Microfossils of the Middle MississippianSalem Limestone: Midwest U.S.A.*
by Alan Goldstein *Originally published in the Digest of the Mid America Paleontological Society (MAPS) in1999, edited 2008.
Introduction
The Salem Limestone contains one of the most famous microfossil faunas in the world. Near Bedford, Indiana, the lithology of this formation allows it to be used as dimension stone. It can be cut and carved into exterior building stones and statuary. One of the most famous buildings covered with Indiana or Bedford limestone (the commercial name) is the Empire State Building. It is covered with billions (if not trillions) of tiny fossils!
Lithology
The Salem Limestone has been called an oolitic limestone by some (see Cumings et al, 1906, p. 1190). However, oolites (ooliths) are tiny pellets formed by CaCO3 forming around a nucleus. They are of chemical, not organic origin (Whitten & Brooks, 1972). In addition, other lithofacies are present that do not contain the abundance of microfossils discussed below.
Once considered a dwarf fauna, the Salem is now considered to be size-sorted (see, for instance, Feldmann, 1984, p. 11). In other words, the ocean currents sorted out the larger fossils elsewhere and the smaller fossils (typically 0.5 - 2 mm) were deposited together, not unlike a sandstone. There are certain layers where larger fossils, particularly fenestrate bryozoans and brachiopods may be found. In addition, even the fine-grain sediment of the dimension stone has the occasional larger fossils within.
Stratigraphy
The Salem Limestone is bounded by the Harrodsburg Limestone beneath and the St. Louis Limestone above. It is Meramecean (Valmeyerian) or Middle Mississippian age, approximately 350 million years old. The type locality is the railroad cut at Spergen Hill, Washington County, Indiana. Exposures are widespread in south-central Indiana, occurring in Monroe, Greene, Lawrence, Harrison and Floyd counties. In Kentucky the Salem Limestone occurs in far-western Jefferson County, Bullitt, Meade, Hardin counties and points south and east to Pulaski County, associated with the top of the Muldraugh Hill escarpment.
In Kentucky and in southern-most Indiana (Floyd County), the lower part of the Salem Limestone is actually a shale facies called the Somerset Shale, named after the city of Somerset in Pulaski County. This shale (less than three feet thick in Indiana) contains large mats of bryozoans and rare complete three-dimension colonies of the bryozoan Cystodictya lineata Ulrich. Pentremites, Platycrinites, extremely rare Tricoelocrinus, shark teeth, and the odd coral Palaeacis can also be found. Microfossils are not easily observed.
The Salem Limestone occurs in surface exposures on the south western part of the Illinois. Baxter (1960) divided the Salem into four members, which in ascending order are the Kidd, Futts, Chaffin and Rocher.
In Jefferson County, Missouri (southwest of St. Louis), the formation collected on State Hwy 20 is a siltstone. It might be the equivalent of the Futts Member in Illinois which is dolomitic. The microfauna occurs with macrofossils and all are molds or steinkerns (internal casts).
Collecting Salem Microfossils
I have collected microfossils on my family's former property near Lanesville, Indiana. Cumings and Beede (1906) noted the Lanesville area as a classic microfossil area. I have also collected material on Highway 37, immediately north of Bedford. It is also possible to collect samples from freeze-thaw debris from buildings with Indiana Building Stone exteriors. I cannot recommend using a rock hammer on buildings and statuary!
While a student at the University of Louisville, I noticed other microfossil collectors. House sparrows were eating microfossils that weathered out of building stone. Presumably they were using these as gastroliths or as a source of calcium.
The best way to collect the microfossils is to locate an exposure where the calcium cement has dissolved leaving a residuum of loose microfossils. They can be swept into a small container or zip-type baggy with a small paint brush. If there is a large quantity of sediment, a spoon or small trowel can be used.
After obtaining a sufficient quantity, the next step is to clean it. Care must be used, or your collection will end up down the drain or in some other equally useless place. I use a fine-mesh screen or cheese cloth. Remember, you are trying to clean fossils that may be smaller than 0.5mm in diameter! Another method is to put the sample in a bucket, fill it partially with water and swish it around to release soil (clay) particles. Slowly decant the water into another container while trying to keep the sand-like microfossils in the first container. Repeat the process until the water is clean, then drain the contents onto a lint free cloth or sheet of newspaper and let it dry completely. The dried fossils will stick to one another, but gently massaging the fossil "sand" with your fingers will separate them.
