 
un·gu·late
(un'gyoo-lit) [L., unguis, a hoof; -atus suffix meaning
provided with]
NOUN: a mammal
having hooves |
Literally, "ungulate" refers to any animal with hooves - a hoof being an
enlarged toenail (see below). However, in practice, the use of the name
"ungulate" has been inconsistent. While it was originally used to refer
to the orders Artiodactyla (even-toed) and Perissodactyla (odd-toed) - the
"true" ungulates - over time the term expanded to seven different extant
Mammalian orders . . . some of which have no hooves whatsoever! This
broadening of the definition was based on presumed family relationships -
relationships that recent advances have shown to be artificial. As a result,
ungulate is now understood to have no taxonomic significance, and
its definition has returned to its original descriptive roots: a mammal
with hooves.
COUNT THE HOOVES: some
ungulates and their feet.
What is a HOOF?
A hoof is really just a modified toenail. Hooves, claws, and nails
are all composed of two structures: the unguis (a scale-like plate;
our finger- and toe-nails) and the subunguis (a softer layer, found
as a very fine layer on the underside of our fingernails) which connect the
unguis to the pad of the digit. Unlike claws and nails, hooves are
the principal point of contact between the legs and the ground - as a result,
ungulates are said to have unguligrade limbs.
In ungulates, the tough unguis encircles the tip of the digit as a cylinder,
enclosing the subunguis within in. Since the unguis is harder than
the subunguis, it does not wear down as quickly, resulting in a firm (sometimes
sharp) leading edge to the hoof. The pads of the digits lie behind
the hooves - these pads touch the ground in perissodactyls and some
cetartiodactyls, while only the hooves bear weight in other ungulate
cetartiodactyls (notably the Suiformes and Ruminantia).
In many cetartiodactyls, only the central two digits touch the ground. The
lateral digits are often reduced (if they are present at all) and only touch
the ground in soft terrain. Because of this, these lateral digits are
called "false hooves" or "dewclaws".
Why hooves?
The development of hooves illustrates a major innovation in the evolution
of a cursorial (running) lifestyle, pushed on early ungulates by fast-running
predators. Hooves have such an extensive structure because they support
all of the body weight of ungulates. Why is this important? Since
only the hooves touch the ground, the rest of the parts of the foot have
essentially become parts of the leg, substantially increasing the length
of stride. Modern ungulates have taken this to the extreme: the
metapodials (the bones between the wrist/ankle and the digits) are often
as long as the other parts of the legs. Raising the heel and digits
off of the ground also increases the number of joints which move the
legs forward - this increases the rate of stride. Coupled together,
these two factors have given modern ungulates the speed needed to survive,
an evolutionary imperative which would not have been possible without
the development of hooves.
UNGULATE: A mammal with HOOFS or
HOOVES?
The 1964 Edition of Webster's Dictionary gives the plural of hoof as
"HOOFS, rarely HOOVES". Over the past
few decades, usage patterns of the plural forms of hoof have changed: the
online Merriam-Webster's Dictionary (2006) gives the plural as
"HOOVES, also HOOFS." Officially, either
is correct! |
What is an UNGULATE?
The Historical Perspective
Anatomy and paleontology have historically been the main tools used to determine
evolutionary affinities. The two orders of true ungulates were grouped
together early on, on account of their similar leg structures and (of course)
their hooves. Even the names of the orders - Artiodactyla and
Perissodactyla - were created in tandem, implying that the number of toes
was the main difference between the even- and odd-toed ungulates.
Since the hoof was the defining character of the ungulates, feet were the
focus of researchers trying to decipher their origins. The large flat
nails of elephants, hyraxes, and sea cows - collectively called the
"paenungulates" ("almost ungulates") - were thought to represent an evolutionary
intermediate between traditional claw-like nails and true hooves.
Originally placed at the base of the ungulate lineage, continued research
suggested that the paenungulates were more specialized than the true ungulates,
nesting them firmly in the ungulate family tree . . . thereby requiring that
the definition of ungulate be expanded.
Anatomical studies and fossil findings connected these five orders (along
with several extinct groups of ungulates) to a single common ancestor - the
early ungulate order Condylarthra from the Paleocene Epoch (65 to 54.8 million
years ago). It also became apparent that Condylarths had evolved into
other mammals as well, namely the aardvark (Tubulidentata) and the nail-less
whales and dolphins (Cetacea). Because of the (presumed) common ancestry
of these orders, a taxonomic infraorder - Ungulata - was created,
encompassing all descendants of Condylarthra . . . and as a result, the
definition of "ungulate" was expanded to include animals without a single
hoof!
