mexico, between 19º–23ºn - Instituto de Geología
Transcription
mexico, between 19º–23ºn - Instituto de Geología
45 Carranza-Castañeda, Óscar, 2006, Late Tertiary fossil localities in central Mexico, between 19°–23°N, in Carranza-Castañeda, Óscar, and Lindsay, E.H., eds., Advances in late Tertiary vertebrate paleontology in Mexico and the Great American Biotic Interchange: Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias, Publicación Especial 4, p. 45–60. Late Tertiary fossil localities in central Mexico, between 19º–23ºN Óscar Carranza-Castañeda* ABSTRACT Only 35 years ago, the continental late Tertiary biostratigraphy of Mexico was almost unknown. Most of the information regarding previous reports came from the Yepómera fauna, located in northwest Chihuahua. From central Mexico, the only known faunas were Tehuichila in the State of Hidalgo, Rancho La Goleta in the State of Michoacán and Rancho El Ocote in the State of Guanajuato, with the inconvenience that these faunas were referred only to the Pliocene age. Recent studies in the sedimentary basins of central Mexico (between latitudes 19°N and 23°N) have shown a wealth of diversity and abundance of Cenozoic vertebrates that represent the Hemphillian and the Blancan mammalian ages. Besides, our results have yielded relevant new information, revealing central Mexico as an important area for mammalian migrations during the Blancan. Key words: stratigraphy, paleontology, mammals, fossils, basins, Hemphillian, Blancan, Mexico. RESUMEN La estratigrafía continental del Terciario tardío de México, hasta los años setenta del siglo pasado, estuvo basada en la información de la fauna de Yepómera en el noroeste de Chihuahua. En el centro de México, la información sobre la estratigrafía y contenido fósil de las cuencas, fue escasa y se basó sólo en la información disponible de las faunas de Tehuichila en el estado de Hidalgo, la fauna de La Goleta en el estado de Michoacán y la fauna de Rancho El Ocote, en el estado de Guanajuato, todas referidas al Plioceno. Los trabajos recientes efectuados en las cuencas sedimentarias del centro de México, entre los paralelos 23° y 19°N, han demostrado la diversidad y abundancia de vertebrados fósiles, que están representando dos edades de Mamíferos, Henfiliano y Blancano, además de proporcionar información acerca de las migraciones de faunas hacia el centro de México, durante el Blancano. Palabras clave: estratigrafía, paleontología, mamíferos, fósiles, cuencas, Hemphilliano, Blancano, México. INTRODUCTION The continental late Tertiary stratigraphy of Mexico for many years was based mainly on the published papers of the Yepómera fauna, northwestern Chihuahua. The *Centro de Geociencias, Campus Juriquilla, Universidad Nacional Autónoma de México, Querétaro, Mexico. E-mail address: [email protected] late Tertiary stratigraphy of central Mexico was poorly known until the second half of the past century; it was biased, incomplete and based on isolated fossil findings without any stratigraphic control, which did not allow to make any correlations between the Mexican faunas. The results of the investigations carried out in the sedimentary basins of central Mexico, between 19º and 23º N, have shown that in these basins there is a large fossil diversity that will contribute to a better un- Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 Carranza-Castañeda 46 derstanding of migrations and the evolution of diverse taxonomic groups. Besides, the fossil material has been important to establish the age of the basins, which for many years were considered filled only by Quaternary alluvium in the geologic maps, and that their origin and sedimentation were the result of a single geologic event. The purpose of this paper is to communicate the recent advances in the knowledge of the better studied basins from the Transmexican Volcanic Belt and the latest fossil discoveries of their faunal content. In the second half of the last century, the Instituto de Geología, UNAM, started the study of the main basin in central Mexico, and now is sponsored by the Centro de Geociencias, UNAM, Campus Juriquilla, Querétaro. Abbreviations COLO GTO HGO HUAN JAL JUCH MICH NAY QRO TEO TECO ZAC Colotlán Guanajuato Hidalgo Huanusco Jalisco Juchipila Michoacán Nayarit Querétaro Teocaltiche