Arsinoitherium (Arsinoe II's beast) is an extinct genus of paenungulate mammals belonging to the extinct order Embrithopoda. It is related to elephants, sirenians, and hyraxes. Arsinoitheres were superficially rhinoceros-like herbivores that lived during the Late Eocene and the Early Oligocene of North Africa from 36 to 30 million years ago, in areas of tropical rainforest and at the margin of mangrove swamps. A species described in 2004, A. giganteum, lived in Ethiopia about 27 million years ago.

Adults of the species A. zitteli stood around tall at the shoulders and in length. A. zitelli has been estimated to have weighed anywhere from , whereas A. giganteum estimates range from . The most noticeable features of Arsinoitherium were a pair of enormous horns above the nose and a second pair of tiny knob-like horns over the eyes. These were structurally similar to the horns of modern bovids. While reconstructions usually show them as similar to the ossicones of giraffes, in life each bony core may have been covered, like the horn cores of bovids, with a large horn of keratin. Both males and females had horns. While some investigators have described a larger and a smaller species from the same site, others have identified the difference in body and tooth size as sexual dimorphism.

Arsinoitherium superficially resembles a species of rhinoceros, but it is not closely related as it is not a perissodactyl; instead, their closest extant relatives are elephants and manatees. Fossils have been found in sediments deposited in coastal swamps and warm, humid, heavily vegetated lowland forests across what is now Africa and Arabia.

Taxonomy

Early history

Arsinoitherium was named in 1902 by Hugh John Llewellyn Beadnell. and the Ancient Greek word θηρίον (thēríon), meaning "beast". The species epithet of the type species, A. zitteli, was given to it in honor of the eminent German paleontologist Karl Alfred Ritter von Zittel, regarded by some as the pioneer of paleontology in Egypt. Arsinoitherium has been considered an aberrant proboscidean historically, and a member of an obsolete group named the Barypoda, composed of the genus and its relatives. The best-known (and first-described) species is A. zitteli. Another species, A. giganteum, was discovered in the Ethiopian highlands of Chilga in 2003. The fossil teeth, far larger than those of A. zitteli, date to around 28–27 million years ago. While the Faiyum Oasis is the only site where complete skeletons of Arsinoitherium fossils were recovered, arsinoitheriids have been found in southeastern Europe, including Crivadiatherium from Romania, and Hypsamasia and Palaeoamasia from Turkey.

Description

thumb|Reconstruction of Arsinoitherium by [[Mauricio Antón]]

Arsinoitherium was a heavily built animal, with robust limbs and anatomical adaptations similar to elephants to support their body weight, a condition known as graviportality. The most noticeable feature of the skull of Arsinoitherium are two large horns atop its skull, structurally similar to those of bovids (cattle, goats, and their relatives). A second pair of smaller knob-like protuberances sat behind these horns.

Skull

thumb|202x202px|View of the skull from the front, showing the narial openings and prenasal bar|left

A proboscis or tapir-like lip has been suggested for Arsinoitherium due to the more posterior (rearward) position (retraction) of the narial openings and the morphology of the incisive foramen. The squamosal bone is enlarged and forms most of the lateral wall of the cranium. In the orbit, on the lacrimal bone, there is a small protuberance which connects with the maxilla. The premaxilla of Arsinoitherium is comparatively small, while the maxilla is enlarged and possesses a large jugal process which links to the lacrimal. The canines and incisors are similar in shape and most likely not used for any combat purposes.

Pelvic girdle and hindlimbs

The ilia of the pelvis are flared, with the inner surface being slightly concave. The shape of the acetabulum is oval, and the pit for the round ligament is remarkably deep. On the pubis, the anterior border is also concave. A. giganteum was larger than A. zitteli, with proportionally longer limbs. These localities have an array of both terrestrial as well as aquatic and even marine fauna, and some authors have posited that Arsinoitherium may have had semi-aquatic habits due to the morphology of the forelimbs. This has been disputed, however, as analysis of stable isotopes reveals the genus was most likely primarily terrestrial.

Communication

Arsinoitherium, like modern elephants, had the ability to communicate via foot drumming and loud, low frequency vocalisations. Its cochlear canal lacked a secondary bony lamina, an indicator of an adaptation for hearing low frequency sounds.

Palaeoenvironment

In Dor-El-Talha, a fossil locality in Libya, the genus is known to have lived alongside many species, including members of the suborder Hyracoidea, apterodontine hyaenodonts, the proboscideans Palaeomastodon, Barytherium and Moeritherium as well as the giant snake Gigantophis.

Fossils of Arsinoitherium have been found in:

; Eocene

  • Aydim Formation, Oman
  • Idam Unit Formation, Libya
  • Djebel Chambi, Tunisia

; Oligocene

  • Malembe, Angola
  • Jebel Qatrani Formation, Egypt
  • Chilga Formation, Ethiopia
  • Erageleit Formation, Kenya
  • Ashawq Formation, Oman
  • Shumaysi Formation, Saudi Arabia

References

  • New fossils from Ethiopia open a window on Africa's 'missing years'
  • Arsinoitherium fact file on BBC Science & Nature: Prehistoric Life
  • Description of Arsinoitherium zitteli from upper Eocene strata in Egypt