Dinosaurs and Other Reptiles from the Mesozoic of Mexico
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Dinosaurs and Other Reptiles from the Mesozoic of Mexico

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203 pages
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Description

Mesozoic reptiles of Mexico and the event that hastened their demise


This overview of dinosaur discoveries in Mexico synthesizes current information about the geography and environment of the region during the Mesozoic when it was the western margin of the ancient continent of Pangea. The book summarizes research on various groups, including turtles, lepidosauromorphs, plesiosaurs, crocodyliforms, pterosaurs, and last but not least, dinosaurs. In addition, chapters focus on trackways and other trace fossils and on K/P boundary (the Chicxulub crater, beneath the Gulf of Mexico, has been hypothesized as the site of the boloid impact that killed off the dinosaurs). Dinosaurs and Other Reptiles from the Mesozoic of Mexico is an up-to-date, informative volume on an area that has not been comprehensively described until now.


Contributors
Acknowledgments
Preface
1. History of the Discoveries of Dinosaurs and Mesozoic Reptiles in Mexico
Jose Ruben Guzman-Gutierrez and Héctor E. Rivera-Sylva
2. Paleogeography and Paleoenvironment of Mexico during the Mesozoic
Wolfgang Stinnesbeck and Eberhard Frey
3. Turtles of the Mesozoic of México
Donald Brinkman
4. Mesozoic Lepidosauromorphs of Mexico: A Review and Discussion of Taxonomic Assignments
Víctor Hugo Reynoso and J. Alberto Cruz
5. Plesiosaurs, Reptiles between Grace and Awe
Eberhard Frey and Wolfgang Stinnesbeck
6. Mexican Ichthyosaurs
Eberhard Frey and Wolfgang Stinnesbeck
7. Overview of Mesozoic Crocodyliforms from Mexico
Gerardo Carbot-Chanona
8. Mexican Pterosaurs—Rare Jewels in the Fossil Record
Eberhard Frey and Wolfgang Stinnesbeck
9. Mexican Saurischian Dinosaurs
Héctor E. Rivera-Sylva and Kenneth Carpenter
10. The Ornithischian Dinosaurs of Mexico
Héctor E. Rivera-Sylva and Kenneth Carpenter
11. A Summary of the Mesozoic Vertebrate Tracks of Mexico
Victor Manuel Bravo Cuevas and Rubén Rodríguez de la Rosa
12. The Cretaceous-Paleogene (K-Pg) Boundary in Mexico
Wolfgang Stinnesbeck and Eberhard Frey
Index

