Muroid rodents are currently represented in South America by three sub-families, with sigmodontines being the dominant group (Patton et al. 2015). Concordantly, the fossil record of cricetids in the subcontinent, although very poor compared with its recent diversity, is almost exclusively composed by members belonging to the subfamily (Pardiñas et al. 2002). As an exception, a form unearthed from Quaternary deposits in northern Ecuador, the genus Copemyodon Fejfar et al. 1996, was attributed to the Neotominae (Fejfar et al. 1996). Represented by a single species, Copemyodon ecuadorensis Fejfar 1996, this small cricetid was interpreted as morphologically close to the fossil genus Copemys Wood 1936, but also to the living one Peromyscus Gloger 1841 (Fejfar et al. 1993, 1996). In this systematic scenario and taking into account its isolated occurrence within a dense assemblage of fossil sigmodontines, Copemyodon was hypothesized as an example of an independent “copemyine-neotomine” attempt to colonize South America (Fejfar et al. 1996).
Despite its putative importance, Copemyodon was almost not discussed again after its original description. Probably coined too late to be listed by McKenna and Bell (1997), the genus was virtually ignored by all subsequent treatments (e.g. Patton et al. 2015), only briefly mentioned as linked to the fossil record of Peromyscus (cf. Musser and Carleton 2005) and in a few reviews about the evolutionary history of the group (e.g. Pardiñas et al. 2002, Zijlstra et al. 2014). As an example of the destiny of a plethora of fossil forms in a universe, currently dominated by molecular systematics, characteristic of the recent record of sigmodontine rodents, two decades later Copemyodon integrates a large number of enigmatic taxa.
In their analysis, Fejfar et al. (1996) posited an erroneous biogeographical scenario when they described Copemyodon; these authors stated “… the copemyine-peromyscine group, previously considered to be restricted to Northern and Central America, and which presence in South America has never before been reported” (Fejfar et al. 1996:138). Certainly the “copemyine-peromyscine group” is a heterodox construction; even in the 1990s because it combines stem cricetids, mostly represented by Copemys with a large array of crown members now subsumed within the Neotominae (Carleton and Musser 1984). Beyond this, according to the authoritative view of Patton (2015:58–59), “The Sigmodontinae comprise the vast majority of cricetid rodents in South America, representing a large radiation. Only single species of Reithrodontomys and of Isthmomys of the otherwise highly diverse and mostly Northern or Middle American Neotominae… and of Tylomys of the lesser diverse Middle American Tylomyinae… extend into South America. Neotomine and tylomyine taxa are limited to western Colombia and adjacent Ecuador”. In brief, Fejfar et al. (1993, 1996) overlooked the occurrence of the neotomine genus Reithrodontomys Giglioli 1874 in Ecuador, formerly recognized by most authors since Thomas (1898), described Reithrodontomys soederstroemi with type locality in Quito.
When described by Fejfar et al. (1996:139), the holotype of Copemyodon ecuadorensis was referred as “Holotype: lower left jaw with m1–3, temporarily stored at the Department of Earth Sciences of the University of Florence”. Now, this material is correctly labeled as holotype under the number MECN 129, in the paleontological collections of the Museo Ecuatoriano de Ciencias Naturales (Quito, Ecuador). The correspondence of MECN 129 with the material figured by Fejfar et al. (1996: figures 2 and 3) is beyond doubt. In addition, several upper and lower toothrows, cranial fragments and isolated molars originally referred by Fejfar et al. 1996 to Copemyodon are also available for study in MECN collections (see Appendix 1).
