Thomasomys igor, Brito & García & Castellanos & Gavilanes & Curay & Carrión-Olmedo & Reyes-Barriga & Guayasamin & Salazar-Bravo & Pinto, 2024

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, Vertebrate Zoology 74, pp. 709-734 : 709-734

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) .

Figures

Figure 1. Cyt b gene tree of the genus Thomasomys and close outgroups. The tree was inferred by maximum likelihood (ML) using a nucleotide substitution model (GTR + F + G 4); it contains 222 terminals belonging to 36 named species of Thomasomys, including the two new species here described (indicated with black arrows), it also contains at least 20 putative new species of Thomasomys, and 20 outgroups of the genera Rhipidomys, Chilomys and Rhagomys. Nodal support statistics for the analyses in IQ-Tree are from: ultrafast bootstrap approximation (UF), SH-like approximate likelihood ratio test (aL), and the aBayes test (aB). White sections of the circles indicate UF and aL bootstrap frequencies <75 % and aB posterior probabilities <0.95, whereas black sections of the circles indicate UF and aL bootstrap frequencies ~ 75 % and aB posterior probabilities ~ 0.95. Insets represent differences in topology and branch lengths of three ML approaches. Inset A: Collapsed gene tree of the main figure. Inset B: Collapsed gene tree inferred with a codon substitution model. Inset C: Collapsed gene tree estimated with the RAxML tree with the CAT approximation; black circles represent bootstrap values ~ 75 %. Terminals are labeled with GenBank accession number, museum voucher number and scientific name — in case that the species have not been described, the sequences are indicated with ‘ sp’ and a geographical proxy — ‘ ONT ’ indicates that the sequence was obtained with MinION Oxford Nanopore Technologies.

Figure 4. Dry skin in dorsal (A, B), lateral (C – E), and ventral (E, F) views of Thomasomys otavalo, sp. nov. (A, C, E; MECN 6912, holotype), and T. igor sp. nov. (B, D, F; MECN 702, holotype). Scale bar: 20 mm.

Figure 5. Selected external and soft anatomical traits of Thomasomys: A, C dorsal and (B, D) plantar surface of right pes (A, B: T. otavalo sp. nov, MECN 5604, paratype; C, D T. igor sp. nov, MECN 708, paratype); E soft palate (T. otavalo sp. nov, MECN 6921, paratype); F, G gross morphology of the stomach (T. otavalo sp. nov, MECN 5700, paratype). Abbreviations: 1–5, digits; b, bordering fold; co, cornified epithelium; d, duodenum; d 1 – d 3, diastemal rugae; ge, glandular epithelium; i, incisura angularis; i 1–15, interdental rugae. Scale bars: 5 mm.

Figure 6. Distal portion of the tail of some species of Thomasomys: A T. otavalo sp. nov. (MECN 5604, paratype); B T. ucucha (MECN 2691); C T. igor sp. nov. (MECN 708, paratype); D T. silvestris (MECN 5054). Scale bar: 10 mm.

Figure 8. Morphological comparisons. Comparison of diastemal and palate region in four species of Thomasomys. A T. otavalo sp. nov. (MECN 6912, holotype); B T. ucucha (MECN 7937); C T. igor sp. nov. (MECN 702, holotype); D T. silvestris (MECN 4928). Arrow in (A) indicates that the anterior border of the M 1 at the same level of the posterior end of the zygomatic plate; the lower dotted line indicates the penetration of the mesopterygoid fossa in relation to the hypoflexus of the M 3; abbreviations: hf, hypoflexid; if, incisive foramina; mpf, mesopterygoid fossa; pal, palate; ppp, posterolateral palatal pit; zp, zygomatic plate. Scale bar: 5 mm.

Figure 9. Alisphenoid-mastoid region comparisons in lateral view in four species of Thomasomys: A T. otavalo sp. nov. (MECN 6912, holotype); B T. ucucha (MECN 7937); C T. igor sp. nov. (MECN 702, holotype); D T. silvestris (MECN 4928). Arrow in (B) illustrated to upper edge of the mastoid that exceed the edge of the subsquamosal fenestra; while the oblique dotted line indicates the suture between the squamosal and parietal. Abbreviations: hp, hamular process of squamosal; exo, exoccipital; ip, interparietal; pa, parietal; lpp, lateral parietal portion; mas, mastoid; mf, mastoid fenestra; pgf, postglenoid foramen; sq, squamosal; ssf, subsquamosal fenestra; zps, zygomatic process of the squamosal bone. For ease of comparison are not included to scale

Figure 10. Comparison of the left anterior portion of the rostrum, viewed from left side, in four species of Thomasomys: A T. otavalo sp. nov. (MECN 6912, holotype); B T. ucucha (MECN 7937); C T. igor sp. nov. (MECN 702, holotype); D T. silvestris (MECN 4928). Thomas’ angles according to incisive and molar planes are indicated as well as the extension of the molar series. Arrow in (A) point to the anterior border of the nasal bone extending beyond the anterior face of the incisors relative to (B); abbreviations: n, nasal; nc, nasolacrimal capsule; p, premaxillary; sf, supraorbital foramen; zp, zygomatic plate. Scale bar: 5 mm

Figure 11. Comparison of the upper right molar series in occlusal view among some species of Thomasomys: A T. otavalo sp. nov. (MECN 6912, holotype); B T. ucucha (MECN 7937); C T. igor sp. nov. (MECN 702, holotype); D T. silvestris (MECN 4928); abbreviations: af, anteromedian flexus; al, anteroloph; ml, mesoloph; mfl, metaflexus; pf, paraflexus; prf, protoflexus. Scale bar: 1 mm.

Figure 12. Comparison of the lower right molar series in occlusal view among some species of Thomasomys: A T. otavalo sp. nov. (MECN 6912, holotype); B T. ucucha (MECN 7937); C T. igor sp. nov. (MECN 702, holotype); D T. silvestris (MECN 4928); abbreviations: ml, mesolophid; prf, protoflexid. Scale bar: 1 mm.

Figure 13. Thomasomys igor sp. nov. (Río Tatahuazo, Bolívar, Ecuador): cranium in (A) dorsal, (B) ventral, and (C) lateral views, and mandible in (D) labial view (MECN 702, holotype). Scale bar: 1 mm.

Figure 14. A relief map of Ecuador is depicted with hypsometric tints that show the elevation variations across the region. The sampling localities of five Thomasomys species are marked on the map.