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New species of postcopemys (Cricetidae: rodentia) from the early pliocene of Lago De Chapala, Jalisco, Mexico.

In Mexico, the Neogene fossil record of rodents is poorly known. Jacobs and Lindsay (1981) and Lindsay and Jacobs (1985) described rodent remains from the well-known late Miocene (late Hemphillian North American Land Mammal Age; NALMA) locality of Yepomera, Chihuahua, including several cricetids (Prosigmodon oroscoi, Prosigmodon chihuahuensis, Calomys elachys, Calomys baskini, Calomys winklerorum, Baiomys kolbi, and Postcopemys valensis). Carranza-Castafieda and Walton (1992) described another late Hemphillian fauna, the Rancho El Ocote local fauna, Guanajuato, with 10 taxa (including three species of Calomys, B. kolbi, three species of Prosigmodon, two species of Neotoma, and an indeterminate cricetid). Also from this area in younger beds of Pliocene age (Blancan NALMA), remains of capybara Phugatherium dichroplax were reported (Carranza-Castafieda and Miller, 1988; Vucetich etal., 2015). Finally, Ferrusquia-Villafranca (2003) mentioned the presence of indeterminate rodents from the Nejapa local fauna, southeastern Oaxaca, of middle Miocene (Barstovian NALMA) age.

Lago de Chapala is the largest natural freshwater lake in Mexico located in a late Cenozoic basin, the Chapala rift, in southern Jalisco south of Guadalajara. Lago de Chapala (Fig. 1) is a remnant of a much larger Plio-Pleistocene lake that Clements (1963) and Mitchell (1965) referred to as Lake Jalisco. Strata exposed on the northern flank of the Chapala rift basin are volcaniclastic to clastic fluvial and lacustrine deposits interbedded with volcanic ash beds and are referred to as the Chapala Formation (Palmer, 1926; Downs, 1958; Clements, 1963; Delgado-Granados et al., 1995; Rosas-Elguera et al., 1997; Ferrari et al., 2000). The Chapala Formation overlies andesitic lava K/Ar dated at 3.4 [+ or -] 0.2 million years ago (Delgado-Granados et al., 1995; Rosas-Elguera et al., 1997; Ferrari et al., 2000). This radiometric date constrains the maximum age of the Chapala Formation to the late Pliocene epoch. Parts of the formation also extend into the Pleistocene, and vertebrate fossils of both Pliocene and Pleistocene age have been collected there, possibly representing three NALMAs.


Fossil vertebrates from Lago de Chapala have been known since the 1950s, and they have been published since then (e.g., Downs, 1958; Clements, 1963; Rufolo, 1998; Lucas, 2003,2008; AlberdiandCorona, 2005). The vertebrate fossils include fishes, reptiles, birds, and mammals. According to Lucas (2008) the fossils collected from the bottom of the lake and in the shorelines represent a Late Pleistocene (Rancholabrean) assemblage, including xenarthrans (Holmesina, Glyptotherium, Nothrotheriops shastensis, Paramylodon cf. P. harlani), carnivores (Canis cf. C. latrans, C. cf. C. lupus, Panthera cf. P. onca, ?P. atrox), small rodents (Neotoma, Sigmodon), a capybara (Neochoerus aesopi), a lagomorph (Lepus), proboscideans (Cuvieronius hyodon, Mammuthus imperator), horses (Equus conversidens, E. cf. E. francisci, E. ?excelsus, E. niobrarensis), a tapir (Tapirus), a peccary (Platygonus compressus), camels (Camelops hesternus, Camelops and an indeterminate lamine), deer (cf. Navahoceras, Odocoileus, Cervus), an antelope (Tetrameryx shuleri), and a bison (Bison).

However, the beds of the Chapala Formation (Fig. 1) exposed in the northern shore of the lake produce some fossil mammals such as the equid Nannippus, which suggests a Blancan (Pliocene) age; a skeleton of Stegomastodon rexroadensis recovered from the bottom of the lake also is considered to be of Blancan age (Lucas, 2008). Remains of Cuvieronius have been collected from the bottom of the lake and could represent the Irvingtonian NALMA (early Pleistocene). Thus, the Chapala basin yields mammalian fossil assemblages of Blancan, Rancholabrean, and possibly Irvingtonian ages (Lucas, 2003, 2008).

In the Lago de Chapala area, Barbour (1973) and Smith et al. (1975) mentioned the presence of the rodents Neotoma and Sigmodon collected near the town of Jocotopec; Peters (1951) identified remains of capybara, which he assigned to the extant species Hydrochoerus magnus, which later Mones (1991) identified as Neochoerus aesopi.

