Among the few extinct cricetid rodents named from Quaternary deposits of Argentina, Wiedomys marplatensis Quintana, 2002 is an obscure form. This taxon was based on a fragmentary lower jaw with the second and third molars in situ unearthed from the San Andrés Formation (Early Pleistocene) exposed in the southeastern Buenos Aires Province, Argentina. The original description provided by Quintana (2002) was mostly devoted to the comparison of the fossil material with another extinct species, Cholomys pearsoni Reig, 1980, previously named by Reig (1980) from the Pliocene strata in the same general area. Therefore, the allocation of this jaw into the recent genus Wiedomys Hershkovitz, 1959 was, in fact, poorly supported. This generic allocation was transitively assumed from the general similarity detected between Wiedomys and the monotypic Cholomys Reig, 1980, a hypothesis which constitutes the seed of the tribe Wiedomyini (Reig 1980). Taking into account that Quintana (2002: 271–272) provided neither metrical comparisons of the holotype of W. marplatensis (Quintana 2002) nor a clear statement of its type locality and given that he only figured the material with a schematic draw (Quintana 2002: figure 8A), our consideration of this form as an “obscure sigmodontine” does not seem to be capricious.
We revised the holotype of Wiedomys marplatensis housed in the paleontological collections of the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (Buenos Aires, Argentina; acronym MACN) by confronting it with other sigmodontines. We also examined the right lower jaw MACN 19727, referred to as Cholomys pearsoni by Quintana (2002) that guided him in his comparisons with W. marplatensis. In order to refine the comparison with Cholomys, we observed directly the holotype of C. pearsoni, a right lower jaw housed in the Museo Municipal de Ciencias Naturales y Tradicional de Mar del Plata “Lorenzo Scaglia” (Mar del Plata, Buenos Aires, Argentina; acronym MMP M). All these fossils were compared with several recent sigmodontines; Appendix 1 contains a list of the materials examined. The terminology employed in this note follows Reig (1977) and the general concepts of Hershkovitz (1962).
The holotype and unique known material of Wiedomys marplatensis (Figure 1) is a tiny fragment of left dentary (MACN 19697) lacking almost all parts except the m2 and m3 and the adjacent labial portion of the bone; the fossil was firmly mounted in a pin probably by the collector in order to facilitate its handling. The field label, written on a small fragment of a newspaper, specified “11-1-90, Santa Isabel”; therefore, the material was collected by Quintana on January 11, 1990, when he was in charge of the paleontological collection of the MMP. It most likely corresponds to “Playa Santa Isabel” (ca. 38.19° S, 57.67° W), a coastal locality situated near Chapadmalal, General Pueyrredón County, Buenos Aires Province, and hence it might be restricted as the type locality of W. marplatensis.
When to hand, the first aspect that calls the attention from the holotype of Wiedomys marplatensis is its enlarged retromolar fossa (Figure 1). Although the ascendant ramus is broken at its base, the remainder portion labially delimits a subrectangular area of the bone adjacent to the posterior side of the m2 and the entire m3. The floor of this fossa is mostly flat and lacks any perforation. The second striking aspect of W. marplatensis is the robust and planate conditions of its molars (Figure 2). A more cautionary inspection of the occlusal surface indicates that the m2 has the lingual corner slightly above the remainder portion of the molar, suggesting a previous bi-level condition; the m3 is almost totally flat (Figure 1). Both retromolar development and molar robustness and planate condition are partially age-dependent traits amplified in older animals. In fact, the degree of wear of the molar occlusal surfaces in W. marplatensis indicates that it represents a full adult individual. The wear effects on the molar design are evident in the m3, where the enamel wall of the main cusps are “fused” with the cingulids. This is particularly evident in the outer margin of the hypoconid and in the anteriormost point of the entoconid (Figure 2).
