This taxonomy of this order is not clear, but it may belong to
Meridiungulata (along with
Notoungulata,
Litopterna,
Pyrotheria and
Xenungulata). In turn, Meridungulata is believed to belong to the extant superorder
Laurasiatheria. Some scientists have regarded the astrapotheres (and sometimes the Meridiungulata as a whole) as members of the clade
Atlantogenata. However, collagen and mitochondrial DNA sequence data analysed in 2015 places at least the notoungulates and litopterns firmly within Laurasiatheria, as a
sister group to the
perissodactyls.[5][6][7]
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.[4]
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.[4]
The most famous member of the order is undoubtedly Astrapotherium, 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 a
gap between their tusks and the
hypsodont cheek teeth. The short and retracted nasal bones indicate a moderately developed tapir-like proboscis. The small Eocene Trigonostylops lacked such retracted nasals and probably also a proboscis. Other astrapotheriids, such as the
CasamayoranScaglia and Albertogaudrya, were between a sheep and a tapir in size and already the largest South American mammals.[4]
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.[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 that Antarctodon is closer to genera classified by previous authors as astrapotheriids (e.g., Albertogaudrya and Tetragonostylops) than it is to Trigonostylops."
Bond et al. 2011, p. 2
Ameghino, Florentino (1887). Apuntes preliminares sobre algunos mamíferos estinguidos del yacimiento de "Monte Hermoso" existentes en el "Museo La Plata". Buenos Aires.
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Burmeister, Hermann (1879). Description physique de la République Argentine : d'après des observations personnelles et étrangères. Vol. 3 Animaux vertébrés, 1. partie, Mammifères vivants et éteints. Paris: Savy. p. 520.
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Cifelli, R. L. (1993). "The phylogeny of the native South American ungulates". In Szalay, F.S.; Novacek, M.J.; McKenna, M.C. (eds.). Mammal phylogeny. New York: Springer-Verlag. pp. 195–216.
ISBN9780387978536.
Johnson, Steven C.; Madden, Richard H. (1997). "Uruguaytheriine Astrapotheres of Tropical South America". In Kay, Richard F.; Madden, Richard H.; Cifelli, Richard L.; Flynn, John J. (eds.). Vertebrate paleontology in the neotropics : the Miocene fauna of La Venta, Colombia. Washington: Smithsonian Institution Press. pp. 355–82.
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Kraglievich, Lucas (1928). Sobre el supuesto Astrapotherium Christi Stehlin, descubierto en Venezuela (Xenastrapotherium n. gen.) y sus relaciones con Astrapotherium magnum y Uruguaytherium Beaulieui. Buenos Aires: La Editorial Franco-Argentina.
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Kramarz, Alejandro G; Bond, Mariano (2009). "A new oligocene astrapothere (Mammalia , Meridiungulata) from Patagonia and a new appraisal of astrapothere phylogeny". Journal of Systematic Palaeontology. 7 (1): 117–128.
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10.1127/0077-7749/2011/0132.
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OCLC12322584.
Paula Couto, Carlos, de (1963). "Um Trigonostylopidae do Paleoceno do Brasil". Anais da Academia Brasileira de Ciências. 35 (3): 339–51.{{
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Paula Couto, Carlos, de (1976). "Fossil mammals from the cenozoic of Acre, Brazil". Congresso Brasileiro de Geologia. 28. Porto Alegre: Museu de Ciências naturais da Fundação zoobotânica do Rio Grande do Sul: 237–249.
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Simpson, George Gaylord (1935). "Descriptions of the oldest known South American mammals, from the Río Chico Formation". American Museum Novitates (793): 1–25.
OCLC44083494.
Simpson, George Gaylord (1957). "A new Casamayoran astrapothere". Revista del Museo Municipal de Ciencias Naturales y Tradicional de Mar del Plata. 1 (3): 11–18.
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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.