Newly discovered Cretaceous mammal was an absolute entity

In the Cretaceous period, dinosaurs were large and mammals were small. At least, that was the general view – until recently, when a slew of larger ancient downs started showing up in the fossil record. However, none come close to the size of Patagomaia chainko: a brand new mammal from the Late Cretaceous that probably weighed as much as 26 kilograms.

Discovered in southern Patagonia, South America, the new species is represented by only fragments of the hindlimb and pelvis. Nevertheless, that is sufficient for experts to estimate its size and likely general appearance: “Estimates of the body mass of Patagomaia were done by taking measurements of the postcranial remains,” the researchers write, “using regressions that have already been used in other fossil mammals.”

The verdict: the remains ‘belong to a medium-sized mammal, comparable in size to […] the canids Lycalopex culpaeus”, the team concluded. That’s the Latin name for the culpeo, or Andean fox – although it’s actually more closely related to a wolf or a jackal than to a true fox – or to put it another way: P. chainko was bigger than a red fox, but smaller than a coyote.

That may seem at odds with the 26 kilo estimate. There is a good reason for this: it is a maximum weight, rather than an average or probable weight for the species. The team even writes: “we estimate a body weight of approximately 14 kg [30.8 pounds] for the holotype specimen,” which they believe “probably represents the maximum this animal could have achieved.”

Yet that is still very remarkable. Most Northern Hemisphere mammals from that era have a body mass of less than 100 grams (0.22 pounds), they point out, with 99 percent not even reaching 1 kilogram (2.2 pounds). Even the largest previously known species from that period – the Early Cretaceous Repenomamus and the Late Cretaceous Vintana – by approximately 10 and 8.9 kilograms (22 and 19.6 pounds), respectively.

Based on the specimen’s femur, tibia, hips and hip sockets, the researchers were also able to glean some clues about the animal’s potential shape. It does not appear to have much in common with monotremes such as the platypus, although it does share some similarities with badgers and porcupines.

Cladogram showing the phylogenetic affinities of Patagomaia chainko, geographical location and paleohistological images, map showing the fossil location, cross-section of the femur (left), the tibia (right) in polarized light with lambda compensator.

Cladogram showing Patagomaia chainko phylogenetic affinities, geographical location and paleohistological images; Map showing the fossil location; Cross section of the femur (left); tibia (right) in polarized light with lambda compensator.

Image credits: Chimento, NR, Agnolín, FL, García-Marsà, J. et al., Scientific Reports, (CC BY 4.0)

In all, the creature is undoubtedly a therian mammal: its remains exhibit a wide range of features almost always found in other known specimens of the class, and almost never outside of it. Overall, though, the creature is quite unique, even for its time – and not just because of its size.

“Although Patagomaia reveals therian affinities, it differs from Paleogene South American representatives of this clade,” the researchers note. “In total, Patagomaia shows no morphological features that could link it to any of the mammalian clades […] often included in Cretaceous and early Paleogene beds from South America.

“This new discovery shows that the Late Cretaceous mammalian faunas of South America were taxonomically diverse, including not only gondwanatherians, dryolestoids and monotremes, but also early therians,” the team concludes. “Patagomaia also reveals that the evolution of large body size among Late Cretaceous mammals was more complex than previously thought.”

The article was published in the journal Nature Scientific Reports.

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