In 2009, D. Mendez-Cardenas, under the direction of M. Benammi, carried on a Master's project in which she described part of the fossil rodent material collected from two stratigraphic sections located in the northern shore of Lago de Chapala. She also obtained samples for radiometric and paleomagnetic dating, which gave 3.44 [+ or -] 0.38 Ma and 3.69 [+ or -] 0.38 Ma. Mendez-Cardenas (2009) identified a small part of the rodent material collected and reported the presence of five forms (Sigmodon minor, Sigmodon medius, two indeterminate forms of Sigmodon, and Neotoma cf. N. sawrockensis). The study of the rodents collected from Lago de Chapala was retaken as a project by one of the present authors (AR). Here we describe a new species belonging to Postcopemys, which also represents the second record for the genus in Mexico. In addition, we make a phylogenetic exploration of the new Postcopemys species using dental characters of some North American cricetid stock.

MATERIALS AND METHODS--The specimens used in this study are housed at Museo de Paleontologia Maria del Carmen Perrilliat M., Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico City.

Phylogenetic Analysis--Data Set (Appendix 1): We performed a cladistic analysis to test the phylogenetic relationships of the new taxa primarily using the molar characters from Weksler (2006). We used characters 54, 55, 57, 58, 59, 60, 61, 62, 63, 64, 65, 70, 71, 72, 73, 74 of Weksler (2006), with the addition of new characters (see character list below; Appendix 2). For occlusal molar terminology, we follow Reig (1977, 1978, 1980). We took measurements with a digital caliper and referred to the maximum dimension for each molar. We compared the Chapala material with taxa closely related systematically, biogeographically, and chronologically, such as Copemys, Postcopemys, Peromyscus, Abelmoschomys, Prosigmodon, Bensonomys, Lindsayimys, Baiomys, and Reithrodontomys (Appendix 3).

Outgroup: We used Copemys longidens as an out-group because the species has been considered an ancestor of North American Cricetidae, and is also closely related with its probable ancestor Democricetodon from Eurasia (Lindsay, 1995).

Search Methods: We treated all characters as nonadditive (unordered); and we treated gaps as missing. To explore clade sensitivity, we conducted analyses implementing 'prior' and 'implied' weighting methods (Goloboff, 1993; Goloboff et al., 2008). For the implied weighting method, the default concavity constant values were 3 (k = 3). We performed heuristic parsimony analyses of 1,000 replicates using the "traditional search option" of TNT 1.1 (Goloboff et al., 2008). The swapping algorithm used was tree bisection reconnection, with 10 trees saved per replication, collapsing trees after search. To measure node stability, we used the absolute frequency and frequency differences (GC) arising from symmetric resampling (Goloboff et al., 2008) based on 1,000 replicates (P = 0.33).

Geological Setting--Two stratigraphic sections were measured along the roads between Chapala-Ajijic and Chapala Guadalajara. In the 80-m-thick Section 1, five layers yielded fossil mammal remains, 22 samples were collected for paleomagnetostratigraphy, and one ash layer was radiometrically dated with the U-Pb method using zircons (Mendez-Cardenas, 2009).

The stratigraphic sequence at Chapala includes two types of deposits that are interbedded: one volcaniclastic unit that is formed by thin layers of fine and coarse sands and conglomeratic sands, where the fossil rodents were collected in five different levels. This unit also includes thick layers of silt and clay with fish remains. The second type of deposit includes ash layers formed by felsic ash, where zircons for radiometric dating were collected. According to Mendez-Cardenas (2009) this section belongs to the Arroyo Hondo Formation, Chapala Group, previously described by Delgado-Granados et al. (1995).

The sand varies from very fine to coarse-grained, and it includes some primary structures such as normal gradation, and parallel lamination, cross-stratification along the section.

The paleomagnetic analysis revealed five magnetic zones, three with reversed polarity and two with normal polarity, equivalent to magnetostratigraphic Chrons C3n.1n (4.493 Ma) and C2An.3n (3.330 Ma). According to the magnetostratigraphy, the time of deposition of this sequence is in the range of 4.3-3.3 Ma. The radiometric dating of zircons provides an age of 3.44 [+ or -] 0.38 Ma (Mendez-Cardenas, 2009). The fossiliferous levels 1 and 2 correlate to Chron C3n.1r, and levels 3, 4, and 5 with C2An.1n. Thus, the fossils described below pertain to the early part of the Pliocene epoch, and the early Blancan NALMA.


SYSTEMATIC PALEONTOLOGY Mammalia Cuvier, 1798 Rodentia Bowdich, 1821 Cricetidae De Rocherbrune, 1883 Postcopemys Lindsay and Czaplewski, 2011

Type Species--Postcopemys repenningi Lindsay and Czaplewski, 2011, MNA locality 701, Buckboard Wash, Verde Fm., Yavapai County, Arizona.

