At present, mammals are represented by placentals, marsupials, and the primitive monotremes, which, unlike the former, lay eggs to reproduce. In the past, however, another lineage of mammals—the Multituberculata—evolved under the shadow of the dinosaurs. They first appeared during the Middle Jurassic, around 170 million years ago, and survived the Cretaceous–Paleogene mass extinction.

Multituberculates were extinct mammals that diversified and spread across a planet very different from today’s. The warmer and more humid climate of the Mesozoic fostered ecosystems unlike those of the present, with vegetation dominated by conifers, cycads, ginkgos, and ferns, and a rich fauna whose most familiar representatives were the herbivorous dinosaurs (sauropods and ornithischians), the carnivorous theropods, and the flying reptiles known as pterosaurs.

Multituberculates were small mammals ranging in size from that of a tiny mouse to a squirrel. They developed a cranial morphology that foreshadowed what would later be seen in rodents, with continuously growing incisors separated from the molariform teeth by a toothless gap (diastema). These molariforms were characterized by parallel rows of multiple cusps or tubercles, a feature that gives the group its name.

In many species, a large, ridged, blade-like tooth—known as a plagiaulacoid tooth—was often present in the lower jaw, just in front of the molariforms. This dental specialization likely served to slice or crush hard items such as arthropod shells, tough fruits, and seeds, or to cut small plant shoots. This tooth worked in concert with a unique chewing motion, in which the mandible moved from front to back (palinal), unlike in most other mammals, where jaw movement is primarily backward–forward or transverse.

The extinct lineage of multituberculates inhabited Earth for nearly 100 million years. Today, they are known from an extensive fossil record across North America, Europe, and Asia. The occurrence of this group in the Southern Hemisphere has long been the subject of debate. Unlike the relatively complete skeletons known from the Northern Hemisphere, in South America and Australia, multituberculates were, until recently, represented only by isolated plagiaulacoid teeth.

One reason their presence was questioned is that, like other traits—such as the existence of a single bone forming the lower jaw (the dentary)—the plagiaulacoid tooth appears to have evolved independently in different mammalian lineages, including some marsupials. Because these teeth were the only possible evidence of the group in South America, their identification kept researchers divided. On one hand, some considered that these plagiaulacoid teeth belonged to multituberculates; on the other, some argued that, in the absence of additional evidence, they were probably attributable to other mammalian lineages. In this context, the leading candidate was the group Gondwanatheria, a lineage of mammals known from the fossil record of South America, Antarctica, Madagascar, and Africa.

The recent discovery of Notopolytheles joelis—represented by an upper molar bearing three aligned rows of cusps—from Upper Cretaceous sediments of the La Colonia Formation in Chubut Province, Argentina, provides conclusive evidence for the presence of multituberculates in South America. This finding opens new questions about the evolutionary history of mammals in the Southern Hemisphere. The study describing this new species was published in Scientific Reports by Javier N. Gelfo and Francisco Goin, researchers at CONICET’s Division of Vertebrate Paleontology, Museo de La Plata (Universidad Nacional de La Plata), together with Nahuel Vega from the Centro Atómico Constituyentes (CNEA).

This tiny tooth, only a few millimeters long, shows features that differ substantially from those of its northern relatives—such as an elongated root supporting one of the cusp rows and a marked difference in height among them. From the same locality, Argentodites coloniensis was known from a single plagiaulacoid tooth. Although it was originally described as a multituberculate, this taxonomic assignment was later questioned, as were records of plagiaulacoid teeth from the Early Cretaceous of Australia, described as Corriebaatar marywaltersae.

The discovery of this new molar, assigned to a new genus and species of multituberculate, indirectly confirms the identity of these previous findings and reinforces the presence of the Cimolodonta lineage in South America and Australia. It is also likely that they were present in Cretaceous Antarctica, when the configuration of the Gondwanan supercontinent allowed terrestrial faunas to disperse and thrive across connected landmasses. This evidence, together with the fossil record of monotremes in Patagonia and the dispersal of marsupials into Australia through Antarctica during the Paleogene, attests to the continuity of these continental ecosystems.

This scenario began to change with the final fragmentation of Gondwana and the emergence of geographic barriers such as the Drake Passage between Patagonia and the Antarctic Peninsula, and the Tasmanian Passage between Australia and Antarctica. The climatic shifts at the Eocene–Oligocene boundary (~34 million years ago), which led to pronounced global cooling, were partly linked to this progressive isolation of Antarctica and the establishment of the Antarctic Circumpolar Current.

