The research on paleontology of the sedimentary basins started at the Instituto de Geología has continued for more than 20 years at the Centro de Geociencias, Campus Juriquilla; both institutions belong to the Universidad Nacional Autónoma de México. The main objective of this project is to study the biostratigraphy of the Late Neogene sedimentary basins in the central region of Mexico.

These basins are in the Transmexican Volcanic Belt and north to it between the parallels 19° and 23°. The results obtained have demonstrated the importance of these sedimentary basins to complement the biostratigraphy of Mexico and North America.

The abundance and diversity of mammals that have been collected, with reliable stratigraphic and geographical information, have shown two faunal associations of different ages, Hemphillian and Blancan NALMA (North American Land Mammals Ages). These records allow us to understand important geological and biological events like the dispersal of mammals from the Temperate North America faunas to the central region of Mexico, the dispersal of the antilocaprids Cosoryx and Plioceros described in the Juchipila fauna in the early-late Hemphillian age, and the equids Calippus martini and the antilocaprid Merycodus, described from the la Nopalera locality in ages of 6.77 ± 0.21 My; these discoveries imply a reconsideration of the concepts of extinction, stratigraphic range and geographical distribution (Carranza-Castañeda et al., 2013; Robles-Rivera, 2015).

Another important result of the work in these basins refers to the most important biological event among the Americas, the Great American Biotic Interchange to (GABI). The recent discoveries of Neotropical immigrants in the Tecolotlán basin in deposits of 4.89 My confirm that there was more than one invasion of Neotropical faunas toward North America, prior to closing of the Panamanian land bridge (McDonald and Carranza-Castañeda, 2017). Additional contributions to these results refer to the exchange and displacement of mammals between the faunas of temperate North America toward the faunas of the central Mexico in both directions. Recent descriptions of neotropical mammals suggest that the western part of Mexico was an important migration route (Sussman et al., 2016; McDonald and Carranza-Castañeda, 2017; Carranza-Castañeda, 2018).

The above-mentioned information is based on an extensive collection of fossil mammals that has been integrated with the results of research in the sedimentary basins that have been studied, however, very little has been mentioned of the advances in the knowledge of the equids that have allowed to complement the biostratigraphic information of the strata that contain neotropical immigrants and mammals from the Great Plains of North America.

The objectives of this work are: to mention the first records of equids collected in the Clarendonian and early Hemphillian deposits, located south of the San Miguel de Allende basin, state of Guanajuato; to comment and describe the geographical distribution of Dinohippus mexicanus in the faunas of the central region of Mexico, defining the great variability that occurs in the facial region and the occlusal surface of upper and lower molars; to describe partially the stratigraphy of the most important faunas where diagnostic material of D. mexicanus was collected. In addition, the associated fauna in the stratigraphic levels where they were collected with the criterion that the age is determined by radiometric dates will be mentioned. The age of the records mentioned are determined to early-late Hemphillian, late Hemphillian (Hh3), and latest Hemphillian (Hh4). the analysis on interbedded volcanic ash and the associate fauna have determined the age of the fossils (Carranza-Castañeda et al., 2013; Figure 1).

All the fossil materials mentioned in this paper are housed in the fossil collection of the Instituto de Geología, Ciudad Universitaria and the Centro de Geociencias, UNAM in Juriquilla. Querétaro.

To avoid the repetition of names, of the associated fauna with records of D. mexicanus is mentioned in Supplementary Table 1.

The terminology used in the description is from Lance (1950), MacFadden (1984, 2016).

The abbreviations used in the text and tables are: Ap, Anteroposterior, DPOF, Dorsal Preorbital fossa; Colo, Colotlán; HC, High crown; IGM, Instituto de Geología; Ii incisive; Jalp, Jalpan; Juch, Juchipila; JalTeco, JT Jalisco Tecolotlán; LACM, Natural History Los Angeles County Museum; L, Left; MPGJ, Museo de Paleontología Geociencias Juriquilla; Mm, molar; MSTHT, Mesostyle Crown Height; NMMNH, New Mexico Museum Natural History; Pp, Premolar; R, Right; Tab, Tabasco; Tr, transverse; Zac, Zacatecas; UNAM, Universidad Nacional Autónoma de México.

