Oecomys mamorae (Thomas)

Oryzomys (Oecomys) mamorae Thomas, 1906b: 445 (type locality— Bolivia, Cochabamba, Yungas, upper Río Mamore´, Mosetenes; holotype —BMNH 1900.8.3.21).

Oecomys mamorae: Osgood, 1916: 206 (name combination).

Oryzomys mamorae: Ellerman, 1941: 358 (name combination).

Oryzomys mamorae mamorae: Cabrera, 1961: 405 (retained as species, ranked as nominate subspecies).

Oryzomys concolor roberti: Hershkovitz, 1960: 559 (part, mamorae allocated as full subspecific synonym).

Oecomys concolor roberti: Anderson, 1985: 12 (part, marmorae listed as full synonym).

EMENDED DIAGNOSIS: A species of Oecomys ( Sigmodontinae: Oryzomyini) characterized by a combination of medium-large size (HBL <130–150 mm, HFL <25–29 mm, ONL <31–33 mm), relatively long tail (TL <150–170 mm), narrow interorbit and weakly developed supraorbital ridges, long and narrow incisive foramina, relatively short bony palate and simple posterolateral palatal pits, alisphenoid struts typically absent, and a derived carotid circulatory pattern (skull lacking squamosal-alisphenoid groove, sphenofrontal foramen, and posterolateral groove on the parapterygoid plate; posterior opening to the alisphenoid canal compressed; stapedial foramen absent; groove dorsally crossing the parapterygoid plate present).

DISTRIBUTION: Subhumid and gallery forests in savanna and Chaco zones of central and eastern Bolivia, contiguous westcentral Brazil, and northern and eastern Paraguay (fig. 6). Known elevational range sea level to 2100 m, most localities within 200–500 m.

REMARKS: The cranium of the type specimen of mamorae (BMNH 1900.8.3.21), an old adult female, exhibits the essential traits of a derived carotid pattern (no sphenofrontal foramen, stapedial foramen minute), yet it does retain shallow traces of the squamosalalisphenoid groove on the inner walls of the braincase. As in most specimens of O. mamorae that we have examined (table 5), the type lacks alisphenoid struts. Thomas (1906b: 446) critically contrasted his new form O. mamorae to marmosurus, here allocated as a junior synonym of O. concolor, and captured the consistent proportional differences that we have observed in the larger samples of each species now available: ‘‘the distinction of this animal [ O. mamorae], which may be separated from its only equal in size, O. (Oe.) marmosurus, by its narrower interorbital region, less developed orbital ledges, and larger palatal foramina.’’ The interorbital constriction (IOB) and long incisive foramina (LIF) correlate strongly with the first canonical variate that discriminates specimens of O. mamorae from those of O. concolor and O. sydandersoni (fig. 3; tables 3, 4, 7).

Following Thomas, O. mamorae had been retained as a distinctive species by most authors until Hershkovitz (1960) submerged it within his expansive view of O. concolor, as a full synonym of O. c. roberti . Hershkovitz’s definition of the subspecies confused two distinctive species, O. roberti Thomas (1903) and O. mamorae Thomas (1906b), which are easily separable from one another and from O. concolor (Wagner) proper. Examples of O. roberti, like most other Oecomys, retain the complete carotid arterial pattern in contrast to the derived condition found in O. mamorae . Although the dorsal pelage of both species tends to be pale ochraceous-tawny and the venter mostly white, the cover hairs are typically shorter in O. roberti (3–5 mm over the rump) than O. mamorae (7–9 mm). In addition, the supraorbital shelf and posterolateral palatal pits are more pronounced in O. roberti . Specimens of O. roberti have been recorded in northern and easternmost Bolivia (Anderson, 1997; Emmons et al., 2006), just peripheral to the range of O. mamorae (fig. 6), but the two species have yet to be discovered in sympatry.

Specimens of Oecomys have been recovered from owl pellets in northeastern Argentina, from the provinces of Chaco (Massoia and Fornes, 1965, as O. concolor) and Formosa (Pardiñas and Ramírez-Llorens, 2005, as O. sp.). By geographic proximity alone, these samples are plausibly referable to O. mamorae, but certain measurements reported for the fragmented crania of the Argentine form seem to run larger than O. mamorae proper from Bolivia (notably IOB, LIF, BIF, LD, WM1— Pardiñas and Ramírez-Llorens, 2005: table 1). Critical review of variation within nominal O. mamorae and comparisons with the Argentine populations are required to verify their specific assignment. Such a review should include those populations documented in eastern Paraguay, to the east of the Paraguay River, reported as O. concolor (Myers, 1982) and here referred to O. mamorae based on our examinations.

