Thomasomys otavalo, 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 otavalo sp. nov.

Holotype.

MECN 6912 (field number JBM 2574), an adult female collected 19 January, 2022, by Jorge Brito, Rubí García and Fausto Rodríguez, preserved as dry skin, skull, postcranial skeleton, and muscle and liver biopsies in 95 % ethanol.

Measurements of the holotype (in mm, mass in g).

HB = 120; TL = 161; HF = 29, E = 19; W = 36; CIL = 28.62; LD = 8.49; LM = 4.7; BM 1 = 1.53; LIF = 5.54; BIF = 2.11; BPB = 3.58; BZP = 2.54; LIF = 5.13; ZB = 16.63; DI = 1.54; BIT = 2.03; LR = 11.12; LN = 11.84; BR = 5.94; OL = 10.02; BMF = 2, 29; BCO = 7, 54; BCB = 14.67; LMN = 19.1; DR = 3.3. External and craniodental measurements of additional specimens are listed in Table 4 .

Type locality.

Ecuador, Provincia de Imbabura, Área de Protección Hídrica Otavalo Mojanda (0.15456, – 78.27536, WGS 84 coordinates taken by GPS at the site of collection, elevation 3,690 m).

Paratypes.

MECN 6909, MECN 6911, adult males, all preserved as dry skins and cleaned skulls, collected in Provincia de Imbabura, Área de Protección Hídrica Otavalo Mojanda (0.15456, – 78.27536, 3,680 m) by J. Brito, M. Yánez and C. Paucar on 19 January, 2022. MECN 6916, adult female, preserved as dry skins and cleaned skull, collected in Área de Protección Hídrica Otavalo Mojanda (0.15824 ° S, – 78.28259, 3,550 m) by J. Brito on 21 January, 2022. MECN 6920, 6921, adult males, all preserved as dry skins and cleaned skulls, collected in Área de Protección Hídrica Otavalo Mojanda (0.15456, – 78.27536, 3,685 m) by J. Brito, R. García and F. Rodríguez on 21 January, 2022. MECN 5603, juvenile female, MECN 5604, juvenile male, all preserved as dry skins and cleaned skulls, collected in Área Protegida Privada Neblina Norte (0.337535 ° S, – 78.432619, 2,990 m) by J. Brito, J. Curay, E. Beltrán and M. Esparza on 23 March, 2017. MECN 5698, MECN 5700, adult males, MECN 5701, adult female, all preserved as cleaned skulls and the rest of the body in ethanol, collected in Tabla Chupa (0.336465, – 78.413066, 3,055 m) by J. Brito, J. Curay, E. Beltrán and M. Esparza on 24 March, 2017. MECN 7869, adult female, MECN 7870, adult male, preserved as dry skin and cleaned skull, collected in Cayapas Chupa (0.54117, – 78.48343, 3,290 m) by J. Brito, A. Yacelga on 28 September 2023. MECN 7873, adult female, MECN 7874, adult male, preserved as dry skin and cleaned skull; MECN 7877, adult female, MECN 7881, 7882, adult males, all preserved as cleaned skulls and alcoholic bodies, all collected in Cayapas Chupa (0.53640, – 78.47506, 3,440 m) by J. Brito, A. Yacelga, L. Rodríguez on 03 October 2023. MECN 7871, adult female, preserved as cleaned skulls and alcoholic bodies, collected in Provincia de Esmeraldas, Cordillera de Cayapas, Loma Negra (0.54574, – 78.49796, 3,230 m) by J. Brito, L. Rodríguez on 01–02 October, 2023.

Etymology.

The specific epithet “ Otavalo ” honors the Otavalo culture, here treated as a noun in apposition. The Otavalo people are recognized for their music and ability for weaving and comercializing textiles. For decades, the Otavalos have been one of the most recognizable and proud indigenous cultures of South America ( Meisch 2002).

Diagnosis.

A species of Thomasomys unique due to the following combination of characters: Head-body length 110–125 mm, very long tail (~ 144 of head-body length), with white apical portion (15–35 mm); narrow interorbital region with rounded supraorbital margins; narrow zygomatic arches; long incisive foramina covering approximately ~ 63 % of the diastema, but not extending posteriorly between molar series; upper first molars aligned with posterior margin of the zygomatic plate; subsquamosal fenestra larger than the postglenoid foramen; upper incisors opisthodont.

Morphological description of the holotype and variation.

