| Toxodontia | |
|---|---|
 | |
| Mixotoxodon | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | †Notoungulata |
| Suborder: | †Toxodontia |
| Families[2] | |
Toxodontia[3] is asuborder of themeridiungulate orderNotoungulata. Most of the members of the five included families, including the largest notoungulates, share several dental, auditory and tarsal specializations.[4] The group is named afterToxodon, the first example of the group to be discovered by science.
Isotemnidae, the oldest and most primitive family of toxodonts, were generally large animals with larger canines than other early notoungulates. The family is probablyparaphyletic orpolyphyletic since only primitive dental features unite the 12 included genera, such as a complete dentition with unreduced canines and nodiastemata in the earliest genera. Likewise, they are only weakly linked to other toxodonts by a few dental features, and their primitive cheek tooth pattern can be basal to all notoungulates exceptnotioprogonians. The oldest of the 12 genera in this family isIsotemnus known from theRiochican-Casamayoran, but other genera are known from the Casamayoran, includingThomashuxleya,Pleurostylodon, andPampatemnus. Of these, the sheep-sizedThomashuxleya is the best known; its skeleton shows that it was relatively robust, had limbs with reduced distal elongation, and five-hoofed feet with a large central digit.[4]
Notohippidae was also present during the Casamayoran but tend to have morehypsodont cheek teeth than isotemnids.Pampahippus, one of the earliest genera in this family was, nevertheless, low-crowned with densely packed cheek teeth. Its primitiveness is suggested by the retained paraconids on the lower molars.Eomorphippus, aMustersan notohippid, was moderately hypsodont,Deseadan genera such asRhynchippus andEurygenium were very high-crowned, andSantacrucian genera had acquiredcementum on the crowns similar to equids.[4]
The last toxodont family from the early Tertiary are theLeontiniidae. They share several derived tarsal and auditory features with the notohippids and are, together with theDeseadanToxodontidae, nested within the paraphyletic Notohippidae. Leontiniids are known from the Deseadan or later, but the oldest and most primitive leontiniid, the MustersanMartinmiguelia, retained a primitivebrachydont dentition without diastemata except around the very small canines.Scarrittia, the best-known leontiniid, hadmesaxonic (enlarged central digit) feet with reduced lateral digits.[4]
Toxodontids first appeared during theOligocene in the form ofProadinotherium. Another well-known toxodontid isNesodon, a medium-sizedMiocene toxodontid descended fromProadinotherium which had converted its second upperincisors into tusks.Toxodon itself evolved during thePliocene alongside the relatedTrigodon, an equally largemammal which possessed ahorn projecting from itsforehead, in the same way as a modernrhinoceros.Toxodon was a hugeherbivore (about the size of a modernblack rhinoceros) with four toes on each foot.
