Thomasomys igor Brito, García, Castellanos, Gavilanes, Curay, Carrión-Olmedo, Reyes-Barriga, Guayasamin, Salazar-Bravo & Pinto, 2024, sp. nov.
Creators: name: Brito, Jorge
name: García, Rubí
name: Castellanos, Francisco X.
name: Gavilanes, Gabriela
name: Curay, Jenny
name: Carrión-Olmedo, Julio C.
name: Reyes-Barriga, Daniela
name: Guayasamin, Juan M.
name: Salazar-Bravo, Jorge
name: Pinto, C. Miguel
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Rodentia
Cricetidae
Thomasomys
Thomasomys igor

description: Thomasomys igor sp. nov.
Thomasomys caudivarius – Salazar-Bravo and Yates (2007: fig. 2); partim, not Thomasomys caudivarius Anthony, 1923.
Thomasomys caudivarius – Steppan and Schenk (2017: fig. 2); partim, not Thomasomys caudivarius Anthony, 1923
Thomasomys sp. 1 – Brito et al. (2019).
Holotype.
MECN 702, an adult female collected 15 July 1994, by Jorge Salazar-Bravo (field number JSB 716), originally catalogued at Museum of Southwestern Biology MSB: Mamm: 70717 (NK 27680), preserved as dry skin, skull and postcranial skeleton.
Measurements of the holotype (in mm, mass in g).
HB = 118; TL = 160; HF = 27, E = 19; W = 30; CIL = 27.44; LD = 8.49; LM = 4.74; BM 1 = 1.6; LIF = 5.77; BIF = 1.75; BPB = 3.53; BZP = 2.14; LIF = 5.19; ZB = 15.15; DI = 1.45; BIT = 1.81; LR = 9.51; LN = 11.61; BR = 5.18; OL = 7.73; BMF = 2.43; BCO = 7, 13; BCB = 13.31; LMN = 15.07; DR = 3.25. External and craniodental of additional specimens are listed in Table 4.
Type locality.
Ecuador, Provincia de Bolívar, Río Tatahuazo, 2.5 km NE de Cruz de Lizo (– 1.716667, – 78.98333, WGS 84 coordinates taken by GPS at the collection site, elevation 2,875 m).
Paratypes.
MECN 700, juvenile male, MECN 701, adult male, preserved as dry skins and cleaned skulls, by Jorge Salazar-Bravo on 15 July 1994. MECN 703, male adult, preserved as dry skins and cleaned skulls, by Nelson Monar. MEPN 6203, adult female preserved as dry skin and cleaned skull, by Igor Castro on 17 and 18 July 1994, respectively, collected at 2.5 km NE de Cruz de Lizo the same locality of the holotype. MSB 70712, juvenile female, collected at Cruz de Lizo (– 1.716667, – 78.95000, 3,000 m) preserved as dry skins and cleaned skulls, by I. Castro on 18 July 1994.
Etymology.
This species is named in honor of Igor A. Castro Revelo (1971–2022), Ecuadorian, prominent collector of rodents and curator of the mammal collection at the Museo Ecuatoriano de Ciencias Naturales (MECN) during the period 1994–2001. The species epithet is formed from the name “ Igor ” taken as a noun in apposition.
Diagnosis.
Species of Thomasomys with a unique combination of characters, as follows: Head-body length 102–122 mm, with long tail (~ 130 % of head-body length); interorbital region narrow with rounded supraorbital margins; zygomatic arches converging anteriorly; long incisive foramina covering approximately ~ 72 % of the diastema, but not extending posteriorly between the molar series; M 1 without reaching the posterior edge of the zygomatic plate; subsquamosal fenestra larger than the postglenoid foramen; upper incisors opisthodont; M 3 with paraflexus and mesoflexus long and fused; M 3 comparatively large; procingulum of m 1 divided.
Morphological description of the holotype and variation.