Largest of the small fossils (view about 2 cm) Somewhat smaller fossils, dominated by crinoid columnals Examining the fossils
The size of these microfossils make them ideal for a binocular microscope with relatively low magnification (10x is sufficient, 5x - 25x zoom is even better). Pour the fossils out into a shallow tray (I use the lid of a 35mm film canister.) Select a minimal amount, so that the fossils are not piled upon one-another. I use a fine artists paint brush dampened slightly so specimens will adhere. A brush with a few hairs allows one to nimbly select a single fossil out of the "masses" for separation and closer examination. I keep a number of containers on either side of the microscope so that I can sort a variety of specimens at one time. Gastropods, foraminfera, ostracods, sponge spicules, crinoids, etc., are each assigned their own container. They can later be subdivided by species, if so desired, or mounted in a micropaleontology specimen slide mount if one is available.
Small fossils (FOV ~1 cm), note echinoid spine fragments Paleontology - Focusing in the microfossils
A variety of tiny fossils can be observed. This section will describe those typically found intact, or in small fragments that can be identified. Larger (>5mm) fossils are occasionally found in the mix and are interesting to examine, too. Abrasion from being tumbled on the sea floor is apparent on some fossils. This includes blunt-tipped gastropods, broken echinoid spines, fragmented ostracods, rounded edges on crinoid calyx plates, etc. Some fossils (i.e. gastropod shells and fringed crinoid columnals) show extraordinary surface detail, indicating fairly rapid burial after death. Most of the preserved fossil remains are as a result of the death of the organism. Only the trilobites molted as they grew, and their fragments are surprisingly rare (although it could be a bias from my collecting areas). For comparison, I dissolved a block of Early Devonian Garra Limestone from New South Wales, Australia. The silicified (quartz-replaced) residuum contains plenty of trilobite pleural segments, cephalon plates and pygidia. Bryozoans and brachiopods are abundant, but mollusks and echinderm remains are scarce. This could be a bias of silicification.
Foraminifera The calcareous foraminiferan Globoendothyra baileyi (Hall) is ubiquitous within the Salem Limestone. Even the smallest quantity of sediment will contain examples. There is some variation in size, but they are readily identifiable with a microscope or a 10x loupe. Other genera of forams may be seen, but I have not found literature describing them.
Corals Fragments of corals may be observed in the sediments. Most juvenile corals are large enough to be sorted out. The most common coral, Hapsiphyllum cassedayi (Milne-Edwards and Haime) is too large to be considered as a microfossil, even though fragments or the tip of a coral may be seen under the scope. Fragments of Cystelasma (another rugose coral) or Cladochonus (an auloporid tabulate) may be observed, although they tend to be on the large size, as well.
Echinoderms Fragments of echinoderms dominate the Salem Limestone. There is a tremendous variety of shapes and sizes of stem and body plates. The variety no doubt greatly exceeds the number of known species by a significant margin. No starfish (asteroids or ophiuroids) have been described from the Salem Limestone, but as they are found in Osagean and Chester-age strata, especially associated with crinoid anal tubes, they are very likely among the disarticulated echinoderm remains found in the sediment.
Crinoids A variety of columns and columnals can be observed. Among the most interesting are those fringed with multiple cirri and oval Platycrinid columnals. Numerous plate fragments can be observed and most cannot be assigned to a particular genus, much less a species! Of those that can, basal and radial plates of Strimplecrinus coxanus Worthen, Dichocrinus simplex Shumard, Hyrtanocrinus ornatus Wachsmuth & Springer and Synbathocrinus swallovi Hall may be identified by their plate ornamentation or shape.
Blastoids By far, the most abundant blastoid in the Salem fauna is Pentremites conoideus Hall (also see Galloway and Kaska, 1957). It is possible to find tiny thecas (bodies) and an assortment of tell-tale plates in the mix. Ambulacral plates are occasionally seen. Columns are indistinguishable from crinoids.
Echinoids Socket plates and spines of echinoids like Archaeocidaris norwoodi Hall are fairly abundant. Spines may be longer than many crinoid columns in the same mix, but are readily apparent by their tapering to a sharp point and lack of segments. Some still contain the terminal ball that connected to the socket plate. The socket attachment is also easily recognized in the sand-size particles with magnification. Juvenile echinoids must have been as small as a pea.
Echinoid plate, spine, and blastoid ambulacral plate (right) Holothuroids Sea cucumbers are perhaps the most abundant echinoderm on the sea floor today. Most holothuroids do not contain the calcified plates like most echinoderms. They contain tiny plates called sclerites. Most are microscopic and readily disperse after death; consequently sea cucumbers are rare in the fossil record. Cumings, et al (1906), attribute some radiating spicules to holothuroids. This writer is does not know if these spicules belong to sea cucumbers or sponges. Many lack the symmetry of other Mississippian sponges (i.e. hexactinellids) I have observed.