The holotype of Copemyodon ecuadorensis is a left dentary with m1–3 (molar nomenclature used here follows Reig 1977) and the horizontal ramus well preserved although both the angular and coronoid processes are broken and the incisor is missing. According to the degree of wear of the molars, it belongs to a moderately adult individual (Figure 1). It was recovered at La Calera (0°30′04.74″N, 77°51′47.33″W, 2710 m) in a profile described near Cantón Bolivar in the province of Carchi, northern Ecuador. The holotype of C. ecuadorensis, as well as abundant additional material referred for this taxon, include a late Holocene small mammal assemblage representing several sigmodontine rodents, lagomorphs, and soricomorphs (see Fejfar et al. 1993). A single specimen also attributed to Copemyodon was recovered at Quebrada Cuesaca (0°31′04.17″N, 77°53′13.53″W, 2640 m), a late Pleistocene-Holocene assemblage recovered very near to La Calera (see Fejfar et al. 1993, 1996).
Fejfar et al. (1996:139) described Copemyodon as a “medium sized cricetid” with lower brachydont molars displaying main cusps in alternate pattern and having bisected anteroconid, a “drop-like” mesostylid and an “ectolophid or ectostylid regularly joining the anterior side of hypolophid”. The upper molars were highlighted by “labially shifted, asymmetric and wide anterocone” and large mesolophs. All these dental traits were considered as “very unique and primitive”. Probably guided by the incorrect presumption that the authors had at the unique South American neotomine, Fejfar et al. (1993, 1996) focused the comparisons to the widespread living genus of the subfamily, Peromyscus. They also compared the fossil Ecuadorean material to the Nearctic but extinct form, Copemys. These comparisons overlooked a basic rule in any taxonomic approach to first check the living assemblage at the local and regional scales to look for potential candidates for comparison. When such is done, Copemyodon is indistinguishable from Reithrodontomys and Copemyodon ecuadorensis is very close to the forms contained in the Reithrodontomys mexicanus species group (reviewed by Arellano 2015; figure 1). Beyond the general agreement that we found between the morphology of the material referred to Copemyodon and several recent specimens of R. mexicanus species group from Ecuador (see Appendix 1), additional confirmation of our taxonomic hypothesis is derived from the incisor morphology. Reithrodontomys, as a genus, is diagnosed by the presence of a well-developed groove, slightly displaced to the labial side, which cross the anterior face of each upper incisor (cf. Hooper 1952). These grooves resemble those of the sigmodontine rodent genus Reithrodon Waterhouse 1837 and are the basis of the generic epithet applied for this neotomine. In the context of the small size of the known species of Reithrodontomys, one of the smallest cricetids, the presence of grooved incisors is diagnostic (Hooper 1952). We searched the numerous isolated incisors recovered from La Calera and detected suitable examples that can be confidently attributed to Reithrodontomys (Supplemental Figure 1).
The holotype of Copemyodon ecuadorensis shares with the species comprised in the Reithrodontomys mexicanus species group several morphological traits (Figure 1, Supplemental Figure 1), including a gracile and slender dentary with a moderately marked masseteric crest, an inconspicuous capsular projection, small brachydont molars, main cusps arranged in alternate pairs, murids anteriorly-posteriorly aligned, procingulum of m1 composed by two conulids, mesostylids and ectostylids present in both m1–2 and a posteroflexid reduced to an enamel ring in the m3. Dental measurements are also in accordance and support the identity between both taxa (Table 1).
Summarizing, Fejfar et al. (1993, 1996) were correct in their assertion that a neotomine representative was present in the Quaternary deposits of northern Ecuador. However, they failed to connect their findings with the single living genus of this subfamily that reaches this country, Reithrodontomys. Copemyodon constitutes a junior subjective synonym of Reithrodontomys (Aporodon). The current taxonomic scenario for this genus in Ecuador (Arellano 2015) embraces a single species, Reithrodontomys mexicanus (Saussure 1860) containing at least three recognized subspecies, Reithrodontomys mexicanus eremicus (Hershkovitz 1941), Reithrodontomys mexicanus milleri (Allen 1912) and Reithrodontomys mexicanus soederstroemi (Thomas 1898). However, Moreno and Román (2013) highlighted relevant craniodental differences among these mentioned forms that suggest that each could be ranked as a species (see also the genetic and morphometric data analyzed by Chávez 2012). For the present, we retain ecuadorensis as a species under the new name combination Reithrodontomys ecuadorensis (Fejfar et al. 1996). This name is therefore available in the context of a much needed revision of South American populations of this genus (see also Gardner and Carleton 2009). Reithrodontomys ecuadorensis differs metrically from R. m. milleri (see Table 1). Dental similarities, such as the well-developed paraflexus shared between R. ecuadorensis and R. m. soederstroemi, while in R. m. eremicus this structure is shorter (see Moreno and Román 2013), support their putative conspecificity (Figure 1).
