An adult male (CMI 7712), including skin, skeleton, and tissues, collected on 28 April 2023 by Raquel Alvarado-Larios, Pablo Cuello, Agustina Ojeda, Andrea P. Tarquino-Carbonell, and Paula Cornejo (original field number RCAL 91) (Figs 4, 5). An 801 base-pair sequence of the cyt b gene has been deposited in GenBank with accession number (OR669674).

Argentina: Mendoza Province, Tunuyán Department, Cajón de Arenales, alongside Ruta Provincial N° 34, 2710 m a.s.l. (33.6213°S, 69.5112°W; Fig. 1: locality 6).

TOTL, 226; TAIL, 70; HFL, 31; EAR, 6.5; TLS, 39.66; CIL, 37.71; NL, 15.58; NW, 5.66; FL, 11.31; RW, 8.91; ZB, 22.26; IOB, 7.44; BB, 15.65; BIB, 23.90; IFH, 8.02; DL, 10.21; PL, 16.89; UIW, 6.06; TRL, 8.71. Weight, 110 g.

An adult male (CMI 7739), including skin, skeleton, and tissues, collected 28 April 2023 by Raquel Alvarado-Larios, Pablo Cuello, Agustina Ojeda, Andrea P. Tarquino-Carbonell, and Paula Cornejo (original field number RCAL 92). An 801 base-pair sequence of the cyt b gene has been deposited in GenBank with accession number (OR669673).

TOTL, 215; TAIL, 64; HFL, 31; EAR, 8; TLS, 38.16; CIL, 36.36; NL, 14.28; NW, 5.81; FL, 11.05; RW, 8.94; ZB, 22.77; IOB, 7.98; BB, 16.35; BIB, 23.86; IFH, 7.65; DL, 10.03; PL, 16.11; UIW, 6.00; TRL, 8.40. Weight, 109 g.

Three adult males (CMI 7735 [original field number RCAL 87], CMI 7737 [RCAL 89], CMI 7738 [RCAL 90]) and one adult female (CMI 7736 [RCAL 88]), including skin, skeleton, and tissues, collected 26 April 2023 by Raquel Alvarado-Larios, Pablo Cuello, Agustina Ojeda, Andrea P. Tarquino-Carbonell, and Paula Cornejo at the Finca Caicayén II (33.3925°S, 69.2007°W; Tupungato Department, Mendoza Province).

We named this species in reference to the region where the type locality lays, the Valle de Uco (Uco Valley), which includes the Tupungato, Tunuyán, and San Carlos Departments in Mendoza Province, central western Argentina; this valley is well known for its fine wines.

A small-sized tuco-tuco of the C. mendocinus species group (TOTL, 215–263 mm; TAIL, 64–79 mm; HFL, 31–36 mm: EAR, 6–8 mm; Weight, 109–138 g); dorsum Drab to Dusky Drab, which turns lighter on flanks; venter Light Brownish Drab to Light Drab, with line separating from dorsum scarcely defined; a patch above nose and forehead, blackish. Skull moderately robust, with rostrum and nasals proportionally long and narrow; premaxillary-frontal suture evident anterior to the naso-frontal suture; interorbital region with posteriorly divergent outer margins. Zygomatic arches thin, slightly divergent backwards in dorsal view; dorsal profile of cranium with a marked ventral inflection in the parietal-occipital region; incisive foramina short and narrow, recessed in a common fossa of posteriorly divergent outer borders; interpremaxillary foramen small or nearly absent; sphenopalatine vacuities nearly tear-shaped; auditory bullae moderately inflated and nearly oval, with salient auditory tubes.