Tecolotlán Zacatecas GUANAJUATO STATE San Miguel de Allende Basin Previous works. The first mention of the San Miguel de Allende area was made by Arellano (1951), who talked about the Rancho El Ocote horses and some glyptodont scutes from Rancho Viejo. Later, Mooser (1958, 1959, 1963, 1964, 1973) and Dalquest and Mooser (1980) describe the fauna of Rancho El Ocote. They suggest the fauna corresponds only to one age, late Hemphillian, and that is younger than the Yepómera fauna. The results of the research done in this basin have demonstrated the importance of it. For better under- standing, in this paper the study area has been separated in the most complete stratigraphic sequences. Rancho El Ocote Area The field work carried out at Rancho El Ocote allowed to establish that in this locality two stratigraphic levels occur, each with a different lithology and faunal content. In addition, an unconformity is present, which separates the strata that bear the early Blancan fauna. The results of the taphonomic analysis made in the GTO 2B locality showed that only in the basal layer, named Rhino Level (because it is the only layer where it is possible to find rhinoceros material), the genera Nannippus minor (=Nannippus aztecus), and Neohipparion are abundant, and Teleoceras, Osteoborus, Agriotherium, Machairodus, and Megalonyx are present. A late Hemphillian age was determined for this layer. In the overlying bed, named White Layer because of its lithology of fine grained sand with volcanic ash, the equids Dinohippus mexicanus and Astrohippus stockii are abundant, also a fibular fragment of Glossotherium was recovered in the top of the section. Based on the equid species, the age of this layer was determined as latest Hemphillian. The volcanic ash lenses present in this sequence were dated by the fission-track method, giving an age of 4.8 ± 0.2 Ma, which corroborates the late Hemphillian age based on the fauna described in previous papers (Table 1) (Carranza-Castañeda and Ferrusquía-Villafranca, 1978, 1979; Miller and Carranza-Castañeda, 1984; Carranza-Castañeda, 1989; Carranza-Castañeda and Walton, 1992; Carranza-Castañeda and Miller, 1996; Miller and Carranza-Castañeda, 1996; MacFadden and Carranza-Castañeda, 2002; Flynn et al., 2005). Los Corrales stratigraphic sequence is at the top of this section in erosional discordance. In it molariforms of Nannippus peninsulatus and molars of Equus simplicidens were found. One complete osteoderm and fragments of other assigned to the pampathere Plaina have been recovered. Radiometric dating of the volcanic ashes of this layer gave an age of 4.70 ± 0.07 by 40Ar/39Ar, and 4.6 ± 0.3 by the fission-track method (Kowallis et al., 1998) (Figure 1). Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias Publicación Especial 4 Late Tertiary fossil localities in central Mexico, between 19°–23°N 47 Table 1. Faunal list for Hemphillian and Blancan mammalian taxa from central Mexico. Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 48 Carranza-Castañeda Table 1. Faunal list for Hemphillian and Blancan mammalian taxa from central Mexico (continued). Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias Publicación Especial 4 49 Figura 1. Stratigraphic sequence of the Rancho El Ocote area. Late Tertiary fossil localities in central Mexico, between 19°–23°N Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 Carranza-Castañeda 50 It is important to point out that the HemphillianBlancan boundaries in these localities are more reduced than in any other research area. Also, in the upper part of this layer, a Pleistocene sandy clay, brown in color, covers all the section (Figure 2). Locality GTO 2 Rancho El Ocote Locality GTO 2B Type section 21°05.28’N, 100°41.01’W Blanco layer Rhino layer Pleistocene latest H late H vore diversity of the Mexican Hemphillian localities (Carranza-Castañeda, 1992; Carranza-Castañeda and Miller, 1996; Miller and Carranza-Castañeda, 1996), also the badger Taxidea was collected recently. The horses are abundant, mainly Dinohippus mexicanus, Astrohippus stockii and Neohipparion eurystyle. The radiometric date gave a Hemphillian age (Kowallis et al., 1998). Locality GTO 43 Rinconada 21°03.44’N, 100°47.81’W H Los Galvanes Area The stratigraphic sequences of the localities from GTO 12 