The “tropical beast” from Argentina



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Most modern mammals are either placentals or marsupials. These two lineages (and their extinct relatives) are collectively known as therian mammals, and achieved global dominance during the Cenozoic. Earlier mammal groups (including most therians outside of Placentalia or Marsupialia) became extinct along with the dinosaurs, thus rendering the term “Age of the Mammals” somewhat ironic.

However, some non-therian mammals endured. Monotremes live on in Oceania, in the familiar platypus and echidnas that still survive today as the sole non-therian mammals. Other groups died midway through the Cenozoic, but nonetheless still diversified into a variety of unique species, even as therians expanded across land, sea and air.

One of the most impressive of these fossils was uncovered in mid-Paleocene deposits in Punta Peligro, Argentina. First described in 1993, this specimen was a lower jaw preserving most of the cheek teeth, which would later be joined by various isolated teeth. Measurements and comparisons with other mammals showed a critter described as “dog-sized”, and probably weighing in excess of 20kg (44lbs).

These teeth belonged to a clear herbivore. The molars and premolars are blocky and have a rather complex surface, and in life were probably used to crush and grind plants. They might have even indicate “side-to-side” (transverse) chewing, much as in a horse or a sheep, though they were less specialised for it and probably mostly relied on the same “normal” chewing as we do. The jaw is thick and robust, as expected from a mammal toughening through hard plants.

Named Peligrotherium tropicalis (“Tropical beast from [Punta] Peligro”), this animal was originally classified as an ungulate. This wasn’t too unreasonable; Punta Peligro has ungulates such as Carodnia, that may be related to modern tapirs, horses and rhinos. Punta Peligro ungulates were generally around the same size as Peligrotherium, adding more reasons to assume it was one.

But something amiss was noted early on. The molars and premolars of Peligrotherium, though superficially similar to those of ungulates, had a very strange cusp anatomy, so much so that it was almost as if it acquired complex cusps independently from any other therian mammal. Furthermore, unlike the other local ungulates its molar and premolar teeth had more than one root.

As it turns out, Peligrotherium wasn’t an ungulate, or even a therian mammal of any sort. Rather, it was a member of Dryolestoidea, an ancient lineage of mammals that lasted across the Jurassic and Cretaceous periods. Dryolestoids were fairly similar to therians, but still differed in a variety of details from tooth shape to brain anatomy.

A group of dryolestoids, Mesungulatidae, flourished in the South American Late Cretaceous, with forms like Mesungulatum and Coloniatherium being somewhat small herbivores with blunt skulls. Peligrotherium was closely related to these animals, and was essentially a much larger version, its molars dwarfing those of its Mesozoic relatives.

Mesungulatids, like many other mammals, became extinct alongside dinosaurs and other groups in the mass extinction at the end of the Cretaceous Period. However, Patagonia seems to have been less affected by it than other regions of South America, and a larger variety of animal species survive the catastrophe here. Among such animals were the ancestors of Peligrotherium, which in the absence of herbivorous dinosaurs and crocodilians grew to enormous sizes quickly.

The exact cause of extinction for Peligrotherium and kin isn’t fully understood. It has been suggested that competition with therian mammals might have corralled South American dryolestoids, but this hasn’t been formally tested, and it is just as likely that therian migrants from the north might simply have taken the niches emptied by the mass extinction event. Peligrotherium coexisted with various ungulates, so this might be of interest for future research.

Dryolestoids would continue to survive in South America for another 50 million years, with the mole-like digging Necrolestes occurring in the patagonian Miocene. More impressively, they might be still alive today in Australia, as one study found marsupial moles (Notoryctes) to not be actual marsupials, but dryolestoids related to Necrolestes. It isn’t peer-reviewed, however, and is yet to be formally addressed.


J. N. Gelfo and R. Pascual. 2001. Peligrotherium tropicalis (Mammalia, Dryolestida) from the early Paleocene of Patagonia, a survival from a Mesozoic Gondwanan radiation. Geodiversitas 23(3):369-379 [P. Mannion/P. Mannion]

G. W. Rougier, A. M. Forasiepi, R. V. Hill and M. Novacek. 2009. New mammalian remains from the Late Cretaceous La Colonia Formation, Patagonia, Argentina. Acta Palaeontologica Polonica 54(2):195-212

Agnolin, F.; Chimanto, N. (2014-12-22). "Morphological evidence supports Dryolestoid affinities for the living Australian marsupial mole Notoryctes". PeerJ Preprints. 2: e755v1. doi:10.7287/peerj.preprints.755v1. Retrieved 2015-12-30.



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