In all the Hemphillian faunas described in the sedimentary basins of central Mexico, there is a large diversity of mammals among which the equids are widely represented. However, C. hondurensis and Astrohippus ansae are present only in the early-late Hemphillian faunas. Neohipparion eurystyle and D. mexicanus are present in all sedimentary sequences, from the early-late and late Hemphillian, although in the deposits of the latest Hemphillian (Blanco Layer of Rancho El Ocote), only D. mexicanus survives until the unconformity of Blancan age (Carranza-Castañeda, 1989, 1992; Carranza-Castañeda et al., 2013).

El Valle del Gigante. At the top of the Sierra Madre Occidental, is the rancheria of El Valle del Gigante, which was initially explored and investigated by Dr. Theodore Downs and Mr. Harley J. Garbani from LACM, in Valle de El Gigante. Years later, the author continued the research in this area and discovered a new locality Nay 1 El Gigante. The continuity of the research in this area, led to the discovery of other localities. In the town of Huajimic Nay 2 El Huichol, there is material of upper and lower molars of D. mexicanus, and molars that were referred to as Rhynchotherium (?). The research in the north part of Nay 1, a new location Nay 3 El Trapiche, was discovered in the rancheria of the same name. Preliminary results are a collection of upper molars of D. mexicanus and fragments of the upper molar of Teleoceras, besides fragments of proboscideans. The age of this fauna has been referred to late Hemphillian, by analysis of the volcanic ash collected in the town of El Trapiche was analyzed by the method of U/Pb and gave an age of 5.50 ± 0.3 My.

All the skulls collected in the early-late and late Hemphillian faunas in central Mexico present a wide range of variability in the DPOF. The skull from Juchipila considered the oldest record of D. mexicanus in central Mexico, was compared with a mold 41478-1 of D. leidyanus housed in the collection of the University of Texas at Austin. The main differences in D. leidyanus cast, the dorsal (DPOF) and malar fossa are well developed. In addition, the protocones are slightly inclined toward the lingual edge.

In the Mexican specimen, the protocone is larger and horizontal to the maxillary lingual border. This is an important characteristic that eliminates the similarity between them (Matthew and Stirton, 1930; Azzaroli, 1988). The comparison with illustrations of D. interpolatus (Matthew and Stirton, 1930), also demonstrate important differences because the DPOF is always well developed and bounded at its edges, which does not occur in the skull of Juchipila which has smooth edges and is very shallow. Also in D. interpolatus the malar fossa is constant and absent in the skull of Juchipila and all the Mexican skulls. Regarding the curvature of the molars, it is difficult to determine this characteristic in the Juchipila skull because the molars are included in the maxilla. However, the isolated teeth of MPGJ 4067 Zac Juch 47 M12 from El Resbalón, are short and curved, the protocone is rounded, small and slightly oriented to the lingual edge. That reminds us of D. interpolatus, but this does not always occur in the molars of Juchipila. Most of the teeth from Juchipila have the anterior border developing a small heel, and the lingual border is flat and horizontal.

The skull from Juchipila was compared with the description of the skull of D. mexicanus collected in the Rhino level of Rancho El Ocote locality GTO 2B, (MacFadden and Carranza-Castañeda, 2002). Some similarities are evident in both specimens.

The nasal notch is in the same position, posterior to P2; the infraorbital foramen is 68 mm above P4. The malar fossa is absent, and the malar crest is strong and ends between P4-M1. The biggest difference is the DPOF. In the Guanajuato specimen it is pocketed posteriorly, deeper and the edges are well defined and is 80-mm anterior to the orbit. The specimen from Juchipila has only a faint depression 58 mm in front the orbit, and is shallow, with poorly defined borders. However, when compared the facial region of the Tecolotlán skull, the fossa is like the Rancho El Ocote specimen which is well delimited posteriorly, deep, and the edges are well defined.

