Isotemnids were the common large (relatively speaking) notoungulates of the Eocene, and are last recorded in rocks of early Oligocene age. As originally conceived, they do not constitute a natural (monophyletic) group; rather, they include a collection of early-diverging members of the suborder Toxodontia and perhaps one or more other traditionally-recognized families (see Billet 2011). Given this, it should come as no surprise that they are recognized by their lack of derived features compared to other notoungulates. For example, they have low-crowned (brachydont) teeth and a complete and closed dentition (i.e., they have not lost any teeth relative to the common ancestor of notoungulates, and they lack conspicuous gaps in the tooth row). Additionally, most isotemnids have large canine teeth, a feature common to many early ungulates.

Reconstruction of Thomashuxleya externa from Simpson (1936:fig. 1), based on specimens referred to Anisotemnus distentus and Pleurostylodon simulis in addition to Thomashuxleya (Shockey and Flynn 2007).

Isotemnids were herbivores that mostly consumed leafy vegetation, twigs, and buds. Although virtually complete, articulated isotemnids are known from the late Eocene of La Gran Hondonada, Argentina, I am unaware of any detailed description or analysis of such remains. Rather, attention has focused on geologically older species from early middle Eocene sites such as Cañadón Vaca (e.g., Simpson 1936). Studies of such remains have indicated that isotemnids probably had a plantigrade (“flat-footed”) stance like a bear rather than a more upright stance typical of modern ungulates. Thomashuxleya, one of the better-known middle Eocene isotemnids, was a bit larger than a sheep, probably about 1.5 m long (5 feet) and weighing 70-90 kg (150-200 lbs.) (Croft 2016).

Nearly complete skeleton of Periphragnis from La Gran Hondonada on display at the Museo Paleontológico Egidio Feruglio (MPEF) in Trelew, Argentina

Classification: Order Notoungulata; Suborder Toxodontia

Stratigraphic Range: early Eocene? (Riochican) to early Oligocene (Tinguirirican)

Recognized Genera: Anisotemnus, Distylophorus, Isotemnus, Pampatemnus, Periphragnis, Pleurostylodon, Rhyphodon, Thomashuxleya

Selected References:

  • Billet, G. 2011. Phylogeny of the Notoungulata (Mammalia) based on cranial and dental characters. Journal of Systematic Palaeontology 9:481-497
  • Bradham, J., J. J. Flynn, D. A. Croft, and A. R. Wyss. 2015. New notoungulates (Notostylopidae and basal toxodontians) from the early Oligocene Tinguiririca Fauna of the Andean Main Range, central Chile. American Museum Novitates 3841:1-24.
  • Croft, D. A. 2016. Horned Armadillos and Rafting Monkeys: the Fascinating Fossil Mammals of South America. Indiana University Press, Bloomington, Indiana, 320 pp.
  • Shockey, B. J., and J. J. Flynn. 2007. Morphological diversity in the postcranial skeleton of Casamayoran (?middle to late Eocene) Notoungulata and foot posture in notoungulates. American Museum Novitates 3601:1-26.
  • Simpson, G. G. 1936. Skeletal remains and restoration of Eocene Entelonychia from Patagonia. American Museum Novitates 826:1-12.
  • Simpson, G. G. 1967. The beginning of the age of mammals in South America. Part II. Bulletin of the American Museum of Natural History 137:1-260.
  • Vucetich, M. G., and M. Bond. 1982. Los primeros Isotemnidae (Mammalia, Notoungulata) registrados en la Formación Lumbrera (Grupo Salta), del noroeste argentino. Ameghiniana 19:7-18.