Description
Oryzomys gorgasi is a medium-sized oryzomyine with small ears and large feet, and is similar to the marsh rice rat in general appearance. The long and coarse fur is brownish above and ochraceous below. At the base of the tail, the upper and lower sides differ in color and at the end is a short tuft of hairs. The scales on the tail are well-developed. As in other Oryzomys, the hindfeet exhibit specializations for life in the water. The plantar (lower) surface of the metatarsus is naked. Two of the pads are very small. Ungual tufts, tufts of hair at the bases of the claws, are poorly developed. Interdigital webbing is present, but extends along less than half of the first phalanges.
In specimens from El Caimito, total length is 220 to 290 mm (8.7 to 11 in), averaging 259 mm (10.2 in) (measured in 6 specimens); tail length is 116 to 138 mm (4.6 to 5.4 in), averaging 130 mm (5.1 in) (measured in 8 specimens); hindfoot length is 30 to 32 mm (1.18 to 1.26 in), averaging 31 mm (1.22 in) (measured in 10 specimens); ear length is 15 to 17 mm (0.59 to 0.67 in), averaging 16 mm (0.63 in) (measured in 7 specimens); and condylo-incisive length (a measure of total skull size) is 26.9 to 31.4 mm (1.06 to 1.24 in), averaging 29.6 mm (1.17 in) (measured in 5 specimens). In the holotype from Colombia, an old male, total length is 240 mm (9.4 in); tail length is 125 mm (4.9 in); ear length is 19 mm (0.75 in); and condylo-incisive length is 32.1 mm (1.26 in). The collector recorded the holotype's hindfoot as being 34 mm (1.34 in) long, but Sánchez and colleagues remeasured it as 33 mm (1.30 in).
The rostrum (front part of the skull) is short. The broad zygomatic plate develops a prominent notch, but not a spine, on its front end, and its back margin is in front of the first molars. The interorbital region, located between the eyes, is narrowest towards the front and is flanked by beadings along its margins. The interparietal bone is relatively long. The incisive foramina, perforations of the palate between the incisors and the molars, are narrow and long and taper towards the end. The palate itself is also long, extending beyond the molars, and includes prominent posterolateral palatal pits near the third molars, which are excavated into deep fossae. The roof of the mesopterygoid fossa, the opening behind the palate, is not perforated by sphenopalatine vacuities. O. gorgasi lacks an alisphenoid strut; in some other oryzomyines, this extension of the alisphenoid bone separates two openings in the skull, the masticatory–buccinator foramen and the foramen ovale accessorius. The squamosal bone lacks a suspensory process that contacts the tegmen tympani, the roof of the tympanic cavity, a defining character of oryzomyines. The subsquamosal fenestra, an opening at the back of the skull determined by the shape of the squamosal, is almost absent.
In the mandible (lower jaw), the upper and lower masseteric ridges come close together below the first molars, but do not fuse. The back end of the lower incisor root is in a capsular process, a raising of the mandibular bone behind the molars. The upper incisors have yellowish enamel and are opisthodont, with the cutting edge inclined backwards. The molars are relatively small and are brachydont (low-crowned) and bunodont (with the cusps higher than the connecting crests). They are similar to those of the marsh rice rat in structural details. The upper and lower first molars have small accessory roots, as in many other oryzomyines, and the second and third lower molar each have two roots only.
Oryzomys gorgasi is distinguished from other Oryzomys species by its short rostrum, the form of its incisive foramina, the absence of sphenopalatine vacuities, and the near absence of a subsquamosal fenestra. Within the species, the Colombian specimen differs from the Venezuelan animals in being larger in some measurements, but having smaller teeth, and in having oddly shaped wear facets of the incisors. The Colombian animal was probably kept in captivity for some time after it was caught, which would explain its large size and odd wear facets. There are no substantial differences between mainland O. gorgasi and material from Curaçao.
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