Apnoctomys conicetorumgen. et sp. nov.

Apn: for the Administración Nacional de Parques Nacionales (APN), which is the Argentinean institution whose objectives are to design, conduct and execute the policies necessary to conserve and manage National Parks, Natural Monuments and National Reserves; octo (Latin): eight, referring to the figure-eight-shaped occlusal morphology of the cheek teeth; mys (Greek): mouse.

Western and northeastern flanks of the Sierra de Guasapampa, Córdoba, Argentina.

Only the type species.

As for the single included species (see below).

An adult female (CMI 7710), including skin, skull, skeleton, and tissues, collected on 1 April 2023 by P. Cuello (original field number AO 476; Fig. S1). A 657 bp fragment sequence of the cyt b gene has been deposited in GenBank with accession number PZ424460.

Argentina: Córdoba Province, Minas Department, eastern slope of the Sierra de Guasapampa, 800 m NE of Casco de la Estancia Pinas, Parque Nacional Traslasierra (–31.1588°, –65.4509°; Fig. 4: locality 4).

An adult male (CMI 7711; Fig. 5), including skin, skull, skeleton, and tissues, collected on 2 April 2023 by P. Cuello (original field number AO 478). A 657 bp fragment sequence of the cyt b gene has been deposited in GenBank with accession number PZ424461.

(in mm) Total length, 357; length of tail, 180; length of hind foot (with claw), 34; length of ear, 25; weight, 112 g; CBL; 39.74; NL, 14.72; NW, 5.31; RW, 7.30; ZB, 22.07; IOC, 10.03; FL, 12.47; MB, 19.47; IFL, 3.83; IFW, 2.74; UDL, 10.10; UTL; 8.18; TBL, 12.84; TBW, 6.21; FMW; 6.50; FMH, 5.94; SCH, 16.83.

(in mm) Total length, 318; length of tail, 150; length of hind foot (with claw), 37; length of ear, 25; weight, 120 g; CBL; 39.57; NL, 14.92; NW, 5.33; RW, 7.98; ZB, 21.97; IOC, 10.24; FL, 12.15; MB, 18.68; IFL, 3.73; IFW, 3.32; UDL, 10.51; UTL; 8.28; TBL, 13.22; TBW, 6.17; FMW; 6.00; FMH, 6.22; SCH, 15.57.

A species of the genus Apnoctomys, family Octodontidae, characterized by a slightly wider skull when compared to Octomys, a feature that is particularly evident in the posterior region of the incisive foramina, the mesopterygoid fossa, and the expansion of the zygoma. In addition, the premaxillary septum separating the incisive foramina is shorter, and their auditory bullae are smaller or much smaller, than those of Octomys, Pipanacoctomys, and species of Tympanoctomys. The frontal bones have a posterior process along the medial sector of the frontoparietal suture. Finally, Apnoctomys conicetorumgen. et sp. nov. has the orbitosphenoid attached to the basisphenoid, rather than the presphenoid, with a suture that remains between these bones; this is a unique arrangement, not observed in other octodontids, where the orbitosphenoid originates from the presphenoid without a persistent suture. The occlusal morphology of the molars in Apnoctomys conicetorumgen. et sp. nov. resembles that of Octomys; however, the labial ends of both lobes of M1–2 and the lingual end of the anterior lobe of m1–2 are more acuminate than in O. mimax and O. rosiae. Diploid number: 2n = 60.

