Paleontologists have discovered a new species of rodent, acarechimys, an extinct genus that was widely distributed across South America, based on partial jaws and teeth found in Brazil.

Red-tailed miracles hunikuini inhabited the Amazon region, present-day Brazil, approximately 10 million years ago during the late Miocene epoch.

This ancient species is part of the Octodontidea, a family of neotropical rodents within the par order Caviomorpha.

“The aquatic rodent families of the Neotropics, specifically Kabiomorpha, are fascinating due to their remarkable taxonomic and ecological diversity, reflecting a complex evolutionary history,” noted Colonia and colleagues.

“These rodents first appear in South American fossil records at the end of the Eocene epoch, according to current data, particularly from the records in Peru’s Amazon region.”

“The early scarcity of these species had already contributed to the diversification seen in the initial representatives of the Crown Group.”

“Today, Caviomorphs comprise 266 species, organized into two major clades and four superfamilies.”

“The superfamily Octodontoidea includes existing groups like Abrocomidae, Echimyidae, Octodontidae, Ctenomyidae, and several extinct lineages.”

“This represents the most diverse and abundant clade within Caviomorpha, both taxonomically and phenotypically.”

The fossilized teeth of Red-tailed miracles hunikuini were collected from outcrops during fieldwork in 2019 and 2022 in the Solimus Formation near the city of Feijó in Brazil’s Acre state.

“Since the mid-19th century, the Solimus Formation has revealed a diverse array of vertebrate fossils,” paleontologists commented.

“Recent dating of the fossil-rich area has suggested that the sediments date back to the Tortonian stage of the Upper Miocene.”

“Mammals represent one of the most diverse fossil groups found here, including bats, sloths, litopterns, marsupials, prominent families of rodents, primates, and Sirenians.”

“Additionally, remains of birds, crocodiles, Chelonians, fish, amphibians, invertebrates, oological bones, and plant materials have also been documented.”

The discovery of Red-tailed miracles hunikuini in this area, alongside various other fauna, indicates the coexistence of multiple superfamilies and highlights the richness of Brazilian Amazonian fauna.

“acarechimys likely emerged following the end of anemia and diversified in the Patagonia region during the early Miocene,” the researchers stated.

“By the mid-Miocene, it expanded beyond Patagonia, spreading into northern South America.”

“In the late Miocene, this lineage persisted only in regions outside Patagonia.”

“This new discovery marks the last known occurrence of the genus, supporting the theory of a late survival of rodent lineages in tropical areas—a pattern also observed in octodontoids, hydrocherines, and neoepimarids.”

“The rapid diversification and radiation of acarechimys, combined with the scarcity of data and discrepancies among studies focused on this group, and the isolated teeth found in the Amazon region, pose significant challenges for research on this genus.”

“Further studies are essential to enhance our understanding of the diversification and temporal variations of this broad octodontoidea lineage.”

The team’s publication has been featured in the Journal of Vertebrate Paleontology.

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Emmanuel Fontora et al. 2024. New Late Miocene Stem Octodontoids (Rodentia: Cabiomorpha: Octatinkiyoi) from the Solimus Formation (Brazil): Paleogeographical implications. Journal of Vertebrate Paleontology 44 (1): e2382822; doi: 10.1080/02724634.2024.2382822