Pelage dense, fine, and silky, about 12–15 mm long over back and rump (Fig. 4); dorsum Drab to Dusky Drab, which turns lighter on flanks; individual hairs Dark Neutral Gray to Olive Gray, except for distal tips, which are lighter and brownish. A patch above nose and forehead are blackish. Sides of head and flanks with marked agouti effect. Color of ventral pelage Light Brownish Drab to Light Drab, with line separating from dorsum weakly defined; individual hairs dark gray basally, with distal tips whitish to cream. Chin, throat, part of chest and inguinal area darker. Fur of fore- and hindlimbs colored like dorsum, except for internal sides which are lighter. Specimens from Tupungato are lighter and brownish, with sides of nose tinged with Cinnamon, while those from Tunuyán are much darker overall (Fig. 4). Mystacial vibrissae surpassing dorsal edge of pinnae when laid back alongside head; superciliary vibrissae sparse, extending to base of pinnae when laid back alongside head. Ears sparsely covered with short, blackish hairs. Manus covered by short blackish hairs, except on sides where they are whitish. Frontal claws, long (ca. 10 mm in third digit) and are sparsely covered by ungual tufts. Pes broad and dorsally covered by blackish hairs; all digits with ungual tufts of whitish stiff bristles, and strong claws. Tail short (ca. 43% of head and body length), darker above than below and sparsely covered by short hairs.

Skull moderately robust with rostrum proportionally long and narrow, interorbital region with posteriorly divergent outer margins, and zygomatic arches thin and slightly divergent posteriad in dorsal view (Fig. 5). Nasals slightly bowed, comparatively long, and narrow, with nearly straight lateral margins; their anterior tips are in–line to level of anterior end of upper diastema. Premaxillo-frontal suture placed slightly anterior to the naso-frontal suture. Supraorbital ridges well defined, without defined postorbital processes on frontals. Interparietal small and nearly triangular in outline. Temporal ridges not developed. Lambdoid crest weakly developed. Rostral masseteric fossa dorsal to alveolar sheath of the I1, deep and ending in curved crest at level with premaxillary-maxillary suture. The bottom of alveolar sheaths of upper incisors visible laterally located at level of DP4. Zygomatic arch broad, with well-developed and pointed postorbital processes of jugal and a slightly expressed jugal ventral process; jugal dorsal fossa well excavated; and jugal longitudinal crest well defined. Dorsal profile of cranium with marked inflection ventrad in parietal-occipital region. Incisive foramina short, narrow, recessed in a common fossa of divergent outer borders posteriad; interpremaxillary foramen small, nearly absent. Palatal bridge with two major palatine foramina at about level of M1. Mesopterygoid fossa “V”-shaped, reaching anterad to middle portion of M2; bony roof of mesopterygoid fossa with two moderate nearly tear-shaped sphenopalatine vacuities. Alisphenoid-basisphenoid bridge thin. Buccinator-masticatory foramen mediumsized, undivided. Paraoccipital processes well developed and axe-shaped. Auditory bullae moderately inflated, nearly oval, with salient auditory tubes (Fig. 5).

Mandible robust, markedly hystricognathous, with coronoid process long and pointed; condyloid process robust, bearing a poorly developed articulation flange. Postcondyloid process with small ventrolateral apophysis. Chin process small and moderately visible in lateral view; bottom of alveolar sheath of p4 weakly protruding (Fig. 5).

Upper incisors large, robust, and orthodont to slightly ophistodont; frontal enamel surface Orange. Maxillary tooth rows slightly divergent posteriad. M3 reduced, with posterolingual face flat with an anterior protruding lobe (Fig. 5).

One female from “Tupungato” has a 2n = 47, FN = 68 (Massarini et al. 1991).

Ctenomys ucosp. nov. can be differentiated from C. coludo by its less inflated and proportionally smaller tympanic bullae, and its proportionally larger rostrum and narrower nasals. In addition, Ctenomys ucosp. nov. has large and wide incisive foramina, while in C. coludo these openings are short and wide.