A large (total length 318–357 mm), heavy-bodied rat with a moderately large tail (Fig. 5). Pelage long and soft; the dorsal coloration is grayish brown, darker toward the midline and clearer along the flanks; individual hairs (12–17 mm) have gray bases and brownish tips. Ventral hairs are entirely creamy white, except toward the sides of the venter and along the internal side of the hindlegs where they have gray bases. A collar of gray hairs with brownish tips is present. Dorsal and ventral colorations are sharply defined. On the head, including the cheeks and nose, the color is similar to that of the dorsum, but the overall appearance is more finely agouti. The proximal portion of the tail is strongly bicolored, dark brown above and creamy white below; the distal half is covered by large and blackish hairs, which surpass the tail tip by approximately 35 mm (see Fig. S2). Ears are brownish, with a conspicuous postauricular patch of cream-colored hairs and a small tuft of grayish-brown hairs at their bases. The mystacial vibrissae are long, reaching the middle portion of the ears; some vibrissae are white, while others are black. Superciliary and genal vibrissae are blackish. Manus and pes (see Fig. S3) are covered by whitish hairs; ungual tufts are large, covering almost entirely the claws. The digits of the manus are relatively short with broad, blunt tips. Digits (D) have moderately deep transverse grooves along their entire length. The three interdigital pads and single hypothenar and thenar pads are large, fleshy, bulbous, and covered by rounded scutelations. Manual D2 and D3 are nearly equal in length, with D3 slightly longer; D1 and D4 are shorter than D2–D3, with D4 slightly longer. Pollex is short with a small nail. The claws of the manus are narrow (0.5 mm) and short (3 mm) but project well beyond the tip of the fingers. There are five non-opposable digits on the pes; they are moderately long and have transverse grooves and rounded tips. D2, D3, and D4 are increasingly elongated, and D5 reaches the middle of the second phalanx of D4. Hallux is short, level with the posterior margin of the second interdigital pad. Pedal claws are relatively narrow (0.77 mm) and large (3.3 mm). The four interdigital pads and single hypothenar and thenar pads are small and rounded and are surrounded by rounded scutelations. The hypothenar pad is slightly smaller than the thenar pad and lies intermediate at a level between the first interdigital and thenar pads. The soles of the feet are covered by small, rounded scutelations. The plantar surface is completely naked; however, long hairs at the sides project over the calcaneum area.

The skull is strongly built, with a long diastema, medium-sized auditory bullae, and a nearly square braincase (Figs 6, S4). The interorbital constriction is broad, with its borders slightly divergent forwards and the lateral ridges well-marked. The nasals extend to the plane of the anterior face of the incisors. The lacrimal bones are small and nearly square in outline. The fronto-parietal suture is widely opened and “V”-shaped. The parietals are slightly inflated and nearly smooth. The zygomatic arch is inserted high; the zygomatic arch descends to the level of the alveoli of the upper molars. The infraorbital foramen is ovate. The antorbital zygomatic bar is nearly vertical. The jugal-maxillary suture is quadrangular. The jugal lateral fossa is shallow, occupying most of this bone. The paraorbital process is low and small in lateral view and formed by the squamosal. In the orbit, the portion of the lacrimal bone posterior to the nasolacrimal canal is wide. The foramen into the nasolacrimal canal is small. The sphenopalatine fissure extends to the anterior level of the alveolar sheath of the M2. The posterior portion of the fissure is wider and longer than the anterior portion. The frontal bone limits the posterodorsal extent of the fissure, and a small ridge of the orbitosphenoid reaches it posteriorly. The alveolar sheath of the M1 partially occludes the sphenopalatine fissure but is not contained within it (Fig. 6). The alisphenoid has a slender anterior apophysis expanded on the maxilla and a frequently combined masticatory-buccinator foramen (Figs 6, 7). The incisive foramina are short and broad. The wall bounding the foramina behind the premaxilla-maxillary suture are visible from the ventral aspect because they do not diverge in the dorsal direction. The premaxillary septum separating the foramina is short and wide, with a strong ventral crest (Fig. 8). The mesopterygoid fossa is deeply excavated, proportionally broad, and “V”-shaped; its anteriormost point is placed at the level of the posterior plane of the M1. The sphenopalatine vacuities are wide and shorter in front of the basisphenoid-presphenoid suture than behind it. The basicranial bones are wide, especially the basioccipital. The orbitosphenoid is attached to the basisphenoid rather than the presphenoid in CMI 7710. In CMI 7711, it exhibits a small contact with the presphenoid and a broader connection via a suture with the basisphenoid. The middle lacerate foramen is large due to the medium-sized auditory bulla, which makes contact with the alisphenoid only at the postero-external margin, adjacent to the squamosal. The auditory bullae are moderately expanded at their anterior portion. They have styliform processes extending long beyond the anterior surface of the bulla. The foramen ovale is large and lacks an external alisphenoid margin. The epitympanic recess of the petrosal bulla is moderate in size. The posterior squamosal process has its origin high and is straight (Fig. S4). Its posterior end develops into a long apophysis that, along with the lateral process of the supraoccipital, extends below the roof of the external auditory meatus (EAM). The roof of the EAM is slightly inflated. The mastoid process is reduced.