As Copemyodon is a junior synonym of Reithrodontomys, the fossil records of La Calera and Quebrada Cuesaca are eloquent to indicate that the latter genus has, at least, Late Pleistocene and Holocene occurrences in Ecuador. Apparently, those from Quebrada Cuesaca (Late Pleistocene) are the oldest record for the subfamily in South America. Neotomines, as well as other major groups of rodents that today are primarily Nearctic in distribution such as Heteromyidae and Geomyidae, probably had an important local evolutionary history. However, to date, in absence of paleontological evidences, it is mostly a speculative matter (see, e.g. the discussion about the history of heteromyids in Alexander and Riddle 2005).
We studied the holotype of Copemyodon and additional fossils recovered in La Calera and Quebrada Cuesaca thanks to the kindly access to paleontological collections granted by the curator G. Medina and the authorities of the MECN. S. Ocaña also helped in many ways to facilitate the travel of UFJP to Ecuador. A. Guevara and E. Criollo (Departamento de Metalurgia Extractiva de la Escuela Politécnica Nacional) provided valuable assistance in obtaining the photographs that illustrate this contribution. D. Voglino masterfully improved Figure 1. Funds for this research were derived from PICT (Agencia) 2014-1039 (to UFJP). We are deeply indebted to the above-mentioned persons and institutions.
Allen, J.A. 1912. Mammals from western Colombia. Bull. Am. Mus. Nat. Hist. 31: 71–95. Google Scholar
Arellano, E. 2015. Genus Reithrodontomys Giglioli, 1874. In: (J. Patton, U. Pardiñas and G. D’Elía, eds.) Mammals of South America, Volume 2: rodents. University of Chicago Press, Chicago, IL. pp. 61–63. Google Scholar
Carleton, M.D. and G.G. Musser. 1984. Chapter 11: Muroid Rodents. In: (S. Anderson and J.K. Jones, eds.) Orders and families of recent mammals of the world. John Wiley & Sons, New York. pp. 289–379. Google Scholar
Chávez, D. 2012. Diferenciación morfológica y genética de las poblaciones del ratón cosechador mexicano Reithrodontomys mexicanus Saussure, 1860 (Rodentia Cricetidae) en los Andes del Ecuador. Graduate thesis, Pontificia Universidad Católica del Ecuador, Quito, Ecuador. pp. 127. Google Scholar
Fejfar, O., G. Ficcarelli, C. Mezzabotta, M. Moreno Espinosa, L. Rook and D. Torre. 1993. New finds of cricetids (Mammalia, Rodentia) from the late Pleistocene-Holocene of Northern Ecuador. Documenta, Laboratoire de Géologie, Lyon 125: 151–167. Google Scholar
Fejfar, O., A. Blasetti, G. Calderoni, M. Coltorti, G. Ficcarelli, F. Masini, L. Rook and D. Torre. 1996. First record of a copemyine-peromyscine cricetid (Rodentia, Mammalia) in South America: hypotheses regarding its ancestry in the Palaearctic. Acta Zool. Cracov. 39: 137–145. Google Scholar
Gardner, A.L. and M.D. Carleton. 2009. A new species of Reithrodontomys, subgenus Aporodon (Cricetidae: Neotominae), from the highlands of Costa Rica, with comments on Costa Rican and Panamanian Reithrodontomys. Bull. Amer. Mus. Nat. Hist. 331: 157–182. CrossrefGoogle Scholar