The discovery of Notopolytheles joelis was made possible through the efforts of many people, particularly students from the Facultad de Ciencias Naturales y Museo de La Plata, who participated in the collection of microfossils, and the initiative of Dr. Rosendo Pascual, the driving force behind a novel and insightful theoretical framework for the evolutionary history of South American mammals. Rosendo was not only a brilliant paleontologist and theorist but also led the fieldwork conducted in the La Colonia Formation at the end of the last century.

During those campaigns, several tons of sediments were screened and washed in search of small mammal teeth. Processed sediment samples are far more likely to yield microfossils, which are then separated under a binocular microscope. This is a slow but much more effective method than attempting to locate such tiny fossils directly in the field, where wind, light, and shadow often conspire against the observer.

Rosendo Pascual proposed that the plagiaulacoid teeth from the Cretaceous of Patagonia provided evidence for the presence of Multituberculata in the region. An alternative explanation attributes these teeth to another major group known from the Cretaceous and Paleogene —the Gondwanatheria. This latter lineage of mammals is also known in South America, mostly from isolated teeth, with the notable exception of the remarkable Sudamerica ameghinoi from the Paleocene levels of the Banco Negro Inferior in Patagonia. The jaw of Sudamerica ameghinoi exhibits a rodent-like incisor and, for the first time in mammalian history, molariform teeth with continuous (hypsodont) growth. Unlike the jaws known for cimolodont multituberculates, Sudamerica ameghinoi lacks a plagiaulacoid tooth.

Rosendo’s hypothesis, therefore, implied the coexistence of multituberculates and gondwanatherians as independent groups in the Cretaceous of Patagonia. The rationale behind this inference rested on the principle of parsimony—or Ockham’s razor—which, from an evolutionary standpoint, favors the simplest explanation. Thus, it was more reasonable to assume that Gondwanatheria never possessed plagiaulacoid teeth than to propose that such teeth evolved in the Cretaceous ancestors of Sudamerica ameghinoi and later disappeared during their evolutionary history.

The discovery of the upper molar of Notopolytheles joelis supports Rosendo Pascual’s interpretation and highlights that the diversity and ecological complexity of Cretaceous mammal communities in Patagonia were greater than previously thought. Multituberculates, a successful group of mammals characteristic of the Mesozoic and Paleogene of the Northern Hemisphere, also inhabited South America, Australia, and most likely the other landmasses that once formed Gondwana.

Our understanding of the evolution and biogeography of these lineages remains an unfinished narrative, with many chapters still to be revisited and others yet to be written. Forged in the shadow of the great dinosaurs, this story continues to be reconstructed today from tiny teeth that reveal a complex and extraordinary past.

Bibliographic information
-Gelfo, J.N., Goin, F.J. & Vega, N.A. First unambiguous evidence of Multituberculata from the Late Cretaceous of South America. Sci Rep 15, 41500 (2025).  
- Kielan-Jaworowska, Z., Ortiz-Jaureguizar, E., Vieytes, C., Pascual, R., & Goin, F. J. (2007). First? cimolodontan multituberculate mammal from South America. Acta Palaeontologica Polonica, 52(2).
- Kielan-Jaworowska, Z., Cifelli, R. L., & Luo, Z. X. (2005). Mammals from the age of dinosaurs: origins, evolution, and structure. Columbia University Press.
- Pascual, R., Goin, F. J., Krause, D. W., Ortiz-Jaureguizar, E., & Carlini, A. A. (1999). The first gnathic remains of Sudamerica: implications for gondwanathere relationships. Journal of Vertebrate Paleontology, 19(2), 373-382.
- Pascual, R., & Ortiz-Jaureguizar, E. (2007). The Gondwanan and South American episodes: two major and unrelated moments in the history of the South American mammals. Journal of Mammalian Evolution, 14(2), 75-137.
-Rich, T. H., Vickers-Rich, P., Flannery, T. F., Kear, B. P., Cantrill, D. J., Komarower, P., ... & Fitzgerald, E. M. (2009). An Australian multituberculate and its palaeobiogeographic implications. Acta Palaeontologica Polonica, 54(1), 1-6.
-Rougier, G. W., Martinelli, A. G., & Forasiepi, A. M. (2021). Mesozoic mammals from South America and their forerunners (p. 388). Cham, Switzerland: Springer.