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Costatoblastus, a Channel Fill Blastoid from the Sappington Formation of Montana

James Sprinkle and R. C. Gutschick
Journal of Paleontology
Vol. 41, No. 2 (Mar., 1967), pp. 385-402
Stable URL: http://www.jstor.org/stable/1302019
Page Count: 19
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Costatoblastus, a Channel Fill Blastoid from the Sappington Formation of Montana
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Abstract

Numerous well-preserved specimens of the blastoid Costatoblastus sappingtonensis, n. gen., n. sp., are found within a small channel fill system in unit G of the Sappington Formation (Devonian--Mississippian) at a single locality in southwestern Montana. This new genus is characterized by having five paired spiracles, two anal deltoids, a rounded-biconical shape, and strong riblike ornamentation. The blastoids are thought to be of earliest Mississippian age. This single known occurrence is unusual because the specimens are found in a channel deposit of fine-grained sandstone. Blastoids are rare in sandstones of any type and have not been previously reported from channel fillings. Although most of the blastoid calyces have been crushed by the compaction of the surrounding sediments, many of the specimens are exceptionally well preserved having attached stems, brachioles, and protective cover plates. The specimens displaying attached appendages show a characteristic type of current orientation, indicating gentle current movement along the channel bottom. The flattened calcite specimens are found on weathered slabs of fine-grained sandstone and were successfully cleaned for study by using hydrofluoric acid. Well-preserved blastoid appendages that were revealed by this cleaning technique are described in detail and provide new information on blastoid morphology. The individual stem segments of this blastoid are rimmed by a prominent flange and can easily be distinguished from those of most fossil crinoids. Paleoecological features that are shown by the blastoids themselves, the sandstone channel fill system, the channel fauna, and the normal sediments of unit G are briefly described in an attempt to reconstruct the original environment of these blastoids. The authors have concluded that the blastoids probably lived in a large colony in the channel bottom within a few hundred feet of their final burial place in a very shallow-water marine environment. Individuals in this colony were apparently torn loose by current action and transported a short distance along the channel bottom before being buried in the sandy sediment.

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