Type species

Proclinodontomys dondasi n. gen. n. sp.

Other species

Proclinodontomys mordax n. comb. (Winge, Reference Winge1887).

Diagnosis

Proclinodontomys n. gen. differs from Clyomys by having much smaller bullae. It differs from Clyomys and Euryzygomatomys by having a more developed fossa for the M. temporalis, much more procumbent upper incisors, more flared and laterally expanded zygomatic arches, frontal less markedly expanded posteriorly, jugals (between the anterior border and the lower jugal process) much deeper anteriorly than posteriorly, with the dorsal border descending more abruptly posteriorly, smaller orbital cavity, and external auditory meatus relatively smaller and slanted upward and backward. It differs from Euryzygomatomys by the absence of lateral flange of the canal for infraorbital nerve, the protuberance formed by the bottom of the alveolar sheath of the upper incisor lateral to DP4, and the narrower posterior portion of incisive foramen.

Occurrence

Early-middle Pleistocene--early Holocene of Argentina and Brazil. It is also represented in the Pleistocene of Bolivia (see below).

Etymology

From Greek pro = anterior, clino = inclination, odonto = tooth, mys = mouse; in reference to the fact that is a rodent with the front teeth inclined forward.

Remarks

Proclinodontomys dondasi n. gen. n. sp., from the early-middle Pleistocene of Central Argentina, here described, and Proclinodontomys mordax n. comb., from the Pleistocene and Holocene of Brazil (Winge, Reference Winge1887; Hadler et al., Reference Hadler, Verzi, Vucetich, Ferigolo and Ribeiro2008), previously assigned to the genus Euryzygomatomys (Winge, Reference Winge1887), share a combination of features (see diagnosis of the genus and comparative description of P. dondasi n. gen. n. sp.), differing from any other known echimyid. This allowed us to identify both species as belonging to a new genus. This study also reveals that among echimyids, both species are closely related to Euryzygomatomys spinosus and Clyomys. These working hypotheses are supported by the results from the phylogenetic analysis (see below).

In addition to Euryzygomatomys mordax n. comb., a second putative extinct species of Euryzygomatomys was listed as integrating the fossil assemblage recovered in the Pleistocene basin of Tarija, Bolivia (as E. hoffstetteri; see Marshall et al., Reference Marshall, Berta, Hoffstetter, Pascual, Reig, Bombin and Mones1984; Marshall and Sempere, Reference Marshall, Sempere and Suarez Soruco1991), but never described. Although E. hoffstetteri must be treated as a nomen nudum, its placement in Euryzygomatomys is controversial (see Loss, Reference Loss2014), and it will not be treated in detail here. However, a preliminary study made by the senior author of the cranial material of Tarija suggests that it could be assigned to Proclinodontomys n. gen. The skull of Tarija (MNHN TAR 51, selected as holotype of E. hoffstetteri by Osvaldo Reig, in schedis) has at least three diagnostic features of Proclinodontomys n. gen., such as the markedly procumbent incisors, strongly developed fossa for the M. temporalis, and the presence of the bottom of the alveolar sheath of the upper incisor lateral to DP4. Extensive studies are necessary to elucidate the status of E. hoffstetteri.

Holotype

MPH-P 173, an almost complete skull with right and left DP4-M3, only lacking the nasals.

Diagnosis

It differs from Proclinodontomys mordax n. comb. by having relatively narrower rostrum, wider interorbital region, slender incisors, M3 relatively large with respect to the M2, and narrower posterior zygomatic root.