Pelage fine, dense, and soft, about 10 – 12 mm long over the back and rump (Fig. 3 ); somberly colored (dark drab) with low countershading. Dorsal coloration Dark Drab (Color 45) along flanks, shading to Hair Brown (color 277) mid-dorsally. Ventral pelage Medium Neutral Gray (Color 298) basally, with superficial of Ground Cinnamon Medium Neutral Gray (Color 270); not sharply demarcated from dorsal coloration (Fig. 4 A, C, E ). Mystacial vibrissae long, extending beyond pinnae when laid back alongside head. Ears sparsely covered with short, blackish hairs, not contrasting conspicuously with color of head. Hairs over metapodials and digits of manus and pes dark, but with white ungual tufts, abundant and extending well beyond the edge of the claws (Fig. 5 A, B ). Tail longer than the combined length of head and body ( LT about 144 % of HB), uniformly dark, with white apical portion (between 15–35 mm, Fig. 6 A ); with short hairs, but end with some larger hairs; it is covered with rectangular scales (13 or 14 rows / cm near the base), with three dark brown hispid hairs emerging from the base of each scale, no longer than 1–1.5 scale rows, ventral hairs are whitish in color. Mammae six in inguinal, abdominal, and postaxial pairs.

Cranium medium for the genus (28.1–30.2 mm of CIL). Long rostrum, somewhat acuminate and narrow with the nasal and premaxillary bones extending beyond the anterior face of the incisors (giving the appearance of an incipient rostral tube); poorly developed gnathic process. Posterior margin of the nasal bone not reaching the plane of the lacrimal bone. Shallow zygomatic notches. Small and rounded lacrimal bones. Narrow interorbital region with smooth outer edges, leaving the alveolar maxillary processes exposed in dorsal view (Fig. 7 A ). Anteriorly narrowed zygomatic arches. Supraorbital region with divergent posterior borders (sensu Steppan 1995). Frontoparietal suture “ V ” - shaped. Broad and rounded braincase, slightly flattened at the outer edges. Small and concave exoccipital. Dorsal profile (in lateral view) distinctively flattened from nasal tips to midfrontal region; anterior margin of zygomatic plate slightly tilted backward. No further development in the ethmoturbinals is distinguished. Small lambdoidal crest. First maxillary molar aligned with the posterior edge of the zygomatic plate (Fig. 8 A ). Zygomatic arches sturdy with long jugals spanning a major segment in the zygomatic process of the maxillary bone. Alisphenoid strut wide and short. Carotid circulation primitive (Pattern 1, sensu Voss 1988), as indicated by the presence of a large stapedial foramen, prominent squamosal-alisphenoid groove, and sphenofrontal foramen. Subsquamosal fenestra larger than the postglenoid foramen; hamular process of squamosal thin, long, slightly curved, and distally applied on the mastoid capsule; tegmen tympani broadly overlapping posterior suspensory process of squamosal. Upper edge of the mastoid not exceeding the edge of the subsquamosal fenestra (Fig. 9 A ). 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 64 % of diastemal length), not approaching M 1. Premaxillary process wide in the middle and narrow at the posterior end, maxillary septum of incisive foramina slender and long. Palate short and broad (sensu Hershkovitz 1962), with mesopterygoid fossa reaching the hypoflexus of M 3. Posterolateral palatal pit small and inconspicuous. Mesopterygoid fossa broad, straight sided, extending anteriorly between third molars; bony roof complete or perforated by narrow, slit-like sphenopalatine openings flanking the presphenoid / basisphenoid suture. Basisphenoid wide with slightly flat edges. Foramen ovale small. Parapterygoid fossae narrow, approximately triangular, with moderately (excavated) anterior limits. Middle lacerate foramen broad. Lateral expressions of parietals present. Auditory bullae small and uninflated with large and wide Eustachian tubes. Dentary moderately long, with long and narrow coronoid process (extends 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 large, broad, and opisthodont (Thomas’s angle of 80 °, Thomas 1919; see Fig. 10 A ), with front enamel Light Chrome Orange (color 76); brachydont and pentalophodont molars (sensu Hershkovitz 1962). Parallel upper molars small, pentalophodont, hypsodont in juveniles, and lacking cingula; coronal surfaces crested when unworn; main cusps slightly opposite and sloping backwards when viewed from side (Fig. 11 A ). M 1 rectangular in outline with procingulum divided by anteromedian flexus into unequal anterolabial and anterolingual conules; thin anteroloph; wide mesoloph. M 2 squared in outline; mesoloph showing the same condition as in M 1. M 3 rounded in outline with conspicuous anteroloph; shallow paraflexus; mesolph, metaflexus and hypoflexus distinctive, when unworn. Lower molars with main cusps alternating and sloping backwards when viewed from side. First lower molar (m 1) with short anteromedian flexid that divides the procingulum into subequal anterolabial and anterolingual conules (Fig. 12 A ); ectolophid thin and short mesolophid. Protoflexid of m 2 slim; thin and short mesolophid; m 3 slightly shorter than m 2.

Tuberculum of first rib articulating with transverse processes of seventh cervical and first thoracic vertebrae; second thoracic vertebra with differentially elongated neural spine; vertebral column composed of 19 thoracicolumbar, 16 th with moderately developed anapophyses and 17 th with little developed anapophyses, 4 sacrals (fused), and 42–45 caudal vertebrae; usually the second and third caudal vertebrae with small but complete hemal arches; 12 ribs.