The first hypothesis to be tested is if Wiedomys marplatensis can be allocated within Wiedomys, which also comprises two living species, Wiedomys pyrrhorhinos (Wied-Neuwied 1821), the type species, and Wiedomys cerradensis Gonçalves et al. 2005, both distributed in the arid lands of northeastern Brazil (Bonvicino 2015). Both extant Wiedomys species are very similar in terms of molar morphology (Gonçalves et al. 2005). Wiedomys marplatensis shares with the living Wiedomys the general morphologies of the retromolar fossa and the molar occlusal pattern. However, the fossil specimen displays several differences including (1) coronal surface mostly planate (W. marplatensis) vs. crested (living Wiedomys) and (2) mesolophids absent (W. marplatensis) vs. typically present (living Wiedomys). Particularly striking is the general proportion of the m3 shown by W. marplatensis with respect to the living Wiedomys species, mainly W. pyrrhorhinos. While the former has an m3 with an anteroposteriorly compressed posterior lobe, the latter is characterized by a larger m3, where the mesoflexid constitutes a broadened valley in order to contain a tiny mesolophid (Figure 2; Supplemental Figure 1). These differences in proportions are also evident regarding the m2 of these taxa and they also involve the orientation of the hypoflexid, more transverse in W. marplatensis than in the living Wiedomys. All these contrasting features are enough to discard the assignment of W. marplatensis to Wiedomys (Table 1).
It is clear that Wiedomys marplatensis represents a morphologically unusual cricetid in the context of those recorded from the Argentinean fossil record, mostly composed of hypsodont or mesodont forms (e.g. Akodon Meyen, 1833, †Panchomys Pardiñas, 1997, Reithrodon Waterhouse, 1837; see Pardiñas et al. 2002). However, it shares with †Cholomys pearsoni this condition. Cholomys pearsoni is a small fossil cricetid described on a single right lower jaw (MMP M-869; Supplemental Figure 2) recovered from the beds of the Vorohué Formation near Chapadmalal (Reig 1972, 1980). Cholomys was allied to Wiedomys to cement the concept of the tribe Wiedomyini, a clade recently enlarged with the addition of Phaenomys Thomas, 1917 and Wilfredomys Avila-Pires, 1960 according to genetic and morphological evidence (Pardiñas et al. 2014a, Machado et al. 2015). Quintana (2002) performed a comparison between W. marplatensis and C. pearsoni, involving the MACN 19727 to accomplish this goal (see above). We studied the latter and concluded that it does not belong to C. pearsoni. Among other salient differential traits, this material is characterized by a low and elongated mandible with a narrow incisor, a small retromolar fossa, brachydont simplified molars with m3 particularly shortened, and a procingulid of the m1 lacking an internal ring (see Quintana 2002: figure 8B; Supplemental Figure 3). On the contrary, C. pearsoni, judged from its holotype, is a small sigmodontine with a high and short mandible with an outstanding broad incisor, enlarged retromolar fossa, brachydont molars with a large m3 and an m1 with complex procingulid. Quintana (2002: 273) remarked putative differences between W. marplatensis and C. pearsoni, including the absence of ectostylid in the m3 and the small retromolar fossa of the latter. However, these traits were retrieved from a material which is, in fact, not referable to as Cholomys and more probably belongs to a sigmodontine such as Calomys Waterhouse, 1837 or †Chukimys (Barbiére et al. 2016). When W. marplatensis and the holotype of C. pearsoni are compared, the similarities are striking (Figure 2). Both taxa share an enlarged retromolar fossa, brachydont small teeth with thick enamel and lacking mesolophids, well-defined posterolophids directed backwards and penetrating hypoflexids in the m3. It is not hard to understand the occlusal morphology of the W. marplatensis as a more advanced stage of wear of those molars of C. pearsoni; the large and produced backwards posterior root of the m3 in W. marplatensis points in the same direction (Figure 1). The most plausible hypothesis is that W. marplatensis Quintana, 2002 is a junior synonym of C. pearsoni Reig, 1980. Accepting this proposal, the biochron of †C. pearsoni runs between the Vorohuean and Sanandresian stages, roughly the Late Pliocene-Early Pleistocene (~3–2 My) according to the current chronological schemes (Cione et al. 2015). As a counterpart, the stratigraphic and geographic ranges of Wiedomys must be restricted to Late Pleistocene-Holocene deposits in Bahia, Brazil (Oliveira and Lessa 1999). In Appendix 2, we provide a summary of the revised content of the tribe Wiedomyini and the fossil cricetids discussed in this paper.