Referred Species--Postcopemys valensis (Shotwell, 1967), Postcopemys vasquezi (Jacobs, 1977), and Postcopemys maxumensis, Lindsay and Czaplewski, 2011.

Diagnosis--Modified from Lindsay and Czaplewski, 2011, using Reig's (1977, 1978, 1980) cusp terminology. Small, brachydont cricetid rodent having protolophule II [paraloph] not aligned with the anterior arm of the hypocone in upper molars (M), but with the entolophid aligned with the anterior arm of the protoconid in lower molars (m). Protolophule is absent in M1, usually present but weakly developed in M2, and strongly developed in M3. The hypocone of M3 is reduced. Upper cheek teeth have three prominent roots and lack accessory rootlets; lower cheek teeth have two prominent roots and lack accessory rootlets.

Postcopemys chapalensis sp. nov. Fig. 2a; Table 1

Diagnosis--The largest species of Postcopemys based on its brachydont bunodont molars; also bears the following unique character combination: median mure connected to protocone on M1; m1 with a wide and strongly bilobed procingulum; ectolophid and ectostylid on m1 present; mesolophid well-developed on m1, but short in m2 and not extending to the lingual cingulum.

Holotype--IGM-4850, isolated left m1 (Fig. 2a, recovered from stratigraphic level 3).

Referred Material--IGM-4851, partial left M1 (Fig. 3a, from level 1), lacking the procingulum; IGM-4852, IGM 4853, two right M2 (Fig. 3b-c, from level 3); and IGM 4854, one left M2 (Fig. 3d, from level 3); IGM-4855, one right m1, lacking the procingulum (Fig. 2b, from level 1); IGM-4856, IGM-4857, two right m2 (Fig. 2c-d, from level 1); and IGM-4858, one left m2 (Fig. 2e, from level 2).

Type Locality and Horizon--Chapala Formation levels 1, 2, and 3, northern flank of the Chapala rift basin volcaniclastic to clastic fluvial and lacustrine deposits interbedded with volcanic ash beds, Lago de Chapala, Jalisco State, Mexico.

Age--Early Pliocene, early Blancan, 3.44 -- 0.38 Ma to 3.69 [+ or -] 0.38 Ma.

Etymology--From Lago de Chapala, a Nahuatl name that means "where the grasshoppers swim."

Description: Upper Molars--These teeth are brachydont and have labial flexi that have the same anteroposterior length but mesiolabially are wider. The specimen IGM4851 (M1) lacks the procingulum, which is broken away; the main cusps are alternated (not aligned opposite one another on the tooth). The posteroloph in M1 is weakly developed and almost fused with the metaloph; posterofossette is ephemeral, disappearing with wear. The mesoloph on M1 is well-developed, extends from the median mure to the labial cingulum, and is fused with mesostyle. The labial portion of the mesoloph curves as it slopes toward the floor of the mesoflexus. In M1 and M2 the paraflexus and metaflexus are strongly curved posteriorly. The protostyle on Ml is absent. The median mure is equal in size to the anterior mure. The mures are ridgelike and anteroposteriorly oriented, and situated more or less along the midline of the molar. An ectostyle is present. A paralophule is present in M1 and M2.


The paracone of Ml is connected with protocone by an enamel bridge situated at the posterior-most end of the protocone. There is a cingulum in the mouths of the labial flexi in M1. The dentine is not exposed on the mures of M1. In the M1 protocone and hypocone, dentine exposure areas are reduced and tend toward the labial side. The median mure in M1 and M2 is connected directly to the protocone and the paraloph in M1 is not aligned with the median mure. In M2, the mesoloph is shorter than in M1. The protoflexus and paraloph are present in M2.

Lower Molars--The lower teeth of the new species have, in general, a complex morphology, they are brachydont and subrectangular in outline, with flexids wider than conids. The m1 has the labial and lingual cusps alignment alternate rather than opposite (as in the uppers), the metaconid is anteriorly positioned with respect to the protoconid, and the entoconid is anteriorly positioned with respect to the hypoconid. The murids are ridgelike and longitudinally oriented along the midline of the tooth. The areas of dentine exposure in protoconid and hypoconid are reduced, and tending toward the labial side.

The entolophid in m1 and m2 is aligned with the median murid. The procingulum in m1 is very welldeveloped, wide, and horizontal with respect to the lophids, and strongly bilobed. The anterolabial-anterolingual conulids are symmetrical, equal in size, and are of the same size as the metaconid and protoconid. The m1 anteromedian flexid is deeply developed as in the other species of Postcopemys. The m1 has an anterolabial cingulum, ectolophid, and ectostylid. The hypoflexid posteroflexid and the protoflexid-mesoflexid in ml are aligned and positioned opposite one another on the occlusal surface. The posterior arm of the hypoconid and posterolophid in m1 are not aligned. The m1 of P chapalensis has an ectolophid closelyjoined to the anterior wall of the hypoconid. The mesostylid in m1 is connected to the mesolophid, and in early wear stages these structures became disconnected. The anterolabial cingulum in m2 is well-developed, and the posteroflexid is wide and long.


DISCUSSION--The isolated upper and lower molars from Chapala Lake share the diagnostic characters of the genus Postcopemys: the major cusps of upper and lower molars are alternate, and the anterior and median mures and murids in M1 and m1 are anteroposteriorly oriented and situated along the midlines of the molars. Also, the Chapala specimens share with Postcopemys a paraloph not aligned with the anterior arm of the hypocone in upper molars, but with the entolophid aligned with the anterior arm of the protoconid in lower molars, and the protolophule is absent in M1.

Jacobsomys differs from Postcopemys in having a distinctly bilobed anterocone on M1; major cusps of upper molars opposite one another rather than alternate; anterior and median mures of M1, M2, and m1 anteroposteriorly oriented and situated along the midlines of the molars; mesoloph absent in Ml and M2; paralophule on Ml and M2 strongly connected to mesostyle. In addition, Jacobsomys has the ml mesolophid-mesostylid and ectolophidectostylid partially coalesced with the entoconid and hypoconid, respectively.


Copemys (sensu Lindsay, 2008, whose terminology differs from Reig's [1980] in part) differs from Postcopemys in having molars with alternation of the labial and lingual cusps incomplete, protolophules (= protolophule and paraloph) on Ml and M2 weakly developed, protolophule II (= paraloph) not aligned with the anterior arm of the hypocone, entolophulids (= entolophids) of ml and m2 weakly developed, not aligned with the posterior arm of the protoconid. The last character is shared with Baiomys and Abelmoschomys; posterior arm of the protoconid on ml and m2 is short, not directed transversely across the midline; incisors lack ridged ornamentation.

Peromyscus differs from Postcopemys (as characterized by Lindsay, 2008) in having upper molars with the protolophule aligned with the anterior arm of the hypocone; lower molars have the entolophid aligned with the posterior arm of the protocone.

There are four species of Postcopemys reported: Postcopemys valensis from the early Pliocene of Oregon (Shotwell, 1967), and from the Hemphillian locality Yepomera, Chihuahua (Lindsay and Czaplewski, 2011); Postcopemys vasquezi from the Hemphillian Quiburis Formation, southeastern Arizona; Postcopemys repenningi from the early Blancan Verde Formation, Arizona, and the Maxum fauna, Tassajera Formation, California; and Postcopemys maxumensis from the Maxum local fauna, Tassajera Formation, California (Lindsay and Czaplewski, 2011).

Postcopemys chapalensis differs from these other Postcopemys in its larger size and in having a very welldeveloped mesolophid and ectolophid in m1; a very deep anteromedian flexid in m1; a very well-developed and symmetrical procingulum in m1; and an ectolophid and ectostylid present in m1 and m2.

Postcopemys vasquezi further differs from P. chapalensis in having a narrow and not bilobed procingulum. Postcopemys valensis from Yepomera, Chihuahua, is smaller than P. chapalensis, the anterimedian flexid in m1 is very small or absent, and the ectolophid and ectostylid is absent.

Phylogenetics--Our preliminary phylogenetic analysis based on molar characters recovered one tree with consistency index = 0.320, and retention index = 0.633. Our results suggest that P. chapalensis and the Postcopemys clade, including J. dailyi, nests between the outgroup C. longidens and Peromyscus difficilis. This result is in agreement with the Lindsay and Czaplewski (2011) hypothesis on the ancestor-descendant relationship of Copemys-Peromyscus via Postcopemys. However, the putative North American clade including Jacobsomys, Calomys, Bensonomys, and the Prosigmodon group, sometimes considered as members of the Sigmodontini or Sigmodontinae rather than Peromyscini-Neotomini (e.g., Baskin, 1978, 1986; Czaplewski, 1987; but see Pardinas etal., 2002 for an alternative interpretation), clusters basal to Postcopemys and Baiomys. The Prosigmodon species appear as a paraphyletic group with Prosigmodon holocuspis in the base of a Sigmodon clade, and Prosigmodon oroscoi, Prosigmodon ferrusquiai, and Prosigmodon chihuahuensis at the base of all of the cricetids analyzed and close to C. longidens (Fig. 4; shows the common synapomorphies).

Another interesting result of our analysis is that Abelmoschomys joins in the base of the Tylomyinae (Central American vesper rats and climbing rats; Steppan et al. 2004; Musser and Carleton, 2005), and shares a strongly bilobate procingulum on M1 and a posteroloph weakly developed or fused with the metaloph.

The species of Jacobsomys sensu Lindsay and Czaplewski (2011), and May et al. (2011) are paraphyletic in our analysis, with J. dailyi clustering with Postcopemys chapalensis, P. repenningi, and P. maxumensis, and probably with P. vasquezi regarding the described morphology in Jacobs (1977); whereas, Jacobsomys verdensis clusters with the base of Calomys and Bensonomys, with all three sharing a long anterolabial cingulum. We believe J. dailyi shows the molar characters of Postcopemys rather than Jacobsomys and should be referred to the genus Postcopemys.

CONCLUSION--Postcopemys is a rare cricetid element in the Hemphillian-Blancan of North American faunas. The new species Postcopemys chapalensis comprises the second record of the genus in Mexico, and also represents the southernmost record of the genus. Phylogenetically P. chapalensis is close to the base of the Postcopemys clade, which includes P. maxumensis, P. repenningi, P. chapalensis, and J. dailyi, which should be referred to Postcopemys on the basis of our analysis and interpretation of the molar characters of the genus. The phylogenetic analysis based on dental characters supports the hypothesis of morphological transition between Copemys and Peromyscus via Postcopemys. Discrepancies between our phylogeny and those of other workers in the relationships among fossil cricetid rodents in the Americas probably reflect the limitations of using isolated molars and molar rows in attempting to reconstruct cricetid phylogeny. it is clear to many researchers on the Cricetidae of the western hemisphere that the fossils of this hyperdiverse group require a much more complete fossil record before the group's phylogeny and biogeographic history can be elucidated.

We thank the Programa de Estancias de Investigation, Direction General de Asuntos del Personal Academico (UNAM), Instituto de Geologia, and Programa de Intercambio Academico of the Coordination de Investigation Cientifica, UNAM, for all the support during the research; we extend our thanks to J. M. Contreras, who took the photographs; the staff of the Colecciton Nacional de Mastozoologia, Instituto de Biologia, and of Museo de Paleontologia Maria del Carmen Perrilliat M., Instituto de Geologia (UNAM) for granting access to study the material under their care; ADR wants to thank Instituto Venezolano de Investigaciones Cientificas for the financial support during the stay. We thank A. Soltorzano for his assistance in the phylogenetic analysis and cleaning the material; E. H. Lindsay, who kindly provided NJC with comparative casts of several fossils; M. L. Mendez-Cardenas for use of her stratigraphic and radiometric data; and U. F. J. Pardinas for his insight on fossil cricetids and helpful comments on an earlier draft of this manuscript. We extend our thanks to two anonymous reviewers; their comments improved the manuscript.
APPENDIX 1--Character and taxa matrix ([dagger] = fossil form) of the
scored characters from Appendix 2. ? = missing

Repomys maxumi              0011011002100220001012122011130020??002?01
[dagger] Copemys            000001101110000111000202200003021010111111
[dagger] Bensonomys         110000101310000?11001002211013122001102110
Calomys hummelinki          201101101200002011010001110113122101102100
[dagger] Prosigmodon        1100001012100120000??????????3022010??2?1?
[dagger] Prosigmodon        2000001013100120000?0000000203022010102?11
[dagger] Prosigmodon        200000101210012000000000000013022010102100
[dagger] Prosigmodon        00?00111?21001??00011100000013?021??102100
[dagger] Abelmoschomys      20000010000000????0??1?11111130000??102?10
[dagger] Sigmodon minor     001111010211111100111201110113002110002100
[dagger] Sigmodon medius    001111010211111100111201110113002110002100
Sigmodon alstoni            001111010211111100111211110113002110002100
Neotoma alleni              010001100210021111001202200013002110002100
Tylomys nudicaudus          201100100100000111001101110013001100111010
Ototylomys phyllotis        201100100000000111001101110010000000010010
Nyctomys sumichrasti        201100100000000111001101110010000101110000
Otonyctomys hatti           101100100000000111001101110010000100110000
Peromyscus difficilis       100000100010000111000000001010000100102000
Nelsonia neotomodon         010100100210022000011211101013002110002100
Neotomodon alstoni          100011100110001111101200010102001111112010
Reithrodontomys megalotis   10001010021000??1110?212210013002110112000
Baiomys musculus            110000101210002011000100010113002110102000
[dagger] Jacobsomys         11000010121000??11000000010013022001113?10
[dagger] Jacobsomys         11000010101110??00000000010010000101100010
[dagger] Postcopemys        ??000010101000??0000000001001?000001113010
[dagger] Postcopemys        100000101011101000000202210003000000102010
[dagger] Postcopemys        100000101011101000000202210003001100101010

APPENDIX 2--List of characters used in the analysis of Postcopemys relationships. We used 58 characters from Weksler (2006) in parentheses (58). We used the Reig (1977) occlusal teeth nomenclature, but the Lindsay (1972) occlusal teeth nomenclature in brackets in characters 41 and 42, for a better understanding.

