Patagomaia chainko: A large Therian Mammal from the Late Cretaceous of southern Patagonia.

Our typical perception of Mesozoic Mammals is of small, Shrew-like creatures, which were probably nocturnal in habit. However, recent discoveries have revealed a more diverse range of ecological strategies, including swimming, gliding, and burrowing Mammals, as well as species growing as large as a small Dog. The majority of known fossil Therian Mammals from the Late Cretaceous come from the Laurasian continents of the Northern Hemisphere, which has led to the assumption that this is where these Mammals originated, and that they subsequently radiated into the Southern Hemisphere following the End Cretaceous extinction. This has been contradicted by both genetic and palaeontological evidence that some Therian lineages diversified in the Southern Hemisphere prior to the End of the Cretaceous.

In a paper published in the journal Scientific Reports on 3 February 2024, Nicolás Chimento of the Laboratorio de Anatomía Comparada y Evolución de los Vertebrados at the Museo Argentino de Ciencias Naturales 'Bernardino Rivadavia', and the Consejo Nacional de Investigaciones Científicas y Técnicas, Federico Agnolín, also of the Laboratorio de Anatomía Comparada y Evolución de los Vertebrados at the Museo Argentino de Ciencias Naturales 'Bernardino Rivadavia', and of the Fundación de Historia Natural 'Félix de Azara' at the Universidad Maimónides, and the Consejo Nacional de Investigaciones Científicas y Técnicas, Jordi García‑Marsà, again of the Laboratorio de Anatomía Comparada y Evolución de los Vertebrados at the Museo Argentino de Ciencias Naturales 'Bernardino Rivadavia', and the Consejo Nacional de Investigaciones Científicas y Técnicas, Makoto Manabe of the Japanese National Museum of Nature and Science, Takanobu Tsuihiji  of the Department of Earth and Planetary Science at the University of Tokyo, and Fernando Novas, once again of the Laboratorio de Anatomía Comparada y Evolución de los Vertebrados at the Museo Argentino de Ciencias Naturales 'Bernardino Rivadavia', and the Consejo Nacional de Investigaciones Científicas y Técnicas, describe a new species of Therian Mammal from the lower Maastrichtian Chorrillo Formation of Santa Cruz Province, Argentina.

The new species is named Patagomaia chainko, where 'Patagomaia' is a combination of 'Patagonia' the region of Argentina where the fossil was found, and 'maia', the Greek word for 'mother', and 'chainko', means 'large bone' in the Aonikenk language, which is indigenous to eastern Patagonia. The species is described from two specimens, the first comprising the distal end of the left ulna, two fragments of the preacetabular wing of the left ilium, the acetabular region of the left hemipelvis, a fragment of the ischial blade, the proximal end of the right femur; the distal end of the left femur, the proximal end of the left tibia, and other indeterminate bone fragments, while the second comprises a partial left acetabulum and ischium and an incomplete right femoral shaft.

Images of Patagomaia chainko holotype remains, MPM-PV-23365: (a) fragments of the left pelvis; (b) silhouette and skeletal scheme with details of the preserved bones; (c) distal end of the left ulna; (d) proximal end of the right femur and distal end of the left femur; e, proximal half of the left tibia. Scale bar is 20 mm. Chimento et al. (2024).

A number of features of the femur and associated bones are indicative of Patagomaia chainko having been a Therian Mammal (the group which includes both the Eutherians, or Placental Mammals, and the Metatherians, or Marsupial Mammals), and not in other Mammalian groups present in the Late Cretaceous, such as the Monotremes, Multituberculates, Morganucodontans, and Docodontans. However, it was not possible to place the species precisely within a phylogenetic analysis, as most fossils from the period are known only from teeth and cranial remains.

Simplified time-calibrated cladogram showing the phylogenetic affinities of Patagomaia chainko, geographic location and palaeohistological images. The simplified cladogram shows our interpretation unifying three analyses conducted. Map showing the fossil locality. The specimen was recovered at the new site located at S 50° 30′ 39.888″ and W 72° 33′ 18.035″, close to the Isasicursor 2 site (marked with a red star) in the Maastrichtian Chorillo Formation. Transverse section of the femur (left) showing the External Fundamental System (white arrowheads); and tibia (right) in polarized light with lambda compensator. Abbreviations: ER, Erosion room; PFB, parallel-fibered bone tissue; TB, trabecular bone; VC, vascular canal. Scale bar is 0.75 mm. Chimento et al. (2024).

Patagomaia chainko is estimated to have had a mass of between 2.6 kg and 26 kg in life, with an  average estimate of 14 kg. Whilst this is far from huge,  even the smallest estimate would have made it one of the largest known Mesozoic Mammals, most of which were smaller than 1 kg, and the average size estimate would have made it larger than the estimated sizes of the two largest previously described Mesozoic Mammals, the Early Cretaceous Chinese Eutriconodont, Repenomamus, which has an estimated mass of 10 kg, and the Late Cretaceous Gondwanatherian, Vintana, which has an estimated mass of 8.9 kg.

Most Late Cretaceous Mammals from the Northern Hemisphere have a mass of less than 100 g, with less than 1% of described species estimated to have been larger than 1 kg. In Patagonia 17 Mammalian taxa have previously been described, belonging to a variety of taxa, including the Monotremata, Gondwanatheria and Meridiolestida, with eight species calculated to have exceeded 1 kg in mass. This implies ecological pressures were present in Patagonia which favoured the evolution of larger species at least 5 million years before the End Cretaceous Extinction. 

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