Discovery and classification in Nectomys In 1913,
Oldfield Thomas of the
British Museum of Natural History (BMNH) in London published the first description of
Mindomys hammondi, using two specimens collected at
Mindo in
Pichincha Province, Ecuador, in the same year by Gilbert Hammond. He named the species
Nectomys hammondi,
classifying it in the genus
Nectomys, which at the time included not only the large water rats currently placed in it, but also
Sigmodontomys alfari and
Oryzomys dimidiatus. He considered the animal to be most closely related to
Nectomys russulus, a species he had himself described in 1897 and which is now recognized as a
synonym of
Sigmodontomys alfari. In his 1941 review
The Families and Genera of Living Rodents,
Sir John Ellerman retained
N. hammondi as a species of
Nectomys, but noted that the features of its teeth were atypical for the genus, as "the cusps appear to show no tendency to become suppressed." Reviewing the genus
Nectomys in 1944,
Philip Hershkovitz listed
N. hammondi among species of
Nectomys incertae sedis (of uncertain position), and considered its placement in
Nectomys as dubious. Characters he listed as conflicting with a
Nectomys identity of the species included the short hindfoot with a long fifth toe, the weakly developed
posterolateral palatal pits (perforations of the
palate near the third molars), and the orientation of the
zygomatic plate.
Classification in Oryzomys Hershkovitz published again on
Nectomys in 1948 after examining additional material, including the
holotype of
N. hammondi. He now considered the latter to be a species of
Oryzomys (at the time a large genus that included most of the current members of the tribe Oryzomyini), but distinctive enough to be placed in its own
subgenus. Noting that the species was "extremely long-tailed", he introduced the subgeneric name
Macruroryzomys for
hammondi. In 1970, Hershkovitz treated the species in another publication and noted that his name
Macruroryzomys was a
nomen nudum ("naked name") because he had not explicitly mentioned characters differentiating it from other taxa in his 1948 publication. Nevertheless, he did not do anything to rectify the situation, and
Macruroryzomys remains a
nomen nudum. Hershkovitz rejected any relationship between
O. hammondi and
Nectomys or
O. aphrastus and instead argued that
O. hammondi was closely similar to
Megalomys and may be close to the ancestor of
Megalomys. In 1982, Steadman and Ray mentioned the animal in passing under the name
Macruroryzomys hammondi and reaffirmed its relationship to
Megalomys. In the 2005 third edition of
Mammal Species of the World,
Guy Musser and Michael Carleton listed
O. hammondi as an
Oryzomys of obscure affinities, but suggested that it may be related to
Megalomys. Traits of
O. hammondi that supported the latter placement include: a relatively short palate that does not extend behind the
maxillary bones; simple posterolateral palatal pits; absence of a
capsular process (a raising of the bone of the
mandible, or lower jaw, at the back end of the incisor); and presence of the
posteroloph on the upper third molar (a crest at the back of the tooth). In these characters,
O. hammondi differs from many or most Oryzomyini and is similar to some species outside Oryzomyini, but all traits of
O. hammondi are present in at least one other member of the tribe. Traits shared by
O. hammondi and
Oecomys included: tail with the same coloration above and below (unicolored);
parietal bones extending to the sides of the skull; narrow zygomatic plate, without a zygomatic notch; posteroloph present on upper third molar;
mesoflexus (a valley in the molar crown in front of the
mesoloph crest) on upper second molar not divided in two. In Weksler's analysis, species placed in
Oryzomys did not form a coherent (
monophyletic) group, but instead were found at various positions across the oryzomyine tree, and he suggested that most of these species, including
O. hammondi, should be placed in new genera. Later in 2006, Weksler and others described ten new genera for species formerly placed in
Oryzomys, including
Mindomys for
hammondi. Noting its "enigmatic distribution" and uncertain but perhaps basal position within Oryzomyini, they labeled the species an "extraordinary rat" worthy of continued inquiry. ==Description==