Edited by: Leonardo Dos Santos Avilla, Universidade Federal do Rio de Janeiro, Brazil
Reviewed by: Spencer G. Lucas, New Mexico Museum of Natural History and Science, United States; Federico Agnolin, Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Argentina
This article was submitted to Paleontology, a section of the journal Frontiers in Earth Science
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In the advances of the research of the sedimentary basins from central Mexico, it is significant the discovery of the first records of equids of Clarendonian and early Hemphillian age that are mentioned. The specimens were collected in southwest deposits of the San Miguel de Allende basin, the ages are verified by radiometric dates. The largest collection of
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
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 (
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
On the map are the locations explained in the paper.
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
The terminology used in the description is from
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.
The continuity of the investigations in southern San Miguel de Allende basin, have demonstrated new records of equids that correspond to Clarendonian and early Hemphillian ages, being the first time that in Mexico there are mentioned equids of unequivocal Clarendonian age, whose age is not inferred, but discovered a few meters below the andesites that have been dated.
In the city of Comonfort, 25 km south of San Miguel de Allende, two new localities have been discovered, the locality with the older fauna is Locality GTO 92 Empalme Escobedo, located 5 km south of the town of the same name. The fossils were collected in a quarry of materials for road construction. The thickness of the sedimentary sequence is greater than 50 m and it is composed of different strata of igneous rocks and gravels at the base, covered by lacustrine sediments and layers of clay with sands. The fossils were collected in these layers. This sequence has a thickness of 5 m, is composed of clay and clays with gravel only 6 m above are the strata of andesites that cover the entire sequence.
The age of the sedimentary sequence is determined by the radiometric dates that were made in the andesites located 6 m above, by the method of 40Ar/39Ar, which produced the result of 10.6 ± 0.3 My (
The fossil material are two teeth, MPGJ 3031 assigned to
The molar MPGJ 3031, M1, is hypsodont and curved, the protocone is small rounded with no heel, and is joined to the protoselene, by the isthmus that is constricted. The prefossette has multiple plications and a well-developed prefossette loop; the postfossette with the anterior border very simple and large plicaballin, with elongated hypocone.
MPGJ 3032 M1 fragment of upper molar, by the wear corresponds to an old adult, the main character for its assignment is the isolated and rounded protocone and simple fossettes without plications.
The second locality, GTO 91 Palmillas, is near the village of the same name. The area is very small; however, the fossils were collected in the deposits of the La Nopalera Formation (
The associated fauna includes remains of the antilocaprid
Upper molars: MPGJ 1713 P2, MPGJ 1706 P34, MPGJ 1707 M2, MPGJ 1705 M3, MPGJ 712 M3, MPGJ 1708 M1, MPGJ 1709 M12, MPGJ 1711 M1, and MPGJ 1710 M3. Lower molar MPGJ 1824 m2. The main characters that justify the assignment to
The comparable
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,
The lake sediments fill the basin and a few sites have fluvial deposits. The study began north of the city of Colotlán to its limit to the south in the village of Tepechitlán where some metapodial fragments were collected and that is at the end of the basin.
The most important is the locality Zac 9 Sanidad Animal (
It is the most relevant area in the state of Zacatecas by the stratigraphy and the diversity and number of fossils collected with an extent of 165 and 15 km in width in the NNE direction. The first works on geology were made by students from the University of New Orleans,
The most important localities are in the central part of the basin, between the Tabasco city in the north and Moyahua in the south. The greatest extent is found in the middle part at the localities: El Mixtón and La Perdida, Cofradía and the Pitahaya area; and the Juchipila area with the localities El Resbalón, La Copa y El Epazote. In the northern part, the Tabasco area, the localities are La Escalera, Patos y Madera and the Wang locality. The associated fauna that has been described in previous works has been referred to the early-late Hemphillian age, was collected and supported by the radiometric date obtained in the analysis of volcanic ashes by the method of U/Pb which gave results of 6.95 ± 0.27 My in the lower and 5.59 ± 011 in the upper ash, ages that bracket the fauna in the early-late Hemphillian age (NALMA) (
The diversity of equids presented in these localities is remarkable.
