Oscar Carranza-Castañeda

Geochronology of Hemphillian-Blancan Aged Strata, Guanajuato, Mexico, and Implications for Timing of the Great American Biotic Interchange

We present new geochronologic (magnetostratigraphy, fission‐track and 40Ar/39Ar radioisotopic dates, biostratigraphy) data constraining the age of three separate sequences and a composite section from Guanajuato, Mexico. Those data make this one of the most complete and precisely age‐calibrated sequences in North America spanning the Hemphillian/Blancan North American Land Mammal “Age” (NALMA) boundary interval, and the data further constrain the timing and pattern of the Great American Biotic Interchange (GABI). In total, 196 samples (77 sites) were used to construct the magnetic polarity stratigraphies, with eight fission‐track analyses and four new 40Ar/39Ar radioisotopic dates. The sections sampled are possibly latest Miocene to Late Pliocene in age, spanning about 2.7 m.yr. (∼5.5–2.7 Ma), from Chrons C3n.4n or C3r to C2An.1n. The radioisotopic dates and magnetostratigraphic correlations indicate the Hemphillian/Blancan NALMA boundary at Guanajuato in central Mexico is ∼4.7–4.8 Ma, and it probably lies within Chron C3n.2r, consistent with most prior age estimates and correlations. Our analyses indicate that a variety of “invaders” (taxa with South American ancestry, including Megalonyx, Glossotherium, and Plaina) clearly were present by at least 4.7–4.8 Ma and therefore much earlier in lower latitude Middle America than in more temperate parts of North America. Others (e.g., Neochoerus, Glyptotherium) were present shortly thereafter but still much earlier than in higher latitudes. Thus, the first appearances of these five immigrant taxa can no longer be used to define the beginning of the late Blancan. This timing significantly predates the earliest documented major influxes of North American forms into South America, is at least 1.5–2 m.yr. earlier than the beginning of the GABI previously recorded in North America (beginning of the Blancan [Bl 2], ∼2.7 Ma), and suggests faunal provinciality and more staggered arrival and dispersal of South American immigrants than indicated in temperate sequences.

Late Cenozoic mammals from central Mexico

ABSTRACT Historically, the knowledge of late Cenozoic mammals from central Mexico has been slow in developing. Most early studies did not include stratigraphic controls or meaningful correlations. At present the geology is incompletely known at most fossil-yielding localities. However, research in the past several years, especially in the State of Guanajuato, has provided much needed stratigraphic control relevant to collected fossils. Of the eleven faunas identified in this paper, only the Cedazo (Pleistocene), Las Tunas, Rancho El Ocote, Coecillo, and Rancho Viejo (Pliocene) have this control. These faunas, then, with several taxa reported here for the first time, are considered most important. They provide significant information about changing climates, a trend from humid, warm conditions in the Pliocene to drier and cooler conditions in the Pleistocene, and mammalian dispersals. Most fossils can be derived from North American ancestors; however, South American constituents can be identified beginning...

New lagomorphs from the Pliocene of central Mexico

ABSTRACT This paper presents the most diverse Pliocene lagomorph fauna yet reported from Mexico. Four genera and five species are herein recognized from several localities in the central state of Guanajuato. These presently discovered taxa were compared with previously described lagomorphs from North America; and three, one genus and two species, are considered new. Additionally, one other species could possibly be new. Taxa which have been reported elsewhere include Hypolagus, Pratilepus ?kansasensis and Notolagus cf. velox. Apparently no published record exists which describes Pliocene lagomorphs as far south in North America as those listed in this study.

Contrasting Silicic Magma Series in Miocene‐Pliocene Ash Deposits in the San Miguel de Allende Graben, Guanajuato, Mexico

The San Miguel de Allende graben, Guanajuato, Mexico, contains numerous rhyolitic volcanic ash beds. Electron microprobe and x‐ray fluorescence analyses of glass shards from 14 localities, combined with mineralogic, stratigraphic, radiometric, and paleomagnetic data, allow us to correlate the ash beds (and the intervening sedimentary strata and fossils), understand the timing of volcanism, date the age of extension, and better understand the tectonic and volcanic evolution of central Mexico. Our analyses reveal that at least six separate eruptions of rhyolitic ash occurred during the Late Miocene and Pliocene (5–3 Ma) while the San Miguel Allende basin was subsiding. The fallout ash beds can be distinguished by phenocryst mineralogy, inferred eruption temperatures, and differences in major and trace element compositions. Two magma series (medium K and high K) are represented in the rhyolitic tephra deposits. The high‐K series is marginally peralkaline, reduced (indicated by high Fe/Mg ratios), dry (paucity of hydrous silicates), hot (≤860°C), and enriched in many incompatible trace elements including Nb, Y, and Zr. They have the characteristics of anorogenic rhyolites formed in rift and hotspot settings. In contrast, the rhyolites of the medium‐K series are similar to those erupted in subduction‐related volcanic arcs. They are calcic to calc‐alkalic with low Fe/Mg ratios and have hydrous phenocrysts, suggesting the water fugacity was also high. Eruption temperatures were less than ∼860°C. However, these medium‐K rhyolites have high Sr/Y ratios and were probably derived by differentiation of adakitic parents. We suggest the contrasting rhyolite series are the result of differentiation of two different parental magma types. During the late Miocene to Pliocene slab breakoff, hot asthenosphere rose through a break in a subducting slab of oceanic lithosphere. Parents of the medium‐K series (adakitic magmas) formed as hot rising mantle caused the edge of the torn plate to melt. High‐K parental magmas were formed by decompression melting as asthenospheric mantle rose through the gap in the slab.

