SDSM 571

This week's
This week’s “What Is It Wednesday” fossil.

This week’s “What Is It Wednesday” fossil is SDSM 571, a left jaw of an animal originally described as Pliocyon walkeri, but now known as Ischyrocyon gidleyi.  Another jaw of the same taxon was featured in the “Private Property” post.

SDSM 571, left dentary of Ischyrocyon gidleyi.
SDSM 571, left dentary of Ischyrocyon gidleyi.

Ischyrocyon belongs to the family Amphicyonidae, more commonly known as “bear-dogs”. Amphicyonids were a family of carnivores phylogenetically positioned between bears and dogs, hence the nickname. Although found worldwide, they are relatively common in North America from the Eocene to the late Miocene. Early amphicyonids were small, dog-like carnivores, but by the middle of the Miocene, they had reached the gigantic proportions of lions or bears. They went extinct at the close of the Miocene, presumably replaced by large felids (cats), ursids (bears), or large canids.

As we mentioned on Wednesday, this specimen has an interesting back story. In 1957 it was collected from a gravel pit in what was then called the Thin Elk Formation, now the Ash Hollow Formation, near Mission, SD. However, it was not collected via “traditional” means. J. R. MacDonald (1969) describes the collection process…

“Most of the collection was made while the pit was being worked for select borrow for highway construction. Sand and gravel were bull-dozed into a hopper and, by means of a conveyor belt, were fed onto a vibrating screen. Trucks and trailers were loaded with the desired grades of material that fell through the screens, and the residue was discarded by means of a chute. In order to retrieve the fossils, a collector stood at the bottom of the chute with a garden rake and salvaged the specimens before they were destroyed by falling rock. Needless to say, lumps and bruises were the fruit of overzealous collecting when a series of specimens came down the chute in a short rush. During later operations, a crusher was used in the pit, and the collector stood on a walkway beside the vibrating screen and grabbed the specimens before they were devoured by the mechanism.”

Satirical drawing of the collection process done by Mary Butler of the L.A. County Museum published in Harksen and MacDonald (1969).
Satirical drawing of the collection process, done by Mary Butler of the L.A. County Museum, published in Harksen and MacDonald, 1969.

The Mission Fauna has since become a very important fossil assemblage from the Clarendonian Land Mammal Age. The majority of the fauna was collected by the means described above, including dogs, proboscideans, horses, rhinos, tapirs, peccaries, oreodonts, camels, and pronghorn antelope.

MacDonald and his colleagues were engaged in an early form of what today is known as “monitoring paleontology.” These paleontologists work with various types of construction crews to recognize and salvage fossil resources before they’re destroyed by development processes. Monitoring paleontology is a huge part of modern paleontology that employs a great number of people. There is a common misconception among the scientific community that monitoring paleontology contributes little to paleontologic discovery. The Mission Fauna is a shining example of how untrue this assertion is.

Be sure to acknowledge and thank your favorite monitoring paleontologist!

Harksen, J.C., and MacDonald, J.R. 1969. Guidebook to the Major Cenozoic Deposits of Southwestern South Dakota. Guidebook 2. South Dakota Geological Survey. 103p.
MacDonald, J.R. 1969. An Early Pliocene Fauna from Mission, SD. Journal of Paleontology. 34(5):961-982.
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