Doctoral Dissertations

Date of Award

12-1989

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Thomas W. Broadhead

Committee Members

Kenneth R. Walker, Robert E. McLaughlin, Edward E. C. Clebsch

Abstract

The Decatur Limestone (Pridolian) and Ross Formation (Geddinnian/Lochkovian), which crop out in the western valley of the Tennessee River, are a series of fossiliferous bioclastic limestones and fine-grained terrigenous clastic rocks which span the Silurian-Devonian boundary. These formations were deposited at a paleolatitude of 20 to 30 degrees south on a shallow shelf bounded by the Illinois Basin to the north, the Black Warrior Basin to the south, the Nashville Dome to the east, and the Ozark Uplift and New Madrid Rift Zone to the west.

The Decatur and Ross contain diverse assemblages of echinoderms with a total of 38 genera and 69 species in 7 classes, with the crinoids comprising the greatest number of taxa. Of the crinoids, camerates are most diverse (16 genera, 33 species) followed by disparids (8 genera, 11 species), flexibles (4 genera, 13 species), and cladids (2 genera, 4 species). Among the other echinoderm classes, blastoids are represented by 3 genera and 3 species; rhombiferans, edrioasteroids, cyclocystoids, stylophorans, and echinoids are each represented by a single genus and species.

Preservation of the echinoderms ranges from nearly complete specimens to completely disarticulated debris. Several genera, particularly among the flexibles, are known vi from nearly complete crowns, whereas the cyclocystoid Sievertsia sp. and the disparid crinoid Pygmaeocrinus sp. are recognized only from isolated ossicles. This style of preservation is interpreted to have been produced by episodic storms. The complete crowns represent living individuals and the cups represent forms that were somewhat resistant to disarticulation, buried by storm entrained sediment. The cups of some of the crinoids, particularly among the camerates, were resistant enough to disarticulation to serve as sites of epibiont colonization. The most abundant identifiable echinoderms occur in several beds of coarse debris within the Decatur Limestone, and in the lower Roc]chouse Limestone and the upper Birdsong Shale (bryozoan zone) members of the Ross Formation.

The echinoderm fauna preserved in the Decatur and Ross formations compare most favorably with those found in Oklahoma (17/32 genera in common), Bohemia (13/29), and the Armorican Massif (7/22). These comparisons indicate a greater degree of interchange among the echinoderm faunas of these regions than has been previously reported. Representatives of the genus Pvomaeocrinus are noted for the first time outside Bohemia. Within the Decatur-Ross interval, Scvphocrinites and Marsuoiocrinus (Amarsupiocrinus) make their last appearance, and Dolatocrinus first appears.

Seven new genera and 13 new species of echinoderms are described herein, including Probalocrinus dianis (Strimple), 1963 n. comb.. Eudimerocrinus hlabsei n. sp., Dimerocrinites (Dimerocrinites) cheilobathron n. sp., Macrostvlocrinus tertibrachialis n. sp., Marsupiocrinus (Aroarsupiocrinus) devonicus n. sp., Paramarsupiocrinus broadheadi n. gen. et n. sp., Parasupiocrinos broweriM/u> n. gen. et n. sp., Eodolatocrinus hlabsei n. gen. et n. sp., Eohalvsiocrinus broweri n. sp., E. bretti n. sp., Icthyocrinus eruaatus n. sp. Parahormocrinus decaturensis n. gen. et n. sp., Eohadroblastus inexpectatus n. gen. et n. sp., and Eodevonocystis marilynni n. gen. et n. sp.. In addition, the blastoid Leptoschisma lorae (Dunbar) is referred to Decaschisma. and the disparid crinoid Phimocrinus americanus Springer is referred to Theloreus. Most previously described taxa have been re-diagnosed in order to emphasize those characters that serve to distinguish them.

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