Buenos Aires is located by the River Plate, a wide river that is formed by the Paraná and Uruguay rivers just upstream from the Argentine capital city. It is loaded with sediments washed down from the subtropical regions of Southern South America.
Here, in 1896 workers were building a port for Buenos Aires and while excavating the river bed to pour concrete for the docks, they came across a strange skull. It was found at the bottom of the excavation, about 12.86 m (42 ft.) below the water level at low tide (and 11 m -36 ft.- below the river bed). Very deep indeed to be the skull of a relatively recent native American.
In 1909, Florentino Ameghino described the skull and believed that it belonged to a distant ancestor of modern man, the Diprothomo platensis.  It was quite nice as it added proof to his theory of an American origin for mankind.
Later it was studied by Ales Hrdlicka and Bailey Willis from the Smithsonian Institution (Washington, U.S.A.), who promoted the theory that modern man entered America through Alaska from Asia. Unsurprisingly both concluded that it was just another modern human skull, perhaps one belonging to an Indian. 
The sediments where it was found, were of Pre-Ensenadian age, and thus would, according to modern dating techniques be at least one million years old. This was well before modern humans roamed the earth.
But, Hrdlicka systematically refuted all of Ameghino’s claims regarding the skull’s primitive aspect. He said that the brows or “[surbarobital] ridges are prominent but not at all comparable with the heavy supraorbital welts of the Neantherdals and Spy skulls or even with those of some of the Australians” .
However he was forced to admit that “the forehead is somewhat low and sloping” but quickly added that it is similar to those of Indian skulls. Note that “low” forehead is associated to “low” intellect (i.e. smaller brain) and, therefore something that would be expected in an archaic hominid such as H. erectus.
The image above shows our Buenos Aires Diprothomo skull. However This is not the only example in Southern South America of an ancient or primitive human skull which could belong to a H. erectus, there is another case in Brazil:
During the 1970s, a Canadian archaeologist, Dr Alan Lyle Bryan from the University of Alberta, found a highly mineralized calotte (skullcap) that had been found at Lagoa Santa in Minas Gerais, Brazil.
This calotte had “very thick walls and exceptionally heavy brow ridges” (i.e. supraorbital ridges). 
The bones seem to have come from the Sumidouro cave and were found by Danish naturalist P. W. Lund in the nineteenth century. 
It was photographed by Byran (see below)  and those anthropologists that saw the photographs believed that it was a fake or a cast or perhaps an Old World fossil that had got mixed up with the Brazilian specimens at the (regrettably unnamed museum where it was kept). Bryan said that he had seen it and that it was no fake. Supposedly it has later “mysteriously disappeared from the museum”. 
Notice how thick the bones are! Definitively primitive and erectus-like. Actually, comparing it with the photograph of Diprothomo they look very similar.
I would love to end the post here but, the source that provides the photograph shown above  goes on to mention another paper Investigation of a fossilized calotte from Lagoa Santa, Brazil, by EDXRF by Anjos, Lopes and Souza (2005) that reproduces a calotte from Lagoa Santa athat is identical to the one in the photographe above and which, disclosed after an X-Ray study that it "had been mounted with pieces from different origins", meaning that it is a fake.
If the missing calotte and the one analyzed with X-Rays is the same one or different ones, is not known. So, there is still a chance that the Bryan skull exists and belonged to an H. erectus.
 Ameghino, Florentino, (1909). Le diprothomo platensis un precurseur de l'homme du pliocene inferieur de Buenos. Aires. Anales del Museo. Nacional de Buenos Aires. 1909. pp. 107-209.
 Hrdlicka Ales, Bailey Willis, W.H. Holmes, (1912). Early Man in South America. Washington Government Printing Office. “Diprothomo platensis”. pp. 318+
 Michael A. Cremo, Richard L. Thompson (1996). Forbidden archeology: the hidden history of the human Bhaktivedanta Book Publishing, Inc., pp. 418.
 Alan Lyle Bryan, (1978). Early man in America from a circum-pacific perspective. Issue 1 of Occasional papers of the Department of Anthropology, University of Alberta. Archaeological Researches International, pp. 318.
 George Weber, Lagoa Santa sites (Minas Gerais, Brazil). Citing:  Beattie O.B., Bryan A.L., (1984). A Fossilized calotte with Prominent Browridges from Lagoa Santa, Brazil. Current Anthropology, vol. 25, no. 3:345-346
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2011 International Year of Forests Copyright 2009-2011 by Austin Whittall ©