Interatheriidae

Skeleton (in lateral view) and skull (in palatal view) of the Santacrucian interatheriine Interatherium robustum (from Sinclair, 1909).   The skeleton of this species is more elongate than that of most interatheriids.

The Interatheriidae ("interatheres") are perhaps the most successful group of notoungulates; they are the longest-ranging of all notoungulate families and nearly two dozen genera have been described (though it is unclear how many of these are valid). Moreover, interatheriids are often very abundant in the faunas in which they are found, suggesting high population densities; they are one of the most common mammals in the early Miocene Santa Cruz Fauna of southern Argentina and are similarly common in the middle Miocene La Venta fauna of Columbia. They have been collected in nearly every well-sampled fauna of appropriate age from northern Venezuela to Patagonia. One conspicuous exception to this pattern is evident in Miocene faunas of northern Chile and Bolivia; interatheriids are apparently absent from the late early Miocene Chucal Fauna of northern Chile and are rare or absent in most Bolivian faunas.

Examples of interatheriid (interatheriine) teeth from the collections of the Field Museum. A. Anterior upper dentition (I1-P3) of Protypotherium from the Santa Cruz beds (Santacrucian) of Patagonia as illustrated by specimen PM 12057.  Note that the enlarged first incisors are perpendicular to the maxilla. B. Left mandible of Cochilius volvens bearing p3-m3 in occlusal view; anterior is to the left, labial side is toward the bottom.  This specimen (PM 14655) was collected from early Miocene Colhuehuapian strata in southern Argentina. C. Anterior lower dentition of Protypotherium from Santa Cruz (PM 13061).  Note that the incisors are procumbent and have a longitudinal groove, creating something resembling a comb; such structures are used for grooming in some modern mammals.

Interatheriids are small animals, in life probably weighing between 500 g and 15 kg. They are generally divided into two sub-families, Notopithecinae and Interatheriinae, though the former probably isn't a "natural" group (see below). All interatheriids share the presence of a unique feature of the skull: the jugal (one of the bones comprising the zygomatic arch) is separated from the orbit by the maxilla (the upper jaw bone; see below). Interatheriids also have characteristically bilobed lower cheek teeth, with strong labial and lingual sulci (see photo above and diagram below). All interatheriids retain a full complement of teeth (three incisors, one canine, four premolars and three molars in each quadrant), though some of these teeth are reduced to very small pegs. The upper incisors are enlarged and are implanted perpendicularly into the maxilla (like most mammals). This contrasts with the condition in some other typotheres (e.g., mesotheriids, archaeohyracids, hegetotheriids) in which they are implanted obliquely. The lower incisors are procumbent and form a comb-like structure that might have been used for grooming.

Skull of Protypotherium australe in right lateral view, approximately 8.5 cm long (modified from Sinclair, 1909 ).  The jugal (which is separated from the orbit by an extension of the maxilla) is highlighted in red. Note the presence of a closed (complete) dentition, with no significant spaces between the incisors and the cheek teeth.  Reconstructions of seated (middle) and walking (right) Protypotherium (from Dixon et al., 1988 and Barnett, 1960, respectively).

The subfamily Notopithecinae includes the oldest members of the family, most of which are small, have low-crowned cheek teeth, and display the "face" pattern of fossettes characteristic of the typothere notoungulates (see the diagrams of Notopithecus teeth to the left). Representatives of the subfamily occur in faunas of Riochican through Tinguirirican age and are quite characteristic of these faunas. Phylogenetic studies suggest that this subfamily is not a monophyletic group; rather, it is likely a paraphyletic one, with the various taxa representing successive outgroups to the Interatheriinae. They are last recorded in Tinguirirican (early Oligocene) faunas of Chile and Argentina.

Upper right dentitions (anterior to the right) of Miocochilius (left, FMNH PM 5424) and Plagiarthrus (right, FMNH PM 13415).  In Plagiarthrus, note the small middle lobe in each tooth and the larger "molarized" premolars.  The specimen of Miocochilius is from the middle Miocene (Laventan) fauna of La Venta, Colombia and includes P2-M2. The spcecimen of Plagiarthrus is from the Deseadan (late Oligocene) of Patagonia and includes P3-M3.

The subfamily Interatheriinae includes the later members of the family; they occur in Tinguirirican through Huayquerian faunas. Many interatheriines exhibit simplified cheek teeth; all internal fossettes are lost and the only distinguishing characteristics are the outlines of the teeth; see photos and diagram above. This primarily results from the evolution of hypselodont (ever-growing) cheek teeth in this group. Due to developmental constraints, it is not possible to maintain isolated fossettes while keeping the roots of the teeth open. Interatheriines are one of four groups of notoungulates to evolve ever-growing cheek teeth. In many respects, the teeth of interatheriines resemble those of hegetotheriids. They can be distinguished from hegetotheriids by the absence of a flat lingual face on the lower molars (the lingual face has a sulcus in interatheriines), and the presence of a pronounced lingual sulcus on the upper molars (most hegetotheriids do not display this sulcus, although there are some exceptions.)  Unlike later hegetotheriids and mesotheriids, many interatheriines retain the primitive trait of a closed dentition (i.e., 44 teeth present in the adult with none lost to create a space in the toothrow.)      Some of the later forms reduced the lateral digits of the feet, creating a paraxonic ("two-toed") foot similar to that seen in modern artiodactyls. The figure to the right demonstrates how the structure of the hind foot in the middle Miocene interatheriid Miocochilius is more similar in overall form to that of a modern artiodactyl (a peccary, Tayassu) than to that of a contemporary interatheriine (Protypotherium). The figure is modified from Stirton (1953).

Stratigraphic Range: Riochican (early Eocene?) to Huayquerian (late Miocene) Geographic Distribution: throughout Argentina, western Bolivia, western Brazil, central and southern Chile, Colombia, Ecuador, Uruguay, northern Venezuela Taxonomy: Order Notoungulata, Suborder Typotheria, Family Interatheriidae Representative Taxa: Subfamily "Notopithecinae": Notopithecus, Transpithecus, Gulielmoscottia; Subfamily Interatheriinae: Eopachyrucos, Santiagorothia, Cochilius, Plagiarthrus, Progaleopithecus, Protypotherium, Interatherium, Miocochilius Selected References: Hitz, R., M. Reguero, A.R. Wyss, and J.J. Flynn. 2000. New interatheriines (Interatheriidae, Notoungulata) from the Paleogene of central Chile and southern Argentina. Fieldiana: Geology (New Series) 42:1-26. Hitz, R.B., J.J. Flynn, and A.R. Wyss. 2006. New basal Interatheriidae (Typotheria, Notoungulata, Mammalia) from the Paleogene of central Chile. American Museum Novitates 3520:1-32. Hitz, R., G. Billet, and D. Derryberry. 2008. New interatheres (Mammalia, Notoungulata) from the late Oligocene Salla Beds of Bolivia. Journal of Paleontology 82:447-469. Reguero, M.A., M. Ubilla, and D. Perea. 2003. A new species of Eopachyrucos (Mammalia, Notoungulata, Interatheriidae) from the late Oligocene of Uruguay. Journal of Vertebrate Paleontology 23:445-457. Simpson, G.G. 1932. Cochilius volvens from the Colpodon Beds of Patagonia. American Museum Novitates 577:1-13. 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. Stirton, R.A. 1953. A new genus of interatheres from the Miocene of Colombia. University of California Publications in Geological Sciences 29:265-348.