Superorder †Meridiungulata Rank Order | Infraclass Eutheria Phylum Chordata Subclass Theria | |
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astrapotheria sister s spread i
Astrapotheria is an extinct order of South American and Antarctic hoofed mammals that existed from the Late Paleocene (Itaboraian SALMA) to the Middle Miocene (Laventan SALMA), 59 to 12 million years ago. Astrapotheres were large and rhinoceros-like animals and are certainly one of the most bizarre orders of mammals with an enigmatic evolutionary history.
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The history of this order is enigmatic, but it may taxonomically belong to Meridiungulata (along with Notoungulata, Litopterna and Pyrotheria). In turn, Meridungulata is believed to belong to the extant superorder Laurasiatheria. However, some scientists regard the astrapotheres (and sometimes the Meridiungulata all together) to be members of the clade Atlantogenata. An example of this order is Astrapotherium magnum. When alive, Astrapotherium might have resembled a mastodon, but was only three meters (ten feet) long.
Description
Their lophodont molars and tusk-like canines became extremely large and ever-growing in later astrapotheres. The upper molars lack an ectocingulum and are dominated by well-developed ectoloph and protoloph. Additional lophs formed in some derived taxa. They had lower molars with two cross-lophs, including a high protocristid, and eventually became almost selenodont. As a result, their dentition is similar to notoungulates, but it seems to have evolved independently. The cheek teeth are similar to rhinocerotoids, including similar microstructure, which indicate they had the same function.
Postcranially, astrapotheres are relatively robust and more or less graviportal but have slender long bones, most notably in the hindlegs, suggesting they were amphibious. In order to support their proboscises and large heads they had relatively long and massive necks in relation to the rest of the vertebral column. Their feet are pentadactyl with short and stout podial and metapodial bones. Most characteristic for the order are the flat astragalus, equipped with a short neck and a flat head, articulating with both the navicular and cuboid bones; and their calcaneus with its enlarged peroneal tubercle.
Three families are recognized: Eoastrapostylopidae from the late Paleocene, Trigonostylopidae from the Paleocene-Eocene, and Astrapotheriidae from the Eocene-Miocene. The Brazilian, Itaboraian Tetragonostylops and the Argentinian, Riochican Eoastrapostylops are the oldest astrapotheres. The latter, with its low-crowned and lophoselenodont cheek teeth, is considered the most primitive astrapothere. Trigonostylopids are distinct from other astrapotheres in their ear anatomy but are included in the order because of otherwise similar characters. Trigonostylops is one of few eutherian taxa found in Antarctica.
The most famous member of the order is undoubtedly Astrapotherium, a 3 m (9.8 ft) long elephant-like beast that had lost its upper incisors and developed ever-growing canine tusks. They had lost their anterior premolars, resulting in a gap between their tusks and the hypsodont cheek teeth. The short and retracted nasal bones indicate a moderately developed proboscis. The small Eocene Trigonostylops lacked such retracted nasals and probably also a proboscis. Other astrapotheriids, such as the Casamayoran Scaglia and Albertogaudrya, were between a sheep and a tapir in size and already the largest South American mammals.
Classification
There is no scientific consensus regarding the classification within Astrapotheria. For example, Paula Couto 1963 originally described Tetragonostylops as a trigonostylopid but Soria 1982 and 1984 transferred the genus to Astrapotheriidae and concluded that the remaining two genera in that family, Trigonostylops and Shecenia, form a basal collateral branch within Astrapotheriidae. According to Cifelli 1993, Trigonostylopidae (including Eoastrapostylopidae) is the stem group of Astrapotheriidae.