La Pantera are in erosional discordance. Here, abundant material of Nannippus peninsulatus has been recovered, a skull fragment and isolated molars of Equus simplicidens as well as undescribed remains of rodents, amphibians and reptiles. The two ash layers present were dated by fission-tracks, giving an age of 3.9 ± 0.3 Ma. At the base of this sequence is the Rinconada fauna (locality GTO 43), which bears the largest carni- Locality GTO 12 La Pantera A and B 21°03.78’N, 100°46.13’W Locality GTO 2 Los Corrales early Blancan Locality GTO 83 (GTO 2D Plaina) 21°05.26’N, 100°41.30’W early Blancan Figure 2. Stratigraphic sequence of the Los Galvanes area. Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias Publicación Especial 4 Late Tertiary fossil localities in central Mexico, between 19°–23°N The localities from GTO 4 Arroyo El Tanque correlate with GTO 12. On different places of El Tanque, abundant fossils of Blancan age have been recovered, such as birds and reptiles, skeletal elements of Hemiauchenia blancoensis and Platygonus, molars and mandibles of Nannippus peninsulatus and Rhynchotherium, Neochoerus cordobai molars and Glossotherium material (Montellano-Ballesteros and Carranza-Castañeda, 1986); the last two represent the first records of South American immigrants integrated with the Holartic faunas (Flynn et al., 2005). Locality GTO 4 Arroyo El Tanque 21°04.03’N, 100°46.12’W The basal bed of locality GTO 47 Arroyo Belén corresponds to the same stratigraphic level of GTO 4. Glyptotherium, Neochoerus elements and the most significant remains of a pampathere tentatively referred to Plaina have been recovered in this locality. They represent the oldest records of South American immigrants in North America; the bearing bed has an age of 3.6 ± 0.04 Ma. This sequence is covered by sandy clay with Equus sp. molars and remains of the rodent Thomomys sp. (Figure 2). Locality GTO 47 Arroyo Belén 21°04.69’N, 100°46.41’W early Blancan Rancho Viejo Area In the base of this sequence is locality GTO 44 La Presa, where molars of Calippus (Grammohippus) castilli and diverse elements of Dinohippus interpolatus have been collected. The locality was assigned to the earliest late Hemphillian (Carranza-Castañeda and Espinosa-Arrubarrena, 1994). Other localities of the same age where these species were recorded are GTO 50 La Cancha EW and GTO 69 Arroyo Sorpresa. Locality GTO 44 La Presa 21°00.86’N, 100°47.19’W earliest late H Locality GTO 50 La Cancha EW earliest late H 51 In the Comonfort area, distant just 35 km, in the La Nopalera locality (Ranchería de Neutla), similar molars to those of Calippus have been recovered. In Rancho Viejo are the localities of GTO 42 Rancho San Martín, where besides Astrohippus stockii and Dinohippus mexicanus molars, a skull, a mandible and different skeletal elements of Canis ferox were collected. This fauna correlates with the one of Arroyo Tepalcates (GTO 52), which bears abundant material of Astrohippus stockii, Dinohippus mexicanus, Machairodus cf. M. coloradensis, Desmathyus brachidontus and Megatylopus matthewi. The age of these localities is late Hemphillian. Locality GTO 42 Rancho San Martín 21°01.36’N, 100°46.00’W late H Locality GTO 52 Arroyo Tepalcates 21°01.25’N, 100°45.46’W late H The early Blancan locality GTO 11 Garbani is just 800 m to the north of GTO 42; here an age of 4.1 ± 0.5 Ma was estimated by a dated volcanic ash (Kowallis et al., 1998). In this locality Nannippus peninsulatus, Equus simplicidens molariforms and undescribed rodent and bird material are abundant. Locality GTO 11 Garbani 21°01.12´N, 100°45.46’W early Blancan Three kilometers to the southeast, the localities GTO 5 Miller Place, GTO 33 Pork Chop and GTO 14 Cuesta Blanca are located, where the largest amount of the capybara Neochoerus cordobai and some elements of Glossotherium have been collected. Locality GTO 14 Cuesta Blanca 21°01.26’N, 100°45.24’W early Blancan Locality GTO 6 Arrastracaballos is towards the south of this sequence, which bears the largest Blancan faunal diversity of the San Miguel de Allende basin. The best collected material are the horses Equus simplicidens and Nannippus peninsulatus, the carnivores Borophagus diversidens, Felis studeri, Pliotaxidea Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 52 