The mandible is large and heavy. The diastema is short and proportionally shallow. The plane defined by the occlusal surface of the molar series is above the incisor alveolus. The notch for the insertion of the anterior medial masseter muscle is moderately expressed and lies just below the dp4. The masseteric crests are moderately demarcated. The capsular projection is well expressed and lies below the upper sigmoid notch. The coronoid process is small and triangular in outline. The condyloid process is broad in lateral view and located slightly above the coronoid process. The angular process is proportionally short and flattened, with a dorsal groove for insertion of the masseter lateralis; it extends posteriorly to the level of the condyloid process. The semilunar notch is well expressed and nearly “C”-shaped.

The upper incisors are orthodont and short, with the base of their alveolar sheath dorsal to the ventral zygomatic root, at the level of the anterior portion of the DP4 (Figs 6, S4). The bottom of the alveolar sheath of the lower incisors is located behind the m3.

Molars are rootless, with an eight-shaped occlusal surface figure formed by two essentially transverse lobes (Fig. 9). These lobes are separated by hypoflexus/id and mesoflexus/id, which face each other and are in medial contact. The M3 and m3 are moderately reduced in comparison to the other molars, particularly regarding their posterior lobe. The labial ends of the anterior and posterior lobes of the DP4–M2 are acuminate. The m1 and m2 are rotated labially in relation to the dp4, with their sagittal axis oriented anterolabially rather than anteriorly.

Skeletal counts include 12 ribs, 19 thoracolumbar (dorsal) vertebrae, 4 sacral vertebrae, and 29 caudal vertebrae. The axis has a large spinous process, almost three times higher than those at the third to seventh cervical vertebrae. The tuberculum of the first rib contacts both the seventh cervical and the first thoracic vertebrae. A conspicuous spinous process, two times higher than those of the other thoracic vertebrae, is present on the second thoracic vertebra. Overall, spinous processes become gradually smaller from the third to fifth thoracic vertebrae.

The scapula is nearly triangular in shape; its lateral surface is divided by the scapular spine into a smaller supraspinous fossa and much larger infraspinous fossa. The cranial border is sigmoid, with its curvature being more marked towards the neck of the scapula; the vertebral and axillary edges are slightly curved. The scapular spine is separated from the scapular body approximately from the posterior half of its lateral surface, has a slight curvature in its extension towards the caudal edge, and extends ventrally beyond the glenoid cavity. The spine is continued in a wide and blunt acromion and a thin, pointed metacromion. The coastal surface presents a slight concavity in the middle of its extension along the line that divides the lateral fossae. The glenoid cavity is oval. The coracoid process extends slightly into an antero-medial direction, having a rounded extreme.

The humerus has a robust diaphysis, nearly cylindrical in cross-section. The head becomes narrower distally and with a posterior extension forming a “peak”. The greater and lesser tubercles are oval and are separated by a deep and narrow bicipital groove. The deltoid crest is located towards the first and second thirds of the diaphysis; it is well-developed, laterally expanded, and ends in a pointed to rounded tip. At the distal epiphysis, the capitulum is flattened and is separated from the trochlea by a marked groove; the trochlea is broader than the capitulum. The lateral epicondylar crest is moderately developed. The supratrochlear foramen is present and rounded, while the entepicondylar is absent. The radial and olecranon fossae are moderately shallow.

The radius has a cylindrical diaphysis, being flattened on the side that contacts the ulna; its proximal portion is slightly curved cranially. The articular fovea is well-developed, oval, and has a concave surface; the neck is well-marked. Distally, the medial styloid process of the radius is small and pointed, with a concave carpal surface. The ulna has a relatively short and robust olecranon and a poorly developed anconeal process. The trochlear notch is wide and concave. The lateral coronoid process is small. The medial coronoid process is anteriorly projected. The radial notch is wide and concave. The medial styloid process of the ulna is well-developed and rounded.

