The Double-Horned Tank — Arsinoitherium

Have you ever seen an animal with two massive horn-like structures on its head resembling those of a rhinoceros, accompanied by two smaller bony horns on the top of the skull? Imagine a creature whose limbs are thick and pillar-like, similar to those of elephants or rhinos, and whose diet consists entirely of plants. Some readers might immediately think of the brontotheres discussed previously. However, the animal introduced here does not belong to Brontotheriidae or Rhinocerotidae. Instead, it represents a distinct lineage within the order Embrithopoda, specifically the family Arsinoitheriidae, and the genus Arsinoitherium.

Figure 1. Replica of the skeletal fossil of Arsinoitherium（圖片來源：Aram Dulyan，採用 CC BY 2.5 授權）

Arsinoitherium was a gigantic herbivorous mammal that lived from the Eocene to the Oligocene. Its appearance was highly unusual. The skull bore two enormous horns extending from the top of the head toward the nasal region. Unlike the horns of rhinoceroses, which are composed primarily of keratin, the horns of Arsinoitherium were made of bone internally. The limbs of this animal were also unlike those of rhinoceroses; structurally they resembled the columnar limbs of modern elephants.

From an evolutionary perspective, embrithopods are more closely related to the orders Proboscidea (elephants) and Hyracoidea (hyraxes) than to the odd-toed ungulates such as horses and rhinoceroses. All three groups belong to the larger clade Afrotheria, a lineage of mammals that originated and diversified in Africa. Since this article is intended as a brief encyclopedia-style introduction, the broader evolutionary relationships within Afrotheria will not be explored in detail here.

Figure 2. Reconstruction of Arsinoitherium（圖片來源：Dmitry Bogdanov，採用 CC BY 3.0 授權）

In the history of life on Earth, Arsinoitherium represents one of the early lineages of mammals that evolved large body size. Individuals reached a shoulder height of approximately 1.75 meters and a body length of about 3 meters, making them comparable in size to modern white rhinoceroses. Despite this similarity in size, their ecological preferences were different. Rather than inhabiting open grasslands, Arsinoitherium appears to have favored humid tropical forests and swampy environments.

The dentition of Arsinoitherium was relatively primitive. Adults possessed a total of forty-four teeth, characterized as low-crowned teeth. This type of tooth structure suggests a diet consisting primarily of broad-leaved plants and tender foliage rather than abrasive grasses.

The discovery of Arsinoitherium also carries a small historical anecdote. The name of the genus derives from Arsinoë II, the queen of Ptolemy II Philadelphus of the Ptolemaic dynasty in ancient Egypt. The first fossils of this animal were discovered near the ruins of the queen's palace. The specimen found there was also one of the most complete known fossils. In honor of the queen, paleontologists named the animal “the beast of Arsinoë,” giving rise to the scientific name Arsinoitherium.

Figure 3. Gold coin bearing the portrait of Arsinoë II（圖片來源：MatthiasKabel，採用 CC BY-SA 3.0 授權）

That concludes this brief introduction to this remarkable prehistoric mammal. If you ever encounter a fossil specimen or a museum reconstruction of Arsinoitherium, you might enjoy sharing the story of this extraordinary creature with friends and family.

Author: Rodrigo

References:

1. Alan Turner & Mauricio Anton.（2004） .Evolving Eden, An Illustrated Guide to the Evolution of the African Large-Mammal Fauna. New York: Columbia University Press.

2. Donald r. prothero. (2016). The Princeton Field Guide to Prehistoric Mammals. Princeton University Press. Court, Nicholas (1992-02-01). "A unique form of dental bilophodonty and a functional interpretation of peculiarities in the masticatory system of Arsinoitherium (mammalia, Embrithopoda)". Historical Biology.

3. Beadnell, H.G.C. (1902). "A preliminary note on Arsinoitherium zitteli, Beadnell, from the Upper Eocene strata of Egypt". Public Works Ministry, National Printing Department.