Carranza-Castañeda sp., Trigonictis sp., and Canis cf. C. lepophagus, the artiodactyls Hexobelomeryx fricki, Hemiauchenia blancoensis, and Platygonus sp. Besides, undescribed rodent material, as well as the lagomorph Hypolagus vetus come from this locality. Locality GTO 6 Arrastracaballoslate H – early Blancan 21°00.92’N, 100°45.31’W It is important to point out that the Pleistocene localities where isolated teeth and a partial skull of Equus material have been collected are only 400 m north of this locality (Figure 3). Coecillos Area (20º59.93’N, 100º46.06’W) Locality GTO 69 Arroyo Sorpresa is situated at the base of the same sequence, and is of early late Hemphillian age. Its fauna is represented by the most complete palate of Dinohippus interpolatus, with both P2-M3 series, and a maxillary fragment of a gomphothere. Figure 3. Stratigraphic sequence of the Rancho Viejo area. Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias Publicación Especial 4 Late Tertiary fossil localities in central Mexico, between 19°–23°N Above these stratigraphic sequences there are different sites within the Coecillos area where abundant material has been collected of Dinohippus mexicanus, Astrohippus stockii, Neohipparion eurystyle, Nannippus minor (=N. aztecus), Teleoceras, a Machairodus incisor, numerous rodent remains, specially few skulls and jaws of Paenemarmota, the lagomorphs Notolagus velox, Hypolagus and the artiodactyls Hemiauchenia, Texoceros, Hexobelomeryx, Megatylopus and Desmathyus. All these collecting sites were named locality GTO 30 Coecillos. Recent work in the ravine named GTO 75 Arroyo de Emilio (20º59.97N and 100º45.88’W), where a diversity of fossils was found, made this locality important. Here were collected a Dinohippus mexicanus skull fragment and mandibles, a complete and articulated skeleton of the peccary Desmathyus. Some jaws, a partial skull, an almost complete skeleton, mandibles, skull and other elements of Paenemarmota, a new indeterminate mustelid, and the badger cf. Taxidea and the first record of the possible skunk Conepatus, a Hemiauchenia mandible and another jaw of a probable Texoceros. Undoubtedly, all the material corresponds to the Hemphillian. Pleistocene sediments, in erosional discordance, are in the same ravine. In them, an Equus mandible associated with a more or less complete Glyptotherium cf. G. floridanus carapace, including the mandible of the same individual, were collected. In the same area, the locality GTO 36 Palma Sola has a similar stratigraphy; Astrohippus molars have been collected in the basal Hemphillian sediments, and molars of Equus sp. in the Pleistocene ones. Recently, Blancan fossils were recovered in the locality GTO 79 La Perdida, consisting of isolated molars of Nannippus peninsulatus. This shows that in the Coecillos area there is a complete stratigraphic sequence from early late Hemphillian, late Hemphillian, Blancan to Pleistocene. Discussion The most complete stratigraphic sequences known at present in Mexico and probably in North America occur in the San Miguel de Allende basin. Of the four 53 sections, the most complete is the one from Rancho Viejo area, with representative faunas of the early late Hemphillian, late Hemphillian, early Blancan and Pleistocene. The age of these localities has been confirmed by radiometric dates of volcanic ashes intercalated with the bearing beds. In all four stratigraphic sections there is evidence of South American immigrants. Without doubt, they represent the first fossil evidences that during the Hemphillian-Blancan boundary the Panamian land bridge had an efficient terrestrial corridor, through which the American faunal interchange began (Miller and Carranza-Castañeda, 1999; Carranza-Castañeda and Miller, 2004). HIDALGO STATE Zacualtipán Basin Accidental fossil findings made by coal miners during their work in the Zacualtipán mines, exploited during the second half of the xix century, are among the first descriptions of fossil material of the Mexican Miocene. This fossil material without stratigraphic control (Leidy, 1882; Cope, 1885, 1886) is known as the Tehuichila fauna. In the first descriptions, the age was assigned to the Miocene. Ferrusquía-Villafranca (1978), in the revision of the Miocene faunas, established their age as Pliocene. During