Concluding that Wiedomys marplatensis is a junior synonym of Cholomys pearsoni resolves partially the obscure condition of this taxon; now the aim is to discuss the affinities of †C. pearsoni. The hypothesis advanced by Reig (1980) is that this taxon is an extinct representative of Wiedomyini. Quintana (2002) tacitly supported this idea when he allied marplatensis to Wiedomys, because Wiedomys was the unique living wiedomyine at the time when Reig (1980) erected the tribe. The morphological traits that we discussed in order to extirpate marplatensis from Wiedomys can be used to separate this latter genus from Cholomys. In fact, both taxa share general similarities such as an enlarged retromolar fossa and brachydont simplified molars. On the other hand, they differ in many features, such as the outstanding robustness of the incisor of Cholomys and the absence of any trace of mesolophid on their molars (Figures 2 and 3; Supplemental Figure 2).
We now have the opportunity to deeply discuss some traits mentioned several times along this contribution. The retromolar fossa is an anatomical feature basically unstudied in Sigmodontinae. To our best understanding it was mostly Pacheco (2003: 85–86) who made observations on this trait highlighting a deep, broad and foraminated retromolar fossa in the genera Abrawayaomys Cunha and Cruz, 1979 and Rhagomys Thomas, 1886. Coincidently, for the latter, this condition was described and figured by Luna and Patterson (2003: figure 8). However, an extensive survey on the morphology of this structure for sigmodontines is still missing, although Weksler (2006: 115) did not detect any variation in Oryzomyini, and Teta (2013: 37) apparently referred to it as “repisa ósea por detrás del m3” (freely translated as “bony shelf behind m3”) when he surveyed Abrotrichini. Our direct inspection of several genera and tribes among Sigmodontinae suggests a moderate morphological diversity in terms of size, foramination and form of the retromolar fossa. Clearly, Abrawayaomys (cf. Pardiñas et al. 2009: figure 6J) and Rhagomys exemplify the condition where the retromolar fossa is developed as a patent basin mainly caudal to the molar series. Contrastingly, in most other sigmodontines, the retromolar area is not expressed as a fossa, but instead as a flat or partially depressed region with or without perforations. The geometry of this area follows two main rough types, one subtriangular in outline and basically adjacent to the m3 and another condition resembling a broadened “groove” and also involving the m2. Apparently, neither the size nor the development of the incisor is involved at producing these types of retromolar regions. For example, the euneomyine Neotomys ebriosus Thomas, 1894, a medium-sized sigmodontine with a specialized and very robust incisor has a triangular and poorly developed retromolar fossa. On the contrary, the phyllotine Calomys callidus (Thomas, 1916), a small sigmodontine with a slender incisor displays the rectangular enlarged type. As a widespread characteristic, age variation is important and older individuals have the retromolar region enlarged contrary to young animals. Cholomys – including the type of Wiedomys marplatensis – has an enlarged “groove” retromolar fossa, a condition shared not only by Wiedomys and Wilfredomys (both assigned to Wiedomyini) but also by many other sigmodontines such as Calomys (phyllotine), Pseudoryzomys Hershkovitz, 1962 (oryzomyine), etc. In †Cholomys pearsoni, the retromolar fossa is anteriorly narrowed probably due to the subadult condition of this individual and it shows two important perforations and a recessed sector caudal to the m3. It highly resembles the condition observed in several species of Calomys. On the contrary, Wiedomyini – also including Phaenomys – have a more rectangular and “grooved” retromolar area.
