Noronhomys

Noronhomys vespuccii, also known as Vespucci's rodent, is an extinct rat species from the islands of Fernando de Noronha off northeastern Brazil. Italian explorer Amerigo Vespucci may have seen it on a visit to Fernando de Noronha in 1503, but it subsequently became extinct, perhaps because of the exotic rats and mice introduced by the first explorers of the island. Numerous but fragmentary fossil remains of the animal, of uncertain but probably Holocene age, were discovered in 1973 and described in 1999. Noronhomys vespuccii was a fairly large rodent, larger than the black rat (Rattus rattus). A member of the family Cricetidae and subfamily Sigmodontinae, it shares several distinctive characters with Holochilus and related genera within the tribe Oryzomyini, including high-crowned molars with simplified crown features and the presence of several ridges on the skull which help anchor the chewing muscles. Although a suite of traits suggest that Holochilus is its closest relative, it is distinctive in many ways and is therefore classified in a separate genus, Noronhomys. Its close relatives, including Holochilus and Lundomys, are adapted to a semiaquatic lifestyle, spending much of their time in the water, but features of the Noronhomys bones suggest that it lost its semiaquatic lifestyle, after arrival at its remote island.

Discovery and taxonomy Italian explorer Amerigo Vespucci may have seen this animal on his fourth voyage, which took him to Brazil; the Lettera di Amerigo Vespucci delle Isole Nuovamente in Quattro Suoi Viaggi recorded that he visited an island just south of the equator on August 10, 1503. On this island, identified as Fernando de Noronha, he saw "very big rats and lizards with two tails, and some snakes". The Lettera purports to be an account of Vespucci's voyages, but it is unlikely that he produced it himself and, additionally, his fourth voyage may never have taken place. The biological details given in the Lettera's account of Fernando de Noronha agree with what is known of the natural history of the island, lending weight to the view that it derives from a visit, whether by Vespucci himself or by another explorer. The lizard is probably Trachylepis atlantica and the record of snakes most likely refers to Amphisbaena ridleyi, which is actually an amphisbaenian instead of a snake. During excavations conducted in 1973, American ornithologist Storrs L. Olson found fossils of a moderately large rat on Fernando de Noronha, which were described as a new genus and species in a 1999 publication by Olson and his colleague, mammalogist Michael D. Carleton. The material is now in the United States National Museum of Natural History in Washington, D.C., and in the museum of the Pontifícia Universidade Católica do Rio Grande do Sul in Porto Alegre, Rio Grande do Sul. The generic name, Noronhomys, combines the name of the island of Fernando de Noronha with the Ancient Greek μῦς mys "mouse" and the specific name, vespuccii, honors Amerigo Vespucci. Noronhomys would have been larger than the black rat (Rattus rattus), which was common on ships and which Vespucci would have been familiar with, consistent with his description of "very large rats".

Noronhomys is a member of the tribe Oryzomyini, which includes over a hundred species distributed mainly in South America, including nearby islands such as the Galápagos Islands and some of the Antilles. Oryzomyini is one of several tribes recognized within the subfamily Sigmodontinae, which encompasses hundreds of species found across South America and into southern North America. Sigmodontinae itself is the largest subfamily of the family Cricetidae, other members of which include voles, lemmings, hamsters, and deermice, all mainly from Eurasia and North America. Carleton and Olson performed a detailed comparison of Noronhomys to members of the mainland genera Holochilus and Lundomys on the basis of both general morphology and morphometrical data, concluding that the Fernando de Noronha rat is distinct from both other animals. They used a cladistic analysis to examine its relationships within Oryzomyini, also including two species of Holochilus, Lundomys, and five other oryzomyines. They found that Noronhomys appeared closest to Holochilus, with Lundomys more distantly related. Eighteen shared characters (synapomorphies) supported the grouping of Noronhomys with Holochilus. Another form described as a species of Holochilus, Holochilus primigenus, may also be related, but is likely to fall outside the Holochilus–Noronhomys clade; it has since been transferred to its own monotypic genus, Reigomys. In 1998, a fragmentary fossil of another species of this group of oryzomyines was found in eastern Argentina. It was initially identified as a possible second species of Noronhomys on the basis of the presence of a crest on the upper first molar, the mesoloph, but the specimen is different from Noronhomys vespuccii in other respects, and in 2008 it was described as a new genus and species, Carletonomys cailoi, related to Noronhomys and associated genera.

Description

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