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What is an UNGULATE?
A simple question with a variable answer!

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:

Molecular Phylogeny of Mammals 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:

Ungulates of
the World
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Family Tree

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. Trends in Ecology and Evolution; 19(8): 430-438.