The skulls differ in the size of the bar separating the orbit of the DPOF margin. The Juchipila skull is ∼58 mm, the specimen from Tecolotlán it is 61 mm, and the Rancho el Ocote is ∼80 mm. The skull from the GTO 56 Rinconada area has differences that distinguish it from the other skulls, the DPOF, is a slight depression with soft edges. In addition, the absence of the hypocone and hypoconal grooves possibly represent a primitive form of Equus (Figure 12).

The skull MPGJ 103 from GTO 75 Arroyo de Emilio, not present evidence of DPOF, as the structures and size suggest a transitional form between Dinohippus and a primitive Equus. However, the upper volcanic ash 4.54 ± 15 My and the associated fauna including Dinohippus mexicanus, N. eurystyle, and N. aztecus with the jaw MPGJ 857 collected in the lower layer, dated 5.32 ± 0.14 My; these ages undoubtedly encloses all the fauna in late Hemphillian age. Also, the C. sanmiguelensis specimen was collected in the same stratigraphic level (Wang and Carranza-Castañeda, 2008).

The comparison between the rows of teeth P2/p2 and M3/m3, shows a similarity in size. When comparing the P2-M3 row in the IGM 7596 from Rancho El Ocote fauna is 157.9 mm, the P2 with a prominent anterostyle. The same measurement in the Juchipila specimen is 150∼, though the anterostyle of P2 is broken. In the skull MPGJ 856 from Rinconada area, the length P2-M3 is 150 mm. The similarity in size of the three specimens is remarkable. The most important differences are the pronounced anterostyle of Rancho el Ocote, and the presence of P1 in the Rinconada maxillary that is not present in the other specimens.

Regarding the protocone, a great variability is also observed in the shape and size of the upper molars. The most noticeable difference is observed when comparing the protocones of the Rancho el Ocote skull where the anterior border is round or very small, the heel is almost non-existent, like the Rinconada specimen. The specimen from Tecolotlán MPGJ 1967 is a young individual, and some differences are observed. The anterior border of the protocone of M1 is rounded, in M2 which is incompletely erupted, the anterior edge is elongated and sharp forming a small heel. The posterior part is elongated and pointed. These variations can be explained because the skull belongs to a young individual. On the contrary, the dentition from Juchipila skull, referred to the early late Hemphillian age, the protocones should be more related to the shape of D. interpolatus and D. leidyanus, with its axis slightly inclined to the lingual border, and the anterior edge without heel. However, in the skull of Juchipila, the shape of the protocones is elongated with a horizontal and concave lingual border and with a prominent and unexpected heel. Nevertheless, the specimen MPGJ 4067 P4-M1 R and L of the locality Zac Juch 47 El Resbalón of the same age, the protocones are small, lobular without vestiges of a heel and are different from those molars of the Mixtón teeth. These differences show the wide variability that exists in the protocone in individuals of the same age and stratigraphic level.

In comparison the Rancho El Ocote skull of late Hemphillian age, the protocones of the maxilla do not present a heel although the shape is larger. However, strong differences with the specimen MPGJ 131 at locality GTO 75 Arroyo de Emilio, collected in the upper part of the stratigraphic sequence of the Coecillos area were present. These teeth have elongated protocones with concave lingual border. The shape is like a wooden shoe and is remarkable in the R and L M1, but the anterior side is oval without a heel.

The curvature is another important characteristic. The specimen MPGJ 5112 of GTO 2, site F, is larger, in comparison to MPGJ 131 from late Hemphillian of Arroyo Emilio. The contrast between them in curvature and size is outstanding. The specimens MPGJ 5112 M1 site F, is high crowned, the axis Ap 30.84 and Tr 29.12 mm, has been compared with IGCU 5652 M1 referred to Equus, an old individual collected in GTO 39 Tlaloc, early Blancan. The measurements Ap 27.56 mm, TR 30.12, show similarity between them.

The Blancan specimen MPGJ 3987 M1 referred to E. simplicidens that correspond to a young individual, collected in the locality JalTeco 7 Las Gravas M1 referred to Equus, share some features with IGCU 11503 of Rancho El Ocote: the curvature and height crown are similar, both has elongated protocone with an evident triangular heel and concave lingual margin.

The specimens MPGJ 5126 P4 from GTO 2B-F, with minimum wear, the molar is straight and long like IGCU 11503 M1, when compared with MPGJ 4067 P4-M1 R and L from the locality Zac Juch 47, that have lobular and small protocone without vestiges of a heel. It is evident that there is great diversity of shape and size in the protocone of D. mexicanus in the faunas of central Mexico, even in young and old specimens of the same locality and stratigraphic level.

Lower Teeth. The jaws, maxillary and isolated teeth, show wide range of variability, but there are no differences from that already mentioned in previous works (Lance, 1950; Quinn, 1955; MacFadden, 1984, 2006, 2016; Carranza-Castañeda, 1992). However, several differences are observed. The metaconid and metastylid are lobular in molars and in premolars, the metastylid is smaller and often the lingual margin is pointed. The entoconid is lobular, the hypoconulid short although it is not constant or very reduced in the jaw MPGJ 857 Arroyo de Emilio and is absent in MPGJ 5136 of Rinconada. The linguaflexid in the MPGJ 5136 from Rinconada is open and shallow, however, the jaw IGCU 6963 that corresponds to a young individual, the linguaflexid is also deep. The specimen MPGJ 857 Arroyo Emilio has a V shape and is more closed, like the mandibula MPGJ 3946 of GTO 2 B level Rhino. The ectoflexid is deep in molars, the protoconid and hypoconid in the jaws GTO 2A MPGJ 5125 and MPGJ 5132 from GTO 43 Rinconada and jaw 310 from GTO 30 Coecillos are horizontal and flat. However, in the specimen MPGJ 5136 and 5132 from Rinconada, the protoconid and hypoconid are horizontal and flat in the premolars. Some of the jaws that are described retain part of the symphysis and the incisors. The differences between them are not significant in relation to their size.

Some of the incisors have an infundibulum limited by enamel and filled with cement, however, others only present a small lake without enamel. The disposition of the incisors is arranged in a circular or flat row, possibly this difference is related to grazing or a mixed feeding (Figure 13 and Supplementary Table 2).

The outstanding result of the research over the material in Rancho El Ocote, without a doubt, is the specimens that have been collected in the GTO 2 sites F and D. These differ in their measurements of the anteroposterior (Ap) and transverse (Tr) axis which are clearly distinguished from those collected in the stratigraphic sequence of the Rhino and White levels. All the teeth from GTO 2 Site F and D, the hypoconid and protoconid have their labial edges more flattened and straight. The ectoflexid is narrow but very deep in the molars, almost touching the inner edge of the linguaflexid and less deep in the premolars. The linguaflexid is wide and shallow. Another difference is the metastylid that tends to be small and more pointed at its lingual edge. In addition, Pli caballinid is frequent but very small.

All the measurements of the teeth in the jaws described in this paper were plotted on a graph. It is remarkable that all the material from GTO 2 site D-F are separated from the typical Hemphillian teeth, and suggest a transitional group between Dinohippus and primitive Equus, as seen in the Figure 6, 14.

(1) Dinohippus mexicanus, has a wide geographical distribution in the central region of Mexico between the parallels 19° and 22°, only the most important localities due to the number of specimens, presence of skull, jaws and isolated upper and lower teeth by the variability they present in their structures and the stratigraphic position have been mentioned in this work. The main localities: early-late Hemphillian of Juchipila Zacatecas, late Hemphillian of Arroyo de Emilio Rinconada, Coecillos and Rhino layer of Rancho El Ocote; and latest Hemphillian in the Blanco layer, at Rancho El Ocote, basin of San Miguel de Allende, Guanajuato, late Hemphillian-early Blancan of Tecolotlán basin, Jalisco. The associate fauna always with stratigraphic indexes of late Hemphillian age and confirmed by radiometric ages.

(2) In D. mexicanus the morphology of the fossa dorsal (DPOF), was considered an important character in the diagnosis and systematics of the Pliohippus group of the Hemphillian age. However, the new records of skull collected in the faunas of early-late Hemphillian and late Hemphillian of the central region of Mexico, have shown that this structure has a wide variation in shape and size.

(3) The greatest differences observed in the skulls are more evident in the MPGJ 103 specimen of Arroyo de Emilio older individual, which has the largest facial region without evidence of DPOF, and the hypocone is not clear because the natural wear. These differences are comparable with the skull MPGJ 856 old individual from Rinconada, where the DPOF is reduced to a narrow and shallow groove and the hypocone is absent. It is possible to argue that these variants are determined by the ontogenetic age and natural wear. However, it is remarkable that these skulls were collected in deposits of 4.5 ± 15 My, (MPGJ 103) and in 4.4 ± 0.3 My (MPGJ 856), and in both the affinity with Equus is noticeable. However, the associated fauna of these two compared specimens, undoubtedly contains stratigraphic indices of the late Hemphillian age (Supplementary Table 1).

(4) The sample of skulls referred to Dinohippus mentioned in this work is the largest that has been described the differences observed in the DPOF induce to reconsider its value in the diagnosis of Dinohippus. The malar fossa is never present or insinuated in the facial region in the sample of skulls.

(5) The height of the crown in upper and lower molars is greater than 82.34 mm (MPGJ 4040, GTO 2B), 76.65 (MPGJ 5143, GTO 43) and in lower molars is 82.73 mm (MPGJ 5116 GTO 2 site F), these values are higher than the Yepomera samples and those mentioned for the fauna of Bone Valley, Florida (MacFadden, 2016). Always the cement covers the crown.

(6) The protocone has a wide variation of shape and size, even in molars collected at the same stratigraphic level vary from rounded to elongated although always the lingual edge is horizontal to the jaw. The anterior edge very often forms a heel that is not constant in all molars inclusive from the same locality, such as the molars of the skull of Juchipila and the molars MPGJ 4607 (ZJ 47).

(7) The MPGJ 856 skull from the Rinconada area, with shallow DPOF, very simple fossettes and absence of hypocone are features that make it distinctive to another skull. Also, the jaws: MPGJ 5136 P2-M3, MPGJ 5132 P3-M2, and MPGJ 6963 P2-M3 of the locality GTO 43 Rinconada, with the wide and shallow linguaflexid, protoconid and hypoconid are more flattened and straight; are features distinguish them from the mandibles of El Ocote and Coecillos. These differences suggest that the skull MPGJ 856 and the mandibles of GTO 43 Rinconada have significant affinity with Equus.

(8) The upper and lower molars collected in the Rhino and Blanco layers, have differences in size compared to those collected in the GTO 2 site D and F, on their AP and Tr axes and the height of the crown, indicating a “Group” with transitional characters between Dinohippus and a primitive Equus.

(9) The Graphs 6 and 14 have been plotted the measurements Ap, Tr and crown height of the upper and lower molars that have been discussed in this work, including the upper and lower molars of the sites GTO 2 site D and F. The result of this analysis suggests that D. mexicanus makes up a distinctive group closer (transitional) to Equus.

(10) In this work it has been shown the variability of the D. mexicanus collected in the Hemphillian localities from Zacatecas, Guanajuato and Jalisco localities: the curvature of the upper molars, the height of the crown in upper and lower molars besides the complexity of the DPOF and the variability of characters of the teeth. All these variations are evidences of the transition from D. mexicanus to Equus, in the faunas of the central region of Mexico in the boundaries Hemphillian-Blancan of Rancho El Ocote and Tecolotlán faunas.

Also, this has been observed too in the fauna of Yepomera (late Hemphillian) and La Concha (early Blancan) in the state of Chihuahua, where some lower molars cannot be reliably referred to Dinohippus (Lindsay, 1984).

(11) The specimens referred to D. mexicanus collected in the boundaries of late Hemphillian-early Blancan age, are the last records of D. mexicanus in Mexican faunas.

OC wrote the manuscript, conceived, interpreted, and collected the data for the graphics, and it is the main collector of the fossil material.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.