Having simplified lower molars, particularly with respect to the upper ones, is a recurrent feature in sigmodontines. A variety of examples can be listed across different genera and tribes (see below). A striking case is when the mesolophid is present in upper molars and their counterpart, the mesolophid is absent (or very small) in lower ones. Hershkovitz (1993) dealt with this case when he described Microakodontomys (considered as an Oryzomyini); in fact he detected examples of Oligoryzomys Bangs, 1900 lacking mesolophids (Hershkovitz 1993: figure 3, note that in that figure the lower molar series of A and B are transposed). According to this author, “the evolutionary process of molar crown simplification … commence with the lower molars, the second usually earliest” (Hershkovitz 1993: 10). Calassomys (Pardiñas et al. 2014a,b) (considered as a phyllotine), a Cerrado endemic, also represents an example where lower molars are mesolophid-free but upper molars display this structure, although poorly developed (Pardiñas et al. 2014b). Wiedomys constitutes a good example of this condition. While the upper molars are characterized by well-developed mesolophs, the equivalent structure in the lower molars is much less expressed. However, mesolophids are not absent in Wiedomys as was stated by Reig (1980: 266) when he proposed the diagnosis of Wiedomyini. Actually, mesolophids in Wiedomys are filiform crests of enamel which link the median mure to the base of the metaconid. A clear indication of their existence is, e.g. in the m3, the projection of the entoconid in its anterolingual corner. Wilfredomys, another living wiedomyine added long after Reig’s definition of the tribe, also shows very narrow mesolophids (Figures 2 and 3), which are more patent in Phaenomys (cf. Pardiñas et al. 2014a: figure 5). Therefore, the living Wiedomyini typically have mesolophids although with variable expression according to the wear condition.
In this context, the absence of any trace of mesolophids in the molars of Cholomys could be an indication that assigning it to this tribe is inappropriate. Furthermore, the marked difference between the hypsodont conditions displayed by the living Wiedomyini regarding Cholomys adds another justification to our proposal. The main cuspids in the former are “mounted” in a molar “shelf” forming typical tubercular hypsodonty (Hershkovitz 1962: 89); this condition is patent in subadult animals (Supplemental Figure 1). On the contrary, main cuspids in the holotype of †Cholomys pearsoni (Figures 2 and 3) follow the type described as coronal hypsodonty (Hershkovitz 1962: 88). Besides, several features on m1 further distinguish Cholomys from the living Wiedomyini (Figure 3), including (1) procingulid well developed with anteromedian fossetid centrally located (Cholomys) vs. procingulid compressed with a small and lingually displaced anteromedian fossetid (living Wiedomyini); (2) protoflexid “single” (Cholomys) vs. “complex” (living Wiedomyini; remember the structure called anterosinusid by Freudenthal et al. 1994); (3) transverse entoconid (Cholomys) vs. oblique entoconid (living Wiedomyini).
The combination (brachydont simplified teeth plus enlarged retromolar fossa plus strong subrectangular incisor plus very small size), i.e. the mosaic that characterizes Cholomys, is unique among sigmodontines. However, this bauplan resembles that of members of two extant tribes, Oryzomyini and Phyllotini. Convergences between a few genera of both clades are classic issues in sigmodontine systematics. An emblematic example is the Pseudoryzomys (see Hershkovitz 1962, Voss and Myers 1991, Braun 1993), but it is to be noted that the simplification in Oryzomyini is related to the development of laminarity (sensu Hershkovitz 1962) such as in Holochilus Brandt, 1835 or Pseudoryzomys. Cholomys does not exhibit such laminarity in its occlusal pattern. The hypothesis of Cholomys as a specialized representative of Phyllotini or a simplified oryzomyine is not only coherent on morphological grounds but also makes sense biogeographically. Wiedomyini are sigmodontines restricted to southeastern Brazil and northern Uruguay, mostly associated with forested humid or dry environments; Oryzomyini and Phyllotini are widespread in mesic, temperate or cold open areas (Patton et al. 2015). In addition, nothing acts as an obstacle to suppose the occurrence of Wiedomyini in the southern Buenos Aires province during the Pliocene-Pleistocene times. However, the evidence accumulated for more than three decades (i.e. after Reig 1980) regarding fossil cricetids in Central Argentina points to limited shifts of the tribes and the adequacy of current distributions in order to address the past biogeography (Barbière et al. 2016).
In brief, we propose here that Wiedomys marplatensis is a junior synonym of Cholomys pearsoni. We also questioned the allocation of Cholomys in Wiedomyini. We suggest that this issue requires further exploration and better fossils; in the meantime, we suggest avoiding the use of Cholomys as a calibration point for the tribe and propose the allocation of this genus within Sigmodontinae incertae sedis (Appendix 2). We are aware of the growing necessity of fossils to set molecular clocks in sigmodontines. However, we think that the unsupported use of problematic or poorly known fossils, such as Prosigmodon Jacobs and Lindsay, 1981 (cf. Leite et al. 2014: table S2; actually, a neotomine instead of a sigmodontine) or “aff. Abrothrix” (cf. Parada et al. 2015: 3; currently, an akodontine instead of an abrotrichine), produce more noise than answers in the complex universe of sigmodontine evolution.