the research carried out in the 1990’s, sponsored by the Universidad Autónoma del Estado de Hidalgo, a new locality, HGO 2 Potrero de Zietla, 20º36.40’N, 98°36.25’W, was discovered. The fauna consists of Dinohippus interpolatus and Calippus (Grammohippus) castilli upper molars, molar fragments of Agriotherium and Teleoceras?, a Hexobelomeryx molar and calcaneum and molar fragments of cf. Prosthennops. This fauna correlates with the one from the San Miguel de Allende basin, GTO 44 La Presa, assigned to the early late Hemphillian (CarranzaCastañeda and Espinosa-Arrubarrena, 1994). The revision of the equid fauna from Tehuichila (MacFadden, 1984; Carranza-Castañeda and EspinosaArrubarrena, op. cit.) showed that in this basin two mammal ages exist: late Hemphillian (late Miocene), Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 54 Carranza-Castañeda represented by the fauna of Potrero de Zietla, and early Blancan (early Pliocene), by the presence of Nannippus peninsulatus. Actopan-Ixmiquilpan Basin This is one of the largest ancient paleolakes in central Mexico. The fluvio-lacustrine sediments extend by thousands of square kilometers, and the thickness in some areas is more than 100 m, covered by early Pleistocene volcaniclastic sediments. At several sites in the surrounding of the Lagunillas village, unidentified mammalian fossil fragments have been collected. In the locality HGO 7 La Suerte, 10º51.35’N and 99º01.36’W, a lower molar fragment of Rhynchotherium and molar fragments of unidentified equids were recovered. In the ravine Bodandha of the Ixmiquilpan region (20°24.22’N, 99°14.56’W), just 2 km south of the ranchería de Xothi, the locality HGO 27 Las Arcinas was discovered. Its fauna consists of upper molar fragments of Nannippus peninsulatus, Equus cf. simplicidens and Platygonus, which are Blancan fossil indexes. In the Santa María Amajac region, 20º18.27’N, 98º42.50’W, there is a sequence of clay, sandy clay and gravel which bears fossils. The ash that is at the base of this sequence was dated, giving an age of 4.2 ± 0.3 Ma by fission-tracks, and of 4.57 ± 0.02 Ma by 40Ar/ 39 Ar, which indicates an early Blancan age. The collected fauna includes Equus simplicidens, Nannippus peninsulatus and Rhynchotherium fragments. These fossils reinforce the early Blancan age assignment. In the upper strata, a Tremarctus maxilla was recovered. Sediments that bear fragments of the rhino Teleoceras, a Hemphillian index fossil, are found to the west of these localities. The sequence has just been preliminary explored (Carranza-Castañeda and Miller, 1998). Tepeji del Río-Tula Basin The Tepeji del Río area, in the southwestern region of the Hidalgo State, is where the lacustrine sequence is best exposed, with a thickness of more than 100 m in some sites. This sequence is covered by volcaniclastic sediments of the locality HGO 20 Tacos y Tortas, where Equus sp. upper molars were collected. The presence of this genus indicates a Pleistocene age. In the Tula area, locality HGO 24 El Bagre, fish remains of the genera Ictiobus (Alvarado-Ortega and Carranza-Castañeda, 2002a, 2002b; Alvarado-Ortega et al., in press) and Ictalurus have been recovered in Blancan age beds. Also, in surrounding sites there are fish fossils that now are under study. Other important localities are HGO 21 Los Fragmentos and HGO 22 Los Tepalcates, where Nannippus peninsulatus material, proboscidean fragments possibly of Rhynchotherium and Platygonus molariforms have been recovered (Figure 4). Locality HGO 25 La Plegaria is southwest of Tepeji del Río city. Its fauna is represented by Astrohippus stocki, Neohipparion eurystyle, Dinohippus mexicanus, Machairodus cf. M. coloradensis, Canis ferox, Megalonyx, Teleoceras fossiger, cf. Megatylopus matthewi, Hemiauchenia vera, Alforjas sp., Hexobelomeryx fricki and cf. Texoceros sp., of late Hemphillian age. It correlates with the faunas of Rinconada, Arroyo Tepalcates and Rancho El Ocote, from Guanajuato State (Padilla-Gutiérrez, 2004). JALISCO STATE Large sedimentary basins occur in this state. The updated results of the prospecting work are given according to the regions where these basins are located. Tecolotlán Basin During the last years an extensive investigation was carried out in this area, located 100 km southwest of