September 18, 2020

how a forgotten fossil solved a ‘crime’ from 13 million years ago

For 15 years it was there, hidden in a drawer, waiting to be rediscovered: an ancient and forgotten fossil that reveals a prehistoric hunting scene, a fight that had as its protagonists one of the largest predators that have existed after the dinosaurs and a giant sloth 13 million years ago in South America.

“For a long time it was considered that there were no fossils in the Amazon,” Palaeontologist François Pujos of the Argentine Institute of Nivology, Glaciology and Environmental Sciences of Mendoza (Argentina) tells SINC. “Actually, there are: they are found in places where vegetation does not grow, on the edges of roads or on the banks of rivers.”

In October 2004, a team of scientists launched a campaign from the Ecuadorian city of Coca to Iquitos in northeastern Peru. “It was then that we found several fossil remains in a sector of the Napo River”, says this researcher from Conicet. “Among them, there was a warm one.”

The bone was badly fractured. To protect this and other fossils and rescue them from the field, paleontologists wrapped them in layers of cloth soaked in water and plaster and transferred them to the Natural History Museum in Lima. “By then remember Pujos I didn’t think that loose bone could be so important. ”

That white package sat in a corner of the laboratory for more than a decade without anyone paying attention until recently a museum technician noticed it and decided to inspect it. When he started cleaning the fossil, he noticed strange cuts. “There is something strange here,” he commented to Pujos with surprise.

It was then that the paleontologist realized. It was the remains of the left leg of a land sloth. —An extinct mammal — that had a unique detail: 46 tooth marks from one of the largest alligators that ever lived on Earth, the Purussaurus.

“It is the first time that indications of predation of a Purussaurus about a mammal, “says the scientist, whose investigation is published today in the magazine Biology Letters of the Royal Society of London. “The find is quite rare. None of the thousands of remains of mammalian specimens found in the Amazon exhibit marks of this type.”

Attack log

13 million years ago a huge inland sea stretched in northwestern South America. Known as the Pebas Mega-Wetland System, it formed a swamp that occupied a third of the South American continent, almost 1,000,000 km². There were also islands there, in which herbivorous mammals, such as glyptodonts and giant sloths, lived in constant struggle with various predators always on the lookout.

“In particular, there was a great monster: the largest land predator after the extinction of the dinosaurs, the Purussaurus“says Pujos.” It had a wide and large skull like that of the Tyrannosaurus rex“.

Scientists estimate that it inhabited South America between 13 and 6 million years ago, during the time known as the Miocene. Some species of Purussaurus they could measure up to 10 meters in length.

Although fossil fragments have been found since the 19th century in Brazil, Peru, Colombia and Venezuela, there is still much that is not known about this striking animal.

Paleontologists assumed that these predators ate giant aquatic turtles, catfish, and mammals, but the physical evidence is extremely rare. He Purussaurus It was so large that it crushed the bones of its prey, digested virtually all of their bodies, and left few marks.

“With this finding we were lucky to find the predation tracks of an alligator of this species that devoured a Pseudoprepotherium, a 78.5 kg adult sloth, the equivalent of a large capybara or capybara, “says the paleontologist.” The tibia is one of the most resistant bones and kept the record of the attack. It was a real crime. ”

After analyzing the bite marks, the direction of the marks and the distance between them in the fossil and comparing them with the shapes of the teeth of the predators of that time and region, Pujos and his colleague, the Peruvian paleontologist Rodolfo Salas-Gismondi -specialist in crocodile evolution and anatomy and a co-author of the study – identified its author, as well as its modus operandi: the markings are consistent with the robust and tapered teeth of a Purussaurus juvenile, about four meters long, similar in size to the current adult black caiman.

“We think the alligator attacked the sloth from behind. It probably threw it into the water,” reveals Pujos. “Something similar to how crocodiles attack large mammals, such as zebras, in Africa today before dragging their prey into the waters of the Nile.”

This discovery from the Peruvian Amazon provides a kind of unusual snapshot of the dietary preferences of these predators: it reveals for the first time that before reaching their XL size, young individuals had a different diet than the Purussaurus Adults. Instead of eating large, armored animals, they fed on smaller land mammals. “They were more opportunistic animals”, warns the researcher. “They were waiting to attack.”

Rise and fall of a predator

He Purussaurus —Whose name was assigned by the Brazilian botanist João Barbosa-Rodrigues in 1892 and means “reptile of the Purus River– It was a huge predator. One hypothesis of its gigantism suggests that the high temperatures would have allowed it to grow to exceed 10 meters in length, almost like a bus.

“He Purussaurus it was the largest non-marine predator that existed on the planet after the extinction of the dinosaurs 65 million years ago, “says Salas-Gismondi, a researcher at the Biogeosciences Laboratory of the Universidad Peruana Cayetano Heredia.” In the modern world, they do not exist. species that exceed 5 or 6 meters in length, so fossil evidence is the only way to know what they fed on when they became giants. What is the Amazon region today must have been an incredible place in the past. ”

It is also likely that the ecosystem in which this animal lived – an environment rich and diverse in resources, as well as stable for millions of years – provided it with enough food and space to develop. He needed to eat more than 40 kilos of meat per day, 20 times more than current alligators and crocodiles.

For that he used his strong teeth. In an investigation published in 2015, the Brazilian paleontologist Tito Aureliano estimated that the bite of the species Purussaurus brasiliensis was twice as powerful as the T. rex: the animal could exert a pressure of up to 11.5 tons, 20 times more than that of a current shark.

Turtle diet

One of his favorite foods was huge turtles. In the Natural History Museum of Lima a fossil turtle shell discovered in Iñapari, Peruvian Amazon and that is about 9-8 million years old is exhibited. “It is 1.3 meters long, but the interesting thing is that in life the tortoise would have lost almost 60 cm of shell, including the left hind leg,” says Salas-Gismondi. “The only animal capable of doing something like this was a Purussaurus gigantic. This was the only evidence for the food preferences of Purussaurus before our discovery. ”

The reign of this predator, however, at one point began to decline. It is not clear when it became extinct, but it probably happened at the end of the Miocene, between 7 and 5 million years ago.

The rise of the northern mountain ranges of South America – the Proto-Andeans – around 12 million years ago marked the beginning of their end. That event slowly reshaped the landscape: little by little, the mega-wetlands disappeared in the Amazon. Lakes and inlets were replaced by rivers and the temperature began to drop.

“In the last millions of years the planet has undergone a constant cooling process”, explains the Peruvian paleontologist. “It is probable that even in the Amazon region the seasonal differences in temperature have been accentuated each year. Purussaurus and other giant crocodiles could have been more affected than other smaller ones by this marked seasonality. We do not know for sure, but possibly the drop in temperature in the cold season started to be cumulatively less favorable for giant species that took longer and longer to reproduce. ”

Fossils are the only window into this distant past. Many of its secrets are still waiting to be discovered deep in the Amazon rainforest.


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