The genus
The only jaw of
MPGJ 1982
At the locality Zac Tab 34, Wang, located 45 km north of the Mixtón, a mandible referred to
MPGJ 1683 of Zac Tab 34 Wang Location, ventral view of the jaw, symphysis and incisors are complete, the incisors have wide infundibulum, the linguaflexid is wide, the metacónid and metastilid are lobular, the lingual edge of the protoconid and hypoconid is rounded.
Zac Juch 30 Mixtón. MPGJ 1982, partial cranium with right region facial well preserved and P3-M3 and fragment of P2. Zac Juch 47 El Resbalón MPGJ 4067 P4-M1, MPGJ 4095 M1, and MPGJ 4067 P4. Lower jaw, Zac Juch 34 Wang MPGJ 1683 belongs to young individual, with p2-m3 and i1-3, both sides and complete symphysis.
The specimen MPGJ 1682 partial skull collected in the locality Zac Juch 30 El Mixtón.
The jaw MPGJ 1683 from the locality Zac Tab 34 Wang, State of Zacatecas, México; early-late Hemphillian age. Collectors, Xiaoming Wang, Jorge Aranda-Gómez and the author.
The skull (MPGJ 1682) on the left side is slightly squashed, the facial region on the right side is complete without deformation. The occipital region is also incomplete, although some of the structures are observed. The nasal notch is above the posterior part of P2, the row of P3-M3 has a length of 126 mm. The infraorbital foramen is located 68 mm above the alveolus of P4. The malar crest is remarkable and strong, it ends between P4 and the anterior part of M1. The malar fossa is absent. The nasomaxillary fossa (DPOF) is shallow, is 58 mm distant from the orbit, is a slight depression not pocketed posteriorly and the anterior end is above the M2. The fossa has poorly defined and rounded borders, the wider part is 38 mm and the infraorbital foramen is included. The distance from the orbit to the DPOF is 57 mm. The height from the alveolus of M2 to the orbit is 114 mm.
The premolar P2 is preserved. It has a prominent parastyle and mesostyle, the enamel is moderately thick and the fossettes are almost without plication by wear.
On the P3 (Ap 26.90 mm and Tr 27.18 mm), the prefossette is very worn and the postfossette is without plications. The hypocone is broad and the hypoconal groove is of moderate depth. They persist in the whole dentition but only reduce their size. The preprotoconal groove is constricted, the protocone is broad and moderately long, the anterior part forms a rounded heel, and the lingual border is flat and parallel to the lingual part of the alveoli. P4 (Ap 24.45 mm and Tr 25 mm), presents the parastyle and mesostyle that are of almost equal widths and flat. The protocone has a remarkable and blunt heel, the posterior part is long and rounded and the lingual border is concave in a “wooden shoe shape.”
M1 (Ap 22.95 mm and Tr 25.44 mm), The fossettes are very worn, the protocone has the “wooden-shoe shape,” posteriorly it is elongated and wide, the anterior with a notable heel, and the lingual border is concave. M2 (Ap 23.7 and Tr 23.06), it has a trapezoidal shape because the parastyle side is longer. The protocone is broad and elongated, with the posterior part triangular, and the anterior part forms an evident heel of rounded shape. On M3 (Ap 24.94 mm and Tr 20.46 mm) the most notable difference of the molar is reduction of the hypocone and hypoconal groove. The protocone in the anterior part forms a prominent heel, posteriorly is elongated and the lingual border is horizontal (
Lower jaw. The specimen MPGJ 1683 preserves the right side, is broken on the horizontal edge, and only has part of the ascendant border of the coronoid process. The symphysis is complete with R and L i1-3, all have a wide infundibulum filled with cement. The canine is very small, which suggests a female. The pre-canine diastema is 8.13 mm, and the post-canine diastema is 74.45 mm. The maximum width between the L I3 and R 13 is 57.16 mm. The dentition p2-m3 is 147.5 length, the Ap of p3-p4 is 54.7 mm and Tr is 18.8; the Ap m1-m2 46.8 and Tr 16.3. The protoconid and hypoconid are rounded in molars and premolars. The metaconid and metastylid in the premolars are rounded almost with similar size and separated by a wide linguaflexid. In the molars, the metaconid is rounded, but the metastylid is smaller and slightly sharp posteriorly. The ectoflexid in the molars is deeper to almost touch the enamel-edge of the linguaflexid; in premolars, it does not penetrate the isthmus. The assigned age is determined associate fauna and analysis of the upper ash the result 5.9 My placed the jaw to early late Hemphillian.