1. (58 modified) Procingulum bilobation in M1:0 = no bilobate, 1 = weakly bilobate, 2 = strongly bilobate.

2. Procingulum symmetry in M1: 0 = asymmetric anterolabial conule more developed than the lingual, 1 = symmetric both conules in the same size.

3. Mures alignment in M1: 0 = aligned, 1 = nonaligned, the anterior mure is displaced to the lingual side.

4. Mures orientation in M1: 0 = parallel respect to the tooth midline, 1 = oblique respect to the tooth midline.

5. Mures development in M1: 0 = both mures on the same length; 1 = anterior mure reduced.

6. Dentine Mure in M1: 0 = absent; 1 = present.

7. Flexus size in M1: 0 = narrow, 1 = wide.

8. Labial flexus relative size: 0 = labial flexi on the same size; 1 = paraflexus reduced.

9. (55) Cingulum in the bottom of the labial flexi in M1: 0 = absent, 1 = present.

10. (62) Mesoloph size in M1: 0 = well-developed to the mesostyle, 1 = weakly developed, a half of the size, 2 = absent, 3 = 1/2

11. Posteroloph size in M1: 0 = well-developed, 1 = weakly developed or fused with the metaloph.

12. Protocone-Paracone alternation in M1: 0 = no alternate, 1 = alternate.

13. Hypocone-metacone alternation in M1: 0 = no alternate, 1 = alternate, metacone anteriorly to hypocone.

14. (54) Molar Hypsodonty: 0 = brachydont, 1 = weakly hypsodont, 2 = highly hypsodont.

15. M3 occlusal surface: 0 = cuspate with a distinct paracone, protocone, and hypocone; 1 = lophate with 3 lophs; 2 = lophate with 2 lophs.

16. Anterolophid size in M3: 0 = short; 1 = long.

17. Protoconid-Metaconid relative position: 0 = alternate, metaconid anteriorly positioned respect to protoconid; 1 = opposite, protoconid and metaconid no alternate

18. Entoconid-hypoconid relative position: 0 = alternate, entoconid anteriorly positioned with respect to hypoconid; 1 = opposite, entoconid and hypoconid not alternate.

19. Flexids size: 0 = flexid wider than conids; 1 = flexids narrower than conids.

20. Anterolabial cingulum size in m2: 0 = well-developed; 1 = reduced weakly developed.

21. Posterior arm of the hypoconid-posterolophid relative position: 0 = not aligned; 1 = aligned.

22. (70) Procingulum bilobation on m1: 0 = deeply bilobed: 1 = weakly bilobed; 2 = no bilobated.

23. Posteroflexid size in m2: 0 = wider and long; 1 = narrow and very short.

24. (57) Hypoflexid-posteroflexid relative position in m1: 0 = aligned and opposite; 1 = not aligned, alternate and with the terminus overlapping; 2 = not aligned but not overlapping.

25. (57) Protoflexid-mesoflexid relative position in m1: 0 = aligned and opposite; 1 = not aligned, alternated and with the terminus overlapping; 2 = not aligned, and not overlapping.

26. Murids relative position: 0 = not aligned, anterior murid displaced labially; 1 = aligned, anterior murid in front of median murid.

27. Posterolophid size in m1: 0 = longer than any lingual lophid; 1 = short.

28. Procingulum orientation in m1: 0 = horizontal with respect to the lophids; 1 = oblique with respect to the lophids, 2 = 0/1.

29. Procingulum size in m1: 0 = smaller with respect to the m1 size; 1 = medium sized with respect to m1 size.

30. (59) Anteroloph on M1: 0 = well-developed and discrete, reaching the labial cingulum, anteroflexus present; 1 = anteroloph present but small, not reaching the labial cingulum, anteroflexus absent; 2 = anteroloph fused with anterocone labially, anteroflexus present as small fossette; 3 = anteroloph and anteroflexus absent.

31. (60) Protostyle on M1: 0 = absent; 1 = present.

32. (61) Paracone of M1: 0 = connected to protocone by enamel bridge situated at posterior-most end of protocone; 1 = paracone connected to protocone by enamel bridge situated at anterior portion of protocone; 2 = protocone and paracone forming single dentine basin without enamel connection.

33. (62) Mesolophs on M1 and M2: 0 = well-developed, extending from the median mure to the labial cingulum, fused with mesostyle; 1 = mesolophs small, not extending to labial cingulum and not fused with the mesostyle; 2 = mesolophs on M1 and M2 absent.

