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Abelmoschomys simpsoni, n. gen. et sp., from the latest Clarendonian of Florida, is the earliest known species of Neotropical sigmodontine rodent. Neotropical sigmodontines initially evolved and diversified in North America in the late Miocene. This group is derived from the North American genus Copemys, and forms the sister group of the peromyscines. The Neotropical sigmodontines entered South America in the Pliocene at about the time of formation of the Panamanian land bridge. This hypothesis is supported by evidence from physiology, karyology, molecular systematics, comparative anatomy, and paleontology, and is not contradicted by parasite data. The present day diversity of sigmodontines in South America is that expected for its continental area. Taxonomic frequency rates necessary to produce the more than 200 species of South American sigmodontines as determined by the geometric growth equation range between 0.68 and 0.82 per million years. Although these rates are higher than those previously reported for Cenozoic mammals, they are comparable to rates for other muroid radiations. These high rates may account for discrepancies between divergence times calculated by the molecular clock versus the fossil record.

“Is it of any interest to anyone but a mouse fancier? (The answer is yes.)”

George Gaylord Simpson (1980, p. 196)
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