Ctenomys ucosp. nov. differs from C. fochi (Figs 6, 7) by its less inflated tympanic bullae, larger rostrum, and larger and narrower nasals, in which the naso-frontal suture is placed behind the premaxillo-frontal suture (vs. both sutures at the same level in C. fochi; Fig. 7). In addition, the nasals of Ctenomys ucosp. nov. have straight and nearly parallel outer margins, while in C. fochi the nasals have a barrel-shapped outline in dorsal view (Fig. 7).

Ctenomys ucosp. nov. can be differentiated from both C. mendocinus (Fig. 6) and C. tulduco by its smaller and less globose tympanic bullae. Additionally, the tympanic bullae are scarcely visible from above in Ctenomys ucosp. nov., while they are visible in the dorsal view in both C. mendocinus (Fig. 6) and C. tulduco.

Ctenomys ucosp. nov. differs from C. verzi by its less robust appearance, narrower nasals and rostrum, and by the presence of an interparietal bone. In addition, Ctenomys ucosp. nov. has orthodont to slightly ophistodont upper incisors, while those of C. verzi are more proodont (Fig. 6).

Pairwise genetic distances with other species of the Ctenomys mendocinus species group range from 1.85 to 3.76 % (Table 1).

Ctenomys ucosp. nov. is known from only two localities, including its terra typica (see above) and Finca Caicayén II, near the small village of Sarmiento (33.3925°S, 69.2007°W; Tupungato Department, Mendoza Province) (Fig. 1). Both localities are separated by ca. 39 km. Two other specimens reported in the literature were referred as from “Tupungato” (see Slamovits et al. 2001, Parada et al. 2011), a reference that could apply both to the town (33.37°S, 69.14°W) or the department of the same name; due to this uncertainty, we do not map these specimens.

Mostly unknown. Field work indicates the species has solitary habits. The two known localities for this species are placed between 1000–2710 m a.s.l. The landscape in this area corresponds to an ecotone between the Low Monte and the Southern Andean Steppe (sensu Olsen et al. 2001), characterized by a mosaic of grasslands, shrublands, and vineyards (Fig. 8A, B). Dominant plants at Cajón de Arenales correspond to grasses and shrubs of the genera Stipa, Adesmia, Mulinum, Nassauvia, Larrea and Chuquiraga (Fig. 8A). Populations at Finca Caicayén II mostly live in anthropogenic plantations of grape-bearing vines, digging their burrows at the foot of these plants (Fig. 8B). This region presented a mean annual temperature between 10°C and 15°C and a mean annual precipitation of 300 mm concentrated in winter months (Norte 2000). Valle de Uco is characterized for its temperate climate with harsh winters, hot summers with cool nights, and its closeness to the Andes.

Ctenomys ucosp. nov. apparently has a small geographic range (Fig. 1), which has a direct influence on its conservation status (Bidau et al. 2012; Caraballo et al. 2023). Even so, most species of tuco-tucos with restricted geographical distributions are traditionally viewed as Data Deficient (DD; see SAREM-MAyDS 2019; but see Teta et al. 2021; Caraballo et al. 2023). Similarly, given our lack of knowledge on C. ucosp. nov., a DD categorization seems to best fit the available evidence. However, considering that C. ucosp. nov. is only known from two localities, one of them laying in an area strongly affected by the wine industry, to consider it as endangered or vulnerable seems to be more realistic. Additional field work is needed to better delineate the species distribution. Notwithstanding, it is possible that C. ucosp. nov. is present within some protected areas, including Reserva Natural Manzano- Portillo Piquenes and/or Reserva Paisajística y Natural Manzano Histórico; as said, additional field work is needed.