The ilium is longer than the ischium, with its wing long, flattened and concave. The anterior end of the ilium is straight with a slightly lateral curvature. The gluteal surface is divided by the gluteal line into a gluteal fossa (dorsal) and an iliac one (ventral); posteriorly, the gluteal line ends in a poorly developed and rounded coxal tuberosity. The ilium fuses distally with the proximal aspects of the ischium and pubis to form the acetabulum. The acetabulum is rounded, interrupted caudo-ventrally by the acetabular notch; the acetabular fossa is deep, with a well-defined lunar surface. The pelvic symphysis is short when compared to the obturator foramen. The obturator foramen is ovoid, with its largest axis cranio-caudally oriented.

The femur is robust, with a straight and cylindrical diaphysis. The femoral head is spherical, with a short neck; its orientation is dorso-medial. The greater and lesser trochanters are well-developed; the greater trochanter extends slightly dorsal above to the head and is dorso-laterally oriented. The lesser trochanter is postero-medially oriented. The trochanteric fossa is well-developed. The third trochanter is poorly developed, lying towards the first and second thirds of the diaphysis. The lateral and medial condyles of the distal epiphysis are nearly equal in their width, being the medial more distally projected. The patellar groove is narrow and bordered by two parallel ridges. The intercondylar fossa is narrow and deep.

The tibia is longer than the femur. The lateral and medial condyles are flattened and nearly equal in size. The lateral condyle is slightly higher than the medial and has a caudal projection. The intercondylar area is narrow and concave. The tibial tuberosity is wide and placed anteriorly, in front, and below the condyles. The dorsomedial ridge is moderately developed and anteromedially extended, placed towards the proximal portion of the bone. The interosseous crest is well-developed, extending from the proximal epiphysis to beyond the middle portion of the shaft. The distal epiphysis has two oval foveae, being the medial fovea smaller than the lateral one. The medial malleolus is larger than the lateral. The posterior process is large and has a groove for the tendon of the flexor digitorum tibialis. The fibula is compressed in its proximal portion and cylindrical in the distal one. The head is flat and fan-shaped. The lateral fibular malleolus is rounded.

The skull of Apnoctomysgen. nov. is slightly broader than to its sister genus Octomys, particularly in the posterior region of the incisive foramina, the mesopterygoid fossa, and the expansion of the zygoma. In addition, the auditory bulla is smaller than that of Octomys, Pipanacoctomys, and species of Tympanoctomys (Fig. 10).

The premaxillary septum of Apnoctomysgen. nov. displays a pronounced ventral crest, which is significantly weaker in Octomys and tends to be less distinct in other genera (Fig. 8). The walls of the incisor foramina behind the premaxillo-maxillary suture are visible ventrally because they do not diverge dorsally. In contrast, Octomys features lateral walls that diverge dorsally, rendering them undistinguishable from the ventral view (Fig. 8). Octodontomys also exhibit this dorsal divergence, whereas other genera display walls that are either straight or moderately divergent (Fig. 8). An interesting trait of Apnoctomysgen. nov. is the attachment of the orbitosphenoid to the basisphenoid, rather than the presphenoid; a suture remains between these bones (Fig. 7). This unique arrangement is not observed in other octodontids, where the orbitosphenoid originates from the presphenoid without a persistent suture. The middle lacerate foramen of Apnoctomysgen. nov. is broad, a characteristic that correlates with the moderately inflated auditory bulla, which only contacts the alisphenoid along the posteroexternal margin (Fig. 7). In Octomys, Pipanacoctomys and species of Tympanoctomys, this foramen is hidden by the bulla, except for a small anteromedial portion near the styliform process. In Spalacopus, and species of Octodon and Aconaemys, this foramen is variable in width but is consistently open through the anterior face of the bulla. The styliform process extends significantly beyond the anterior face of the bulla. This process is similarly extended in Spalacopus and species of Aconaemys, which also exhibit poorly inflated auditory bullae, whereas in Octomys, Pipanacoctomys, and species of Tympanoctomys the styliform process does not extend beyond the anterior face of the bulla. Species of Octodon exhibit a styliform process that varies in length but is always extended beyond the anterior face of the bulla (Fig. 10). The foramen into the nasolacrimal canal in Apnoctomysgen. nov. is situated more posteriorly relative to the alveolar sheath of the incisor than in Octomys, Pipanacoctomys, and species of Tympanoctomys. The sphenopalatine fissure extends to the anterior level of the alveolar sheath of M2 in Apnoctomysgen. nov., whereas it does not exceed the alveolus of M1 in Octomys, Pipanacoctomys, and Tympanoctomys. In species of Octodon, the fissure extends to the level of the alveolar sheath of M1–M2, while in Spalacopus and species of Aconaemys, it is positioned anterior to M1. This fissure is surrounded posteriorly by the maxilla, frontal, and a narrow flange of the orbitosphenoid, with the frontal bone providing a wide limit to the entire posterior aspect of the fissure. In contrast, the fissure is primarily confined by the maxilla in Octomys, Pipanacoctomys and Tympanoctomys, with only a small portion of the frontal bone contributing to the posterodorsal boundary. The fissure opens in the maxilla in Aconaemys and Spalacopus. The frontal forms a posterior process in the medial part of the suture with the parietal in Apnoctomysgen. nov. There is some asymmetry at this level. In CMI 7711 this apophysis is more marked on the right side; similarly, there is a small posterior process on the right side in Octomys mimaxCMI 6855 and BMNH 20.5.11.33 (holotype). The auditory bulla is significantly less inflated than in Octomys, Pipanacoctomys, and species of Tympanoctomys (Fig. 10). Correspondingly, the recess of the petrosal bulla in the roof of the braincase is considerably smaller than in these latter taxa, and the posterior process of the squamosal remains broad without any narrowing. The anterior end of the squamosal on the cranial dorsum is positioned almost at the level of the frontoparietal suture in Apnoctomysgen. nov. In the remaining octodontids, this portion of the squamosal is located anterior to the frontoparietal suture (Fig. 10).

The mandible of Apnoctomysgen. nov. has a longer and shallower body than in Octomys mimax. The prominence of the bottom of the m2 alveolus in the masseteric fossa exhibits a less defined dorsal border compared to that of O. mimax and Octomys rosiae. In addition, the bottom of the incisor alveolus is positioned closer to the upper sigmoid fossa than in these species.

The occlusal morphology of the molars in Apnoctomysgen. nov. resembles that of Octomys (Fig. 9). However, the labial ends of both lobes of M1–2 and the lingual end of the anterior lobe of m1–2 are more acuminate compared to those in O. mimax and O. rosiae.

Western and northeastern flanks of the Sierra de Guasapampa, Córdoba, Argentina (Fig. 4). Pardiñas et al. (2021) referred to these sierras as “Sierra de Guasayán”, a name that corresponds to a different mountain range located to the north, in the province of Santiago del Estero. Voucher specimens were caught within the limits of the Parque Nacional Traslasierra. Other records correspond to visual sightings (1–3, and 5), some of them documented through photographs (e.g., Fig. S5).

We dedicated it to the people who are part of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), including researchers, students, scientific technicians and office workers.

2n = 60; FN = 110 (Fig. S6).

The general landscape at the western and northeastern flanks of the Sierra de Guasapampa is characterized by abrupt metamorphic-rocky slopes, densely covered by rather undisturbed Chacoan forests (Fig. S7). Based on indirect signals, we deduce that Apnoctomys conicetorumgen. et sp. nov. mostly feeds on the abundant bromeliad plants that densely cover the forest floor. Observations of bromeliad cuts were found along the runways where the feces of these rodents were collected. In addition, the stomach contents of one individual were filled with finely chopped green plant material. Fresh feces are cylindrical, approximately 5 mm long, and brilliant green, while the old ones are blackish and opaque (Fig. S8). Overall, most feces were found near rock crevices, sometimes located inside ravines. We found no signs (e.g., mounds of fresh dirt or rocks) that these animals use digging to modify these places. Only two collected individuals were caught after two and three consecutive nights of trapping, using Tomahawk-like traps baited with apples. The only other species caught in this trapline was the Chacoan Leaf-Eared Mouse, Graomys chacoensis.

Apnoctomys conicetorumgen. et sp. nov. has a very restricted distributional range, which could directly influence its conservation status. To the best of our knowledge, this rodent occupies a strip approximately 120 km long and 3–5 km wide. This strip is covered by montane chacoan forest in a relatively pristine situation. Part of its distribution is protected by the Traslasierra National Park and the Parque Provincial y Reserva Forestal Natural Chancaní. Globally, this situation could suggest a conservation situation without major problems for the species. However, given our limited knowledge about this taxon and its natural history, perhaps the IUCN category that best fits the available evidence is that of Data Deficient. Additional field work is much needed to determine with more accuracy the conservation status of Apnoctomys conicetorumgen. et sp. nov., especially taking into account the situation of other related octodontid species. In fact, the only species in the genus Pipanacoctomys is critically endangered, while Tympanoctomys includes one species classified as Near Threatened (T. barrerae) and another as Vulnerable (T. kirchnerorum); finally, Octomys mimax is currently considered as Vulnerable (SAyDS-SAREM 2019).

The underlying causes of these categories are largely the same, including restricted geographical distributions, patchily distributed populations, low densities, and fragmentation and degradation of their habitat

Molecular evidence suggests that the origin of the family Octodontidae dates back to the Late Oligocene or Early Miocene (Upham and Patterson 2015; Álvarez et al. 2017). However, while the age inferred from fossil stem representatives is debated (Reig 1986; Arnal and Vucetich 2015; Verzi et al. 2016), there is general agreement that the differentiation of this clade by acquisition of apomorphies such as rootless molars, and the emergence of its crown group is no older than the Late Miocene (Upham and Patterson 2012, 2015; Suárez-Villota et al. 2016; Verzi et al. 2016; Álvarez et al. 2017). Initial diversification within the crown group of each extant genera would have occurred during the Pliocene, with the notable exception of Tympanoctomys. However, no Pliocene fossils have yet been linked to any living genera, apart from possible evidence for Pithanotomys (Reig 1986; McKenna and Bell 1997). The discovery of Apnoctomys conicetorumgen. et sp. nov. in the living fauna aligns with this pattern; this species cannot be classified within the known extinct modern, euhypsodont genera, which include Abalosia, Pithanotomys, and Pseudoplataeomys (Rovereto 1914; Reig and Quintana 1991; Verzi 2001). This bias may be taphonomic in nature (Verzi et al. 2024). Beyond this, and despite the incomplete fossil record and our still partial knowledge of living species, information regarding the timing of diversification and associated paleoenvironmental context, and the evidence arising from the discovery of Apnoctomysgen. nov. enhances our comprehension of octodontid evolution. Phylogenetic-based reconstructions indicate that the ancestral area of the crown Octodontidae is the current north portion of the Monte biome in western and northwestern Argentina (i.e., the High Monte; Ojeda et al. 2012, 2015). This area is currently inhabited by the desert-adapted species of the cis-Andean clade (i.e., species of the genera Octomys, Pipanacoctomys, and Tympanoctomys; Ojeda et al. 2015). However, at the time when the origin of the crown clade Octodontidae would have taken place, the environments of this area were presumably much like that of the present-day Chaco (Marshall and Patterson 1981; Pascual 1984; Pound et al. 2012; Anzótegui et al. 2019), with a predominance of temperate forests, dry woodlands, and tropical savannas (Pound et al. 2012). It was not until the Late Pliocene-Pleistocene that a shift towards prevalent global cooling and drying climatic conditions became established (Shackleton 1995; Denton 1999; Miller et al. 2020). This climatic change, regionally exacerbated by the rain shadows resulting from the final uplift of the central Andes and oriental mountain systems (Pampean and subandean ranges) during the late Pliocene Diaguita Phase, led to the xeric conditions characteristic of the current Monte desert (Solbrig 1976; Marshall and Patterson 1981; Pascual et al. 1985, 1996; Ortiz-Jaureguizar and Cladera 2006). The caviomorph fauna found in the Early Pleistocene of the San Andrés Formation (approx. 2.5 mya) along the central coast of Argentina suggests that by that time, the fauna of the Monte biome had become distinct and included an extinct desert-adapted representative of the cis-Andean clade, Abalosia castellanosi (Verzi 2001; Verzi and Quintana 2005). In contrast, no evidence of xeric adaptations has been found in caviomorph faunas, including octodontids, from the Late Miocene–Early Pliocene of central and western Argentina (Deschamps and Tomassini 2016). It is therefore reasonable to assume that the habitats occupied by the ancestor of the cis-Andean octodontids were more similar to those of Apnoctomysgen. nov. today. In light of the initial divergence time of this clade, estimated to have occurred around 6 mya during the Late Miocene, and the split between Apnoctomysgen. nov. and Octomys, estimated at approximately 4 mya during the Early Pliocene, it is likely that the desert adaptations observed in Octomys and the Pipanacoctomys–Tympanoctomys lineages evolved independently rather than being inherited from a common ancestor of the cis-Andean clade.

It is noteworthy that, whereas the initial diversification of the crown group Octodontidae dates to the Late Miocene (approx. 9.0 Ma), the origin of the crown group of the sister family Ctenomyidae has been estimated to be as recent as the Early Pleistocene (approx. 1.8 Ma; De Santi et al. 2024). Although older ages have been proposed for crown Ctenomyidae under alternative node-calibration schemes in divergence-time analyses (see Upham and Patterson 2012; Álvarez et al. 2017), we argue that the contrasting ages inferred for crown Octodontidae and Ctenomyidae reflect differential extinction. Indeed, whereas crown Ctenomyidae comprises only the genus Ctenomys, crown Octodontidae includes eight extant genera as well as at least three extinct genera (Verzi et al. 2016).

At least 35 living rodent genera have been proposed since 2000 (D’Elía et al. 2019). Most of these were recognized and named as results of revisionary work showing that an already recognized genus was not monophyletic. A paradigmatic case is the sigmodontine Oryzomys, as species traditionally allocated to it are now placed in 13 genera, of which 12 were proposed since 2006 (Weksler et al. 2006; Voss 2024). In turn, genera described from newly collected specimens in the field are much less common. In South America, there are some cases involving Sigmodontinae, such as Calassomys and Pattonimus (Pardiñas et al. 2014; Brito et al. 2020); within caviomorph rodents, the last genera described as new, based on specimens taken from the field, were Pipanacoctomys and Salinoctomys (the last one now a synonym of TympanoctomysOjeda et al. 2022), two taxa closely allied to Apnoctomysgen. nov. The discovery of Apnoctomysgen. nov. is noteworthy for several reasons. First, it was found in an area that is located less than 130 km from the second largest urban center in Argentina (the city of Córdoba and its surroundings), peripheral to one of the busiest tourist routes in the country. Second, it is a taxon occupying mesic environments despite being part of a small clade of rodents that mostly occupy arid to semiarid conditions. However, its discovery only now is less unexpected if one takes into account how apparently restricted its distribution is and its possible rarity in the field (in three campaigns, only 2 specimens were captured). In any case, this record draws attention to continue with sampling efforts, especially in topographically complex areas, such as those that occur in western Argentina (cf. Teta et al. 2021), and that has still been little surveyed. Finally, the finding of Apnoctomysgen. nov. provides significant information for interpreting the evolutionary pattern of octodontids. The ecomorphological diversification of extant octodontids has been considered to be phylogenetically and biogeographically structured into an originally arid-adapted clade inhabiting cis-Andean deserts and semideserts, and another clade predominantly adapted to mesic trans-Andean environments. Notably, the genus Octodontomys presents an exception; despite belonging to the mesic clade, it is adapted to arid regions on both sides of the Andes (Reig 1986; Ojeda et al. 2012). Apnoctomysgen. nov. compels a reconsideration of this generalization. It suggests that mesic conditions, rather than arid environments, may have functioned as a “cradle” for the cis-Andean clade during the Late Miocene, analogous to their trans-Andean counterparts. Subsequent episodes of aridification that initiated during the Late Pliocene-Early Pleistocene would have independently driven desert adaptations in both clades, finally determining the differentiation of the lineages of Octodontomys, Octomys, Pipanacoctomys and Tympanoctomys.