34. (63) Median mure: 0 = connected to protocone on M1; 1 = median mure not connected to protocone.

35. (64) Protoflexus of M2: 0 = present; 1 = absent.

36. (65) Paracone on M2: 0 = without accessory loph; 1 = accessory loph present posterior to paracone.

37. (71) Anterolabial cingulum on m1: 0 = absent; 1 = long anterolabial cingulum present.

38. (72) Ectolophid and ectostylid on m1: 0 = absent; 1 = present.

39. (73) Mesolophids: 0 = present and well-developed on m1 and m2; 1 = mesolophids present in unworn dentition but small, not extending to lingual cingulum; 2 = mesolophids completely absent; 3 = well-developed on m1, but half-developed in m2, not extending to the lingual cingulum.

40. (74) Anterolabial cingulum on m2; 0 = present; 1 = absent.

41. Paraloph [protolophule II]-anterior arm of the hypocone alignment: 0 = aligned; 1 = not aligned.

42. Entolophid [Entolophulid]-posterior arm of the protoconid alignment: 0 = Aligned; 1 = not aligned.

APPENDIX 3--Material examined for comparison, and in part used in the phylogenetic analysis: (IGM, Museo de Paleontologia Maria del Carmen Perrilliat M., Instituto de Geologia, Universidad Nacional Autonoma de Mexico; UF, Florida Museum of Natural History; Gainesville; UNAM-IB, Coleccion Nacional de Mastozoologia, Instituto de Biologia, Universidad Nacional Autonoma de Mexico; IGCU, Instituto de Geologia, Universidad Nacional Autonoma de Mexico; USNM, United State National Museum; OMNH, Sam Noble Oklahoma Museum of Natural

History; MNA, Museum of Northern Arizona; M, upper molars; m, lower molars; f = fossil form).

[dagger] Abelmoschomys simpsoni: M1 (UF 61327, UF 61328, UF 61329, UF 61330, UF 613301, UF 61332, UF 61333, UF 61334, UF 213505, UF 213506, UF 213507, UF 213509, UF 213508), m1 (UF 61335, UF 61336, 61339, UF 61 340, UF 61342, UF 213503, UF 213504. f Baiomys kolbi: M1-M2 (IGM 5793); M1 (IGM 5794), M1 (IGM 5795), m2 (IGM 5789), m2 (IGM 5790).

[dagger] Baiomys pallidus: modern individual cranium and mandibles (UN AM-IB-2725).

Baiomys musculus: modern individual cranium and mandibles (UNAM-IB-8431, 45262).

Baiomys taylori: modern individual cranium and mandibles (UNAMIB-11165, 36447).

[dagger] Bensonomys elachys: M1 (IGM 5768, Type), M1 (IGCU 1167), M1 (IGCU 1169), m1 (IGM 5769, IGM 5771, IGM 5773), m2 (IGM 5770, IGM 5774).

[dagger] Bensonomys baskini, m1 (IGM 5775, Type; IGM 5780), m2 (IGM 5776, IGM 5777, IGM 5778), m3 (IGM 5779).

[dagger] Copemys pagei, [dagger] Copemys tenuis, [dagger] Copemys russelli, [dagger] Copemys longidens, [dagger] Copemys barstowensis, and [dagger] Copemys esmeraldensis: Lindsay, 1972. [dagger] Jacobsomys dailyi: Lindsay and Czaplewski, 2011; May et al., 2011. [dagger] Jacobsomys verdensis: Czaplewski, 1987, and cast of the original type (MNA 4849, M1, M2, and m1, m2, cast housed in OMNH, 2722). Nyctomys sumichrasti: modern individual cranium and mandibles (UNAM-IB-29531, 29532, 29533, 33756, 46262, 46263).

Ototylomys phyllotis: modern individual cranium and mandibles UNAM-IB-33071, 33072, 36678, 37349.

Otonyctomys hatti: modern individual cranium and mandibles (UNAM-IB-30792, 38008, 46817, 46818).

Peromyscus boylii: modern individual cranium and mandibles UNAMIB-38182.

Peromyscus difficilis: modern individual cranium and mandibles UNAM-IB-43033.

Perosmyscus aztecus: modern individual cranium and mandibles UNAM-IB-44864.

[dagger] Postcopemys: Lindsay and Czaplewski, 2011; May et al., 2011. fCopemys near C. valensis: M1 (IGM 5764), m1 (IGM 5765, IGM 5767).

[dagger] Prosigmodon oroscoi: M1 (IGM 5708, Type; IGM 5807, IGCU 1212, IGCU 37, IGM 5805, IGM 8227, IGM 8228, IGM 8241, IGM 5807), M2 (IGCU 150, IGM 5806), M1-M2 (IGCU 1244), M3 (IGM 5709).

