Hyracodon

Hyracodon ('hyrax tooth') is an extinct genus of perissodactyl mammal from the White River Formation.

Skeletal reconstruction of Hyracodon

Hyracodon Temporal range: Middle Eocene–Late Oligocene PreꞒ Ꞓ O S D C P T J K Pg N
Skeleton in Natural History Museum of Los Angeles
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	†Hyracodontidae
Genus:	†Hyracodon Leidy, 1856[1]
Species
†H. browni †H. leidyanus †H. medius †H. nebraskensis †H. petersoni †H. affinis †H. eximus †H. modestus †H. primus †H. princeps †H. priscidens

Description

It was a lightly built, pony-like mammal of about 1.5 m (5 ft) long. Hyracodon's skull was large in comparison to the rest of the body. Hyracodon was a much smaller animal than modern Rhinocerotoides and differed very little in appearance from the primitive horses of which it was a contemporary (32–26 million years ago). It had a short, broad snout and its long, slender limbs had three digits.^[2] When put next to its comtemporary, Mesohippus, Hyracodon has a proportionally long neck, with robust vertebrae. Compared to earlier hyracodontids from the Uinta Formation, like Triplopus, Hyracodon has more a more gracile manus structure. The median toe is enlarged whilst the lateral toes are greatly reduced. The molars of Hyracodon are similar to those of modern day Rhinoceros, with the last upper molar assuming a triangular shape. The canines and incisors, however were quite different. In form, the anterior teeth of Hyracodon were small and similar in shape, being pointed and curved.^[3]

H. nebraskensis skull, found near Hermosa, South Dakota.

Like the primitive horses, hyracodonts inhabited open forests and wooded steppes and turned from browsing foliage to grazing grass. They died out without leaving any descendants and they mark the end of the phylogenetic branch of hornless, running rhinocerotoids.

Illustrations

1896, Charles R. Knight

1913, Robert Bruce Horsfall

1920, Heinrich Harder