According to the stratigraphic age the skull of Juchipila, early late Hemphillian, a protocone like
The first reference to
In the new investigations in Rancho El Ocote and other Hemphillian localities, the material of
The first mention of horses from Rancho El Ocote was made by
The results of new field work in this locality and studies of taphonomy have demonstrated two stratigraphic levels occur, each one with its own lithology and faunal content that corresponds to the late (Hh3) and latest (Hh4) Hemphillian; covered in erosional unconformity by deposits that contain a fauna of early Blancan age (
The lower layer is named the Rhino layer (GTO2B), because its contains:
The Rhino level is covered by the Blanco layer (GTO2 A), the lithology is a fine sand deposit and volcanic ash lenses, which contain a special distribution of fauna. The
Above the Blanco layer, there are two-three meters of early Blancan sediments where
In the upper part of the Blanco level, there are small deposits of clay containing upper and lower molars that have the axes Ap and Tr larger than those collected in the Rhino level. The fossils from these sites have been cataloged as GTO 2 site D and GTO 2 site F (
Map of Arroyo La Carreta, where the fossil localities of Rancho El Ocote are located. Note that the GTO 2 site D and F are on top of Blanco layer, the stratigraphic column show the sites in the Blanco layer (Latest Hemphillian).
Skull IGM 7596, skull with R I3, P2-M3, L I2-I3, P2-M3.
Lower Jaw. MPGJ 3739 P2-M3 GTO 2A, MPGJ 3872 L p2-p4 GTO 2b, Rhino layer, MPGJ 3946 R p2-m3, symphysis Ri 1-3 L 2-3, MPGJ 5125 GTO 2 A, Blanco layer, with p2-m1, canine and l 2-3 incisive the R I 2-3 broken, MPGJ 3018 R i1-3 Li 1-3.
Upper Molars, GT0 2A Blanco Layer: MPGJ 5131 M3, MPGJ 5151 M1, MPGJ 5132 P4, MPGJ 5153 P34. GTO 2B Rhino layer; MPGJ 510 M3, MPGJ 650 M1, MPGJ 2056 M1, MPGJ 2075 P4, MPGJ 2915 M1, MPGJ 3793 P4, MPGJ 3822 GTO M2, MPGJ 3841 P4, MPGJ 5128 P4, M3, MPGJ 5131M3, MPGJ 5135 M1, MPGJ 11500.
UPPER MOLARS GTO 2 site D, MPGJ 2825 p4, MPGJ 5111 P34, MPGJ 5113 P3, MPGJ 5126 P4, MPGJ 3947 M1.
UPPER MOLARS GTO 2 site F: MPGJ 4040 M1, MPGJ 5146 P4IGCU 5180 M2, IGCU 11503 P4, IGCU 11504 M1.
LOWER MOLARS GTO 2A; site D: MPGJ 5109 m3; MPGJ 2825 p4.
LOWER MOLARS GTO 2 SITE F, MPGJ 495 p3, MPGJ 2285 p34, MPGJ 2285 p43, MPGJ 3548 p3; IGCU 3858 p3; MPGJ 5116 p4, MPGJ 5117 p34, MPGJ 5118 p4, MPGJ 5119 m2, MPGJ 5120 p34, MPGJ 5129 p4, MPGJ 5130 p2, MPGJ 5134 p4, MPGJ 5144 p3, MPGJ 5145 p4; MPGJ 5147 p4, MPGJ 5149 m2, MPGJ 5150 m2.
The skull has been described and discussed extensively by
Lower Jaw referred. The jaw MPGJ 3739 from GTO 2 A, collected from the upper part of Rhino layer, conserves the left side, the horizontal border is complete. The maximum depth is at the beginning of the angle of the mandible below the m3 with 114 mm. The Ap length of p2-m3 is 166 mm. To determine the relationship between jaw depth and the crown height at p4 and m2, an X-Ray of the jaw was made. It is remarkable that there is no relationship between jaw depth (m2) and the height of the crown, which justifies the larger size of the molars of the GTO 2 D and F sites.