Hemphillian and Blancan felids from central Mexico

ABsTRACr-Late Tertiary Carnivores are rare as reported in the fossil record for Mexico. The felids discussed in this paper from the state of Guanajuato represent the most diverse assemblage of this taxonomic group thus far known from the country. They include Machairodus cf. M. coloradensis, Pseudaelurus? intrepidus, and Felis cf. F. studeri. The last-named taxon has not previously been recognized from the late Tertiary of Mexico, and Pseudaelurus has only questionably been identified in the country (but not described or discussed). None of these taxa have yet been reported from areas further south. While age determinations for the above felids from Guanajuato were based on associated faunal constituents, especially abundant and diverse equids, the felids themselves can be useful as chronostratigraphic indicators.

Blancan Camelids from San Miguel de Allende, Guanajuato, Central México

Abstract During the Pliocene, the diversification of the tribes Lamini and Camelini of the Family Camelidae took place in most of North America, but at present in Mexico the systematics of Pliocene Camelidae are poorly known. Fossil material described in this paper was recovered from Blancan I and Blancan III age floodplain and point bar deposits of the San Miguel de Allende basin, Guanajuato state, central Mexico, which approximately spans a time frame from 4.7 to 3.0 Ma. The identified taxa include the lamines Hemiauchenia blancoensis (Meade) 1945, Hemiauchenia gracilis Meachen, 2005, Blancocamelus meadei Dalquest, 1975 and Camelops sp., while the camelines are represented by Megatylopus sp. The records of H. gracilis and B. meadei in the Pliocene of central Mexico are the oldest in North America. Previous studies of the probable feeding strategies of these taxa indicate that they were browsers or browser-like intermediate feeders and just one was an intermediate feeder. The records of these species in the Early Blancan of Guanajuato extend their geographic distribution from the southern USA to central Mexico.

Ontogeny and Sexual Dimorphism of Glyptotherium texanum (Xenarthra, Cingulata) from the Pliocene and Pleistocene (Blancan and Irvingtonian NALMA) of Arizona, New Mexico, and Mexico

North American glyptodonts originated from South American ancestors during the Great American Biotic Interchange no later than early Blancan North American Land Mammal Age (NALMA). A substantial expansion in population samples from the late Blancan 111 Ranch fauna of southeastern Arizona, several late Blancan faunas in New Mexico, and the early Blancan–Irvingtonian faunas of Guanajuato, Mexico, permit, analysis of sexual dimorphism and ontogeny of Glyptotherium texanum Osborn, 1903. Growth of carapacial osteoderms was allometric, including changes of the external sculpturing. Overall anatomy of the carapace changed with growth, with development of distinctive pre-iliac and post-iliac regions in lateral profile of adults. Skulls of adults possess a unique boss on the anterior surface of the descending process of the zygomatic arch that is not present in juveniles. Sexual dimorphism involves differences in anatomy of lateral and posterior osteoderms. Glyptotherium arizonae Gidley, 1926, is a junior synonym of G. texanum. The temporal distribution of G. texanum extends from early Blancan NALMA to Irvingtonian NALMA, with geographical distribution from Central America and Mexico to southern United States.

A PLIOCENE RECORD OF CAPROMERYX (MAMMALIA: ANTILOCAPRIDAE) IN MÉXICO

Abstract The Antilocapridae was a diverse artiodactyl family present in some late Tertiary faunas of North America. In México, its Tertiary fossil record is poorly known. The antilocaprid material described in this paper was collected from the early Blancan fluvial deposits of the San Miguel de Allende Area, state of Guanajuato, México. It includes isolated upper and lower premolars and molars, dental series, and some rami fragments. The material is assigned to Capromeryx tauntonensis. The presence of this species in the early Pliocene of central México represents the oldest record in North America and extends its known geographic distribution from the northwestern United States to central México.

Rediscovered type specimens and other important published Pleistocene mammalian fossils from Central Mexico

ABSTRACT Four type specimens and three other important fossils, all mammalian, which were apparently lost to science, have recently been rediscovered in an old building located in Mexico City. Recovered types include Glyptodon mexicano, Mastodon oligobunis, Equus conversidens, and Equus (Hesperohippus) mexicanus. The three important nontypes are a lower jaw of Arctodus simus, a partial skull of Euceratherium collinum, and a partial skull of Bison antiquus. All the specimens have been recataloged and are now in the Museum of Paleontology, Institute of Geology, at the University of Mexico.

Increased xenarthran diversity of the Great American Biotic Interchange: a new genus and species of ground sloth (Mammalia, Xenarthra, Megalonychidae) from the Hemphillian (late Miocene) of Jalisco, Mexico

Abstract. A new genus and species of megalonychid sloth, Zacatzontli tecolotlanensis n. gen. n. sp., is described from the late Hemphillian of Jalisco, Mexico. Comparison and analysis of the type specimen, a mandible, with other megalonychid sloths shows a closer relationship to South American taxa than those from North America or the Caribbean. This suggests that during the early stages of the Great American Biotic Interchange there were two separate dispersal events of megalonychid sloths—an earlier one represented by Pliometanastes and the later one by Zacatzontli n. gen. While the morphology of the spout of Zacatzontli more closely resembles that of Megalonyx, based on the current record, Zacatzontli does does not enter North America until after the evolution of Megalonyx from Pliometanastes. The role of the northern neotropics in South America as a staging area for South American taxa that entered North America is discussed.