The shorebird-duck mosaic bird, Presbyornis, is best known from the Eocene Green River Formation where it often occurs in local monospecific bonebeds, suggestive of mass mortality events. The sedimentology, paleontology, taphonomy, XRD mineralogy, and the carbonate stable isotope signature (δ18O) of one of these bonebeds was studied on the southern margin of Eocene Fossil Lake.

A series of 408 standardized radiographs were used to construct the first large scale radiograph aided taphonomic map of a vertebrate fossil quarry (3,874 bones were recorded in 11.5 m2). A small group of 100 bones (including a complete skull and sternum) were removed from the micrite matrix to provide the first nearly complete composite skeleton of Middle Eocene Presbyornis. This is the first reported taphonomic map of an avian fossil quarry, the strongest evidence of associated Presbyornis skeletons, and the strongest linkage of mass avian death with Clostridium botulinum neurotoxin in the paleontologic literature.

At this Presbyornis site, two distinct mass mortality events are evident. The first event (lowest in the section) is a widespread aquatic gastropod mortality with localized eggshell and a few Presbyornis bones. This gastropod mortality event occurred on a surface of nondeposition. The second event (2 cm higher) is a broad strandline deposit of at least 66 Presbyornis carcasses (MNI=66).

This surface of nondeposition may represent a time when the gastropods colonized the lake bottom. This colonization was likely followed by a rapid lake regression, which exposed the carbonate mudflat, killed aquatic invertebrates, and enhanced the incubation of C. botulinwn within the decaying invertebrates. The exposed mudflat accumulated eggshell fragments and a few bones, as neurotoxins killed a large number of Presbyornis. Disarticulation of the avian carcasses preceded another freshwater transgression of the lake which deposited the avian bones along strandlines and preserved them within the next unit of rapidly deposited ostracodal calcimicrite.

Analogous modem avian botulism epizootics are often preceded by a mass mortality of aquatic invertebrates, which are exposed and killed by rapid shoreline regression. Many mechanisms have been proposed for mass mortality of waterfowl. The evidence from this site is most consistent with an interpretation of mass avian death due to ingestion of C. botulinwn neurotoxin.




Graduate School

First Advisor

H. Paul Buchheim

Second Advisor

Leonard R. Brand

Third Advisor

Robert A. Cushman, Jr.

Degree Name

Master of Science (MS)

Degree Level


Year Degree Awarded


Date (Title Page)




Library of Congress/MESH Subject Headings

Avian botulism -- Wyoming -- Lincoln County; Birds Fossil -- Wyoming -- Lincoln County; Presbyornis



Page Count

ix; 114

Digital Format


Digital Publisher

Loma Linda University Libraries

Usage Rights

This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.


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Loma Linda University. Del E. Webb Memorial Library. University Archives