The jaw MPGJ 3946 collected in GTO 2 B from the Rhino layer, preserves the symphysis, the canine is partially broken, the incisors form an almost curved row, R i1-3, and L i2-3. The i2, is wider. The infundibulum limited by enamel and filled with cement. The post-canine diastema is 89.74 mm, the pre-canine diastema is ∼7 mm. The length R i2- L i2 is 43.70 mm; the length Ap of p2-m3 is 156 mm, the premolars and molars are straight and hypsodont, p2 67 mm long, p4 is 76 mm high, m2 47.4 mm. The metaconid and metastylid are rounded almost the same size. All the teeth with a remarkable cover of cement, protoconid and hypoconid roughly flat.
MPGJ 5125 is a fragment of left jaw collected in the GTO 2 A White layer, retains a robust symphysis with strong canines that are broken and the incisors with small infundibula without enamel. The jaw has its maximum depth of 101.3 mm, from the linguaflexid of p4 to the horizontal edge. The metaconid is lobular, and metastylid slightly pointed toward the lingual margin, V-shaped linguaflexid in molars, the ectoflexid is deep into the isthmus, the protoconid and hypoconid with well-defined flat edge in the labial border.
MPGJ 3872, is a fragment of left jaw with p2-p4. The most significant features are the metastylid pointed and the slightly rounded protoconid and hypoconid.
The jaw MPGJ 3946 from GTO 2B is very deep, the maximum depth is in p4 and m1. To determine the length of these molars
MPGJ 3739 Jaw fragment from GTO 2A collected in the Blanco layer. The jaw is very deep with full dentition. The X-Ray was taken to know the HC of p4-m1 and to determine its real size in the alveolus, to compare with the height of the crown of the molars of the F site of GTO2.
In the same stratigraphic sequence, at the Rhino and Blanco layers, numerous isolated molars have been collected. The analysis show similarities with the description mentions in lower molars that have been described in the jaws. However, at the top of the stratigraphic sequence, in clay deposits named GTO 2 site D and GTO 2 site F, lower and upper molars have been collected. The preservation is deficient, especially in length of the molars, however, the differences in size of the anteroposterior and transverse axis on the occlusal surface is remarkable.
The metaconid and metastylid are rounded and almost of the same size, the entoconid is round, the linguaflexid is open in V shape and wider in molars. Specimen MPGJ 5134 presents a pli caballinid, the protoconid and hypoconid large and flat in the lingual edge. The upper molars also differ from those collected from the Rhino layer: the crown is higher and straight, and the protocone developed. The Ap and Tr measurements of the p3-m2 of the mentioned jaws from GTO 2A and B have been plotted versus the molars of GTO 2 site D-F. It is remarkable the separation they have with the specimens collected in the sites GTO 2 D-F.
The MPGJ 5145 p4 from GTO 2 site F, was compared with MPGJ 5136 p4 of
The locality is situated 12 km southeast of Rancho El Ocote. The basal layer GTO 30, is a clay layer covered by ashy material, dated 5.32 ± 0.34 My. Above there is a sequence of clays of yellow color that contain
At the top of these deposits, the ash layer that covers the sequence was dated by the U/Pb method; the result is 4.54 ± 1.5 My. This means that the records of
GTO 75 Arroyo de Emilio MPGJ 103, skull fragment with palate and pat of the facial area. MPGJ 131 R P4-M3 and L P4-M1 from GTO 75 Same individual. MPGJ 857 R jaw p2-m3, L i1-3, Ri, 2-3. MPGJ 729 M1, MPGJ 1559 P4, MPGJ 5439. P4.
Lower teeth. GTO 30: MPGJ 310 L p2-m3, IGCU 5201 p2-m1 -m3, c, L i1-3, R i2-3. Isolated lower teeth, GTO 30 5306 p4, IGCU 5308 m2, IGCU 5593 m1, GTO 30. Upper teeth: IGCU 5437 P4, IGCU 5593 M1MPGJ 5438 P3, MPGJ 5578 M1, IGCU 5580 P4.
In deposits of the Coecillos area. GTO 30 Coecillos and GTO 75, Arroyo de Emilio localities, late Hemphillian age.
The skull MPGJ 103 from GTO 75 Arroyo de Emilio of the Coecillos area, consists of a fragment that preserves the complete palate and molars of both sides, the maximum length P3-M3 is 121 mm. The maxillary in the right side is the best preserved part including the wall of the orbit; however, there is no evidences of DPOF. The malar crest is well developed and ends in the anterior part of M1; there is no evidence of a malar fossa. The palate is well preserved in the ventral part, the posterior nasal orifices, the palatine bones, vomer, the anterior palatine holes and the palatal grooves. The maximum length Ap of the choana is 108 mm, the maximum transverse on the back of M3 is 49 mm. The maximum width of the palate between the lingual edge of M3 is 72 mm, maximum width between lingual edge of the protocone of P4 is 62 mm.
