Rhizocyon is an early member of the subfamily Borophaginae

Borophaginae

The subfamily Borophaginae is an extinct group of canids called "bone crushing dogs" that were endemic to North America during the Oligocene to Pliocene and lived roughly 36—2.5 million years ago and existing for approximately .-Origin:...

, an extinct subgroup of canids that were endemic to western North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 during the Whitneyan

Whitneyan

The Whitneyan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 33,300,000 to 30,800,000 years BP, a period of . It is usually considered to fall within the Early Oligocene...

 and Arikareean

Arikareean

The Arikareean North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 30,600,000 to 20,800,000 years BP, a period of . It is usually considered to overlap the Oligocene and Miocene epochs...

 stages) of the Oligocene

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

 epoch, living from ~33.3—20.6 Ma., existing for approximately .

Rhizocyon was similar to a contemporary species, Archaeocyon leptodus

Archaeocyon

Archaeocyon is a small extinct genus of the Borophaginae within the family Canidae which inhabited most of North America during the Whitneyan stage through Geringian stage of the Oligocene epoch 33.3—26.3 Ma Archaeocyon existed for approximately .Species of Archaeocyon are among the earliest...

, from the Great Plains

Great Plains

The Great Plains are a broad expanse of flat land, much of it covered in prairie, steppe and grassland, which lies west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S...

, but it shows a few subtle differences in the structure of the skull

Skull

The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 and dentition

Dentition

Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...

 that indicate that Rhizocyon may be close to the ancestry of later borophagines. Only a single species, R. oregonensis, is known and all fossils come from the John Day Formation

John Day Formation

The John Day Formation is series of rock strata exposed in the John Day River basin of north-central Oregon in the United States. The strata, which vary in age from 39 million years to 18 million years, were formed mainly from ashfalls from volcanoes to the west...

 in Oregon

Oregon

Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...

.

Morphology

Fossil specimens of two individuals' body mass were examined by Legendre and Roth.

• Specimen 1: 1.51 kg (3.3 lb).
• Specimen 2: 1.57 kg (3.5 lb).

Fossil distribution

• Foree Site, John Day Formation
  John Day Formation
  The John Day Formation is series of rock strata exposed in the John Day River basin of north-central Oregon in the United States. The strata, which vary in age from 39 million years to 18 million years, were formed mainly from ashfalls from volcanoes to the west...
  
  , Wheeler County, Oregon
  Wheeler County, Oregon
  As of the census of 2000, there were 1,547 people, 653 households, and 444 families residing in the county. The population density was 1 people per square mile . There were 842 housing units at an average density of 0 per square mile...
  
   ~30.8—20.6 Ma.
• Longview Ranch Airport, John Day Formation, [Wheeler County, Oregon ~30.8—20.6 Ma.
• North Blue Basin Site, John Day Formation, Grant County, Oregon
  Grant County, Oregon
  Grant County is a county located in the U.S. state of Oregon. It is included in the 8 county definition of Eastern Oregon. In 2010, its population was 7,445. It is named for President Ulysses S. Grant, who served as an army officer in the Oregon Territory, and at the time of the county's creation...
  
  ~33.3—30.8 Ma.
• Kimberly Member, John Day Formation, Grant County, Oregon ~30.8—20.6 Ma.
• Roundup Flat (aka South Haystack), Grant County, Oregon ~30.8—20.6 Ma.