Gondwanatheria is an extinct group of Mammaliaformes that lived in the Southern Hemisphere, including Antarctica, during the Upper Cretaceous through the Miocene. Until recently, they were known only from isolated teeth, a few lower jaws, two partial skulls and one complete cranium. Because of this fragmentary knowledge, their placement is not clear.
Classification
For several decades the affinities of the group were not clear, being first interpreted as early xenarthrans, or "toothless" mammals similar to the modern anteater. A variety of studies have placed them as allotheres related to multituberculates, possibly even true multituberculates, closer to cimolodonts than "plagiaulacidans" are. However, a more recent study recovered them as nested among haramiyidans, rendering them as non-mammalian cynodonts. A more recently described specimen has since recovered them as allotheres closely related to multituberculates, but this was soon after followed by a study recovering them as part of Euharamiyida, remaining inconclusive to whether they belong in crown-group Mammalia. There are three known families within Gondwanatheria. The family Sudamericidae was named by Scillato-Yané and Pascual in 1984, and includes the vast majority of named taxa. The family Ferugliotheriidae was named by José Bonaparte in 1986, and includes one genus, Ferugliotherium, and possibly a few other forms like Trapalcotherium. Groeberiidae, originally interpreted as paucituberculatemarsupials, has since been understood as gondwanatherians, though only the type genus, Groeberia, has been examined as such. Further fossils have come fromIndia, Madagascar and Antarctica. A possible Ferugliotherium-like species occurs in Maastrichtian deposits of Mexico, extending the clade to North America. The Miocene genus Patagonia is the youngest known representative.
Biology
Gondwanatheres known from cranial remains almost universally have deep, robust snouts, as befitting their specialised herbivorous lifestyle. Groeberia and Vintana both possess bizarre jugal flanges similar to those of xenarthrans like ground sloths, though they had a palinal chewing method as in most allotheres and unlike almost any therian. Most gondwanatheres are specialised grazers, even being among the first mammals to have specialised for grass-eating long before any therians did, with the exceptions of Groeberidae and Ferugliotheriidae, which lack hypsodont teeth and therefore had more generalistic herbivorous habits. An articulated specimen found in the Maevarano Formation offers insight to the postcranial skeleton of these animals. Among the bizarre and unique features are a mediolaterally compressed and antero-posteriorly bowed tibia, a double trochlea on the astragalus, a fully developed humeral trochlea, and an unusually high number of trunk vertebrae. The new taxon has at least 19 rib-bearing and 11 non-rib-bearing vertebrae. Aside from these derived features, the Malagasy mammal has a mosaic pectoral girdle morphology: the procoracoid is lost, the coracoid is extremely well developed, the interclavicle is small, and the sternoclavicular joint appears mobile. A ventrally-facing glenoid and the well-developed humeral trochlea suggest a relatively parasagittal posture for the forelimbs. Remarkable features of the hind limb and pelvic girdle include a large obturator foramen similar in size to that of therians, a large parafibula, and the presence of an epipubic bone. The fully described animal, now named Adalatherium hui, is a comparatively large sized mammal, compared in size to a large cat. It has more erect limbs than other allotheres.
