Tuesday, December 6, 2016

Anthracobunids: Amphibious Mammals of Eocene Asia

Anthracobunids (Anthracobunidae) are an extinct group of herbivorous mammals from the Eocene of southern Asia. These animals were formerly considered to be members of Tethytheria, the group of mammals which contains elephants, hyaxes, sirenians, and their relatives. However, a recent study in 2014 has shown that anthracobunids were actually basal members of Perissodactyla, related to tapirs, rhinos, and horses.

Defining Characteristics
Anthracobunids have a complete dentition of 44 teeth. The incisors were relatively small and simple, with relatively large canines. The cheek teeth were low-crowned with bunodont* or bilophodont* cusps, suggesting that these animals fed on relatively both soft to moderately hard, non-siliceous plant matter. Based on tooth measurements, anthracobunids have been estimated to be about the size of modern tapirs, with body masses ranging from 100 to 275kg (220 to 610lbs). Unfortunately, complete skeletal remains are virtually unknown for this family. All described anthracobunid fossils are limited to skull and mandibular fragments, isolated teeth, and incomplete postcranial elements. 

Figure 1 from Cooper et al., 2014, showing much of the known fossil material for
the entire family Anthracobunidae.
A-B: the crushed skull of Anthracobune pinfoldi.
C-D: second premolar of A. wardi.
E-F: skull fragment of A. wardi.
G-I: complete mandible of A. wardi.
J-L: proximal phalanges of A. wardi.
M: head of a metapodial from A. pinfoldi.
N: phalangeal fragment of A. pinfoldi.
O: terminal phalanx of A. pinfoldi.

Stable isotope values and long bone geometry studies have suggested that anthracobunids fed on land and spent considerable amounts of time in water. The postcranial bones were hyperostotic, meaning that they were much denser and more compacted than those of most terrestrial mammals. This condition, which increases buoyancy, is an adaptation associated with aquatic or partly aquatic mammals which spend most of their time in shallow water such as hippopotami, early whales, tapirs, and rhinos. All evidence suggests that anthracobunids shared behavioral and ecological similarities with tapirs and certain species of rhino. These animals likely had relatively restricted home ranges which overlapped with a permanent source of freshwater such as a river or lake in which they would frequently wallow and wade. They would forage in the forested environments which bordered these water sources and would readily retreat to water to escape predation, for thermoregulation, relief from terrestrial parasites, or simply to rest.

Tentative reconstruction of Anthracobune wardi based on specimen H-GSP 96258,
an exceptionally complete mandible missing only its incisors. The cranium was
approximated by referencing the skulls of early rhinos and tapirs. Although no
postcranial material is known for this animal, its body proportions and much of
its ecology and behavior would have been similar to that of modern tapirs.

Genera & Species
As of 2014, the Anthracobunidae contains 3 genera with 4 species between them. Two other monotypic genera, Ishatherium (I. sabathuensis) and Nakusia (N. shahrigensis), have been excluded from the family based on differences in dental morphology.

Anthracobune (1940)
The name Anthracobune is derived from the Greek anthrakos, meaning “coal”, and bune, meaning “mound”. Thus, the genus name translates into “coal mound”. Members of this genus lived during the middle Eocene of Pakistan. 2 species are known; A. wardi and A. pinfoldi. A. wardi, the type species of the family, was originally placed in Anthracobune by Pilgrim in 1940, only to be placed in the now defunct genus Lammidhania wardi by Gingerich in 1977. It was later moved back into Anthracobune 37 years later by Cooper et al. in 2014. 

Jozaria (1983)
Jozaria is represented by a single specimen belonging to the only known species of this genus (J. palustris). Geological evidence indicates that the animal inhabited a brackish marsh environment and it may have fed on the soft aquatic vegetation found there.

Obergellia (2014)
Containing the singular species, Obergellia occidentalis, this animal from the middle Eocene of India and Pakistan is the most recently described member of the Anthracobunidae. Although its fossil remains were first discovered in 1980, the genus Obergellia was erected by Cooper et al. in 2014. The name honors the late married vertebrate paleontologists Friedlinde Obergfell and A. Rango Ral. It differs from other anthracobunids in a suite of dental and mandibular characters.

Bilophodont: cheek teeth with two transverse crests or ridges.
Bunodont: referring to cheek teeth with rounded, bumpy cusps.

References & Further Reading
Cooper LN, Seiffert ER, Clementz M, Madar SI, Bajpai S, et al (2014). Anthracobunids from the middle Eocene of India and Pakistan are stem perissodactyls”. PLoS ONE 9(10): e109232 <Full Article>

Kumar K (1991). “Anthracobune aijiensis nov. sp. (Mammalia: Proboscidea) from the Subathu Formation, Eocene from NW Himalaya, India”. Geobios 24(2): 221-239 <Full Article>

Wells NA, Gingerich PD (1983). “Review of Eocene Anthracobunidae (Mammalia, Proboscidea) with a new genus and species Jozaria palustris, from the Kuldana Formation of Kohat (Pakistan)”. Contributions from the Museum of Paleontology 26(7): 117-139 <Full Article>

Sahni A, Kumar K (1980). “Lower Eocene Sirenia, Ishatherium subathuensis, gen. et nov. sp. From the type area, Subathu, Simla Himalayas, H. P.” Journal of the Paleontological Society of India 23 & 24: 132-135 <Full Article>