Newly Discovered Prehistoric Mammal Species Among Largest of Its Time

Paleontologists have uncovered a previously unknown prehistoric mammal species that was among the largest of its time.

The partial remains of the newly identified species were originally collected from a farm in Santa Cruz Province, Argentina, in the vast Patagonia region, which makes up the southernmost tip of South America.

The mammal, named Patagomaia chainko, lived about 70 million years ago during the Maastrichtian age of the Late Cretaceous period (100.5 million to 66 million years ago). The find, described in a study published in the journal Scientific Reports, sheds new light on the evolution of mammals, according to researchers.

Analysis of the fossils revealed that the creature belongs to a group known as Theria, which includes the vast majority of mammals, namely, the lineage of species that give birth to live young. The group excludes the rare examples of mammals that lay eggs, such as the platypus and echidnas.

Based on the available fossils, the researchers suggest that Patagomaia was a medium-sized mammal, comparable in size to the modern-day Andean fox, which can grow to about 4 feet in length.

The researchers estimated that the body mass of the Patagomaia uncovered in Santa Cruz Province ranged between roughly 6 pounds and 57 pounds, with an average estimate of about 31 pounds.

Even the smaller estimates would place Patagomaia among the larger mammals of the Mesozoic Era, which lasted from about 252 million to 66 million years ago and includes the Cretaceous Period.

Meanwhile, the average and higher body mass estimates far exceed those of the largest Mesozoic mammals previously known, according to the study. These include Repenomamus from China and Vintana, found in Madagascar, which had an estimated body mass of 22 pounds and 20 pounds, respectively. The higher estimates for Patagomaia would make it the largest known Mesozoic mammal.

During the Mesozoic, mammals were generally small-bodied, with the majority comparable in size to shrews, rats and mice. But the discovery of Patagomaia supports the view that mammals in the Southern Hemisphere reached relatively large body sizes by the Late Cretaceous, a few million years before their relatives in the Northern Hemisphere.

Northern Hemisphere mammals did not reach similar body sizes until after the mass-extinction event around 66 million years ago, which famously wiped out the dinosaurs, along with much of life on Earth.

The study "reveals that the evolution of large body size among Late Cretaceous mammals was more complex than previously understood," the authors wrote.

The discovery of Patagomaia supports the view that the Southern Hemisphere was "a cradle" of the evolution of modern mammal groups.

Because the fossil record of Mesozoic mammals is much richer in the Northern Hemisphere, it has traditionally been argued that the early evolution and origin of therians occurred primarily on landmasses in the north.

In this view, therian mammals were rare—if not entirely absent—in the landmasses that once made up the ancient supercontinent of Gondwana. (This consisted of most of the landmasses in today's Southern Hemisphere, including Antarctica, South America, Africa, Madagascar and Australasia, as well as the Arabian Peninsula and the Indian subcontinent.)

But some research, including the latest study, has suggested that some therian lineages, at least, evolved and diversified in the Southern Hemisphere in the later stages of the Mesozoic.

The "present discovery is of high relevance, because it supports the idea that southern landmasses constituted an important theater for the early evolution of modern mammals," the authors wrote in the study. "It is also indicative of the persistence of wide gaps in the fossil record of the Southern Hemisphere, up to now representing about 5 percent of the global fossil record of Mesozoic mammals.

"In this context, Patagomaia constitutes an important addition to the meagre record of Mesozoic therians from Gondwana and indicates that some stages of early therian evolution occurred in the Southern Hemisphere."

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