Name:
Hyracotherium
(Hyrax like beast).
Phonetic: Hy-rak-o-fee-ree-um.
Named By: Richard Owen - 1841.
Classification: Chordata, Mammalia,
Perissodactyla, Palaeotheriidae.
Species: H. leporinum
(type).
Diet: Herbivore.
Size: Average 60 to 75 centimetres long.
Known locations: Across the Northern hemisphere,
But best known from Western Europe and North America.
Time period: Ypresian to Lutetian of the Eocene.
Fossil representation: Hundreds of specimens.
For
most of
its history Hyracotherium has been associated with
the genus Eohippus
(dawn horse) that was based upon fossils described by Othniel
Charles Marsh in 1876. This is mostly due to the commonplace
presentation of the name Eohippus towards the
public and subsequent
mention in fiction, the pinnacle of which was the appearance of a
stop motion animated Eohippus in the 1969 film
‘The Valley of
Gwangi’. However some study of Eohippus remains
have
led
palaeontologists to the conclusion that these remains are actually the
same as those of an even earlier named horse called Hyracotherium.
Because Hyracotherium was the first name
established it has, under
international rules governing the naming of animals, priority over
Eohippus which was named thirty-five years later.
Since this time though there has been further evaluation of associated
with these fossils which has led to Eohippus being
preserved as
separate to Hyacotherium.
The
first Hyracotherium
remains were discovered in England and described by Richard Owen, one
of the most important palaeontologists of the day. Owen initially
thought that he might be dealing with a hyrax, a small mammal that
still exists today, but were far more numerous during the Eocene.
However Owen only observed this similarity in the teeth, and much of
the skeleton of this specimen was missing, which resulted in the name
which means ‘Hyrax -like beast’. Marsh’s later described
specimen was virtually complete, and while it was named in the early
period of the ‘bone wars’, Marsh may have only had the
description of the incomplete remains of Hyracotherium
to compare his
discovery against.
Marsh’s
specimen, as well
as other later and more complete discoveries revealed the true nature
of Hyracotherium as one of the most primitive
horses so far known,
which often sees this animal placed in the staring position of the
evolution of horses. Some palaeontologists have even gone one step
further however by suggesting that Hyracotherium is
the form that is
ancestral to the perissodactyl (odd-toed ungulate) mammals that
also include tapirs and rhinos. This extension of the evolution
theory is not supported by all however, and currently most
palaeontologists only refer to Hyracotherium in
terms of horse
evolution rather than wider mammal groups.
Although
Hyracotherium is
seen as a primitive horse, it actually still had toes rather than
hooves, with four on the front feet and three on the rear feet.
Later horses including those we know today support their weight on a
single well developed toe that ends in a hoof. Hyracotherium
was
probably not on open plains runner though, and would have been better
off roaming the undergrowth around forested areas where it could easily
hide its small body from the eyes of the predators of the day.
However as the Eocene progressed the landscapes of the time began to
change from forest to a predominance of open plains that necessitated a
change to a faster running form.
The
teeth of Hyracotherium
are low crowned, a trait that is indicative of a browser of certain
plant parts like leaves and fruits rather than a grazer of grass.
There are forty-four teeth in total with a small gap called a diastema
near the front of the mouth similar to many mammals. This diastema
separates the collecting teeth at the front like the incisors that snip
off plant parts from the processing teeth at the back like the molars
that grind and mash food before it is swallowed. The molars also show
the beginning of the progression towards the molar form of modern
horses. Overall Hyracotherium is generally
thought to have lived and
had a similar ecological niche to that of a small forest dwelling deer.
Hyracotherium
is considered
to have been quite an intelligent animal for its time with a brain that
was proportionately larger in terms of comparison to its body size than
it mammalian contemporaries. It’s thought that this extra brain
size was an advancement towards its senses such as sight, smell and
hearing so that it could detect the presence of other animals,
particularly potential predators. With its lightweight build the
best chance that Hyracotherium had of staying alive
was to use speed
and agility to outrun predators before they got too close to be a
threat.
Further reading
- Description of the Fossil Remains of a Mammal (Hyracotherium
leporinum) and of a Bird (Lithornis vulturinus) from the
London Clay. -
Transactions of the Geological Society of London, Series 2, VI:
203-208. - Richard Owen - 1841.
- The beginning of the equoid radiation. - Zoological Journal of the
Linnean Society 112 (1–2): 29–63. - J. J. Hooker - 1994.
- Quo vadis eohippus? The systematics and taxonomy of the early Eocene
equids (Perissodactyla). - Zoological Journal of the Linnean Society
134 (2): 141–256. - D. J. Froehlich - 2002.
- Fossil Horses--Evidence for Evolution. - B. J. MacFadden - 2005.
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