Worms Three species of Spirorbus worm (S. annulatus Ulrich, S. imbricatus Hall and S. nodulosus Hall) are commonly found in the Salem. Usually this tiny coiled worm is attached to a substrate, such as a shell or bryozoan, but they are found loose (and often incomplete) in the "oolite." Species are distinguished by their surface ornamentation. One species of Cornulites, C. blatchleyi (Beede) has been described. It appears as wrinkled and somewhat annulated, narrow funnel-shaped shell and can be found attached to bryozoans and brachiopods.
Worms Bryozoans Bryozoans are usually considered too large to be microfossils, but fragments of fronds such as Worthenopora or Rhombopora often observed in the remains. The fenestrate bryozoans are particularly interesting. Distinguishing the numerous species of Polypora is extremely challenging. Differentiating that from Hemitrypa or Fenestella is only slightly less difficult. One of most interesting is Pinnatopora with its off-set branching.
Bryozoans Brachiopods There are a great variety of brachiopods - productids, rhynchonellids, spiriferids, and terebratulids. Most of these are juveniles - larger specimens may also be found in the formation where depositional characteristics were different.
Brachiopods (bryozoan on left) Mollusks Perhaps the most recognizable fossils are the mollusks, particularly the snails. Of the rarer mollusks, two species of chiton have been recognized.
Gastropods Snails are common in the microfauna, Cumings et al (1906) illustrate some 40 species. Their diminutive size makes identification of some species difficult. Whether the name of the genera are still accurate is unknown by this writer.
Gastropods Polyplacophora (Chitons) Rare in the fossil record, chitons are represented by head, tail and intermediate valves. The writer has not observed any in Lanesville sediments.
Rostroconchs Cumings & Beede et al (1906) illustrate five species under the name "Conocardium." Whether this is accurate is questionable. Similar rostroconchs from the New Providence Shale have been identified by Dr. Richard Hoare as Pseuomuloscens. Although the Salem rostroconchs are small, they rarely occur as microfossils.
Pelecypods (Bivalves) With only a quarter of species compared to the snails, the pelecypods are not as abundant. Without magnification, the shell symmetry of some genera is difficult to distinguish between some brachiopods.
Trilobites Instars of young trilobites have not been documented in the Salem Limestone. Trilobite molt fragments are also scarce, probably too delicate to survive the grinding action of the calcareous sand-sized particles composed of other fossils on the sea floor.
Ostracods Ostracods are fairly common in the sediment. Most are smooth like Paraparchites carbonaria (Hall). Single and articulated carapaces are found in equal numbers. Geis (1932) and Brayer (1952) described the ostracods from this formation.
Ostracods Conodonts A number of conodonts have been reported from the Salem by Nicoll and Rexroad (1975). The dark colored phosphatic elements are in marked contrast with the other calcified fossils. The writer has not observed any in sediments near Lanesville. They are most often collected using techniques developed for conodonts.
Acknowledgements** The writer thanks Dr. Alan Horowitz for reviewing and updating the faunal list. Larry Osterberger provided some stratigraphic information. Margaret Kahrs motivated me to write this paper. ** All deceased (2009). Any reader aware of more recent fossil names for any species on the list, please contact me: park@fallsoftheohio.org.
Bibliography
Baxter, J., 1960. Salem Limestone in southwestern Illinois. Illinois State Geological Survey, Circular 284, 32 p. Brayer, R. C., 1952. Salem Ostracoda in Missouri. Journal of Paleontology, v. 26, p. 162-174. Cumings, E. R., Beede, J. W., Branson, E. B., and Smith, E. A., 1906. Fauna of the Salem Limestone of Indiana. Indiana Dept. of Geol. and Nat. Resources, 30th Annual Report, p. 1187 – 1486. Feldmann, H. R., 1984. Paleontology and paleoecology of the Somerset shale member of the Salem limestone (Mississippian) in central Kentucky. M.A. Thesis, Indiana University, 112 p. Galloway, J. J. and Kaska, H. V., 1957. Genus Pentremites and its species. Geological Society of America Memoir 69, 104 p. Geis, H. L., 1932. Some Ostracodes from the Salem Limestone, Mississippian, of Indiana. Journal of Paleontology, v. 6, p. 149 - 188. Whitten, D.G.A. and Brooks, J.R.V., 1972. Dictionary of Geology, Penguin Books, London, 516 p. Willman, H. B. and 7 other authors, 1975. Handbook of Illinois Stratigraphy, Illinois State Geological Survey, Bulletin 95, 261 p. Appendix 1: Salem Limestone General Faunal List Updated January 12, 2012
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