Date of Award

1971

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Geology

First Advisor

Dr. Frank R. Karner

Abstract

From 12 collecting sites in the Knife River, southwestern North Dakota, at least 15 species of aquatic mollusks were found. From 4 fossil sites on the same river, at least 17 species of aquatic mollusks were found in addition to 11 species of fossil terrestrial gastropods. From 18 collecting sites in the Heart River, southwestern North Dakota, at least 18 species of aquatic mollusks are presently living. From 6 fossil sites on this river, at least 10 species of aquatic mollusks were found in addition to 4 species of fossil terrestrial gastropods. The unionid family was represented in the Knife and Heart River by each having 6 species living presently and 4 species as fossil. The pisidiid family was represented in both rivers by two genera. The Knife River contained 7 species of aquatic gastropods living presently and the Heart River contained 10 species living presently. As aquatic gastropod fossils, however, there were 11 species from the Knife River and 4 species from the Heart River.

Of the unionids, Leptodea laevissima (Lea) occurs alive in the lower reaches of both rivers but does not occur as a fossil. One specimen of Lampsilis ventricosa (Barnes) establishes the existence of this species in the Missouri River drainage presently; this species does occur as fossil. Anodontoides ferussacianus (Lea) was not found in the fossil assemblage of the Heart River.

The living aquatic gastropod faunas of the two rivers are approximately equivalent to the aquatic gastropod fossil fauna. The fossil assemblage of the Heart River did contain Probythinella lacustris (Baker), a species that was not found in the Knife River. The fossil presence of Armiger crista (Linnaeus, Stagnicola caperata (Say), and Lymnaea stagnalis (Linne) indicate that most of the fossil sediments studied were deposited in a pond or slow-water environment.

The fossil terrestrial gastropods do not indicate any different environments than those found presently on the two rivers. The requirements of Oxyloma retusa (Lea), preferring moist conditions, and Discus cronkhitei (Newcomb), preferring woodland conditions, are adequately met.

Therefore, a significant change in the regiman of the rivers is not evident as the total species are essentially the same, living or fossil.

The molluscan faunal fossil finds are post-5,440 years B. P. ± 200 years or late Holocene.

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