The Coral Ridge Fauna
By Alan Goldstein
(This was originally published in the EXPO XIV Edition of the MAPS Digest in 1992. This article has been updated.)
Introduction
The Coral Ridge member of the New Providence Formation (Osagean, Middle Mississippian) provides a fascinating array of mollusks, echinoderms and other creatures. Yet, because of the paucity of collecting localities coupled with the low abundance, the fauna contains many poorly described species. This article will acquaint readers with the depositional environment, faunal abundance and collecting tips for the fossils which may be preserved in exquisite detail.
The Coral Ridge member and fauna were first described by Conkin (1957) from the Coral Ridge quarry of the General Shale and Brick Company, Jefferson Co., Kentucky. While some fossils are found in road cuts and small outcroppings, there are really only four documented localities where this fauna occurs. One is in Clark County, Indiana. The type locality has the most diverse fauna. More than 80 percent of the species are mollusks. Indeed, over 70 percent of the fossils (excluding traces) belong to a single species!
Stratigraphy and Paleogeography
According to Conkin (1957, 1972), the New Providence Formation consists of three members, in ascending order they are: the Coral Ridge member, the Button Mold Knob member, and the Kenwood Siltstone member. The formation is part of the Borden Group (Figure 1.). Some geologists list the Borden at formational level and assign the New Providence a ranking of member. The Coral Ridge and Button Mold Knob members are not differentiated because the lithology is similar (Figure 2.). The Coral Ridge is considered to be earliest Middle Mississippian age.
During the early Middle Mississippian, sedimentation in the east-central United States was dominated by deltaic deposition. Called the Borden delta, the sedimentary rock show evidence of conditions on the basin floor, the prodelta or foot of the delta, the delta slope, and the delta platform or top. Each portion of the delta had a different environment which supported various faunas.
The Coral Ridge fauna is associated with the basin beyond the delta’s farthest edge in the deepest water. During the earliest Mississippian, the environment was anaerobic, forming black carbon-rich shale. Conditions gradually changed to an oxygen-poor sea floor, as indicated by bioturbated greenish clay shale. This allowed life to exist in patchy communities where conditions were best-suited under a less-t犀利士 han-ideal situation.
Pyritic steinkerns (internal casts) are the most common form of fossil preservation. Trace fossils of animals that burrowed in the mud may be preserved as pyrite in three-dimensions. The occasional fossil with the external shell ornamentation preserved shows incredible detail. The oxygen-poor conditions were likely anoxic beneath the top few centimeters of sediment (Kammer, 1985) allowing microenvironments for the sulfate-reducing bacteria. They would react with detrital iron to in-fall the empty aragonite exo-skeletons with pyrite. In altered to marcasite, siderite, goethite (“limonite”) and quartz. Fossils may be found partially geodized. In addition, conularids are preserved as phosphatic mineralization.
Paleoecology
The Coral Ridge fauna does not have the diversity of other Osagean faunas. The communities were likely of low abundance or only an extremely low number of exoskeletons were preserved. The fossils are dominated by small individuals. Stunting is thought to be the primary mechanism. This cannot be proven in most mollusks, but can be seen in the goniatites. With the exception of the corals (primarily a single species, Amplexus fragilis) the bulk of the fossils are smaller than one cubic centimeter. Very rarely, large gastropods, goniatites and nautiloids are found, but these make up less than one percent of the population. Examples include: Glabrocingulum – 5 cm, Loxonema – 7 cm, Sinuitina – 5 cm, Michelinoceras – 30 cm, and an unidentified coiled nautiloid – 8 cm.
Epifaunal deposit feeders, animals that can move around on the sea floor, dominate. Most are archaeogastropods (Kammer, 1985) – see Table 1. They were likely ingesters of detrital organic material. The actual depth of the basin is unknown. Kammer (1985) indicates that it was likely below the photic zone, however the abundance of tabulate corals indicate that light was present, though at very low levels. (Most tabulates were attached to crinoid columns, elevating them above the muddy sediment.)
The most abundant archaeogastropod is Glabrocingulum ellenae (Conkin), making up 72 percent of the fossils found and 92 percent of the epifaunal deposit feeders! It ranges in size from one millimeter to about three centimeters across. About one in 100 shows external ornamentation, many are steinkerns.
Less common is Trepispira, similar in form to Glabrocingulum which requires close examination to distinguish as a steinkern. The bellerophont gastropod Bucanella is much rarer, as is platycerid Orthynochia (as listed by Conkin, 1957). This snail is coprophagous, situating itself over the anal opening of crinoids.
The monoplacophoran Sinuitina annaea Conkin is uncommon when compared to Glabrocingulum populations, but this mollusk is typically scarce.
Trilobites are uncommon epifaunal deposit feeders. The Coral Ridge fauna is represented by two species. Phillibole conkini Hessler is the more common form, but it is still very rare. Brachymetopus spinosus (Herrick) is less common. Occasion phosphatic nodules are found composed of trilobite fragments – some of them are from large individuals.
