This is Mazon Monday post #127. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Today, we have an article called "The Inarticulate Brachiopods of Pit 14" by Earl Hoffman. This piece appeared in the July/August 1972 edition of the ESCONI newsletter "Earth Science News". Earl was Historian and later Micro-mount study group chairman for a few years.
Brachiopods are animals that look similar to bivalve mollusks, but are actually belong to their own Phylum - Brachiopoda. There are two types of brachiopods, inarticulate and articulate. Inarticulatr brachiopods are missing the tooth-and-groove structures of the valve-hinge present in the articulate group.
Articulate ("jointed") brachiopods have a tooth and socket arrangement by which the pedicle and brachial valves hinge, locking the valves against lateral displacement. Inarticulate brachiopods have no matching teeth and sockets; their valves are held together only by muscles. (R. C. Moore, 1952)
All brachiopods have adductor muscles that are set on the inside of the pedicle valve and which close the valves by pulling on the part of the brachial valve ahead of the hinge. These muscles have both "quick" fibers that close the valves in emergencies and "catch" fibers that are slower but can keep the valves closed for long periods. Articulate brachiopods open the valves by means of abductor muscles, also known as diductors, which lie further to the rear and pull on the part of the brachial valve behind the hinge. Inarticulate brachiopods use a different opening mechanism, in which muscles reduce the length of the coelom (main body cavity) and make it bulge outwards, pushing the valves apart. Both classes open the valves to an angle of about 10°. The more complex set of muscles employed by inarticulate brachiopods can also operate the valves as scissors, a mechanism that lingulids use to burrow.[9]
Each valve consists of three layers, an outer periostracum made of organic compounds and two biomineralized layers. Articulate brachiopods have an outermost periostracum made of proteins, a "primary layer" of calcite (a form of calcium carbonate) under that, and innermost a mixture of proteins and calcite.[9] Inarticulate brachiopod shells have a similar sequence of layers, but their composition is different from that of articulated brachiopods and also varies among the classes of inarticulate brachiopods. The Terebratulida are an example of brachiopods with a punctate shell structure; the mineralized layers are perforated by tiny open canals of living tissue, extensions of the mantle called caeca, which almost reach the outside of the primary layer. These shells can contain half of the animal's living tissue. Impunctate shells are solid without any tissue inside them. Pseudopunctate shells have tubercles formed from deformations unfurling along calcite rods. They are only known from fossil forms, and were originally mistaken for calcified punctate structures.[10][11]
Pit 14 was located southwest of Pit 11, just south of the South Wilmington. It is currently private property. It was known for having a large black shale deposit. Back in Mazon Monday #34, we posted an article about Pit 14 fossils.
Here is a satellite picture from google maps.
THE INARTICULATE BRACHIOPODS OF PIT 14 by Earl Hoffman
Pit 14 of the Northern Illinois Mine of the Peabody Coal Company is located Win Kankakee Co. (N.W. 1/4, Sec. 16, T 31 N, R 9 E) between Essex and South Wilmington, Illinois. The strip mining process deposits on the surface of the spoil piles a black, laminated shale which originally lay either directly above the coal or was separated from the coal by a rather thin layer of gray shale. This black shale appears to correlate with the Mecca Quarry Shale Member (Zangerl & Richardson, 1963). It is a member of the Liverpool Cyclothem of the Carbondale Formation of the Kewanee Group and is of Pennsylvanian age (Kosanke, et. al., 1960).
Fossils are collected by splitting the pieces of shale on the spoil piles and examining all surfaces for the remains of living organisms. Microscopic examination usually reveals specimens too small to be located by the unaided eye. At the north end of the pit (as it was being worked in 1971) the black shale consisted of a laminated, fissle shale and a compact non-fissle gray and black banded component. This non-fissle component yields a distinctly different fauna consisting of brachiopods, gastropods, and pelecypods while the harder, fissle shale yields conodont, shark, and fish remains.