We studied the discussed fossils along many years and visits to MACN and MMP repositories; we are indebted for hospitality and free access to the curators A. Ramos and A. Kramarz (MACN) and A. Dondas, T. Scaglia, L. Scaglia and M. Taglioretti (MMP). Loans of Wiedomys were kindly authorized by J. Oliveira, curator of Museu Nacional (Rio de Janeiro, Brasil); C. Cañón helped us with photographs of Wiedomys recently obtained in the MN. Funds for this research are those of PICT (Agencia) 2014-1039 (to UFJP). We want to express our gratitude to the above-mentioned persons and institutions and also to the associate editor F. Catzeflis and the two anonymous reviewers that largely improved the manuscript. This is GEMA (Grupo de Estudios de Mamíferos Australes) contribution #20.
Appendix 1: Specimens studied in this contribution belong to the following collections: Colección de Mamíferos del Centro Nacional Patagónico (CNP; Chubut, Argentina); Colección de Material de Egagrópilas y Afines “Elio Massoia,” Centro Nacional Patagónico (CNP-E; Chubut, Argentina); Colección Nacional de Paleontología Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (MACN; Buenos Aires, Argentina); Museo Municipal de Ciencias Naturales y Tradicional de Mar del Plata “Lorenzo Scaglia” (MMP-M; Mar del Plata, Buenos Aires, Argentina); Museu Nacional (MN; Rio de Janeiro, Brasil).
Cholomys pearsoni.– MMP M-869 (holotype), incomplete right lower jaw; Baliza San Andrés, Buenos Aires, Argentina, Vorohué Formation.
Wiedomys marplatensis.– MACN 19697 (holotype), incomplete left lower jaw; Playa Santa Isabel, Buenos Aires, Argentina, San Andrés Formation.
Wiedomys pyrrhorhinos. – CNP 3643, CNP 4728, Sítio Marimbondo, Caruaru, Pernambuco, Brasil; MN 68601, Riacho da Ressaca, Piripá, Bahia, Brasil; MN 71607, MN 71608, Parque Nacional da Chapada Diamantina, Salinas, Morro do Chapéu, Bahia, Brasil; MN 73419, Berilo, Minas Gerais, Brasil; MN 73520, Usina Hidroelétrica Irapé, Berilo, Minas Gerais, Brasil.
Wilfredomys oenax.– CNP 2378, CNP 2379, Cerro Colorado, Soriano, Uruguay.
Appendix 2: Revised suprageneric framework, geographical and chronological distribution for the cricetid rodents discussed in this paper.
Sigmodontinae incertae sedis
Genus Cholomys Reig, 1980
Cholomys pearsoni Reig, 1980
Cholomys pearsoni Reig, 1980: 266; holotype: MMP M-869, incomplete right lower jaw with incisor and m1-3.
Wiedomys marplatensis Quintana, 2002: 271; holotype: MACN 19697, incomplete left lower jaw with m2-3.
Geographical distribution: Atlantic slopes near Mar del Plata, Buenos Aires Province, Argentina.
Stratigraphical and chronological distribution: Vorohué and San Andrés Formations; Late Pliocene-Early Pleistocene.
Tribe Wiedomyini Reig, 1980
Type genus: Wiedomys Hershkovitz, 1959 (by original designation).
Included genera (arranged in alphabetic order): Phaenomys Thomas, 1917, Wiedomys Hershkovitz, 1959 and Wilfredomys Avila-Pires, 1960.
Geographical distribution: Eastern Brazil and northern Uruguay.
Chronological distribution: Late Pleistocene-Holocene (for Wiedomys in Brazil)-Recent.
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The online version of this article (DOI: https://doi.org/10.1515/mammalia-2016-0105) offers supplementary material.
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Published Online: 2017-02-11
Published in Print: 2017-12-20