This skull MPGJ 103, is an old individual that does not present evidence of DPOF. According to the stratigraphic position almost at the top of the Coecillos sequence, the deposits where it was collected are only one meter below the ash that was analyzed and gave the result of 4.5 ± 1.5 My. It is possible to argue that corresponds to a primitive
Located almost in middle of the San Miguel Allende basin, limited by the Rancho Viejo hills in the east and the Tequisquiapan in the west. The fossiliferous deposits are 7 m thick.
The basal layer is unconsolidated sandy clay, covered by a lacustrine deposit about 1–2 m thick. This layer represents the bottom of the ancient lake, around 25 km long. The lithology is a sandy clay layer cemented by calcite and silica with a lens of volcanic ash and paleochannels of fine sand and gravel. All the fossils have been collected in this layer. A sandy clay layer covers the lake, and the top of the sequence is overlain by the unconformity of Hemphillian-Blancan age with a thickness of 1–2 m.
The equids collected in these sediments,
The faunal association is a typical of the late Hemphillian fauna Hh3. The result of radiometric analysis of the volcanic ash by the method of fission track of zircons is 4.4 ± 0.3 confirming this age (
GTO 56 MPGJ 856 of skull fragment with two Maxillaries with P2-M3, corresponds to an old adult individual by the wear of the molars. Preorbital dorsal fossa (DPOF), is in the nasal and maxillary bones.
The locality GTO 56, late Hemphillian age, is located about 700 m south of the Rinconada Locality.
Locality GTO 43 Rinconada upper teeth: MPGJ 5137 M1, MPGJ 5138 P4, MPGJ 5139 P4, MPGJ 5140 P4, MPGJ 5141 P4, MPGJ 5142 M1, IGCU 8180 m2. IGCU 8879 P3, IGCU 9590 P4, MPGJ 5143 P4-M1, MPGJ 3764 flat Incisor row i1-3 R and L.
MPGJ 856, two maxillaries with P2-M3, and fragment of nasal-maxillary bones with a dorsal fossa (DPOF), not pocketed posteriorly, like a shallow wide groove, the edges are smooth and poorly defined, with a maximum length of 60 mm.
The right maxillary is the most complete. The crest of the malar bone ends 25 mm above the anterior part of M1. The dentition is complete with P1-M3, (P2-M3, 159 mm, P1 is not included). The teeth are simple without folding in the fossettes, the hypocone and the groove are not present, the protocone is round and small in the premolars and slightly longer in M2-M3. On M1 it is worn away (
Lower Dentition. GTO 43 Rinconada locality, MPGJ 5132 jaw fragment Rp3-m2, MPGJ 5133 Rp2-m2 jaw fragment, 5136 p2-m3, MPGJ 10071 p4-m3. Isolated molars, GTO 43 Rinconada locality IGCU 8180 m2.
The jaws of this locality present some characters that differentiate them from the jaws collected in Rancho El Ocote. Among the important differences is the depth of the mandibular ramus. The specimen MPGJ 5136 of GTO 43, the maximum depth between p4-m1 is 72.43 mm, and on MPGJ 5132 with only p2-m2, the depth between p4-m1 is 75.56. In contrast, the mandible MPGJ 3749, of GTO 2A, the depth is 11.41 cm, and in MPGJ 3946 it is ∼101.48 cm. Besides the linguaflexid in the jaws, MPGJ 5132, IGCU 5133 and MPGJ 5136 is wide and shallow. The metaconid is lobular and the metastylid is rounded with on acute and small lingual border. The entoconid is lobular with an open hypoconulid. In IGCU 6963, p3 and p4, have a small pli caballinid. The ectoflexid is deeper in m1 and m2. The protoconid and hypoconid are flat in premolars and relatively rounded in molars. On MPGJ 5136, MPGJ 5132, and MPGJ 5133, lower jaws, the protoconid and hypoconid are variable from flat to slightly rounded, and the linguaflexid is shallow and open (
Jaws from GTO 43 Rinconada locality.