Epifaunal suspension feeders including brachiopods, corals and echinoderms, make up about half of the species, but comprise less than 13 percent of the fossils found (table 1). Apparently circulation permitted enough food into the environment to allow a variety of epifaunal suspension feeders, but they did not thrive.
Favosites corals are found surrounding crinoid columns. The soft, muddy seafloor wouldnot allow larva to get established. In addition, the elevated colony could feed a few centimeters higher above the basin floor. Colony distribution on the crinoid columns is asymmetrical, indicating a growth preference, likely facing nutrient-bearing currents.
The tiny Crurithryis? sp. is the most common brachiopod. At one to five millimeters in width, this diminutive suspension feeder was the most successful animal living on the seafloor. It is more common than the infaunal suspension feeders which, while buried in mud, fed essentially from the same zone.
Echinoderms are highly diverse, but identifiable plates and calices are very rare. Blastoids with fused plates may be found as a complete head or theca. Granatocrinus kentuckyensis (Conkin) is the most common blastoid. Crinoid plates are usually bound by soft tissue. Upon death they disarticulate quickly. The depth of the Coral Ridge fauna precluded rapid storm burial, as a result, crinoidal material is typically column sections, single plates and rarely arm sections or basal cups. Crinoid holdfasts are the type with cirri spreading away in all directions at regular or irregular intervals along the length preserved. The longest crinoid column found by the writer is about 20 cm.
A substantially smaller number of fossil were infaunal deposit feeders, consisting of at least three genera of bivalves and a rostroconch, Psueomucelens cancellata (Hyde) (table 1). The variety of size and shape of the non-siphonate clams (Ctenodonta sp., Nuculopsis sp. and Phestia sp.) suggest a division of food resources in the sediment (Kammer, 1985). Soft-body infaunal deposit-feeders were abundant, as indicated by bioturbation of the shale and numerous pyritized trace fossils, including Scalarituba missouriensis Weller.
Unlike other Borden delta communities, the Coral Ridge fauna is relatively rich carnivorous cephalopod. Four goniatites and several nautiloids have been report (Work and Mason, 2004). The small goniatites Polaricyclus conkini Work & Mason and Polaricyclus ballardensis Gordon make up five percent of the fossil collected for this report. Other Cantabricanites? greenei (Miller)and Winchelloceras knappi Work & Mason are considerably less common. A larger goniatite is occasionally found at the Clark Co. locality. The distance above the seafloor that these cephalopods lived is not known.
Although the nature of Paraconularia sp. is not well understood, it is found in phosphatic exoskeletons with the Coral Ridge fauna. It is typically associated with the siderite nodules and in the double cone-in-cone nodules. These nodules may consist of numerous fragments or contain a single specimen preserved three-dimensionally. Opercula preservation is very rare.
Table 1
Species Number** % Fossil Feeding Notes
* = Photograph of this fossil at end of the article.
** Number in original article. Some species are now known with additional specimens (i.e., Barycrinus body plates).
Glabrocingulum ellenae (Conkin) 1024 72.2 MG ED Trepispira not listed in original publication. It is probably 2- 4% of the number.
Rugose Corals undifferentiated 141 9.4 CR ES Amplexus fragilis dominates, with Cyathaxonia and Baryphyllum in smaller numbers
Polaricyclus* (both species) 79 5.3 MC C This study done before the species were named.
Sinuitina annaea Conkin 42 2.8 MM ED
Loxonema sp. 36 2.4 MG ED Conkin (1957) lists L. delphincola
Crurithryis? sp. 22 1.4 BA ES
Michelinoceras sp.* 16 1.1 MC C
Cantabricanites? greenei (Miller)* 12 0.8 MC C
Nuculopsis sp. 12 0.8 MB ID
Phestia sp. 10 0.7 MB ID
Cyrtina-like brachiopod* 10 0.7 BA ES
Psueomucelens cancellata (Hyde) 8 0.5 MR ID
Winchelloceras knappi Work & Mason* 8 0.5 MC C
Ctenodonta sp. 7 0.5 MB ID
Granatocrinus kentuckyensis (Conkin)* 6 0.4 EB ES Highly ornate
Rhynchopora beecheri (Greger)* 6 0.4 BA ES
Phillibole conkini Hessler 4 0.3 AT ED
Punctospirifer? subelliptica (McChesney) 4 0.3 BA ES
Orbiculoidea sp.* 4 0.3 BI ES
Paraconularia sp. 4 0.3 CC ES
Bucanella sp. 3 0.2 MG ED
Sponge spicules, indeterminant 3 0.2 P ES
Synbathocrinus dentatus* 2 0.1 EC ES Conkin (1957)
Cyathocrinites australis Kammer* 2 0.1 EC ES Isolated plates
Magnumbonella? sp. 2 0.1 BA ES
Catillocrinus tennessensis 2 0.1 EC ES
Hadroblastus kentuckyensis?* EB ES Conkin (1957) lists Codaster jessieae; Xenoblastus sp. was in Conkin & Conkin (1976)
The following specimens were represented by a single specimen during the original study, although additional specimens have been found since.