UNGULATE: The Modern
Definition
For many years, "ungulates" were grouped together based on a relationship
to an extinct ancestor, not on the basis of hooves. The morphological
family tree (above) was the accepted classification of mammals, and was
continually being fine-tuned as new fossil evidence was uncovered. (The
position of cetaceans next to Artiodactyla was only resolved in the late
1990s). However, recent advances in molecular technology have painted
a vastly different picture of how the different groups of mammals are related,
relying on similarities in genetic codes rather than appearances. While
the first few studies were greeted with skepticism, the consistency of genetic
markers has caused a paradigm shift in modern mammal taxonomy (see the Molecular
Tree below). This has had two major implications for ungulates:
While the order Cetacea was thought be from a Condylarth lineage, the strength
of this tie was not realized until genetic research found a strong affiliation
with hippopotamuses. In fact, based on genetic markers from several
genes, it has been determined that Cetacea is actually a SUBSET of the order
Artiodactyla. Merging two well-defined orders together challenged many
established beliefs, and only recently has the result - the order
Cetartiodactyla - been widely accepted.
Molecular research has also caused the Ungulata to evaporate from mammalian
phylogenies. As seen in the family tree to the left, the members of
the former infraorder Ungulata are now widely spaced apart. The "true"
ungulates - Cetartiodactyla and
Perissodactyla - remain closely allied,
although these groups are far more closely related to pangolins (Pholidota),
bats (Chiroptera), carnivores (Carnivora), and insectivores (Eulipotyphla)
than they are to the paenungulates. Based on molecular AND fossil evidence,
the branch of the tree containing the true ungulates arose on the continent
of Laurasia, and hence, they are titled the "Laurasiatheria". As for
the paenungulates, although they are no longer associated with the true
ungulates, they DO have a strong evolutionary relationship with each
other. These four orders, along with the the elephant shrews
(Macroscelidea) and tenrecs and golden moles (Afrosoricida), form the
"Afrotheria", so named due to an evolutionary history in Africa.
But what of the Ultimate
Ungulate Page?
When this website first started, Ungulata (without cetaceans) was still accepted
as a taxonomically-based (if paraphyletic) grouping. Even as the close
ties between cetaceans and ungulates were established, their specialized
nature meant that there were never any plans to add these "distant relatives"
to the Ultimate Ungulate roster. Cetaceans still represent a
highly specialized group of mammals, and, even though they are now included
within an ungulate order, their presence here would make very little
practical sense. Similarly, there is little justification for including
the (unrelated and un-hoofed) paenungulates on an ungulate website (perhaps
the Paenultimate Paenungulate Page will come into being in the future!?).
Which leaves us with the true ungulates: "Artiodactyls" and Perissodactyls;
two groups of large, herbivorous animals equipped with hooves. Although
they no longer form a good taxonomic group, their similar biologies makes
it convenient to continue using the name "ungulate" in a descriptive sense.
UltimateUngulate.com is thus focussing in on TRUE ungulates with HOOVES,
returning to the initial definition of ungulate, first put in place
when humans started classifying animal life!
Now that you know what an ungulate is, keep
exploring this site:
  
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Literature Cited
Guralnik, D. B. 1964. Webster's New World Dictionary, Concise
Edition. The World Publishing Company: United States of America.
Martin, R. E., R. H. Pine, and A. F. DeBlase. 2001. A Manual
of Mammalogy. Third Edition. McGraw-Hill.
Merriam-Webster Online. 2006. http://www.m-w.com/netdict.htm
Nikaido, M., A. P. Rooney, and N. Okada. 1999. Phylogenetic
relationships among cetartiodactyls based on insertions of short and long
interspersed elements: Hippopotamuses are the closest extant relatives of
whales. Proceedings of the National Academy of Science USA; 96:
10261-10266.
Rohrs, M., and E. Thenius. 1990. Ungulates: Introduction and
Phylogeny. In Grzimek's Encyclopedia of Mammals. Volume
4. Edited by Parker, S. P. New York: McGraw-Hill. Pp. 440-448.
Springer, M. S., M. J. Stanhope, O. Madsen, and W. W. de Jong. 2004.
Molecules consolidate the placental mammal tree. Treds in Ecology
and Evolution; 19(8): 430-438.
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