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The mastodon and the macrauchenia

Dating the teeth helps in the reconstruction of the Northeast's megafauna

from Recife

Two years ago a joint team from the Federal University of Pernambuco (UFPE) and the Federal University of Paraíba (UFPB) had in their hands three teeth from two gigantic mammals that had inhabited the Northeast region, but they weren’t able to precisely place them on the geological timescale. As the search for answers sometimes involves the collaboration of other specialists, the paleontologists got in touch with a physics group at the University of Sao Paulo (USP) in the town of Ribeirão Preto.

Submitted to dating by way of the quantity of radiation that they had accumulated, the fossils revealed when the mastodons and the distant parents of the llama, the American representative of the camel family, lived. One of the teeth of the mastodon, an animal related to the elephant knows as Haplomastodon waringi, belonged to an individual that lived in the Northeast 49,000 years ago; the other tooth is from an example of this same species that had lived in the region some 40,000 years ago, according to the study published in the magazine Applied Radiation and Isotopes. However, the relative of the llama – the macrauchenia or Xenorhinotherium baiense – is a little older: it lived some 52,000 years ago, as the tooth that resisted with time revealed.

The fossils were buried about one meter down in the lake called Lagoa de Dentro, a slight depression that accumulated water and stones in Puxinanã, in the rural part of state of Paraiba. At these locations, known as water hole depositaries, these animals quenched their thirst at the end of the Holocene era and at the start of the Pleistocene era, the period during which they lived. This transition, marked by the passage from a glacial period to an inter-glacial period, changed the vegetation and the climate of the Northeast to the point of leading to the extinction of the megafauna, which included these two species.

Generally the animals of the megafauna were herbivores. They grazed in the sparse vegetation of the savannah that gave way to the current Caatinga (shrubland) and they searched for water at the watering holes. “Already weakened by hunger and thirst, many of these animals died there of exhaustion or were attacked by a saber-toothed tiger”, suggests Alcina Barreto, a paleontologist from UFPE who dug in the area with her colleague José Augusto Costa de Almeida, from UFPB. “It is for this reason that the deposits of the water holes are rich in fossils of the megafauna.”

The mastodon was larger than the current elephant. They had tusks of up to 1.5 meters in length, directed upwards, went around in herds and fed themselves on sprouts, shrubs and grass. Their teeth grew continuously and were substituted by others whenever they were worn out through chewing. The macrauchenia, also a herbivore, was a little larger than a horse of today.

Similar to Argentina
The British naturalist Charles Darwin found fossils of a similar animal in Tierra del Fuego, in Argentina, during one of his stops on his journey around the world aboard the Beagle. Darwin placed it in the group of the ungulates, hoofed quadrupeds, but he dealt with it as a species restricted to South America from which today direct or indirect descendents did not exist. The animal weighed around one ton, three times more than a tapir, currently the largest mammal in South America, and had legs similar to those of the llamas. Its body was robust like that of a horse and was as long as three meters, including its neck and head. It had a snout longer than a tapir, but shorter than the trunk of an elephant.

The species from Patagonia was a little different from that found in Brazil. The two belonged to the order of the litopterns, but were of different species. The one found to have lived in Argentina got its scientific name during the 19th century: Macrauchenia patachonica, meaning something from Patagonia with a huge bull-neck (auchenia is the Greek term that means a long neck). The macrauchenia brasileira is equally long necked, but has nostrils further back – behind its eyes – than the corresponding Argentinean species.

Teeth and the atomic bomb
The teeth of the mastodons and the macrauchenia were dated at USP in Ribeirão Preto by the physicist Oswaldo Baffa and by his then doctorate student Angela Kinoshita, in collaboration with Ana Figueiredo, from the Nuclear and Energy Research Institute – IPEN, of Sao Paulo.

The method used, namely Electron Spin Resonance (ESR), consists in the determination of the radiation dose, called archeological dose, of the fossil sample. The archeological dose comes from the radiation emitted principally by radioactive chemical elements in the soil and received by the fossil during the period in which it was buried. The average annual dose is around 1 milli-Gray (a Gray is the measured unit of the radiation dose). “In a first evaluation, a fossil with an archeological dose of 20 Grays is 20,000 years old”, says Angela.

Invented in Russia some sixty years ago, the ESR technique can evaluate bones, teeth (fossils) and pre-historic ceramics. Teeth are easier to date because of the high level of mineralization. The pioneer in the use of this technique in Brazil was the physicist Sérgio Mascarenhas, from USP in the town of Sao Carlos, who some thirty years ago used it to determine the dose of radiation received by the victims of the atomic bomb that exploded in 1945 on the city of Hiroshima. Mascarenhas began to carry out archeological dating in 1980, in conjunction with Baffa and researchers from Japan.