Orthonychia sp. MG CO
Favosites sp. (F. divergens?) CT ES Conkin (1957)
Barycrinus – B. sculptis? EC ES Additional collecting revealed this to be more common than Synbathocrinus or Cyathocrinites.
Taxocrinus sp. EC ES
Platycrinites hemisphericus EC ES
Euryocrinus veryi (Rowley)* EC ES Found after initial study
Dichocrinus* EC ES Found after initial study
Dielasma? sp. BA ES Poorly preserved specimen, Conkin (1957) lists Girtyella.
Eumetria sp. BA ES
Brachymetopus spinosus (Herrick) AT ED
Crinoid columns and trace fossils are not included in this survey.
For collectors, there is one fossil considered abundant among this fauna – Glabrocingulum ellenae (Conkin). Trace fossils have been excluded because a single Scalarituba missouriensis Weller make break into many fragments. As a group, corals are relatively common. Amplexis fragilis (White & St. John) is often geodized and may be enlarged to as much as 15 cm in length. Multiple visits to the collecting sites does not change the percentage of the most common eight species, but the rarest (percentages <0.5 percent) have the changes from trip to trip. It is these fossils that add “spice” to the collecting trips.
The Collecting the Fauna
There are only four published collecting localities (Conkin, 1957; Kammer, 1985). This writer has collected from two – the quarry of the General Shale and Brick Company and the old Louisville Cement Company quarry near Sellersburg, Indiana. Both are private property and not accessible by non-scientists. The Indiana locality has more abundant Winchelloceras goniatites.
The sparse and patchy nature of this fauna means collectors must be very thorough. Without a systematic sweep of the outcrop, it is quite easy to miss the only Granatocrinus or Phillibole that has weathered out. It is also easy to miss a cluster of mollusks. Glabrocingulum is usually found in clumps of four or more within a 30 square centimeter area.
The soft greenish shale weathers rapidly. Unlike some formations where a single collector can stripe an outcrop for decades with a single visit, the nature of the lithology is self-sustaining. A couple of months (or several good downpours) between collecting trips is sufficient to re-concentrate the fossils. The rarest fossils are uniformly (and widely) distributed.
This article is not designed to aid the collector to gain access to known collecting sites, only provide information about an unusual Middle Mississippian fauna. Pyrite-replaced fossils are found in similar-aged formations throughout Kentucky. They are never abundant, particularly when compared to the faunas of the Glen Dean and other Upper Mississippian formations.
References
Ausich, W. I., Kammer, T. W., and Lane, N. G., 1979, Fossil communities of the Borden (Mississippian) delta in Indiana and northern Kentucky. Journal of Paleontology, v. 53, p. 1182 – 1196.
Conkin, James E., 1957, Stratigraphy of the New Providence Formation (Mississippian) in Jefferson and Bullitt Counties, Kentucky, and Fauna of the Coral Ridge Member. Bulletins of American Paleontology, no. 168, p. 109 – 157.
Conkin, J. E. and Conkin, B. M., 1972, Guide to the Rocks and Fossils of Jefferson County, Kentucky, Southern Indiana, and Adjacent Areas. University of Louisville Printing Service, 331 pp.
Conkin, J. E. and Conkin, B. M., 1976, Guide to the Rocks and Fossils of Jefferson County, Kentucky, Southern Indiana, and Adjacent Areas, second edition. University of Louisville Printing Service, 239 pp.
Kammer, T. W., 1982, Fossil communities of the prodeltaic New Providence Shale Member of the Borden Formation (Mississippian), north-central Kentucky and southern Indiana. PhD Dissertation. Indiana University, Bloomington, IN, 301 pp.
Kammer, T. W., 1982, Basinal and prodeltaic communities of the Early Carboniferous Borden Formation in northern Kentucky and southern Indiana (U.S.A.). Palaeogeography, Palaeoclimatology, Palaeoecology, v. 49, p. 79 – 121.
Weller, S., 1914, The Mississippian Brachiopoda of the Mississippi Valley Basin, Illinois State Geological Survey, Mon. 1, 508 p. (2 volumes).
Work, D. M., and Mason, C. E., 2004, Mississippian (Late Osagean) ammonoids from the New Providence Shale Member of the Borden Formation, north-central Kentucky. Journal of Paleontology, v. 78, p. 1128 – 1137.
Photo Album of Fauna
Brachiopods
Bryozoans
Cephalopods
Echinoderms – Blastoids (All pyrite-replaced)
Echinoderms – Crinoids (All pyrite-replaced)
