The taxonomy of this order is not clear, but it may belong toMeridiungulata (along withNotoungulata,Litopterna,Pyrotheria andXenungulata). In turn, Meridiungulata is believed to belong to the extant superorderLaurasiatheria. Some scientists have regarded the astrapotheres (and sometimes the Meridiungulata as a whole) as members of the cladeAtlantogenata. However, collagen and mitochondrial DNA sequence data analysed in 2015 places at least the notoungulates and litopterns firmly within Laurasiatheria, as asister group to theperissodactyls.[5][6][7]
Theirlophodont molars and tusk-like canines became extremely large and ever-growing in later astrapotheres. The upper molars lack anectocingulum 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 almostselenodont. As a result, their dentition is similar tonotoungulates, but it seems to haveevolved independently. The cheek teeth are similar torhinocerotoids, including similar microstructure, which indicate they had the same function.[4]
Postcranially, astrapotheres are relatively robust and more or lessgraviportal but have slenderlong bones, most notably in the hindlegs, suggesting they were amphibious. In order to support theirproboscises 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 stoutpodial andmetapodial bones. Most characteristic for the order are the flatastragalus, equipped with a short neck and a flat head, articulating with both thenavicular andcuboid bones; and theircalcaneus with its enlargedperoneal tubercle.[4]
The most famous member of the order is undoubtedlyAstrapotherium, a 3 m (9.8 ft) long elephant-like animal that had lost its upper incisors and developed ever-growing canine tusks. They had lost their anterior premolars, resulting in agap between their tusks and thehypsodont cheek teeth. The short and retracted nasal bones indicate a moderately developed tapir-like proboscis. The small EoceneTrigonostylops lacked such retracted nasals and probably also a proboscis. Other astrapotheriids, such as theCasamayoranScaglia andAlbertogaudrya, were between a sheep and a tapir in size and already the largest South American mammals.[4]
There is no scientific consensus regarding the classification within Astrapotheria. For example,Paula Couto 1963 originally describedTetragonostylops as a trigonostylopid butSoria 1982 and1984 transferred the genus to Astrapotheriidae and concluded that the remaining two genera in that family,Trigonostylops andShecenia, form a basal collateral branch within Astrapotheriidae. According toCifelli 1993, Trigonostylopidae (including Eoastrapostylopidae) is the stem group of Astrapotheriidae.[9]
^ab"The uruguaytheriine Astrapotheriidae from the rich middle Miocene Honda Group of the upper Magdalena River valley in Colombia (...) are the youngest securely dated remains of that order in South America."Johnson & Madden 1997, p. 356
^"Phylogenetic analysis suggests thatAntarctodon is closer to genera classified by previous authors as astrapotheriids (e.g.,Albertogaudrya andTetragonostylops) than it is toTrigonostylops."Bond et al. 2011, p. 2
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Soria, M. F.; Powell, J. E. (1981). "Un primitivo Astrapotheria (Mammalia) y la edad de la Formación Río Loro, Provincia de Tucumán, República Argentina".Ameghiniana.18 (3–4):155–68.
Soria, M. F. (1982). "Tetragonostylops apthomasi (Price y Paula Couto, 1950): su asignación a Astrapotheriidae (Mammalia; Astrapotheria)".Ameghiniana.19 (3–4):234–238.
Soria, M. F. (1984). "Eoastrapostylopidae: diagnosis e implicaciones en la sistemática y evolución de los Astrapotheria preoligocénicos".Actas 2° Congreso Argentino de Paleontología y Bioestratigrafía:175–182.
