Wade E. Miller

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...

Upper Jurassic Dinosaur Egg from Utah

The Upper Jurassic egg described here is the first known egg from the 100-million-year gap in the fossil record between Lower Jurassic (South Africa) and upper Lower Cretaceous (Utah). The discovery of the egg, which was found mixed in with thousands of dinosaur bones rather than in a nest, the pathological multilayering of the eggshell as found in modern and fossil reptilians, and the pliable condition of the eggshell at the time of burial indicate an oviducal retention of the egg at the time of burial.

Mammut americanum , Utah's first record of the American mastodon

A sinkhole locality in the Wasatch Plateau has yielded Utahs first record of the American mastodon. At nearly 3,000 m this fossil-bearing site denotes the highest elevation yet reported for Mammut americanum , and demonstrates that mountainous terrain was not a barrier to dispersals. A presumably preferred habitat of spruce-fir open forests, similar to that in the area today, was present during the time of the mastodons reported here. Two incomplete individuals of M. americanum are the only fossils that were found in the sinkhole. Entrapment appears to have been the cause of death. Since the record of M. americanum in western North America is sparse, the present find has added significance in providing more information about the nature of this animal. A possible association of a recovered spear point fragment near the fossils indicates the possibility that man and mastodon were contemporaneous here. Three independent datings were run on mastodon bone. Two based on carbon-14 yielded dates of 7,090 ± 200 yr B.P. and 7,590 ± 100 yr B.P., while the third, using the racemization process, indicated an age of less than 10,000 yr B.P.

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.

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.

A new species of pantodont, cf. Haplolambda simpsoni (Mammalia) from Utah

North American pantodonts have mostly been recovered from the Rocky Mountain states. However, none has previously been accurately reported from Utah. Reference to Coryphodon was made by Cope in 1872 under the name Bathmodon , to which others (e.g., Marsh, 1876) later referred, but this probably was in error. The reported material apparently came from southwestern Wyoming rather than northeastern Utah. An undescribed coryphodontid jaw, though, has been collected from Eocene deposits in the Uinta Basin.