Soft palate with 3 slightly arched diastemal palatal rugae and 5 pairs of interdentals (Fig. 5 E ). The second and third interdental wrinkles are well arched. The interdental wrinkles are short and leave a conspicuous midline “ channel ”. Large gall bladder present in all specimens examined (n = 11). The stomach is of the unilocular and hemiglandular type. The cornified epithelium dominates the corpus and is characterized by its spongy surface; the glandular epithelium is widely distributed over the mostly smooth antrum (Fig. 5 F, G ). The boundary between the two epithelia (= bordering fold, sensu Carleton 1973) is manifested by a thick ridge and exceeds the level corresponding to the esophageal opening to the left. While the angular incisura is barely marked, both the esophageal canal and the angular plica are conspicuous.

Comparisons.

Thomasomys otavalo sp. nov. is most closely related to T. ucucha , and a candidate species from Zuleta (Fig. 1 : Part A). To construct this section, we used specimens referred to Thomasomys ucucha (sensu stricto) near Papallacta, Provincia de Napo, type locality of the species ( Voss 2003). Thomasomys otavalo sp. nov. is a medium-sized species (Table 5 ), differing from T. ucucha (features in parentheses) in having shorter dorsal hairs between 10–12 mm (13–15 mm); tail noticeably long ~ 144 % of HB (~ 128 %); white tail tip 15–35 mm (<5 mm). Qualitative craniodental differences between the two species are conspicuous: the nasal and premaxillary bones in T. otavalo sp. nov. are long, extending well in front of the anterior face of the incisors (short, just beyond the anterior surface of the incisors); upper incisors opisthodont (orthodont, slightly procumbent, see Fig. 10 B ); Thomas angle 80 ° (86 °); M 1 goes beyond the posterior edge of the zygomatic plate (not so); M 1 with short paraflexus and indistinct mesoflexus (short and short); mesolophid of m 1 very short (large).

Other Thomasomys species that could be confused with T. otavalo sp. nov. are T. silvestris and T. igor sp. nov. However, T. otavalo sp. nov. can be differentiated from these species by the white tail tip between 15–35 mm (<5 mm in T. silvestris and T. igor ). Regarding the cranium, T. otavalo sp. nov. can be distinguished by the incisive foramina (64 % of LD), while T. silvestris and T. igor are longer (65 and 71 % of LD, respectively); in T. otavalo the alisphenoid strut slender while in T. silvestris and T. igor they are wide. As for the molars, in T. otavalo sp. nov. and in T. silvestris the M 3 paraflexus short and mesoflexus indistinct, while in T. igor they are long and fused giving the appearance of a horseshoe. Another species with which Thomasomys otavalo sp. nov. could be confused is Thomasomys vulcani (sympatric species), however they are easily distinguished because the new species has noticeably long tail ~ 144 % of HB (~ 90 %).

Distribution.

Known so far from less than 10 localities, all between the Otavalo-Mojanda Hydric Protected Area (Área de Protección Hídrica Otavalo Mojanda), from the highlands of Intag in Provincia de Imbabura, up to the Cordillera de Cayapas at the provincial boundary between Esmeraldas and Imbabura, at elevations of 2,290 –3,685 m. Thomasomys otavalo sp. nov. is geographically delimited by the basins of the rivers Mira (north), Guayllabamba (south), to the west by the inter-Andean valley and to the east by the tropical rainforest.

Natural history.

Thomasomys otavalo sp. nov. is, thus far, endemic to the temperate and high Andean zoogeographic areas ( Albuja et al. 2012) of the montane forest ( Ministerio del Ambiente del Ecuador 2013), an environment characterized by trees with abundant orchids, ferns, and bromeliads. Thomasomys otavalo sp. nov. was collected in mature and disturbed forest where the understory is visually dominated by herbaceous families such as Poaceae ( Chusquea sp. and Neurolepis sp. ), Araceae and Melastomataceae . Visually the canopy is dominated by encino ( Weinmannia spp. ), árbol de papel ( Polylepis spp. ) and guandera ( Clusia spp. ). The species was collected in sympatry with the shrew-opossum, Caenolestes fuliginosus (Tomes, 1863) , the shrew Cryptotis equatoris ( Thomas, 1912) , and the rodents Akodon mollis Thomas, 1894 , Nephelomys moerex (Thomas, 1914) , Neomicroxus latebricola (Anthony, 1924), Microryzomys minutus (Tomes, 1860) , T. vulcani ( Thomas, 1898) , Thomasomys sp. Mojanda ( cinereus group), and Thomasomys sp. Imbabura-Mojanda ( aureus group).

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 3. Live specimen of Thomasomys otavalo sp. nov. (Cordillera Cayapas, Imbabura, Ecuador), in its natural habitat (MECN 7869 paratype). Please note the color of the apical portion of the tail. Photographed by J. Brito on September 28, 2023.

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 7. Thomasomys otavalo sp. nov. (Área de Protección Hídrica Otavalo Mojanda, Imbabura, Ecuador): Cranium in (A) dorsal, (B) ventral, and (C) lateral views, and mandible in (D) labial view (MECN 6912 holotype). 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.