The Deseado Formation of Patagonia

by Frederic Brewster Loomis

The Typotheria with their chisel-like front teeth, lack of canines, and their permanently growing grinders evidently ate a hard type of vegetation. Deep and permanently growing molars are characteristic of the eaters of grass, a form of vegetation which is especially hard on the grinding teeth, on account of the silica in the stems and leaves. This however would scarcely necessitate the development of permanently growing incisors. They are typical of gnawing animals, eaters of bark, twigs, and possibly also leaves, the wood and bark being also a hard type of vegetation to grind. In the case of these forms I believe they were feeders on grass and bark. Their feet are developed either for running or hopping and would suggest hard ground for their habitat.

The Litopterna are typically plains animals, paralleling in their development the horses. The cropping teeth and the grinding molars become progressively longer. The limbs are progressively elongated, the animals walking more and more on the tips of the toes. With this, the metapodials especially and the other limb bones to a less degree, are progressively lengthened. At the same time the side toes are progressively reduced. The teeth indicate grass eating; the limbs life on the plains.

The Rhynchippidae, while not as advanced as the Litopterna, show cropping front teeth, and the molars developing in depth. The locomotion is semidigitigrade, the feet small, and the number of toes reduced to three. They too must be interpreted as grazing or grazing and browsing animals, living on hard ground.

The Leontinidae are heavier forms, but with much the same features as Rynchippidae, though less specialized. On account of the broad upper molars and the less specialization of the dentition, I should feel that these forms were browsers and lived among bushes, but the feet were three-toed and semidigitigrade and they seem to have walked on hard ground.

The Nesodontidae belong to the same type of adaptation as the foregoing family, but have the grinding teeth more complicated, indicative of a more advanced adaptation to hard vegetation. The feet were also adapted to hard ground.

The Homalodontotheria, the Astrapotheria, and the Pyrotheria were all very large animals, known mostly by their dentition, which is adapted to browse. Whether they lived on soft or hard ground is not known, as the feet are not known in any case but the Homalodontotheridae, where they are five-toed and adapted to soft ground. Such large animals were probably inhabitants of some river bank.

The rodents do not contribute much in the determination as to the type of the country, for they could have lived in the open or in the wooded country, but their relative abundance is rather typical of open country.

The birds are all running birds, and indicative of the country having been an open one.

Of our fauna 11 per cent were flesh or insect eating, and for the purpose of determining the type of country may best be omitted. The rodents could have been either forest or open country forms. Of the remaining 54 per cent, the typotheres, the litopternas, the Rhynchippidae, the Leontinidae, the nesodonts and the birds (46 per cent) were distinctly adapted to live on hard ground; the other 8 per cent being evidently suited to living near a river. All 54 per cent ate either grass or browse. The litopternas are grass eaters; the typotheres were specialized to eat grass or bark; nesodonts, Leontinidae, and Rhynchippidae are grass and browse eaters. Even the Pyrotherium has a pair of gnawing tushes. The picture arising from these considerations is a bush covered prairie, a country not unlike the upland bush pampas of Patagonia today.

There is not an aquatic form (fish or turtle) in the whole list, so it is evident that the stream which deposited these Deseado beds was not abundantly inhabited. To me it looks like so many of the streams in an arid country, dry through a considerable part of the year, and so uninhabited. In the whole list I see nothing to indicate forests or swamps. The arid bush covered plain alone seems to suit the requirements.