[dagger] Prosigmodon chihuahuensis: M1 (IGM 5815, IGM 5809, IGM 5812), M2 (IGM 5811, IGM 5816), M3 (IGM 5810) m1 (IGM 5808, type; IGM5817), m3 (IGM 5813, IGM 5814, IGM 5818).

[dagger] Prosigmodon ferrusquiai: M1 (IGM 8254, type; IGM 8245, IGM 8246, IGM 8247, IGM 8248, IGM 8249, IGM 8250, IGM 8251, IGM 8254, IGM 8256, IGM 8258, IGM 8259, IGM 8260), M2 (IGM 8262), M3 (IGM 8285, IGM 8286), m1 (IGM 8287, IGM 8288, IGM 8293, IGM 8294, IGM 8295, IGM 8290).

[dagger] Prosigmodon holocuspis: Czaplewski, 1987, and cast of some specimens in that work, m1 (MNA V4831), m2 (MNA V4832), m3 (MNAV4834), casts housed in OMNH 2721.

Reithrodontomys megalotis: modern individual cranium and mandibles UNAM-IB-892.

Tylomys nudicaudus: modern individual cranium and mandibles UNAM-IB-11529, 11571, 12222, 19333, 19334, 29534.

[dagger] Sigmodon minor: m3 (UF 224698), mandible with m1-m3 (UF 232185), m1 (UF 232192), m1 (UF 232193), mandible with m1m3 (UF 237831), m2 (UF 23 2199), maxilla with M1-M2 (UF 232191), maxilla with M1-M3 (UF 237832), maxilla with M1-M2 (UF 237833), maxilla with M1-M2 (UF 239721).

[dagger] Sigmodon medius: Czaplewski, 1987, and cast of some specimens in that work, M1-M3 (MNAV4835, cast housed in OMNH 2719) and m1-m3 (MNA V4834 cast housed in OMNH 2720).

Sigmodon alstoni: modern individual cranium and mandibles (USNM 406.257, 406.256, 415.024, 406.260, 415.021, 415.016).

Neotoma alleni: modern individual cranium and mandibles (UNAMIB-5224).

Nelsonia neotomodon: modern individual cranium and mandibles (UNAM-IB-1528, 19699).

Neotomodon alstoni: modern individual cranium and mandibles (UNAM-IB- 23101, 23104, 23106, 23113, 23115).

Calomys hummelinki: modern individual cranium and mandibles (USNM 442441, 460437, 460438, 460440, 460441, 460443, 460445).


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Submitted 9 November 2015. Acceptance recommended by Associate Editor, Troy Ladine, 19 February 2016.

Ascanio D. Rincon, * Nicholas J. Czaplewski, Marisol Montellano-Ballesteros, Mouloud Benammi

Instituto Venezolano de Investigaciones Cientificas, Laboratorio de Paleontologia-Centro de Ecologia, Km 11 de la Carretera Panamericana, Edo. Miranda. Aptdo. 21.827, Caracas Cod. Postal 1020-A, Venezuela (ADR) Oklahoma Museum of Natural History, 2401 Chautauqua Avenue, University of Oklahoma, Norman, OK 73072 (NJC) Instituto de Geologia, Universidad Nacional Autonoma de Mexico. Ciudad Universitaria, Deleg. Coyoacan, Mexico D.F. 04510, Mexico (MMB) Institut de Paleoprimatologie et Paleontologie humaine: Evolution et Paleoenvironnements--iPHEP CNRS UMR 7262--Universite de Poitiers, 86073 POITIERS cedex 9, France (MB)

Laboratorio de Paleomagnetismo, Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Circuito de la Investigacion Cienttfica, 04510 Mexico DF, Mexico (MB)

* Correspondent:
Table 1--Measurements of upper (M) and lower (m) teeth of
Postcopemys chapalensis from Lake Chapala, Mexico. Specimens
are deposited in the Museo de Paleontologia Maria del Carmen
Perrilliat M., Instituto de Geologia (IGM), Universidad Nacional
Autonoma de Mexico. Catalog number 4850 is the holotype.

Catalog number (IGM)   Tooth position   Length (mm)   Width (mm)

4851                   M1 left          --            1.83
4852                   M2 right         1.93          1.70
4853                   M2 right         1.80          1.67
4854                   M2 left          1.93          1.63
4850                   m1 left          2.57          1.63
4855                   m1 right         -             1.40
4856                   m2 right         1.93          1.63
4857                   m2 right         1.93          1.67
4858                   m2 left          2.17          1.77
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Date:Jun 1, 2016
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