The upper and lower molars were widely discussed and compared between the localities of San Miguel de Allende and the material of the Yepomera fauna that is housed in LACM, reaching the conclusion that they correspond to the same species. However, in this work it has been observed that in the lower molars, the linguaflexid, protoconid and hypoconid are different and that separates them from the specimens collected in the Rhino level of Rancho el Ocote and from the specimens at the base of the sequence of Coecillos (
This basin is 90 km from the city of Guadalajara and is filled with fluviolacustrine deposits contain abundant North American Mammalian faunas, which represent the late Hemphillian age. The stratigraphic sequence has a thickness of 60 m and several sites have been mentioned where the late Hemphillian material has been collected. However, the stratigraphic sequence has been identified in two areas of main importance: Santa María sequence at the base and San José deposits that cover them. Resting in erosional unconformity are found the San Buenaventura deposits, thought to be of Blancan age (
The mammalian fauna between the two areas, Santa María and San José, is almost the same.
The age of the fauna has been determined by radiometric dating of the ashes that are interbedded with the fossiliferous strata. The ash found in the upper part of the San José area, was dated by the method of 40Ar/39Ar, and gave an age of 4.95 ± 0.02 My and (
In erosional discordance are the deposits of the San Buenaventura sequence which has a thickness of ∼30 m of gravels and paleochannels of sand and clays. The contact with the deposits of San Jose is not exposed. The age of the sequence of San Buenaventura has been referred to the Blancan (
This sequence is important in the transition
MPGJ 1967 JT 35 late Hemphillian age skull fragment of young individual, with region facial and brain case of the right side well preserved. The M1-M2 first stages of wear, P4-M3 in the alveolus without wear.
Late Hemphillian. Isolated teeth: Jal Teco 1, MPGJ M1 Fragment. JalTeco 48: MPGJ 4044 M1, MPGJ 4045 M1, MPGJ 4046 P4.
Jal Teco 37, MPGJ 5025 M2, MPGJ 5029 p4, JalTeco 26 MPGJ 3541 M1; JalTeco 12 MPGJ 5001 M3, MPGJ 5002 P4. MPGJ 5003 p4, MPGJ 5004 m3. JalTeco 33 MPGJ 5027 p4, MPGJ 5028 m2, MPGJ 5026 M1. JalTeco 35, MPGJ 5102 M1. JalTeco 47 MPGJ 5072 m1-2. MPGJ 5069 m2.
Isolated Teeth. Jal Teco 7 Las Gravas locality Blancan age: Upper Teeth: MPGJ 3227 M1, MPGJ 3698 M3, MPGJ 3699 M12, MPGJ 3987 M1, MPGJ 4021 M1, MPGJ 5057 M3, MPGJ 5022 M1, MPGJ 5058 P4, MPGJ 5059 M3.
Lower Teeth, MPGJ 3246 p4, MPGJ 3226 p34, MPGJ 3985 m3, MPGJ 5052 m2, MPGJ 5049 m1, MPGJ 5050 m3, MPGJ 5051 m1, MPGJ 5052 m1, MPGJ 5053 m2, MPGJ 5054 p4, MPGJ 5136 p4.
MPGJ 1967 skull fragment, JalTeco 35 Locality, late Hemphillian age collector Mr. Harley Garbani and the author.
Specimen MPGJ 1967 is a skull fragment of a young individual that conserves only the left facial part with the frontal bones and part of the parietals. In the left facial part, in the anterior of the nasal and maxillary bones a well-defined deep dorsal fossa with the delimited edges, pocketed posteriorly with delimited edges; the fossa is wider above the P4. The orbit is complete, the zygomatic apophysis of the temporal bone, the temporal bone and the zygomatic apophysis of the malar are complete. The malar crest is prominent and ends between P4 and M1.
The dentition is reduced to M1 and M2 because P4 and M3 are erupting from the alveolus. The fossettes are simple, almost without plications, the prefossette of M1 presents a small loop, the postfossette has with small plications in the anterior edge and small pli hypostyle. The hypocone is wide with an open and deep furrow. A small and thin plicaballin is present in M1, and the protocone is elongated, without a heel (
MPGJ 1967 from JalTeco 35, left side, the facial fossa is pocketed with well-defined borders and strong malar crest.