Occurrence

The holotype occurs in coastal cliffs located 2.5 km NE of the mouth of Chocorí stream (38°23'13.9''S, 58°04'34.83''W, Locality 1, see Figs. 1, 2.1), General Alvarado County, Buenos Aires Province, Argentina. The holotype was found in a deposit of tabular geometry located at the base of the cliff, consisting of compact silts and fine-grained sands of red to dark-brown coloration (see Fig. 2.2). The characteristics of the fossil-bearing deposit allow its inclusion in the Facies Association 1 (FA1) described by Cenizo (Reference Cenizo2011) for the cliffs of Centinela del Mar locality, located ~10 km SW of the type locality. The FA1 emerges in the basal sector (sometimes also at medium sector) of the cliffs that extend almost continuously from Centinela del Mar to Mar del Sur localities (Cenizo, Reference Cenizo2011, p. 30; see also Cenizo and Tassara, Reference Cenizo and Tassara2013, p. 21). In addition, the FA1 has been recognized (in similar facies succession, although not equal) in the Chocorí stream area as “Lower Interval” by Beilinson et al. (Reference Beilinson, Gasparini, Soibelzon and Soibelzon2015). The FA1 is characterized by a predominance of fluvial-alluvial depositional events of great homogeneity and amplitude, with the development of massive carbonate banks and “grid-like” carbonate infiltrations as main post-depositional facies, especially on its upper contact. The greater consolidation of its sediments, as well as the tabular appearance and the large horizontal extension of its deposits with respect to the overlying facies, are some of the main features that allow a rapid identification of FA1 in the field (Cenizo, Reference Cenizo2011; Beilinson et al., Reference Beilinson, Gasparini, Soibelzon and Soibelzon2015). Such facies association has been correlated with the Middle Allomember of the Punta San Andrés Alloformation described by Zárate (Reference Zárate1989), ~40 km NE of the type locality (Cenizo, Reference Cenizo2011; Beilinson et al., Reference Beilinson, Gasparini, Soibelzon and Soibelzon2015). Magnetostratigraphic studies performed in the Centinela del Mar outcrops indicate that the deposition of FA1 occurred during a period of reverse polarity assigned to chron Matuyama (Heil et al., Reference Heil, King, Zárate and Schultz2002; Soibelzon et al., Reference Soibelzon, Prevosti, Bidegain, Rico, Verzi and Tonni2009; Cenizo, Reference Cenizo2011), therefore, it would be older than 780 ka AP. This is consistent with the finding of remains of the native ungulate Mesotherium cristatum (Serres, Reference Serres1867) in outcrops of the FA1 located 1.9 km (38°23'42.2''S, 58°05'48.5''W, MPH-P 0210) and 2 km South (38°23'43.5''S, 58°05'50.1''W, MPH-P 0223) of the locality where the holotype was recovered. Mesotherium cristatum is a characteristic mammal of the homonymous South American Biozone (biostratigraphic base of the Ensenadan Stage/Age, see Cione et al., Reference Cione, Gasparini, Soibelzon, Soibelzon and Tonni2015), so that its occurrence in the FA1 supports the early-middle Pleistocene age of this unit.

Figure 2.

(1) Geographical references of the localities mentioned in the text, including the type locality of Proclinodontomys dondasi n. gen. n. sp., and (2) their respective stratigraphic profiles.

Additional material occurs in coastal cliffs located 2.7 km SW of Rocas Negras locality (SW limit of Mar del Sur town, 38°22'19.62''S, 58°02'20.15''W, Locality 2, see Fig. 2.1), General Alvarado County, Buenos Aires Province. The specimen MPH-P 0174 was found ~3.7 km NE of the locality where the holotype was recovered (see Fig. 2.1), also from a deposit corresponding to FA1 (Fig. 2.2; see above). MPH-P 0174 was included within a concentration of microvertebrate remains, probably corresponding to metabolic waste of raptor birds, among which were identified some passerine birds and cricetid rodents.

Comparative description

Specimens described below are illustrated in Figures 3–5, with measurement data given in Tables 1 and 3.

Figure 3.

Skulls of Echimyidae. (1–3) Proclinodontomys dondasi n. gen. n. sp. (MPH-P 0173, holotype) in dorsal (1), ventral (2), and lateral (3) views; (4–6) P. mordax n. comb. (ZMUC 1/1845:13368) in dorsal (4), ventral (5), and lateral (6) views; (7–9) E. spinosus (MACN 18103) in dorsal (7), ventral (8), and lateral (9) views. Abbreviations: bs, basisphenoid; eam, external acoustic meatus; fm, foramen magnum; fr, frontal; I, incisive; if, incisive foramen; iof, infraorbital foramen; ipf, interpremaxillar foramen; ju, jugal; lps, lateral process of supraoccipital; M1, first molar; M2, second molar; M3, third molar; mx, maxilla; na, nasal; oc, occipital; pa, parietal; P4, fourth premolar; pl, palatine; pmx, premaxilla; sq, squamosal; tb, tympanic bulla. Scale bars = 10 mm.

Figure 4.

Cheek teeth of Proclinodontomys dondasi n. gen. n. sp. (MPH-P 0173, holotype), right (1) and left (2) upper cheek teeth, and (MPH-P 0174) left lower cheek teeth (3). Scale bar = 5 mm.

Figure 5.

Mandibles of Echimyidae. Left hemimandible of Proclinodontomys dondasi n. gen. n. sp. (MPH-P 0174) in lateral (1) and occlusal (2) views; right hemimandible (reversed) of E. spinosus (MACN 18103) in lateral (3) and occlusal (4) views. Abbreviations: cp, coronoid process; mc, masseteric crest; m1, first molar; m2, second molar; m3, third molar; mn, masseteric notch; p4, fourth premolar. Scale bars = 10 mm.

Cranium (holotype MPH-P 173).—The rostrum is relatively longer (Fig. 3.1–3.3) than in the extant Euryzygomatomys spinosus (Fig. 3.7–3.9) and in Proclinodontomys mordax n. comb. (Fig. 3.4–3.6). The upper incisors are much more procumbent than in E. spinosus and Clyomys, as in the type of P. mordax n. comb. The diastema is similar in length to that of E. spinosus and slightly longer than in P. mordax n. comb. (see Table 1). The interpremaxillary foramen is much reduced, smaller than in the type of P. mordax n. comb. (Winge, Reference Winge1887, pl. 7, fig. 8). The incisive foramina are narrower than in E. spinosus, and, as in this species, do not show a constriction at the suture between premaxilla and maxilla (Fig. 3.2). This suture is closer to the anterior end of the incisive foramina, like in E. spinosus and the lectotype of P. mordax n. comb. (Winge, Reference Winge1887, pl. 7, fig. 8; Hadler et al., Reference Hadler, Verzi, Vucetich, Ferigolo and Ribeiro2008). As in E. spinosus, the premaxillary septum is wider in its anterior portion, narrowing gradually backward, and it is not joined to the anterior portion of the palatal bridge (Olivares et al., Reference Olivares, Verzi, Vucetich and Montalvo2012). The anterior portion of the palatal bridge is narrow, forming a sub-vertical plate with a medial crest, and is located dorsal with respect to the alveolar margin of DP4 (Olivares et al., Reference Olivares, Verzi, Vucetich and Montalvo2012).

The rostral masseteric fossa (for the origin of the infraorbital part of the M. masseter medialis) is posterior to the premaxillary-maxillary suture and dorsal to the incisive alveolus. The base of the alveolar sheath of the upper incisor forms a marked prominence lateral to the DP4, as in specimens of P. mordax n. comb., being more posteriorly located than in E. spinosus. The antorbital zygomatic ramus is subvertical and at level of DP4. The maxillary fossae are well defined. The anterior zygomatic root is anteroposteriorly broader than that of E. spinosus; it has its anterior margin markedly convex and its posterior one slightly ahead of the DP4. The masseteric tuberosity is well developed, projecting a well-defined ridge similar to that in E. spinosus and specimens of P. mordax n. comb. As in P. mordax n. comb., there is no lateral flange for the canal for infraorbital nerve. The palate is widening slightly posteriorly, where it is incised by a V-shaped mesopterygoid fossa extended until level M2-3. The posterior palatine foramina are at level of M1.

The zygomatic arches are very robust and more flared and laterally expanded than in E. spinosus (Fig. 3.1) and Clyomys. Jugals are very high and extended on the antorbital zygomatic ramus, and largely expanded dorsoventrally in its anterior portion until level paraorbitary process (Fig. 3.3), like E. spinosus, Clyomys laticeps, and Carterodon sulcidens (Emmons, Reference Emmons, Lacey and Myers2005, p. 266). In addition, the jugals (between the anterior border and the lower jugal process) are much deeper anteriorly than posteriorly (Winge, Reference Winge1887, p. 73), more than in C. laticeps (Olivares et al., Reference Olivares, Verzi, Vucetich and Montalvo2012), and much more than in E. spinosus (Fig. 3.9), with the dorsal border descending more abruptly posteriorly than in the latter species.

The jugal fossa is narrower, slightly deeper, and less dorsally extended on jugal than in E. spinosus and, like this species, it is relatively wide anteriorly, not coming to a sharp point. The inferior jugal process is well developed and is behind the paraorbitary process, like E. spinosus.

The lacrimal does not form the margin of the lacrimal foramen, which is located in the dorsal portion of the orbit, like in the living species. Like E. spinosus and Clyomys, the lateral wall of the nasolacrimal canal is strongly convex. The foramen exposing the nasolacrimal canal is dorsal to the palatine foramen, being both located at level of the M1; sphenopalatine foramen is large and anteroposteriorly posterodorsally elongated. The sphenopalatine and palatine foramina are arranged in a posterodorsally oblique groove. Ethmoid foramen is located in the frontal; it is small, rounded and posterodorsal to the sphenopalatine fissure. Palatine foramina are in a long depression of the maxilla at the level of M1. Dorsally the premaxilla extends up to level the dorsal root of the antorbital zygomatic ramus. In lateral view, the latter is at the level of DP4, like in E. spinosus. Postorbital processes of frontals are more developed than that of E. spinosus. Orbital cavity is smaller than that of E. spinosus. Large oval sphenopalatine vacuity is present ventral to presphenoid-basisphenoid suture. The interorbital region is wider than in the living species and the type of P. mordax n. comb. Frontal are less markedly expanded posteriorly than E. spinosus and Clyomys, with their lateral borders slightly concave over orbits. The entire portion of parietals corresponding mainly to the posterior region of the cranial vault is shorter than in E. spinosus. Temporal fossae for M. pars principal and M. pars orbitalis of M. temporalis are deeper than in E. spinosus and bounded by more marked crests, like in the type of P. mordax n. comb. The zygomatic root of the squamosal is more expanded anteroposteriorly than that of E. spinosus. The bullae (unknown in P. mordax n. comb.) are moderately inflated in lateral view, less than in E. spinosus, and moderately elongated in ventral view, as in E. spinosus, being much smaller than that of Clyomys laticeps. The external auditory meatus is placed higher than in E. spinosus, close to the squamosal suture, and it is smaller than that of E. spinosus and C. laticeps. In P. dondasi n. gen. n. sp. it is raised and slanted upward and backward, whereas in E. spinosus it is more laterally directed. The postglenoid foramen is large, elongated, and opens into an anteroposterior tube-like fenestra, between the squamosal and the dorsal portion of the bulla. The mastoid is moderately extended anteroposteriorly between paraoccipital apophysis and the lateral process of the supraoccipital, and it is smaller than in E. spinosus. The lateral process of the supraoccipital is more ventrally extended below level of the lower edge of the external auditory meatus than in E. spinosus. Foramina ovale (only preserved the right one) is relatively small and posteriorly located with respect the pterygoid fossae-alisphenoid canal (only partially preserved of the left side), which is narrow. The pterygoid apophysis (preserved in left side) is long and its end contacts the anteromedial margin of the bulla. The lambdoid crest is well noticeable. Unlike the latter species, the preserved portion of the paraoccipital process reaches the ventral border of the bulla (a feature better observed in lateral view). Nuchal crests are well developed, running continuously from the preserved portion of the paraoccipital processes of the exoccipitals, which are broken at its bases. The supraocciptal has a medial ridge, the occipital external crest, which is long, as in E. spinosus (unknown in the type of P. mordax n. comb.), overhanging the foramen magnum, forming the posteriormost portion of the skull.

Upper dentition

Based on the tooth wear degree, the specimen can be typified as an “old adult,” so there is no labial flexi and the hypoflexus merges into hypofossette in left DP4 and M1, whereas this flexus persists lingually open in right M2 and M3 (Fig. 4.1, 4.2). All teeth are subquadrangular in occlusal outline, with flexi and fossettes less elongated than that of E. spinosus, a condition similar to that of species of Clyomys. The DP4 is smaller than the M1 (see Table 3) with the hypofossette subcircular and smaller than the only persistent labial fossette. In the left M1 the hypoflexus is transformed into a hypofossette, whereas in the right M1 the hypoflexus is still connected with the lingual border of the tooth. In both M1, the hypoflexus/hypoffossette turns anteriorly forward with the bottom opposite to the anterior fossette; the metafossette is smaller than the anterior fossette, especially in the right M1, a feature similar to that of the Clyomys species. The M2 are larger than M1 and, as in these teeth, the hypoflexi are oriented towards the anterior fossettes, which are somewhat larger than the metafossette; the hypocone is slightly labially located with respect to the protocone. The M3 are subequal in length to the M2, unlike Clyomys species, with the hypocone more labially located with respect to the protocone of the M2; the metafossette is smaller than the anterior fossette, such as in the species of Clyomys, and displaced towards the posterior border of the teeth.

Dentary

The left hemimandible (MPH-P 0174) is very similar in size to that of the specimens of Euryzygomatomys spinosus and Proclinodontomys mordax n. comb. The notch for the insertion of the tendon of M. masseter medialis pars infraorbitalis is well developed (Fig. 5.1), such as in E. spinosus (Fig. 5.3) and P. mordax n. comb., and it is horizontally oriented. As in E. spinosus, P. mordax n. comb., and Clyomys this notch is between dp4 and m1, located above of middle mandibular height and united to the masseteric crest, forming a well-developed ridge. The masseteric fossa is relatively deep. The coronoid process (Fig. 5.1) is more slender than in E. spinosus (Fig. 5.3), and does not project posteriorly as in the species of Clyomys. This process rises laterally to the m3, limiting a relatively deep and wide retromolar fossa. The incisor is posteriorly projected to the base of m3. The lower diastema has a step anterior to the dp4 much more marked than in E. spinosus and C. laticeps. This latter feature is similar to that of the fossil specimen MLP 91-IV-25-135 (Clyomys sp.) from Argentina (Vucetich et al., Reference Vucetich, Verzi and Tonni1997).

Lower dentition

Lower cheek teeth of the specimen MPH-P 0174 have a similar general size to those of the holotype of P. dondasi n. gen. n. sp. (Table 1), being the lower cheek teeth rows some longer than the upper one, as it occurred in caviomorphs (Fig. 4), and both lower and upper cheek teeth fit in occlusion. Based on the tooth wear degree, the specimen is considered as a “young adult.” The lower cheek teeth are similar to those of E. spinosus and P. mordax n. comb., but with the flexids more persistent and more transverse lophids.

The dp4 is subquadrangular in occlusal outline, and smaller than the m1 and m2 (Fig. 4.3; Table 3). As in E. spinosus, P. mordax n. comb., and Clyomys species, the dp4 is trilophodont, with the most anterior lophid, the metalophulid I, anteriorly convex, giving a subcircular outline to the anterior border of the teeth. The second lophid in position (the hypolophid) is wide anteroposteriorly, approximately twice the anteroposterior diameter of the metafossettid. This lophid extends lingually from the anterior ramus of the hypoconid-ectolophid up to the lingual border of the tooth. The posterolophid is convex and delimits the posterior border of the metaflexid, which is transversally oriented and open lingually, so the metaflexid is more persistent than the anterofossettid. Differing from E. spinosus and P. mordax n. comb., the anterofossettid is subequal in their anteroposterior diameter to that of the metaflexid, whereas in E. spinosus and P. mordax n. comb., the anterofossettid is more anteroposteriorly elongated. The hypoflexid is located opposite to the metaflexid. All lophids are relatively wider and the flexids more persistent than those specimens of E. spinosus and P. mordax n. comb. with similar stage of wear.

The m1-m3 are subrectangular in occlusal outline, and trilophodont, with lophids transversally oriented. The hypoflexid is deep and turns posteriorly. The lingual flexids are less reentrant than in E. spinosus, P. mordax n. comb., and Clyomys species. The mesofossettid-mesoflexid is more transversally oriented than that of P. mordax n. comb. and E. spinosus, in which this fossettid turns anteriorly. The mesoflexid is less reentrant than the metaflexid. In m1 (the most worn molar), the mesofossettid is the first to be closed. The m2 is larger than m1, and the m3 is smaller than the m2, and relatively larger with respect to remaining teeth compared with species of Clyomys and P. mordax n. comb. (Table 3). Relative size of m3 is more similar to that of the MLP 91-IV-25-135 of Clyomys sp. (Vucetich et al., Reference Vucetich, Verzi and Tonni1997; Table 1), having the m3 larger than the dp4. In the m3, the metaflexid is divergent labially. In the three molars, the hypoflexid is the widest and deepest flexid of the teeth.

Etymology

The species name honors Alejandro Dondas (1948–2016), colleague and friend, tireless finder of fossils at the Atlantic cliffs of Buenos Aires Province and for many years head of the Paleontology Department of the Museo Municipal de Ciencias Naturales de Mar del Plata “Lorenzo Scaglia.” The specific epithet is formed from the surname “Dondas,” taken as a noun in the genitive case, plus the Latin suffix “i” (ICZN, 1999; Article 31.1.2).

Material

MPH-P 0174, left hemimandible with dp4–m3.

Remarks

One of the characteristic features of the MPH-P 173 is its broadly dorsoventrally expanded jugals, a trait shared with members of the tribe Euryzygomatomini (sensu Emmons, Reference Emmons, Lacey and Myers2005; i.e., the semi-fossorial Euryzygomatomys spinosus, Clyomys laticeps, and Carterodon sulcidens). However, the set of features of MPH-P 173 (see comparative description) justifies its assignment to a new species of echimyid. For example, the bulla size of this specimen, which is proportionally much smaller than in the species of Clyomys, permits discarding its inclusion in the latter. The zygomatic arches are more flared and laterally expanded than in E. spinosus and species of Clyomys. The jugals are much deeper anteriorly than posteriorly and show the dorsal border descending more abruptly posteriorly than in E. spinosus and Clyomys species. Another distinctive feature of MPH-P 173, shared with the type of P. mordax n. comb., is the great procumbency of the upper incisors, which are much more procumbent than in E. spinosus and species of Clyomys. The MPH-P 173 also share several features with P. mordax n. comb. (see diagnosis of the Proclinodontomys n. gen.), which allows taxonomic location of both species in a new genus, different from Clyomys and Euryzygomatomys, a hypothesis supported by the results of the phylogenetic analysis.

We assigned the MPH-P 0174 to P. dondasi n. gen. n. sp. based on the cranio-mandibular and dental morphological correlation between this specimen and the MPH-P 0173 (holotype), as well as taking into account the morphological correspondence observed in the living E. spinosus and Clyomys laticeps. The holotype and the MPH-P 0174 are coincident in size (Table 1, e.g., length of the cheek teeth series) and general robustness. Upper and lower cheek teeth show similar relative height of the dental crowns, thickness of the enamel, and proportions of the size of the teeth (Table 3). In both dental series, upper and lower, the third molar is relatively large with respect to the remaining teeth (Table 3). Note that in fossil and extant species of Clyomys, the M3/m3 is reduced. In MPH-P 0174, the m3 is longer than the dp4, differing from E. spinosus and the species of Clyomys. In the holotype and MPH-P 0174, the flexi/flexids are less reentrant than in Euryzygomatomys spinosus, forming less elongate fossettes/fossettids, being in these features more similar to Clyomys laticeps and C. bishopi Avila-Pires and Wutke, Reference Avila-Pires and Wutke1981 (Thomas, Reference Thomas1916; Verzi et al., Reference Verzi, Vucetich and Montalvo1995; Hadler et al., Reference Hadler, Verzi, Vucetich, Ferigolo and Ribeiro2008). However, the flexi and flexids are more persistent than those of the species of Clyomys. So that, for example, the metafossette in the M1 in MPH-P 0173 persists even with advanced state of wear of the Dp4. In agreement with this feature, the flexids of the lower cheek teeth are more persistent than in Clyomys species (the anterior and posterior flexids of the m1 and m2 persist even when the m3 is completely erupted). This indicates that the closing time of the flexi and flexids is coincident in upper and lower cheek teeth, being more persistent than in any species of Clyomys. All these features allow assignment of MPH-P 0173 and MPH-P 0174 to the same species. In addition, the set of features of MPH-P 0174 (such as proportions of the m3, morphology of the coronoid process, and persistence of the flexids) allow us to discard the inclusion of this specimen in any previously known species.

Lectotype

ZMUC 1/1845:13368, skull from undifferentiated Quaternary deposits of Lapa da Escrivania Nr. 5., Lagoa Santa, state of Minas Gerais (Brazil).

Remarks

Winge (Reference Winge1887, p. 72–73) erected E. mordax (as Mesomys mordax) based on two fragmentary skulls found in Lapa da Escrivania Nr. 5., Lagoa Santa, state of Minas Gerais (Brazil), recovered from undifferentiated Quaternary deposits. One of the skulls studied by Winge (Reference Winge1887, originally labelled as “skull 1”) was illustrated with a drawing (Winge, Reference Winge1887, pl. 8, fig. 7), and later by photographs (Hansen, Reference Hansen2012, figs. 35a–c). This material, recently numbered in the collections of the Zoological Museum, Natural History Museum of Copenhagen (ZMUC; Copenhagen, Denmark) as ZMUC 1/1845:13368, is here designated the lectotype of E. mordax according to Article 74.1 of ICZN (1999). Hadler et al. (Reference Hadler, Verzi, Vucetich, Ferigolo and Ribeiro2008) referred to E. mordax several cranial fragments recovered from the archaeological site Afonso Garivaldino Rodrigues (Sitio RS-TQ-58; Holocene; Rio Grande do Sul state, Brazil). In this same contribution, Hadler et al. (Reference Hadler, Verzi, Vucetich, Ferigolo and Ribeiro2008) offered an emended diagnosis of E. mordax that largely increased the knowledge of the species, which is in great part used as basis in our comparative study.

Euryzygomatomys mordax was assigned to Euryzygomatomys by having very deep reentrant flexi/flexids, forming elongate fossettes/fossettids, and by the anterior fossette and metafossette subequal in size (Thomas, Reference Thomas1916; Verzi et al., Reference Verzi, Vucetich and Montalvo1995; Hadler et al., Reference Hadler, Verzi, Vucetich, Ferigolo and Ribeiro2008). However, E. mordax has several differences with respect to E. spinosus (e.g., degree of procumbence of incisors, width relative of the interorbital region at frontals, absence of having a smaller orbital cavity with respect to extant E. spinosus; see diagnosis of the new genus) and no cranial character of E. mordax allows including undoubtedly this species within the same genus as E. spinosus, a taxonomic decision that is supported through the phylogenetic analyses (see below).