Giglioli, E.H. 1874. Ricerche intorno alla distribuzione geografica generale o corologie degli animali vertebrati. III. Regione Boreo- Americana. Boll. Soc. Geogr. Ital. 2: 321–366. Google Scholar
Gloger, C.W.L. 1841. Gemeinnutziges Hand- und Hilfsbuch der Naturgeschichte. Für gebildete Leser aller Stände, besonders für die reifere Jugend und ihre Lehrer. Breslau: U. Schulz und Co., 1:xxxxiv + 496. Google Scholar
Hershkovitz, P. 1941. The South American harvest mice of the genus Reithrodontomys. Occas. Papers Mus. Zool., Univ. Michigan 441: 1–7. Google Scholar
Hooper, E.T. 1952. A systematic review of harvest mice (genus Reithrodontomys) of Latin America. Misc. Publ. Mus. Zool., Univ. Michigan 77: 1–255. Google Scholar
McKenna, M.C. and S.K. Bell. 1997. Classification of mammals above the species level. Columbia University Press, New York. pp. 631. Google Scholar
Moreno, P. and J.L. Román. 2013. Clasificación del género Reithrodontomys en el Ecuador y comentarios sobre la alimentación de la lechuza de campanario (Tyto alba) en los alrededores de Quito. COBIOFRAG, Boletín Técnico 11, Serie Zoológica 8–9: 16–23. Google Scholar
Musser, G.G. and M.D. Carleton. 2005. Superfamily Muroidea. In: (D.E. Wilson and D.M. Reeder, eds.) Mammal species of the world: a taxonomic and geographic reference, 3rd ed. Johns Hopkins University Press, Baltimore, MD, USA. pp. 894–1531. Google Scholar
Pardiñas, U.F.J., G. D’Elía and P.E. Ortiz. 2002. Sigmodontinos fósiles (Rodentia, Muroidea, Sigmodontinae) de América del Sur: estado actual de su conocimiento y prospectiva. Mast. Neotrop. 9: 209–252. Google Scholar
Patton, J. 2015. Suborder Myomorpha Brants, 1855. In: (J.L. Patton, U.F.J. Pardiñas and G. D’Elía, eds.) Mammals of South America, Volume 2: Rodents. University of Chicago Press, Chicago, IL, USA. pp. 58–60. Google Scholar
Patton, J.L., U.F.J. Pardiñas and G. D’Elia. 2015. Mammals of South America, Volume 2: Rodents. University of Chicago Press, Chicago, IL, USA. pp. xxvi + 1336. Google Scholar
Reig, O.A. 1977. A proposed unified nomenclature for the enameled components of the molar teeth of the Cricetidae (Rodentia). J. Zool., London 181: 227–241. Google Scholar
Saussure, H. de. 1860. Note sur quelques mammifères du Mexique. Rev. Mag. Zool. Pure Appl., Ser. 2, 12:1– 11, 53– 57, 97– 110, 241–254, 271–293, 366–383, 425–431, 479–494, 4 pls. Google Scholar
Thomas, O. 1898. On seven new small mammals from Ecuador and Venezuela. Ann. Mag. Nat. Hist., Ser. 7, 1: 451–457. Google Scholar
Waterhouse, G.R. 1837. Characters of new species of the genus Mus, from the collection of Mr. Darwin. Proc. Zool. Soc. Lond. 1837: 15–21, 27–29. Google Scholar
Wood, A.E. 1936. The cricetid rodents described by Leidy and Cope from the Tertiary of North America. Amer. Mus. Nov. 822: 1–8. Google Scholar
Zijlstra, J.S., D.A. McFarlane, V.L.W. den Hoek Ostende and J. Lundberg. 2014. New rodents (Cricetidae) from the Neogene of Curaçao and Bonaire, Dutch Antilles. Paleontology 57: 895–908. CrossrefGoogle Scholar
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Published Online: 2017-03-23
Published in Print: 2017-12-20