Fine, dense, and soft coat, about 11–13 mm long on the back and rump. Dorsal coloration Prouts Brown (Color 47), along the flanks changing to Drab (color 19). Ventral coat Medium Neutral Gray (Color 298) basally, with superficial Raw Umber (Color 24); not clearly separated from the dorsal coloration (Fig. 4 B, D, F). Mystacial vibrissae long, extending beyond the pinnae when placed backward along the head. Ears covered with short, blackish hairs not contrasting with the color of the head. Hairs on metapodials and fingers and toes white and dark giving a gray-haired appearance; ungual tufts white, abundant and extending beyond the edge of the claws (Fig. 5 C, D). Tail longer than the combined length of head and body (about 130 % of HB), uniformly dark, but with white tips (<5 mm, Fig. 6 C); with short sparse hairs, giving a naked appearance at least up to the proximal half, while the distal half is somewhat more hairy; tail end with some larger hairs (<5 mm); tail covered with rectangular scales (14 or 15 rows / cm near base), with three dark brown hairs emerging from base of each scale, no longer than 1.5–2 scale rows, ventral hairs with whitish tips. Mammae six in inguinal, abdominal, and postaxial pairs.
Skull medium for the genus (25.9–27.4 mm CIL). Rostrum long and narrow (Fig. 13 A), with nasal and premaxillary bones extending beyond anterior face of incisors; gnathic process poorly developed. Posterior margin of nasal bone reaches and exceeds the plane of the lacrimal bone. Shallow zygomatic notches. Enlarged lacrimal bones, triangular in appearance. Narrow interorbital region with smooth external borders, leaving the maxillary alveolar processes and labial part of the molars exposed in dorsal view. Zygomatic arches narrow anteriorly. Supraorbital region with divergent posterior borders (sensu Steppan 1995). Frontoparietal suture “ V ” shaped. Broad and rounded braincase, slightly flattened at the outer edges. Broad and concave exoccipital. Dorsal profile (in lateral view) distinctly flattened from nasal tips to middle frontal region; anterior margin of zygomatic plate slightly sloping backward. No further development of the ethmoturbinals. Narrow infraorbital foramen. Not distinguishable lambdoidal crest. M 1 without reaching the posterior edge of the zygomatic plate (Fig. 8 C). Slender zygomatic arches, with long jugals spanning a similar segment in the zygomatic process of the maxillary and squamosal bone. Alisphenoid strut wide and short. Carotid circulation primitive (Pattern 1, sensu Voss 1988), as indicated by large stapedial foramen, prominent squamosal-alisphenoid groove, and sphenofrontal foramen. Postglenoid foramen approximately twice as large as the subsquamosal fenestra; hamular process of squamosal thin, long, slightly curved and applied distally over the mastoid auditory capsule; tegmen tympani broadly overlapping the posterior suspensory process of squamosal. Upper edge of the mastoid does not exceed the edge of the subsquamosal fenestra (Fig. 9 C). Bullae small; pars flaccida of tympanic membrane present, large; orbicular apophysis of malleus well developed. Paraoccipital process small. Hill foramen very small; long and narrow incisive foramina (averaging 72 % of diastemal length), not approaching first molar alveoli. Narrow premaxillary process, maxillary septum of the incisive foramen very thin and long. Short and broad palate (sensu Hershkovitz 1962), with the mesopterygoid fossa entering between the molars and reaching the protocone of M 3. Posterolateral palatal pit small and inconspicuous. Wide, slightly divergent-sided mesopterygoid fossa, extending anteriorly between third molars; bony roof perforated by wide, slit-like sphenopalatine openings flanking the basisphenoid. Basisphenoid wide with slightly flat edges. Small foramen ovale. Parapterygoid fossae narrow, approximately triangular, with shallow (unexcavated) anterior limits. Middle lacerate foramen narrow. Lateral expressions of the parietals present, small (Fig. 9 C). Auditory bullae small and uninflated with short and wide Eustachian tubes. Dentary moderately long, with long and narrow coronoid process (extending beyond upper edge of condylar process); postcondylar and mental processes poorly developed; deep sigmoid notch. Semilunar recess symmetrical, with lower edge ends pointed. Capsular projection of the root of the incisor inconspicuous.
Upper incisors small, slender, and opisthodont (Thomas’s angle of 80 °, Thomas 1919; see Fig. 10 C), with front enamel Light Chrome Orange (color 76); brachydont and pentalophodont molars (sensu Hershkovitz 1962). Upper molars in left and right parallel series, small and pentalodont; hypsodont and cingulate in juveniles; coronal surfaces crested when unworn; main cusps slightly opposite and inclined backward when viewed from the side. M 1 rectangular in outline with procingulum divided by the anteromedian flexus into subequal anterolabial and anterolingual conules; anteroloph large; mesoloph short and / or segmented. M 2 squared in outline; mesoloph showing the same condition as in M 1. M 3 enlarged relative to M 2, rounded in outline with conspicuous anteroloph; paraflexus and metaflexus long and fused (Fig. 11 C); mesoloph present in juvenile specimens; hypoflexid conspicuous. Lower molars with alternate, posteriorly inclined main cusps viewed from the side. First lower molar (m 1) with anteromedian flexid dividing procingulum into subequal anterolabial and anterolingual conules (Fig. 12 C); mesolophid short; mesolophid of m 2 short, hypoflexid deep; m 3 slightly shorter than m 2.
Tuberculum of first rib articulates with transverse processes of seventh cervical and first thoracic vertebrae; second thoracic vertebra with differentially elongated neural spine; vertebral column composed of 19 thoracolumbar, 4 sacral (fused), and 30–39 caudal vertebrae; usually the second and third caudal vertebrae with small but complete hemal arches; 12 ribs. Details of soft anatomy and genitalia unknown.
Comparisons.
Thomasomys igor sp. nov. is retrieved as the sister species to T. silvestris (Fig. 1: Part A). To construct this section, we used specimens referred to Thomasomys silvestris (sensu stricto) from western Provincia de Cotopaxi and Pichincha (Anthony 1924 a, 1924 b; Brito et al. 2019). Thomasomys igor sp. nov. is a medium-sized species (Table 4), which differs from T. silvestris (characteristics in parentheses) by having slightly shorter tail ~ 130 % of HB (~ 140 %). Craniodentally, qualitative differences between the two species are conspicuous: M 1 without reaching the posterior edge of the zygomatic plate (beyond); M 3 with paraflexus and metaflexus long and fused (short); procingulum of m 1 divided (undivided or indistinct).
Other Thomasomys species who could be confused with T. igor sp. nov. are T. otavalo and T. ucucha. However, T. igor sp. nov. can be differentiated from these species by the white tail tip <5 mm (15–35 mm in T. otavalo and <5 T. ucucha). Regarding the skull, T. igor sp. nov. can be distinguished by the incisive foramina (72 % of LD), while T. otavalo and T. ucucha are shorter (63 and 57 % of LD, respectively). As for the molars, in T. igor sp. nov. in M 3 the paraflexus and metaflexus long and fused, while in T. otavalo and T. ucucha they are short. Thomasomys igor sp. nov. was previously referred to as T. caudivarius by Salazar and Yates (2007), Pacheco (2015), and Steppan and Schenck (2017), however, T. caudivarius is larger in size (see Brito et al. 2019: table 2) and has a disjunct distribution on the southwestern slope of the Andes. Another species with which Thomasomys igor sp. nov. could be confused is T. aureus (sympatric species), however they are easily distinguished because the new species is notoriously smaller ~ 113 % of HB (~ 336).
Distribution.
Thomasomys igor sp. nov. is known only from one locality, near to Cruz de Lizo, Provincia de Bolívar, in the intersections of the Bosque Protector Cashca Totoras, at elevations of 2,875 –3,000 m. The new species is geographically delimited by the basins of the rivers Angamarca (north), Chanchán (south), to the east by the inter-Andean valley and to the west by the tropical rainforest (see Fig. 14).
Natural history.
Thomasomys igor sp. nov. is found within the temperate and high Andean zoogeographic areas (Albuja et al. 2012). The ecosystem corresponds to montane forest (Ministerio del Ambiente del Ecuador 2013), which is characterized by trees with abundant orchids, ferns and bromeliads. The species was collected in sympatry with the shrew-opossum, Caenolestes fuliginosus, the shrew Cryptotis equatoris and the rodents Akodon mollis, Nephelomys albigularis (Tomes, 1860), and Thomasomys aureus (Tomes, 1860).
description: Published as part of Brito, Jorge, García, Rubí, Castellanos, Francisco X., Gavilanes, Gabriela, Curay, Jenny, Carrión-Olmedo, Julio C., Reyes-Barriga, Daniela, Guayasamin, Juan M., Salazar-Bravo, Jorge & Pinto, C. Miguel, 2024, Two new species of Thomasomys (Cricetidae: Sigmodontinae) from the western Andes of Ecuador and an updated phylogenetic hypothesis for the genus, pp. 709-734 in Vertebrate Zoology 74 on pages 709-734, DOI: 10.3897/vz.74.e128528