SPECIMENS EXAMINED: 128, as follows. Bolivia — Beni, Baures (FMNH 117063–117066); Boca del Río Ibaré (AMNH 211749) ; Río Ibare´, 26 km from mouth (AMNH 211718, 211719, 211750–211752) ; Busurucucú, Yacuma Prov., 200 m (AMNH 263491) ; Lúcuma, 6 leagues S San Ramón (USNM 460430) ; Magdalena (FMNH 117055–117058) ; Río Beni, El Consuelo, 196 m (NHMS 153) ; Río Beni, Puerto Salinas, 226 m (NHMS 62–64, 74–77, 79, 80); Mamoré River (AMNH 211753) ; Río Mamore´, Marbán, 240 m (AMNH 129254, 129255) ; Mamore´, San Joaquín (FMNH 117053, 117059, 117067, 117068; USNM 391302) ; Río Tijamuchi, sea level (AMNH 262012) ; Yuatre (FMNH 117060, 117061) ; Km 35, NW of Yucumo, 253 m (AMNH 264769; MSB 68481) . Chuquisaca, 2 km E Chuhuayacu, 1200 m (MSB 63355) ; Tomina Province, 40 km from Padilla, Tola Orko, 2100 m (USNM 271581, 271582, 271584– 271587) ; Río Limón, 1300 m (MSB 63354) ; Tomina Province, Tihumayu (USNM 290906) ; Ticucha, Río Capirenda (FMNH 72890) . Cochabamba, Boca Río Chapare, 825 ft (ANSP 19405) ; Río Moile Ichilo (Inst. Roy. Sc. Nat. Belgium 20103, 25297); Todos Santos, 1300 ft (AMNH 38520–38523, 38561, 38563–38566, 38568–38578, 40782–40786; FMNH 21520–21524) ; Yungas Prov., upper Río Mamore´, Mission Mosetenes (BMNH 1900.8.3.21 [holotype of mamorae], 1900.8.3.24) . La Paz, 1 mi W Puerto Linares (MSU 33018) . Santa Cruz, Andrés Ibáñez Prov., Ayacucho (USNM 390655) ; Cordillera Prov., Basilio (USNM 390654) ; Buena Vista, 350, 450, and 500 m (AMNH 61776; BMNH 26.12.4.52, 28.2.9.39–28.2.9.42, 51.5.23.6; FMNH 25267, 25268, 51907, 51913, 51915) ; Cordillera Province, 5 km S Choreti, Camiri, 1000 m (CAS 13805; USNM 276602) ; 3.5 km W Estación Pailón, 300 m (AMNH 260420; MSB 55313) ; 3 km N and 7 km E Ingeniero Mora, 490–580 m (AMNH 247757) ; Punta Rieles (AMNH 263101–263104, 263366) ; San Rafael de Amboró, 400 m (AMNH 262013, 262014, 262119, 262099; MSB 56072) ; Velasco Prov., Santa Ana (USNM 390656, 391301) ; 15 km S Santa Cruz, 400 m (MSB 58648); Santa Cruz de la Sierra, 410 m (CM 2146, 2749); Río Yapacaní (FMNH 51914) . Brazil — Mato Grosso, Caiçara (NMW B475); Descalvados, Uca (FMNH 26643) . Mato Grosso do Sul, Fazenda Acurizal (USNM 531278); Río Paraguai, Urúcum de Corumbá (FMNH 26811) . Paraguay — Amambay, Bella Vista, Colonia Sargento Dure´, 3 km (by road) E Río Apa (MSB 70699, 70739) . Chaco, 50 km WNW Fortín Madrejón, Cerro León (UMMZ 125456) ; 28 km WNW Mayor Pablo La Gorenza, edge Río La Gorenza (= Río Timane), San Alfredo (UCONN 19187–19189) . Misiones, 40 km S San Ignacio (AMNH 234787) . Paraguari, 17 km SW Piribebuy, Saltos de Pirareta (UMMZ 133793) .