Guadalajara city. The stratigraphy of the basin is composed of two sedimentary sequences; the lower was determined as late Hemphillian because of the associated fauna and 40Ar/39Ar ash dating, which yielded an age of 4.89 ± 0.016 Ma, and the upper, Buenaventura, assigned to the late Blancan also by its faunal content and the ash dating that gave an age of 2.6 Ma. The Hemphillian sequence consists of three sections. The lower, named JAL TECO 9 Santa María, is composed of red oxidized clay and sandy clay, as well as gravels that represent paleochannels. The abundance Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias Publicación Especial 4 Late Tertiary fossil localities in central Mexico, between 19°–23°N 55 Figure 4. Main basins in central Mexico. of Nannippus minor (=N. aztecus) is notable; it exceeds any locality from central Mexico, including the localities from Guanajuato. In contrast with other Mexican Hemphillian localities, in this layer few specimens of Astrohippus stockii have been collected. The sequence is interrupted by lacustrine sediments that crop out in almost all the southwestern part of the basin and cover this sequence; they do not have fossil mammals, but in the upper bed some crystallized gastropods have been recorded. Covering these lacustrine sediments, the upper section consists of clay bed sequences named JAL TECO 20 La Hacienda, JAL TECO 26 Kowallis, where the ash gave an age of 4.89 ± 0.16 Ma by the 40Ar/39Ar method. In this stratigraphic level, numerous micro vertebrate remains have been collected, such as rodents, lagomorphs, amphibians, reptiles and birds, which have not yet been described. It is interesting to point out that Astrohippus stocki is very rare in this part of the section and Nannippus minor has never been collected. Unconformably, the sequence of gravel and finegrained sand, whose radiometric date gave an age of 2.6 Ma, allows to situate it in the late Blancan. From this sequence, in the locality JAL TECO 7 Las Gravas, equid molars were found with less progressive characters than those of Equus simplicidens, besides Platygonus, Hemiauchenia blancoensis. In JAL TECO 44 Las Placas and the most important locality JAL TECO 53 Lic. Preciado, proboscidean tusks, horse teeth fragments and the noteworthy South American immigrants Glyptotherium, Neochoerus and pampathere remains of cf. Plaina have been recovered. It is important to note that all along the sequence there are fossils of crocodilian and turtle material, but no remains of Nannippus peninsulatus were observed (Kowallis et al., 1998, 2003, in press; Miller and Carranza-Castañeda, 1998a, 1998b, 1999, 2001a, 2001b; Carranza-Castañeda, 2002; Carranza-Castañeda et al., 2002) (Figure 5). The known fauna is listed in Table 1. Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 Carranza-Castañeda 56 Figure 5. Composite stratigraphic sequence of the Tecolotlán Basin. Colotlán-Tlaltenango Basin The locality JAL COLO 17 Los Vélez is located eight kilometers south of Colotlán city. Here the Neohipparion eurystyle, Astrohippus stockii and Desmathyus fossils have been collected; these taxa establish a late Hemphillian age. In the state border of Jalisco and Zacatecas, just 3 km south of Los Vélez locality, the locality ZAC 18 Santo Domingo is found, where also the late Hemphillian equids Nannippus minor, Neohipparion eurystyle and Astrohippus stockii were recorded. In the locality JAL COLO 9 Los Corrales (= ZAC JUCH 9 Sanidad Animal, 21º57.56´N, 103º18.27´W), molars of Dinohippus interpolatus and Calippus cf. C. castilli have been recovered, which represent the earliest part of the late Hemphillian. It is important to point out that these equid species represent the oldest horses in central Mexico. Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias Publicación Especial 4 Late Tertiary fossil localities in central Mexico, between 19°–23°N 57 Teocaltiche-Xalostitlán Basin QUERÉTARO STATE In different places located between these two cities, different specimens were collected. In the Teocaltiche area, Osteoborus cyonoides (=Borophagus secundus), Astrohippus stockii, Dinohippus mexicanus and cf. Hexobelomeryx fricki fossils were found. This fauna is of late Hemphillian age (Montellano-Ballesteros, 1997; Aguirre-Díaz and Carranza-Castañeda, 2001). In recent field work carried out by the author, a new locality was discovered, JAL TEO 2 El Libramiento, that allowed to recover an equid upper molar with close characters to those of Calippus, a carnivore tibia, a second molar of probable gomphothere and bird remains. The age of this fauna is the earliest part of the late Hemphillian. There have been several reports of Pleistocene fossils from different places in this state. The only locality with late Tertiary fossils is QRO 1 Landa de Matamoros, which is in the Sierra Gorda of Querétaro. The recovered fossils include a gomphothere mandible and maxilla, a Nannippus minor lower molar and an astragalus of a possible Hexobelomeryx. The fauna was assigned to the late Hemphillian. MICHOACÁN STATE Late Tertiary localities from the Michoacán State are very scarce, but there are several reports of Pleistocene fossils. Charo Depression The town of Charo is located 14 km south of Morelia city. In the locality MICH 1 Rancho La Goleta is found the only late Tertiary fauna. It has been mentioned since the second half of the past century, but the age assignment always was ambiguous (Reppening, 1962; Howard, 1965; Ferrusquía-Villafranca, 1978). New investigations carried out with UNAM’s support, allowed to prospect the area and to collect early Blancan fossils (Carranza-Castañeda, 1976). Besides, there are abundant unpublished rodent remains. The radiometric date gave an age of 3.6 Ma. Unconformably, late Pleistocene sediments with Equus sp. molars cover the Blancan sediments. The locality MICH 2 Cinqua, 19°45.43’N, 100°14’32”W, was discovered 160 km southeast of Morelia city. Its associated fauna consists of Neohipparion eurystyle, Dinohippus interpolatus (?), Teleoceras fossiger, Desmathyus and Megatylopus cf. M. matthewii, with an age corresponding to the earliest late Hemphillian (Figure 4). NAYARIT STATE In the 1960’s, Dr. Theodore Downs and Harley J. Garbani collected material from the locality Valle de Los Gigantes that is now housed in Los Angeles County Natural History Museum; the locality is recorded in Centro de Geociencias, UNAM as NAY 1. Los Gigantes Basin (21º27.69´N, 104º04.70´W) This fauna has not been described, but a preliminary study of the present author allowed to identify Dinohippus mexicanus, Neohipparion eurystyle, Agriotherium, Megalonyx, Rhynchotherium, Megatylopus, and a felid, probably Machairodus. During the spring field work of 2005, a lower molar of Dinohippus and turtle (probably Geochelone) carapace fragments were collected. The associated fauna suggests a late Hemphillian age. As part of the recent prospecting work, two new late Hemphillian localities were discovered in the top of the southwestern Sierra Madre. The most interesting one is found in the Huajimic basin, where some fossils were collected. This locality was named as NAY 2 El Huichol, and Dinohippus mexicanus and Rhynchotherium fossils were identified. It is important to note that in all these localities volcanic ash is intercalated with the bearing beds (Figure 4). ZACATECAS STATE Juchipila-Jalpan-Tabasco Basin This is a large geologic structure about 120 km long with a NE orientation, where lacustrine sediments have Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 58 Carranza-Castañeda a thickness greater than 70 m, and in some areas the clay sediments bear abundant root traces and other evidences of fossil plants, such as in locality ZAC JUCH 29 Raíces, in the ranchería of Mezquituta. In other areas only the lacustrine sediments with ostracodes crop out, as in localities ZAC JUCH 24 La Cofradía (21º37.53’N, 103º01.85´W) and ZAC JUCH 27 La Chimenea (21º27.25’N, 103º05.54W). Along this basin, several localities with fossil mammals have been discovered. Among the most important is locality ZAC JUCH 30 Las Lomas del Valle, where upper molars of a possible Calippus and antilocaprid molars were collected; this large area with exposed sediments has diverse fossil mammals. In the locality ZAC JUCH 25 Arroyo Los Fragmentos, Dinohippus mexicanus molars and a tibia, and Astrohippus stockii molars were recovered; the abundance of fragments also suggests a diverse fauna. Other localities are ZAC JUCH 22 Camino de Piedra and ZAC JUCH 23 El Pájaro (21º37.53´N, 103º01.85´W), where bird elements, specially coracoids