Date of Award
Master of Science (MS)
Soft-shelled turtle fossils (Family: Trionychidae) are common in Late Mesozoic and Early Cenozoic freshwater terrestrial deposits throughout western North America. Taxonomy of the Trionychidae is confusing for two reasons: 1) there are differences of opinion as to which characters are important in defining the lower systematic groups; 2) trionychid fossil material is often incomplete and fragmentary and has been used as the basis for numerous taxonomic descriptions.
The Ash Coulee Quarry is a fossil turtle assemblage located within the lower portion of the Sentinel Butte Formation (Paleocene) in Billings County, North Dakota. This quarry has produced at least 40 individual trionychid turtles. The turtle fossils occur within a 10 - 20 cm thick layer of poorly indurated carbonaceous claystone that also contains a large number of log fragments, abundant molluscan fossils, and rare remains of other vertebrates.
The completeness and abundance of the fossil specimens allowed for a detailed description of the fossil trionychid material. Fifty-nine osteological characters used in studies of extant trionychid turtles were identified for the Ash Coulee trionychids. Taxonomic affinity of the Ash Coulee taxon within the Trionychidae was determined through cladistic analysis with the program PAUP 3.1.1 rt.1. The taphonomic conditions of the fossil turtle assemblage were analyzed through comparison with modem examples of turtle death assemblages. The stratigraphy of the site was described, and thickness of units were measured using a Jacobs StaflTM and Brunton™ compass. A sedimentological analysis was made of the lithologic units at the site.
The Ash Coulee Quarry is the largest assemblage ofthis type of trionychid turtle recorded in North America. Morphological analysis and comparison with similar fossil taxa, along with cladistic analysis preliminarily indicate that the Ash Coulee trionychid is a new genus. This new taxon is referred to here as Unnamed Trionychid A (UTA). The most closely related extant trionychids appear to be a clade of tropical African and Southeast Asian soft-shell turtles known as the cyclanorbines.
The Ash Coulee Quarry is interpreted as either a mass mortality assemblage, or a composite time averaged concentration because of the large number of individuals occurring within a restricted stratigraphic interval. The turtle carcasses floated for no longer than 0.5 years and came into contact with a number of logs where they eventually settled. The completeness of the fossils suggests that they were buried rapidly. The cause of death is unknown. The sedimentology and paleontology of the site indicate that the turtles inhabited a calm, quiet-water setting with abundant plant material.
Kays, Glenn B., "Description and taphonomy of a Turtle Assemblage (Chelonia: Trionychidae) in the Sentinel Butte Formation (Paleocene), Billings County, North Dakota" (1999). Theses and Dissertations. 156.