are apparent and separable by distinct unconformities. The lower four members correspond to dinoflagellate zone DN 8, while the uppermost member corresponds to DN 9.

 

Revision of the Ebenezer to Formational Rank from Weems and Edwards (2001)

image.thumb.png.e490ea5d25c9e419609e84f965555a8a.png

 

According to the dinoflagellate zonation of de Verteuil and Norris (1996), DN 8-9 aligns with the Little Cove Point Member (DN 8) and the Windmill Point Member (DN 9) of the St Mary’s Formation of Maryland and Virginia. 

 

Alignment of the Ebenezer to St Mary's Formation of MD and VA from Weems, Self-Trail and Edwards (2004)

image.thumb.png.b8181140c50b54d6538ce96e78e2ad10.png

 

 

All specimens display similar characteristics which include an acute byssal notch, differentiated byssal fasciole, slightly inflated right valve, and a hinge size in adult specimens that is relatively small for adult chesapecten with the exception of Chesapecten covepointensis (DN 8 St Mary’s Formation) and in some cases Chesapecten santamaria (DN 9 St Mary’s Formation).

 

Also, these shells could possibly be divided into two distinct variants although issues with preservation which appears to be somewhat better outside the Savannah River region may exaggerate these differences. Nevertheless, the Chesapecten collected outside of the Savannah River Region exhibit stronger, more raised ribs and have thicker, heavier shells compared to the specimens collected within the Savannah River region whose shells are thinner and ribs are lower and less pronounced. This is especially true of Specimen 1.  Possibly that these variants originate from different members of the Ebenezer Formation. According to Weems and Edwards, “outside of the Savannah region, beds no older than dinoflagellate zone DN 9 occur”. This suggests that the shells collected outside of the Savannah River Region likely belong to Bed 5 of the Ebenezer Formation. Figure 3 of Weems and Edwards (2001) [shown below] suggests that someone scuba diving for fossils in the Savannah River is likely to collect in Bed 4. Therefore, it is possible that the Chesapecten specimens recovered from the Savannah River belong to Bed 4 of the Ebenezer Formation. This stratigraphic information aligns with the observed morphological differences among the specimens and tentatively supports the significance of these variations.  Needless to say, more specimens are needed to confirm. 

 

Lateral Gradation of the Ebenezer from Georgia to Florida - Fig. 3 from Weems and Edwards (2001) 

image.thumb.png.14164765ebda2bd8e8494bd7590c26b7.png

 

Ward (1992) has remarked that the period between Chesapecten santamaria (DN 9) and Chesapecten middlesexensis (DN 10) represents a considerable loss of the fossil record in the stratigraphic succession of chesapecten. These Chesapecten, which bear a strong overall resemblance to Chesapecten middlesexensis while displaying traits of preceding species (smaller hinge, more differentiated byssal fasicole), could help bridge this apparent gap.

 

Notably, no other Chesapecten in this age range outside of Maryland and Virginia have been reported in the literature.

 

Personal Remarks
The equivalency of these shells to the St Mary’s Formation, not the Eastover formation is surprising to me given the strong resemblance to C. middlesexensis. If anyone knows of any findings correlating DN 8-9 to the Eastover, or of the Ebenezer to DN 10 please let me know. Also, if anyone has any additional samples of similar shells from similar sites, even in SC please let me know. Thank you!

 

References

  • de Verteuil, L., and Norris, G., 1996, Miocene dinoflagellate stratigraphy and systematics of Maryland and Virginia: Micropaleontology, vol. 42 (Supplement), 172 p.
  • Huddlestun, P.F., 1988, A revision of the lithostratigraphic units of the coastal plain of Georgia; the Miocene through the Holocene: Georgia Geologic Survey Bulletin, no. 104, 162 p.
  • Ward, L.W, 1992, Molluscan biostratigraphy of the Miocene, Middle Atlantic Coastal Plain of North America, VMNH Memoirs, no 2, 152p.
  • Weems, R.E, Edwards, L.E., 2001, Geology of Oligocene, Miocene, and younger deposits in the Coastal Area of Georgia: U.S. Geological Survey, no 131, 129 p.
  • Weems, R.E, Self-Trail J., Edwards, L.E., 2004, Supergroup stratigraphy of the Atlantic and Gulf Coastal Plains (Middle? Jurassic through Holocene, eastern North America): Southeastern Geology, volume 42, p 191-216

 

 

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Edited by mbeyer747

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