Phenacodus
Encyclopedia
Phenacodus is an extinct genus
of mammals from the late Paleocene
through middle Eocene
, about 55 million years ago. It is one of the earliest and most primitive of the ungulate
mammals, typifying the family Phenacodontidae
and the order Condylarthra.
The typical Phenacodus primaevus was a relatively small ungulate, of slight build, with straight limbs each terminating in five complete toes, and walking in the digitigrade
fashion of the modern tapir
. The middle toe was the largest, and the weight of the body was mainly supported on this and the two adjoining digits, which appear to have been encased in hoofs, foreshadowing the tridactyl type common in perissodactyls and certain extinct groups of ungulates. The skull was small, with proportionately minute brain; and the arched back, strong lumbar vertebrae
, long and powerful tail, and comparatively feeble fore-quarters all proclaim kinship with the primitive carnivores Creodonta
. All the bones of the limbs are separate, and those of the carpus and tarsus do not alternate - each one in the upper row is placed immediately above the corresponding one in the row below. The full series of forty-four teeth was developed; and the upper molars were short-crowned, or brachyodont, with six low cusps
, two internal, two intermediate and two external, so that they were of the typical primitive bunodont structure.
In habits, the animal was cursorial and herbivorous, or possibly carnivorous. In the early Paleocene of North America, the place of the above species was taken by Tetraclaenodon puercensis, an animal only half the size of Phenacodus primaevus, with the terminal joints of the limbs intermediate between hoofs and claws, and the first and fifth toes taking their full share in the support of the weight of the body. These two genera may be regarded as forming the earliest stages in the evolution of the horse, coming below Hyracotherium
(see Equidae
). As ancestors of the artiodactyl section of the Ungulata, we may look to forms more or less closely related to the North American Lower Eocene genus Mioclaenus, typifying the family Mioclaenidae. The species of Mioclaenus were five-toed, bunodont Condylarthra, with a decided approximation to the perissodactyl type in the structure of the feet. A second type of Condylarthra from the North American Lower Eocene is represented by the family Meniscotheriidae, including the genus Meniscotherium
.
These, it is suggested, may have been related to the ancestral Hyracoidea. Teeth and jaws probably referable to the Condylarthra have been obtained in European early Tertiary formations. All Ungulata probably originated from Condylarthra.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of mammals from the late Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
through middle Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
, about 55 million years ago. It is one of the earliest and most primitive of the ungulate
Ungulate
Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...
mammals, typifying the family Phenacodontidae
Phenacodontidae
An extinct family of large herbivorous mammals in the order Condylarthra.Dentition shows that species like Pleuraspidotherium and its relatives were probably browsers....
and the order Condylarthra.
Description
Digitigrade
A digitigrade is an animal that stands or walks on its digits, or toes. Digitigrades include walking birds , cats, dogs, and many other mammals, but not plantigrades or unguligrades...
fashion of the modern tapir
Tapir
A Tapir is a large browsing mammal, similar in shape to a pig, with a short, prehensile snout. Tapirs inhabit jungle and forest regions of South America, Central America, and Southeast Asia. There are four species of Tapirs: the Brazilian Tapir, the Malayan Tapir, Baird's Tapir and the Mountain...
. The middle toe was the largest, and the weight of the body was mainly supported on this and the two adjoining digits, which appear to have been encased in hoofs, foreshadowing the tridactyl type common in perissodactyls and certain extinct groups of ungulates. The skull was small, with proportionately minute brain; and the arched back, strong lumbar vertebrae
Lumbar vertebrae
The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body...
, long and powerful tail, and comparatively feeble fore-quarters all proclaim kinship with the primitive carnivores Creodonta
Creodonta
The creodonts are an extinct order of mammals that lived from the Paleocene to the Miocene epochs. They shared a common ancestor with the Carnivora....
. All the bones of the limbs are separate, and those of the carpus and tarsus do not alternate - each one in the upper row is placed immediately above the corresponding one in the row below. The full series of forty-four teeth was developed; and the upper molars were short-crowned, or brachyodont, with six low cusps
Cusp (dentistry)
A cusp is an occlusal or incisal eminence on a tooth.Canine teeth, otherwise known as cuspids, each possess a single cusp, while premolars, otherwise known as bicuspids, possess two each. Molars normally possess either four or five cusps...
, two internal, two intermediate and two external, so that they were of the typical primitive bunodont structure.
In habits, the animal was cursorial and herbivorous, or possibly carnivorous. In the early Paleocene of North America, the place of the above species was taken by Tetraclaenodon puercensis, an animal only half the size of Phenacodus primaevus, with the terminal joints of the limbs intermediate between hoofs and claws, and the first and fifth toes taking their full share in the support of the weight of the body. These two genera may be regarded as forming the earliest stages in the evolution of the horse, coming below Hyracotherium
Hyracotherium
Hyracotherium , also known as Eohippus or the dawn horse, is an extinct genus of very small perissodactyl ungulates that lived in the woodlands of the northern hemisphere, with species ranging throughout Asia, Europe, and North America during the early Tertiary Period and the early to mid Eocene...
(see Equidae
Equidae
Equidae is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus Equus...
). As ancestors of the artiodactyl section of the Ungulata, we may look to forms more or less closely related to the North American Lower Eocene genus Mioclaenus, typifying the family Mioclaenidae. The species of Mioclaenus were five-toed, bunodont Condylarthra, with a decided approximation to the perissodactyl type in the structure of the feet. A second type of Condylarthra from the North American Lower Eocene is represented by the family Meniscotheriidae, including the genus Meniscotherium
Meniscotherium
Meniscotherium is an extinct genus of dog-sized mammal which lived 54-38 million years ago. It was a herbivore and had hooves. Fossils have been found in Utah, New Mexico. and Colorado. Many individuals have been found together, indicating that it lived in groups....
.
These, it is suggested, may have been related to the ancestral Hyracoidea. Teeth and jaws probably referable to the Condylarthra have been obtained in European early Tertiary formations. All Ungulata probably originated from Condylarthra.