Paleontologists have identified a new species of rodent in Acarechimys, an extinct genus with a widespread South American distribution, from a partial jaw and teeth unearthed in Brazil.
Hypothetical reconstruction of Acarechimys hunikuini. Image credit: Márcio Castro.
Acarechimys hunikuini lived in what is now Brazilian Amazonia around 10 million years ago (Late Miocene epoch).
The ancient species belongs to Octodontoidea, a family of Neotropical hystricognathous rodents within the parvorder Caviomorpha.
“Hystricognathous rodents of the Neotropics, i.e., Caviomorpha, are a fascinating mammal group because of their astonishing taxonomic and ecological diversity, which reflects a complex evolutionary history,” said Dr. Emmanuelle Fontoura, a paleontologist with the Universidade Federal de Santa Maria and CAPPA-Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia, and colleagues.
“These rodents appear suddenly in the South American fossil record at the end of the Eocene epoch, at least from what we know from the available record, especially that of Peruvian Amazonia.”
“By the Early Oligocene, they had already diversified, with the first representatives of the crown groups.”
“Present‐day caviomorphs comprise 266 species, structured into two major clades and four superfamilies: i) Erethicavioi grouping Cavioidea (e.g., guinea pig and related forms) and Erethizontoidea (New World porcupines), and ii) Octochinchilloi with Chinchilloidea (chinchilla and pacaranas) and Octodontoidea (e.g., spiny rats and related forms).”
“The superfamily Octodontoidea includes the extant families Abrocomidae, Echimyidae, Octodontidae and Ctenomyidae, plus extinct lineages.”
“This is the richest and most diverse clade among Caviomorpha, both taxonomically and phenotypically through time.”
The dental remains of Acarechimys hunikuini were collected during field trips in 2019 and 2022 at the PRE06 outcrop of the Solimões Formation near the city of Feijó in the Brazilian state of Acre.
“The Solimões Formation has yielded a great diversity of vertebrate fossils since the mid‐19th century,” the paleontologists said.
“Fossil‐bearing localities from this formation were recently dated, revealing maximum ages of deposition within the Tortonian, i.e., Upper Miocene.”
“Mammals are among the most diverse fossils found there, including bats, cetaceans, litopterns, marsupials, notoungulates, primates, rodents, sirenians, and xenarthrans.”
“In addition, fossil remains of birds, crocodilians, chelonians, fishes, amphibians, invertebrates, ichnofossils, and plants have also been documented.”
The discovery of Acarechimys hunikuini shows that several superfamilies of caviomorphous rodents co-existed in this region, in addition to many other animal groups, highlighting the existence of a rich fauna in the Brazilian Amazonia.
“Acarechimys possibly emerged during the Late Oligocene and diversified in the Patagonian region during the Early Miocene,” the researchers said.
“During the Middle Miocene, it spread to extra‐Patagonian regions, towards the north of South America.”
“During the Late Miocene, the lineage remained only in extra‐Patagonian regions.”
“This new record, which is the last appearance of the genus, supports the hypothesis of the late survival of rodent lineages in the tropics, a pattern that has been documented in octodontoids, hydrochoerines and neoepiblemids.”
“The rapid diversification and radiation of Acarechimys, as well as the scarcity of data, divergence of opinions between studies dedicated to this group, and the many isolated teeth reported for the Amazonian region, make the research of this genus challenging.”
“Further research is required to enhance our comprehension of the diversification and temporal variation of this broad lineage of Octodontoidea.”
The team’s paper was published in the Journal of Vertebrate Paleontology.
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Emmanuelle Fontoura et al. 2024. A new Late Miocene stem Octodontoid (Rodentia: Caviomorpha: Octochinchilloi) from the Solimões Formation (Brazil): paleobiogeographic implications. Journal of Vertebrate Paleontology 44 (1): e2382822; doi: 10.1080/02724634.2024.2382822