and an upper molar probably of Dinohippus were recovered. To the north of Jalpan city are localities ZAC JUCH 28 La Canica, where a Neohipparion eurystyle molar and other fragments were collected, and locality ZAC HUAN 5 Yerbaniz, with fragments of a possible Rhynchotherium. Locality ZAC HUAN 6 Hilda, with Equus sp. upper molars, is north of Tabasco city, where only Pleistocene sediments crop out (Aguirre-Díaz and Carranza-Castañeda, 2000; Carranza-Castañeda and Aguirre-Díaz, 2000, 2001). BAJA CALIFORNIA SUR In the southern part of the Baja California Peninsula, the late Tertiary Las Tunas fauna was for several years the only extensively described fauna (Miller, 1980). Prospecting work done north of San José del Cabo allowed to recover Blancan fossil mammals. This area is being systematically worked now, and abundant Equus simplicidens, Rhynchotherium, a canid material, a Felis lacustris partial jaw, and lagomorphs, rodents and bird specimens have been collected (Carranza-Castañeda and Miller, 1999; Miller and Carranza-Castañeda, 2001a, 2001b, 2002). CONCLUSIONS The carried out research of the continental late Tertiary basins of central Mexico has shown the importance of this region due to the abundance of fossils and the faunal diversity. The faunal associations represent two mammal ages: late Hemphillian (late Miocene) and Blancan (Pliocene), although Pleistocene mammals are also present. In some basins, the superposition of late Hemphillian-Blancan faunas has demonstrated that the San Miguel de Allende and Tecolotlán basins are the most complete stratigraphic late Tertiary continental sequences in Mexico. This superposition opens the possibilities of studying the extinctions that occurred in the boundary between the mammal ages. Also, the abundance of rodent fossils, of which most part has not been described, constitutes an opportunity to study different biological events, such as the evolution of cricetid rodents, which are common in these basins. Collaterally, the boundaries of large lakes that were present in central Mexico during late Tertiary have been defined. The continuity of researching in basins of Mexico will contribute to precise the migrating routes of the first South American immigrants from the early Blancan faunas of central Mexico to the late Blancan faunas of Texas, Arizona, New Mexico and Florida. ACKNOWLEDGMENTS The research of the sedimentary basins from central Mexico began with the support of the Instituto de Geología, UNAM, and continues with that of the Centro de Geociencias, UNAM, Campus Juriquilla, Querétaro. Mr. Harley J. Garbani, since the beginning, has been working in the San Miguel de Allende Basin. I am very grateful to Dr. Wade Miller and Dr. Bart Kowallis, from Brigham Young University; their support and participation in the research have been significant in the success of the results here presented. In the last years, Miss Hilda Troncoso has been prospecting and collecting. I want to express my special thanks for their financial support to the investigations to the following institutions: Centro de Geociencias, Campus UNAM, Juriquilla; National Geographic (Grant 344386); CONACYT (Project 1466 and Project 47071); Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias Publicación Especial 4 Late Tertiary fossil localities in central Mexico, between 19°–23°N National Science Foundation (Grants EAR 8620155, EAR 9316895, and EAR 9902898), and The Earth Watch Institute. Finally, I acknowledge Luis Burgos and Fernando Vega for drawing the illustrations. Also, I thank all participants during the field work, Gerardo Álvarez-Reyes, students and researchers, which have collaborated in different phases of the studies of central Mexico. BIBLIOGRAPHICAL REFERENCES Aguirre-Díaz, Gerardo, and Carranza-Castañeda, Óscar, 2000, OligoMiocene basins of central Mexico and the misinterpretation of their sedimentary fills as Quaternary alluvium: Journal of Vertebrate Paleontology, v. 20, no. 3, p. 25 (abstract). ――― 2001, La cuenca miocena de León-San Juan de los LagosTeocaltiche―un amplio paleolago en el sector centro-occidental del Cinturón Volcánico Mexicano: Unión Geofísica Mexicana, Resúmenes y Programa, Reunión Anual 2001, Geos, v. 21, no. 3, p. 320 (abstract). 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