Teleoceras

Teleoceras Temporal range: Barstovian-Hemphillian ~15.97–4.9 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Specimen at the Natural History Museum of LA
1913 T. fossiger illustration by Robert Bruce Horsfall.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	Rhinocerotidae
Subfamily:	†Aceratheriinae
Genus:	†Teleoceras Hatcher, 1894
Type species
†Teleoceras major
Species[1]
†T. aepysoma †T. aginense †T. americanum †T. brachyrhinum †T. hicksi †T. fossiger †T. guymonense †T. major †T. medicornutum †T. meridianum †T. proterum
Synonyms
†Mesoceras (Cook, 1930)[2] †Paraphelops Lane, 1927[3]

Teleoceras (Greek: "perfect" (teleos), "horn" (keratos)^[4]) is an extinct genus of rhinocerotid. It lived in North America during the Miocene and Pliocene epochs during the Hemingfordian to the end of Hemphillian from around 17.5 to 4.9 million years ago.^[5][6] It grew up to lengths of 13 feet (4.0 meters) long.^[7]

Teleoceras went extinct in North America alongside Aphelops at the end of the Hemphillian, most likely due to rapid climate cooling, increased seasonality and expansion of C4 grasses, as isotopic evidence suggests that the uptake of C4 plants was far less than that in contemporary horses.^[6] The Gray Fossil Site in northeast Tennessee, dated to 4.5-5 million years ago, hosts one of the latest-known populations of Teleoceras, Teleoceras aepysoma.^[8]

T. fossiger, NMNH.

T. proterum and Barbourofelis loveorum, FLMNH.

Teleoceras had much shorter legs than modern rhinos, and a barrel chest, making its build more like that of a hippopotamus than a modern rhino. Based on this description, Henry Fairfield Osborn suggested in 1898 that it was semi-aquatic and hippo-like in habits. This idea persisted for about a century, but has recently been discounted by isotopic evidence.^[6]

Some species of Teleoceras have a small nasal horn, but this appears to be absent in other species such as T. aepysoma.^[9] Teleoceras has high crowned (hypsodont) molar teeth, which has historically led to suggestions that the species were grazers. Dental microwear and mesowear analysis alternatively suggest a browsing or mixed feeding (both browsing and grazing) diet.^[10] Carbon and oxygen isotope analysis of tooth enamel suggests hippo-like grazing habits, but not aquatic.^[11] However, δ¹⁸O measurements from Ashfall suggest that the species T. major was semi-aquatic.^[12]

Teleoceras was sexually dimorphic. Males were larger, with larger tusks (lower incisors), a more massive head and neck, and significantly larger forelimbs. As a result of bimaturism, females matured and stopped growing before males, which is often seen in extant polygynous mammals. Males may have fought for mating rights; healed wounds on skulls have been observed, and healed broken ribs are not uncommon (although not all have their sexes determined). This is further supported by the breeding age female-to-male ratio in the Ashfall Fossil Beds being 4.25:1. There is also a rarity of young adult males preserved at Ashfall, which may be accounted for if they formed bachelor herds away from females and dominant bulls. ^[11]

Skulls

T. aginense

T. fossiger, Kansas.

Young female T. major, Nebraska.

Baby.

Teleoceras major is the most common fossil in the Ashfall Fossil Beds of Nebraska. Over 100 intact T. major skeletons are preserved in ash from the Bruneau-Jarbidge supervolcanic eruption. Of the 20+ taxa present, T. major was buried above the rest, being the last of the animals to succumb (small animals died faster), several weeks or months after the pyroclastic airfall event. Their skeletons show evidence of bone disease, ie hypertrophic pulmonary osteodystrophy (HPOD), as a result of lung failure from the fine volcanic ash.^[13]

Most of the skeletons are adult females and young, the breeding age female-to-male ratio being 4.25:1. There is also a rarity of young adult males. If the rhinos at Ashfall represent a herd, this may be accounted for if young adult males formed bachelor herds away from females and dominant bulls. The age demographic is very similar to that of modern hippo herds, as amongst the skeletons, 54% are immature, 30% are young adults, and 16% are older adults. ^[11]

The greatest concentration of Ashfall fossils is housed in a building called the "Rhino Barn", due to the prevalence of T. major skeletons at the site, of which most were preserved in a nearly complete state. One extraordinary specimen includes the remains of a Teleoceras calf trying to suckle from its mother.^[14]

T. major from the Ashfall Fossil Beds

(1) 8-9 year old young adult male "Tusker". Wisdom teeth emerging. (2) Cormohipparion occidentale

(3) Adult female "Sandy" (4) Calf "Justin".

10-12 year old adult male "Dr. Marie", with all permanent teeth.

Fetus within birth canal of the pelvis.

• McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8
• Prothero, Donald R. 2005. The Evolution of North American Rhinoceroses. Cambridge University Press, Cambridge, 218 pp. ISBN 0-521-83240-3

Media related to Teleoceras at Wikimedia Commons