Hyaenodon horridus

Click here to see the skeleton of Hyaenodon

The Creodonts (flesh-tooth) are a long extinct group of carnivorous mammals whose rise as meat eaters is an example of parallel evolution to modern carnivores; the Fissipedia.

Creodonts are thought to have evolved from primitive insectivores of the late Cretaceous. The variety of sizes and forms that Creodonts achieved rival that of the Fissipeds. Sometime during the Paleocene, the Creodonts branched into two families; the Oxyaenidae, which were rather catlike in form, and the Hyaenodontidae, which were more dog like. The order became extinct in the Pliocene epoch, perhaps unable to compete with the rapidly evolving felids, canids and ursids. Not to mention the increasing adaptation towards speed in prey species, most notably in the Artiodactyls ( Even-toed herbivores).

Creodonts are characterized by relatively unspecialized features in comparison to today's carniores. The teeth, which are obviously adapted for flesh eating, are rather morphologically conservative when comparing them to modern Fissipeds. They did have carnassial ( flesh-shearing ) molars, but these were placed further back in the tooth row than in modern carnivora.  Click here to see the skeleton

Other features of many Creodonts were the short stocky limbs, and the five toed feet (pentadactylus) which ended in short, blunt, hoof-like claws. Though not terribly speedy creodonts were probably very active hunters.

Hyaenodon horridus,  is considered to be a cursorial type, and therefore was an effective predator. But still unable to survive past the Pliocene. 

Hyaenodon horridus is a common predator from the Oligocene epoch of western North America. Its remains are found in the Chadron and Brule formations of the Nebraska and South Dakota badlands. Hyaenodon was wolf sized. It had a long, low lumbar ( back ) region, a primitive feature that it shared with many Creodonts and other groups of ancestral early mammals.

Hyaenodons skull was long and massive and filled with large bone-crushing teeth. But its brain case was very small, so the huge skull must have served mostly to anchor large jaw operating muscles. Like all Creodonts, the skull had no ossified ( bony ) auditory bulla. These bulbous structures are found on the rear underside of the skull in many modern mammals, and housed the delicate bones of the inner ear. This structure was probably cartilaginous in Creodonts and thus was not fossilized.

This painting depicts two Hyaenodon approaching the carcass of a Merycoidodon. An Oreodont whose numerous fossil remains tell us that these herbivores must have roamed the pre-badlands plains in large numbers.

The scavenging bird shown here is a hypothetical crow-like opportunist. 

Because of the fragile nature of their hollow bones, bird fossils are very rare in any time period of the geologic record since their appearance in the Jurassic. Avian species that are known to exist during the Oligocene world include; rails, Grebes, flamingos, woodpeckers, hawks, eagles, vultures,  limpkins, and cranes. This evidence shows that many modern forms were already well established at this time. Though this record so far does not include crows or other smaller scavenger types, there surely must have been species to fill this niche during the Oligocene. This is how I justify the depiction of an animal whose existence is not yet supported by fossil evidence.