The Deseado Formation of Patagonia

by Frederic Brewster Loomis

THE DESEADO FORMATION
OF PATAGONIA

CHAPTER I
Introduction

The material described and the conclusions drawn in the following pages are the results of the Amherst Expedition to Patagonia in 1911; an expedition organized and sent out by the Class of ’96 as a part of their fifteenth reunion. The party consisted of Frederic B. Loomis ’96, Phillip L. Turner ’11, Waldo Shumway ’12, and William Stein of St. Joe, Wyoming, and left Amherst July 1, 1911, returning the first of February the ensuing year, having spent its time collecting in the early Tertiary beds of Patagonia, as exposed in the Territories of Chubut and Santa Cruz, the aim being to secure from the earlier periods a fuller knowledge of the vertebrate animals, such as the Princeton Expeditions obtained for the Patagonian and Santa Cruz formations. The narrative of the expedition has been told in “Hunting Extinct Animals in the Patagonian Pampas.”

Material was found in various beds, from the Cretaceous up to the Lower Miocene; but the major part of the fossils, and most of the facts new to science came from the work in the Deseado Formation. The collections from the horizon were so complete and interesting that this report of the expedition has assumed the form of a monograph of the Deseado Formation, otherwise known as the Pyrotherium beds.

The first work in this formation was done by Carlos Ameghino who at various times between 1889 and 1894 collected for his brother, Florentino Ameghino, the latter studying and describing the collections of Carlos, whose trips covered the country from Chubut down to the Straits of Magellan, and the various formations from the Lower Cretaceous to the Pampean or Pleistocene. Carlos Ameghino and his brother, Florentino, for years explored in Patagonia, going summer after summer at their own expense, and in the meantime maintaining a small book and stationery store in La Plata, the profits of which gave the two brothers a living and furnished the funds for the continual expeditions. In the back of the store was the workshop from which came the continuous stream of knowledge in regard to these strange faunas. One of the best pieces of work done by the brothers was the collecting and describing of the fauna of the Pyrotherium beds the bulk of which is contained in two papers entitled, Première Contribution à la Connaissance de la Fauna mammalogique des Couches à Pyrotherium, and Mammifères Crétacés de L’Argentine, Deuxième Contribution, etc., both published in the Boletin del Instituto Geográfico Argentino, tomes 15 and 18 respectively. These two papers give names to most of the forms which we found, but the genera and species are based on very fragmentary and incomplete material. It has been a pleasure to find the accuracy with which these descriptions were made; and our part has been chiefly to supplement and increase the knowledge of the various forms, and to determine from the more complete material the relationships of these strange forms. In some cases we have been able to assemble all the parts of the animals, and in the others to add more or less to the completion of the knowledge of the forms. There is one peculiarity of Ameghino’s descriptions, namely the absence of data as to the localities where the forms were found.

About 1900 Tournier, in the interests of the Paris Museum, made a series of expeditions (5) to Patagonia, on some of which he found a Pyrotherium, or as he has termed it Deseado, locality just south of the Deseado River, from which he gathered a considerable collection which has been described by Albert Gaudry in various papers mostly in the Annales de Paléontologie.

These two collections and their collaborations represent all the work thus far done on the Deseado beds and fauna. Our collection is the first one of any considerable size to be brought to North America, and it seems to be by far the most complete, the various animals being represented by more complete skeletons than in any of the previous collections.

The beds were first designated as the Pyrotherium beds, and are always so referred to by F. Ameghino. Tournier and Gaudry, feeling the prejudice which is fairly general among Palaeontologists against names based on any contained animal (which may or may not be present at other localities, which may extend through more than one period, and whose name may be changed as a result of further knowledge) used the term Deseado formation, as his collections came from the neighborhood of this river. This is a geographical name and avoids the chance for confusion; so I have adopted it throughout this paper, it being understood as an equivalent of the term Pyrotherium beds.

Ameghino never gave the exact, or anywhere near the exact, localities from which his Deseado specimens came. It was not until 1906, when his Formations Sedimentaires[1] appeared, that any localities were designated, and there on a sketch map he indicates as Deseado exposures, about a dozen points, scattered between the upper part of the Chubut River to some 25 miles south of the Deseado River. These are included in an oval area some 500 miles long by 150 miles wide. Ameghino also suggests on this occasion that the Deseado formation originally extended over at least the whole of this area. As will be seen in the next chapter, I believe that the deposits of this age and character have always been local and isolated. We sought for several of these localities and failed to locate them, especially those near Mazaredo, and the northern one on the Gulf of St. George. The point where we did find our material I believe was one of Ameghino’s localities, though the settlers of that region had never heard of anyone hunting for fossils there; but the settlement had been practically all within the previous six years, which was much later than the time when Carlos Ameghino worked in the region.

Beside the foregoing, an exposure of this age is reported by A. A. Romero, just above the fork of the two branches of the Rio Negro, which is some 500 miles north of the first group of localities mentioned. Ameghino also refers to another locality in the Province of Misiones which would be 1,500 miles north of the typical localities.