Fossil turtles in the Eocene Bridger Formation Wyoming

Fossil turtles in the Eocene Bridger Formation, Wyoming: How and why were they preserved? Research by: Leonard Brand, Ph. D, Loma Linda University Tom Goodwin, Ph. D, Andrews University

Research approach: • The Bridger Formation, in SW Wyoming, is an extensive set of Eocene sediments very rich in vertebrate fossils. • The mammal fossils have been collected since the 1870 s, but the abundant turtles have been mostly left alone • The conventional geology explanation for the Bridger Formation is accumulation of river, lake, and flood plain sediments over several million years

Research approach: We studied it to answer the following questions: What can the fossil turtles tell us about how the Bridger formed? Was it formed at least somewhat catastrophically? How large an area do the individual limestone or mudstone deposits cover? How does it relate to the Genesis flood? Formed during the flood? Formed after the flood?

The lowest part of the Bridger, Bridger A, is the green area on this map. We studied the sediments above this – Bridger B

Three layers have abundant turtle fossils – labeled on this photo of Bridger B All these sediments are volcanic ash that has been transported here by wind or water, and chemically altered

Above - fossil turtles Left – collecting turtles

First study site: the central unshaded area at left is a 22 acre exposure of turtle-rich mudstone. It has close to 400 fossil turtles. Below: the turtle-rich mudstone

Turtles are abundant in the mudstones right above a limestone The limestone accumulates in water – a lake or other water body Other sediments were deposited mostly by flowing water Turtle-rich mudstone Limestone

The thousands of turtles are generally complete shells, with almost no skulls and few limb bones Taphonomy = study of how and why organisms become fossilized Taphonomy research indicates that when turtles die, their heads disarticulate early (in ~ a week), then limb bones. Shells last several months This seems to explain the condition of the fossil turtles

The thousands of turtles are generally complete shells, with almost no skulls and few limb bones If many fossils are all in the same stage of decay, as the turtles are, this indicates they all died at the same time, not at many different times through the years The Bridger turtle-rich layers represent mass mortality events, apparently caused by episodes of volcanic eruptions, which killed and buried the turtles

From 1869 to 1990 the geological deposits in the Bridger had not been mapped Such mapping is important in geological and paleontological research – to know which deposits, in different areas, formed at the same time The geological literature said the limestones were from local ponds and lakes, and thus the limestones could not be mapped over large areas Limestones

We wished to know if that was true, to know how catastrophic and continuous the Bridger really was We began mapping the limestones, and found that almost all could be mapped over the basin – several hundred sq. miles Map of limestones in one topographic quadrangle – the Needle Reservoir quad. Colored lines are the limestone marker beds, where they are exposed along hillsides

After completing the mapping, we chose 21 study sites (black dots) across the Black Mountain turtle layer for comparison of turtle abundance

Fossil turtles were not in local concentrations as would be expected from local ponds and lakes, but the number of turtle bones showed a basin-wide pattern The deaths – mass mortality events – and burial of the turtles represented large-scale events over hundreds of square miles

Conclusions Various types of evidence convince us that the Eocene Bridger represents events after the global flood, not in the flood At each of the three layers studied, large numbers of turtles in extensive lakes were killed and buried within weeks to a few months by volcanic events As each volcanic episode stopped, another extensive lake formed, with more turtles These sediments and fossils appear to have formed in a fairly short time, but longer than a few days or weeks

Conclusions This research resulted from a desire to understand how to relate the Bridger Formation to flood/postflood processes Research guided by a biblical worldview was successful, resulting in publications in scientific research journals