The phylum Brachiopods is divided into two classes--Inarticulata and Articulate. The inarticulate brachiopods are further divided into two orders--the Atremata and the Neotremata (Shrock and Twenhofel, 1953 and Fenton and Fenton, 1958). The inerticulate brachiopods are distin guished from the articulate brachiopods by the lack of hinges between the two valves or shells; the valves being held together only by muscles.
Both orders of the in articulate brachiopods are present in Pit 14. The Atremete (meaning without a strictly defined pedicle opening, the opening being shared by both valves) are represented by the ancient and is still living genus Lingula of the family Lingulidae.
I have identified 32 specimens as Lingula carbonaria Shumerd (dunbar & Condre, 1932 and Sturgeon & Hoere, 1968). The shell is nearly elliptical in outline, sometimes with nearly straight and parallel lateral margins and sometimes with a rather pointed posterior (Figure 1). The specimens range in size from 2.8 mm wide by 4.1 mm long to 8 mm wide by 13 mm long. The outer surfaces of the vzlves often have well defined concentric growth lines numbering from 50 to 80 per mm but "the exceedingly fine radial strise" noted by Dunbar and Condra (1932) were not observed with magnifications up to 80x. A few specimens exhibit brown and white concentric banding which is coarser then the growth line spacing. This pattern may be representative of the color pattern in the living specimens.
Figure 1: Lingula species--exterior view
The order Neotremate is represented by two genera of the family Discinidae which are characterized by circular and flattened conical valves; the pedicle valve containing a pedicle opening in a modified pedicle slit (Figure 2).
Figure 2: Exterior view of the pedicle valve of, an Orbiculoidea species.I identify 25 specimens as Orbiculoides missouriensis Shumard (Dunber & Condra, 1932 and Sturgeon & Hoere, 1968). The size varies from 4 mm to 11 mm in diameter with the length slightly greater then the width. The shells show concentric growth lines numbering 4 to 8 mm depending on the specimen and the distance from the margin of the shell. A few specimens show concentric brown bands..
A single specimen is identified as Lindstroemella patule Girty (Dunbar & Condra, 1932). The specimen is a brachial valve interior 17 mm in diameter. The diagnostic "y shaped muscle scars (Figure 3) described and pictured by Dunbar and Condra (1932) are well defined. these scars are not present in any Orbiculoidea species. Dunbar and Condra (1932) reported that this species had not been found north of Oklahoma.
Figure 3: Interior view of a brachial valve of Lindstroemella patula.
A single distorted and broken pedicle value of diameter 17 mm may be either L. patula or 0. cepliformis McChesney. O. missouriensis is ruled out on the basis of size. Dunbar and Condra (1932) indicate that the pedicle valves of the first two named species are distinguishable only by the vertical profile which is strongly deformed in this specimen. The brachial valve of O. capuliformis would be distinguishable from L. patula by the absence of the "Y" scar.
I would welcome the opportunity to examine specimens from this locality and offer my opinion on their identification. I am especially interested in specimens which do not appear to fit any of the above described species and specimens larger than 10 mm (about 3/8 inch) in diameter or specimens showing distinct muscle scars.
My specimens are available for examination.
Dunbar, C. O. and Condra, G. E. 1932, Brachiopoda of the Pennsylvanian System in Nebraska, Nebraska Geol. Surv., Bulletin 5, 2nd Series.
Fenton, C. L. and Fenton, M. A., 1958, The fossil book, Doubleday and Company, Inc., New York.
Kosanke, R. M., Simon, J. A., Wanless, H. R., and Willman, H. B., 1960, Classification of the Pennsylvanian strata of Illinois, Ill. State Geol. Surv., Rpt. 214.
Shrock, R. R. and Twenhofel, W. H., 1953, Principles of invertebrate paleontology, 2nd ed., McGraw-Hill, New York.
Sturgeon, M. T. and Hoare, R. D., 1968, Pennsylvanian brachiopods of Ohio, Ohio Geol. Surv., Bulletin 63.
Zangerl, R. and Richardson, E. S., Jr., 1963, The paleoecological history of two Pennsylvanian black shales, Fieldiena: Geology. Memoirs, Vol. 4, Chicago Natural History Museum.