Taxonomic work with Ctenomys is far from complete. Ongoing studies suggest that some nominal forms will likely fall into synonymy with others, while there are still many candidate species whose distinction needs to be tested (e.g., the “Quijadas” lineage; see Mapelli et al. 2017; Teta et al. 2023), while additional field work would likely reveal additional undescribed species (e.g., Mapelli et al. 2022). There are only two other groups of Neotropical mammals undergoing a similar scenario, in which more than 8–10 species have been described in the last decade. These are species in the bat genus Myotis (see Novaes et al. 2023 and the references therein) and the sigmodontine rodents of the genus Neacomys (see Tinoco et al. 2023 and the references therein). Much of the recently discovered diversity in these two later genera is mostly cryptic and somewhat unexpected (for example, nobody would have predicted that the species richness of Neacomys would increase from six recognized species by 2015 to the currently 23 recognized species; Weksler and Bonvicino 2015; Mammal Diversity Web 2023). Meanwhile, the case of Ctenomys is distinct as its taxonomy has been largely considered as chaotic and unstable (Sage 1986; D’Elía et al. 2021), with several parts of the extensive adaptive diversification far from been resolved in terms of species boundaries (e.g., the so-called Corrientes and Boliviensis groups). Fortunately, the use of integrative approaches, together with a renewed interest in the exploration of the mammal faunas of little-known and understudied geographic areas in the southern Neotropics, have begun to reveal even more hidden diversity in species of Ctenomys (e.g., Gardner et al. 2014; Tammone et al. 2021; Sánchez et al. 2023; Teta et al. 2023). It is expected that, in this context, renewed interest in the taxonomy of these animals would cause new taxonomic arrangements, including the proposition of new species with additional species being recognized as either endangered or data deficient in terms of the IUCN categories.

The case of C. fochi, the sister species to Ctenomys ucosp. nov., is an eloquent example of the limited ecological and systematic knowledge that characterizes most species of Ctenomys. During almost a century, this species was only known from four specimens, including its holotype, all coming from the species type locality (Teta and D’Elía 2019), and flipped between being considered as a distinct species or as a subspecies of C. mendocinus (e.g., Cabrera 1961; Bidau 2015). During the last year, individuals of this species were found in two additional localities, enlarging its known distribution ca. 200 km SE and 250 km NW (Teta et al. 2023; Jayat et al. in press), and was included for the first time in a phylogenetic analysis, recovering it as a distinct species within the C. mendocinus species group (Teta et al. 2023). Finally, herein we report a fourth locality (El Chepical, Valle del río Gualcamayo, Jáchal, San Juan), which enlarges its distribution ca. 240 km SW from the previous nearest known locality. Thus, when all these localities are considered together, in only one year, our knowledge about the distributional range of C. fochi extended from a single geographical point to an area of more than 61,900 km². This situation likely would change the conservation status of this nominal form, which was considered as Data Deficient by Sánchez and Tomasco (2019; see Jayat et al. in press).

Based on the results of the phylogenetic analysis conducted here, in which haplotypes of specimens assigned to C. johannis are nested within the clade corresponding to C. mendocinus and the morphological evidence provided by Teta et al. (2023) that indicates that phenotypically C. johannis is not distinct from C. mendocinus, we formally include C. johannis as a synynom of C. mendocinus. Under this assumption, the northern distribution of C. mendocinus extends to southern San Juan province, Argentina. Additionally, in that area, the species C. tulduco Thomas, 1921 is also known. Although this last species has not been so far included in any phylogenetic study, the description provided by Thomas (1921), including its external coloration and size of the tympanic bullae, is indicative that this nominal form is likely also a synonym of C. mendocinus. In fact, according to Sanchez et al. (2023), some samples from Pedernal (San Juan), a locality from which Thomas (1921) referred some specimens to C. tulduco in the original description of this species, are indistinguishable from C. johannis (= C. mendocinus; this work). Beyond the case of C. tulduco, whose synonymy under C. mendocinus has yet to be formalized, in just two years, the following nominal forms have been placed in the synonymy of C. mendocinus: C. azarae, C. johannis, C. porteousi, and C. validus (D’Elía et al. 2021, Teta et al. 2023; this study), indicating that just as there is still diversity to be described at the species level and the distinction of other nominal forms needs to be tested. As such, we expect that the taxonomy of Ctenomys will remain unstable during the next few years as more diversity is discovered and more descriptions result.