| Trachytherus | |
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| Skull and mandible ofTrachytherus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | †Notoungulata |
| Family: | †Mesotheriidae |
| Subfamily: | †Trachytheriinae |
| Genus: | †Trachytherus Ameghino, 1889 |
| Type species | |
| †Trachytherus spegazzinianus | |
| Species | |
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| Synonyms | |
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Trachytherus is anextinctgenus ofmesotheriidnotoungulate that lived from the Late Oligocene to the Early Miocene in what is nowSouth America.
Trachytherus had a relatively low, compact skull with an elongated snout, longer than inMesotherium but very similar. Thepostorbital process was also smaller and the nasal bones did not extend as far as inMesotherium. The most notable difference between the two genera was in the zygomatic arch; in the anterior region, the insertion surface of the lateral masseter was highly developed, but not as large than inMesotherium.
The first upper incisor was highly developed, continuously growing (hypsodont) and arched, with an enamel band only on the labial side. The following incisors, canines and premolars were greatly reduced or vestigial. The last three premolars were molar-like, with a very complex pattern rapidly disappearing with wear. The molars were similar to those ofPseudotypotherium andCochilius, but did not possess the three-lobed internal structure characteristic ofMesotheriinae.[1]
The genusTrachytherus was first described in 1889 byFlorentino Ameghino,[2] based on fossil remains found in theSarmiento Formation, a LateOligocenegeological formation inPatagonia. The type speciesTrachytherus spegazzinianus, was also found in several other Oligocene deposits across South America. It was originally assigned to the family Typotheriidae, but Ameghino moved it in 1892 to its own family, Trachytheriidae. In 1894Lydekker moved the genus back to Typotheriidae, regarding it as the taxon that evolutionarily connectedNesodon andTypotherium.[3] In 1895, Ameghino refuted these affinities, although considering thatTrachytherus andTypotherium were related, although not closely enough to belong to the same family.[4] In 1897, for unknown reasons, Ameghino changed the genus name toEutrachytherus, with its own family, Eutrachytheridae.[5] Later, working on its astragalus, he admitted that the similarities between the bones of Eutrachytheriidae and Typotheriidae were so important that their differences were insignificant to him.[6] In 1913,William Berryman Scott placedTrachytherus back in the Typotheriidae, and from there there was a consensus from the researchers.[7][1] Typotheriidae was the name Lydekker assigned to the family containing the genusTypotherium. This genus was later synonymized withMesotherium, meaning the name "Typotheriidae" lost its validity and is now synonymous withMesotheriidae.
Several other species have been assigned toTrachytherus, such asT. medocensis,T. ramirezi,T. alloxus,T. subandinus,T. modestus,T. grandis, found inBolivia,Peru, andArgentina. However, several studies indicates that the genusTrachytherus is paraphyletic and represents a series of derived and basal forms of other typotheres.[8]
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Several species ofTrachytherus are known from the mid latitudes of western South America, in an area called theBolivian Orocline. A paleobiogeographical reconstruction indicate that this region is the possible ancestral area of the Mesotheriidae, and therefore an important region for the diversification of these animals. Later, the mesotheriids, including 'Trachytherus, dispersed to more southern areas.[1] Though at times considered a grazer,Trachytherus had a complex dietary habit that was not exclusively or predominantly based on grasses.[9]
