Isectolophus

Taxonomy

Class: Mammalia

Order: Perissodactyla

Family: Isectolophidae (Peterson 1919)

Genus: Isectolophus (from the latin, equal outside crest)

Species: I. latidens,(Osborn et al, 1878) I. annectins (Scott and Osborn, 1887)

When and where Isectolophus lived: Isectolophus latidens is found in the Bridger Basin, Wyoming (50-46 Ma) and I. annectins is found in the Unita Basin, Utah, and the Washakie Basin, Wyoming (46-40 Ma). The basins contain extensive lacustrian and fluvial deposition of erosional material from surrounding mountains that arose 66 Ma to 54 Ma (the Laramide orogeny). Global temperatures from 50 to 40 Ma were elevated 6 to 10 degrees above present day temperature. Elevation of the Rocky Mountain west basins were at approximately 1000 feet at this time, much lower than today. Because of the lake and stream deposits, low elevation, and high temperatures we can conclude that Isectolophus lived in an environment similar to the tropical to paratropical forest environment that characterizes similar regions in South America, Africa and Asia today.

General Description

The Isectolophidae is a family of perissodactyls that emerged near the early middle Eocene boundary about 50 Ma and is very closely related to modern tapirs..  Perissodactyls are odd-toed ungulates (hoofed mammals that walk on their toes) characterized by having feet that are symmetric about the third digit with generally one, three, or five digits, a single pulley astragulus (ankle bone), and simple hind-gut intestinal digestion. They were the dominant large mammal herbivores during the Eocene, but with the emergence of more difficult to digest grasses and forage, perissodactyls were replaced by artiodactyls during the late Eocene and Oligocene.  The artiodactyls are even-toed ungulates which have more efficient stomach digestive systems and a double pulley astragulus that enable more rapid locomotion.

Present day perissodactyls consist of the horses (Equidae), tapirs (Tapiridae), and rhinoceroses (Rhinocerotidae).  Because very little postcranial material is known, the present day tapir is our best analog to Isectolophus habits  Isectolophus has the tapir characters of bilophodont teeth, a prehensile nose, four hooves on the forelimb and three hooves on the hind limbs (Figure1). The tapir is territorial and has a gestation period of 400 days from which there is typically only one offspring.  The teeth of Isectolophus are bilophodont and have two parallel lophs (ridges, or crests), one uniting the two anterior cusps (paraloph), another uniting the two posterior cusps (metaloph) with a wall on the outside of the tooth (ectoloph) that connects the two ridges (Figure 2).  Like modern tapirs, Isectolophus has a convex metacone.  All rhinoceroses have a concave metacone.  The lophs enable the shearing of plant material by transverse jaw movement. Isectolophus had a full complement of mammalian teeth, with dental formula 3/1/4/3

The body confirmation of Isectolophus probably was a miniature of today's tapir. The length of the living tapir skull is about 2.5 times that of I. latidens. The weight of I. latidens was about 10 to 23 kg (22 to 50 lb).  From the short nasal incision we may conclude that it had a prehensile nose, but the incision is much less than that of the modern tapir.
 

References
Holbrook, L. T., Lucas, S. G. and Emery, R. 2004. Skulls of the Eocen perissodactyls (Mammalia) Homogalax and Isectolophus. Journal of Vertebrate Paleontology, 24:951-956.

Hooker, J. J. Perissodactyla. p. 199-214. in Rose, K. D. and Archibald, J. D. (Eds.) The Rise of Placental Mammals, Johns Hopkins Univ. Press, Baltimore.

Hooker, J. J. and Dashzeveg, D. 2003. Evidence for direct mammalian faunal interchange between Europe and Asia near the Paleocene-eocene boundary; pp 479-500 in S. L. Wing, P. D. Gingerich, B. Schimtz, and E. Thomas (Eds.) Causes and Consequences of Globally Warm Climates in the Early Paleogene. Special Paper 369. Geological Society of America, Boulder, Colorado.

McKenna, M. C. and Bell, S. K. 1997. Classi¯cation of Mammals above the Species Level. Columbia
University Press, New York.

Radinsky, L. B. 1963, Origin and early evolution of North American Tapiroidea. Bulletin of the Peabody Museum of Natural History 17:1-106.

Radinsky, L. B. 1969, The early evolution of the perissodactyla, Evolution 23:308-328.

Web sites: Animal Diversity Web http://animaldiversity.ummz.umich.edu/site/topics/mammal anatomy/ tooth diversity.html Provided by the University of Michigan.
Digital Morphology, http://digimorph.org/index.phtml, Overview Pages, Tapirs, http://digimorph.org/resources/tapirs.phtml Provided by the University of Texas.
Wikipedia: Tapir http://en.wikipedia.org/wiki/Tapiridae.

Authored by James L. Cornette
Content approved by Department of Earth Sciences, DMNS
 

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