Isolated teeth, the molar MPGJ 3987 M1 has a high crown, but the curvature is pronounced, the protocone larger the lingual border is concave with “wooden-shoe shape,” and present a small pointed heel. MPGJ 5022 M1, that correspond to mature individual, the curvature is similar but the protocone is small rounded and without heel. MPGJ 3698 M3 although is an M3 the protocone is larger and the tooth is curved like MPGJ M3 5001 late Hemphillian from locality JT 12 Hilda’s late Hemphillian locality of San Jose sequence in Tecolotlán basin.
The specimen MPGJ 5058 P3 from JalTeco 7 Ap 30.32 Tr 26.67 HC 54.25 mm in comparison with MPGJ 5141 P4 GTO 43 Ap 28.08 Tr 26.16, and HC 65.43 mm, they are similar in the structures and the size is less than 10%, however the protocone has the same shape, concave in the lingual border, developed heel, pli caballin in both and fossettes with same plications.
It is important to point out that the JalTeco 7 molars present the overlap of progressive characters, of a primitive
Lower Teeth. The specimen that have been collected are scarce and damaged, correspond to adult individuals only one is a young premolar. In general, the structures of the occlusal surface have small affinities in comparison with lower molars from the GTO 2 site F, from the Blanco layer in Rancho el Ocote. Hemphillian localities. The linguaflexid is shallow and width in specimens MPGJ 3246 p4, 5050 m2, 5049 p4, 5053 p43, and 5059 p3, all these teeth the most significant difference is the protoconid and hypoconid flat, metaconid lobular and metastylid is smaller and pointed lingually. Deep ectoflexid in molars, short not passing the isthmus in premolars, the paralophid is high until the middle of the metaconid, and some teeth presents a small pli caballinid.
La Plegaria Locality. In the southwestern region of the state of Hidalgo, southwest of the city Tepeji Del Río, several localities with fossil remains were found. The most important is La Plegaria.
The mammal fossils are at the top of the lake sediments that have a thickness greater than 100 m and are evidence of the huge lakes that existed in central Mexico during the late Miocene. The fluvial sediments are at the top of the sequence. This is an area of little extension that is covered by vegetation, however, the fossils are found in clay, sand channels and gravel.
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
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
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 (
The skull from Juchipila was compared with the description of the skull of
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
Comparison of the ventral view of the Dinohippus skulls.
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
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
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
The Blancan specimen MPGJ 3987 M1 referred to
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
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 (
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 (
The structures of the symphysis are compared,
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
All the teeth p3-m2 of the jaws are plotted in the graph, including those of GTO2 site F. Is remarkable the separation all the teeth from site F. The result suggests that the molars of GTO 2 F and D correspond to a group of individuals different from those found in the late Hemphillian localities.
(1)
(2) In
(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
(4) The sample of skulls referred to
(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 (
(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
(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
(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
(10) In this work it has been shown the variability of the
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
(11) The specimens referred to
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.
The research of the sedimentary basins in the central region of Mexico began at the Instituto de Geología and have continued with the support of the Centro de Geociencias (Universidad Nacional Autónoma de México). I thank DGAPA, Project PAPIIT IN 102817, Dr. Xiaoming Wang Curator of LACM for the facilities provided to study the Yepomera collection, Dr. Jesús Alvarado, Director of the Collection Nacional de Paleontología, housed in the Instituto de Geología, for his assistance, Dr. Spencer Lucas of the MNHNM and Dr. Federico Angolin, by the review of this manuscript and their comments, Eduardo Jimenez for his comments in the review of this work, Ing. Jesus Silva-Corona for the photographs, graphics that illustrate this work, and edition, Biologist Hilda Troncoso-A. who made a large part of the preparation, assistance during the preparation of this manuscript, and her collaboration during the field work, MVZ Gerardo Carranza-Troncoso for the X-Ray radiograph, Dr. Carlos Ortega-Obregón of the Laboratorio de Estudios Isotópicos for the determination of radiometric ages of the area of Coecillos, Ing. Cesar Alfredo Contreras-Zamora for his collaboration in the solution of the problems of computation. Dr. Mariano Cerca, Centro de Geociencias, for his comments about the Clarendonian age, and the Earth Watch volunteer for the contributions in the Mexican Megafauna and Fossils of the Sierra Madre Projects.
The Supplementary Material for this article can be found online at: