Name Meaning:  Western Dog

Geologic Era:  Eocene

Location Found:  South Dakota

Estimated Range:  North America

Size:  .8 meters long

Extinction:   Mid-Oligocene

This little fragment came to me under the name Cynodictis gragarius, which initiated a bit of detective work.  As it turns out, Cynodictis is a synonym for the animal more properly named Hesperocyon gregarius.  This was a happy coincidence, as I had been after a Hesperocyon specimen for some time.  This piece is from the right side of the lower jaw (mandible) and has a full tooth and pieces of two others still attached.  Hesperocyon was a small animal (under a meter in length), but has the distinction of being one of the first, if not the first, true canids.   

In terms of appearance, Hesperocyon rather resembled a cross between a fox and a ferret.  This was an animal probably better suited to life in the trees than on the open ground.  Hesperocyon probably hunted smaller animals in the tree limbs, or in short chases on the ground.  The teeth show Hesperocyon to be a very early canid, especially the reduced number of teeth from other, earlier mammals.  This pattern of tooth reduction continued into modern canines.  

Hesperocyon also has inner ears that were encased by bone, as opposed to cartilage as in earlier animals.  Like modern canines, Hesperocyon was primarily carnivorous, though as any dog owner can state, dogs will eat other food items if the opportunity presents itself.  Hesperocyon’s short legs were ill-equipped for long chases, so a forested environment would have been ideal for it.  This is in contrast to modern canines such as wolves, who are excelled pursuit predators.

While Hesperocyon and its kin were very successful animals, changes in their landscape ultimately proved the end to this group.  Starting in the Oligocene, Earth’s climate transitioned from a warm forest to a cooler, drier, and more open landscape.  Animals who relied on concealment to hunt, such as Hesperocyon, were at a great disadvantage compared to animals with longer legs and faster running speeds.  Hesperocyon’s short legs allowed to accelerate quickly, but also made it too slow for a long chase.  The animals who could sustain a chase were the ones who survived the climate transition.  Other animals that went extinct near this time were titanotheres such as Megacerops, which also relied on more forested environments.  While Hesperocyon itself might be gone, many of its descendants remain, some in people’s backyards or living rooms.  

Image Credits:


Skeleton:  By Claire H. from New York City, USA – Hesperocyon gregarius (Dog), CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=5311426


Life Restoration:  By Robert Bruce Horsfall – http://www.archive.org/details/ahistorylandmam00scotgoog, Public Domain, https://commons.wikimedia.org/w/index.php?curid=12808852