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Date de parution 15 avril 2014
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Dinosaurs and Other Reptiles from the Mesozoic of Mexico
DINOSAURS AND OTHER REPTILES FROM THE MESOZOIC OF MEXICO
EDITED BY H CTOR E. RIVERA-SYLVA,
KENNETH CARPENTER,
AND EBERHARD FREY
Life of the Past James O. Farlow, editor
Indiana University Press Bloomington and Indianapolis
This book is a publication of Indiana University Press
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1320 E. 10th St.
Bloomington, IN 47405 USA
iupress.indiana.edu
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2014 by Indiana University Press
All rights reserved
No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and recording, or by any information storage and retrieval system, without permission in writing from the publisher. The Association of American University Presses Resolution on Permissions constitutes the only exception to this prohibition.
The paper used in this publication meets the minimum requirements of the American National Standard for Information Sciences-Permanence of Paper for Printed Library Materials, ANSI Z39.48-1992.
Manufactured in the United States of America
Library of Congress Cataloging-in-Publication Data
Dinosaurs and other reptiles from the Mesozoic of Mexico / edited by H ctor E. Rivera-Sylva, Kenneth Carpenter, and Eberhard Frey.
pages cm. - (Life of the past)
Includes bibliographical references and index.
ISBN 978-0-253-01183-1 (cl : alk. paper) - ISBN 978-0-253-01271-5 (ebk.)
1. Dinosaurs-Mexico. 2. Paleontology-Mexico. 3. Paleontology-Mesozoic. 4. Paleogeography-Mexico. 5. Paleogeography-Mesozoic. I. Rivera-Sylva, H ctor E., [date] II. Carpenter, Kenneth, [date] III. Frey, Eberhard, [date]
QE861.9.M49D56 2014
567.90972-dc23
2013036587
1 2 3 4 5 19 18 17 16 15 14
This volume is dedicated to Antonio del Castillo (1820-1895), founder and director of the National Geological Institute, who led the way in the study of Mexican vertebrate paleontology.
Contents
C
Contributors
Acknowledgments
Preface
1
History of the Discoveries of Dinosaurs and Mesozoic Reptiles in Mexico
Jose Ruben Guzman-Gutierrez and H ctor E. Rivera-Sylva
2
Paleogeography and Paleoenvironment of Mexico during the Mesozoic
Wolfgang Stinnesbeck and Eberhard Frey
3
Turtles of the Mesozoic of Mexico
Donald B. Brinkman
4
Mesozoic Lepidosauromorphs of Mexico: A Review and Discussion of Taxonomic Assignments
V ctor Hugo Reynoso and J. Alberto Cruz
5
Plesiosaurs, Reptiles between Grace and Awe
Eberhard Frey and Wolfgang Stinnesbeck
6
Mexican Ichthyosaurs
Eberhard Frey and Wolfgang Stinnesbeck
7
Overview of Mesozoic Crocodyliforms from Mexico
Gerardo Carbot-Chanona
8
Mexican Pterosaurs-Rare Jewels in the Fossil Record
Eberhard Frey and Wolfgang Stinnesbeck
9
Mexican Saurischian Dinosaurs
H ctor E. Rivera-Sylva and Kenneth Carpenter
10
The Ornithischian Dinosaurs of Mexico
H ctor E. Rivera-Sylva and Kenneth Carpenter
11
A Summary of the Mesozoic Vertebrate Tracks of Mexico
Victor Manuel Bravo-Cuevas and Rub n Rodr guez-de la Rosa
12
The Cretaceous-Paleogene (K-Pg) Boundary in Mexico
Wolfgang Stinnesbeck and Eberhard Frey
Index
Contributors
C
Victor Manuel Bravo-Cuevas
Museo de Paleontolog a
Universidad Aut noma del Estado de Hidalgo
Ciudad Universitaria S/N
Pachuca, HID 42184
Mexico
Donald B. Brinkman
Royal Tyrrell Museum of Palaeontology
Box 7500
Drumheller, AB T0J 0Y0
Canada
Gerardo Carbot-Chanona
Museo de Paleontolog a Eliseo Palacios Aguilera
Direcci n de Paleontolog a Secretar a de Medio
Ambiente e Historia Natural Calzada de Los Hombres Ilustres S/N
Antiguo Parque Madero
Tuxtla Guti rrez, CHP 29000
Mexico
Kenneth Carpenter
Prehistoric Museum
Utah State University Eastern
155 East Main Street
Price, UT 84501
J. Alberto Cruz
Museo de Paleontolog a rea Acad mica de Biolog a
Universidad Aut noma del Estado de Hidalgo
Ciudad Universitaria S/N
Carretera Pachuca-Tulancingo km 4.5
Pachuca, HID 42184
Mexico
Eberhard Frey
Staatliches Museum f r Naturkunde
Erbprinzenstra e 13
76133 Karlsruhe
Germany
Jose Ruben Guzman-Gutierrez
Unidad Acad mica de Ciencias Biol gicas
Universidad Aut noma de Zacatecas
Edificio de Biolog a Campus II
Zacatecas, ZAC 98000
Mexico
V ctor Hugo Reynoso
Departamento de Zoolog a Instituto de Biolog a
UNAM Ciudad Universitaria
Mexico, DF 04510
Mexico
H ctor E. Rivera-Sylva
Departamento de Paleontolog a Museo del Desierto
Carlos Abedrop D vila 3745
Parque Las Maravillas
Saltillo, COA 25022
Mexico
Rub n Rodr guez-de la Rosa
Unidad Acad mica de Ciencias Biol gicas
Universidad Aut noma de Zacatecas
Edificio de Biolog a Campus II
Zacatecas, ZAC 98000
Mexico
Wolfgang Stinnesbeck
Institut for Earth Sciences
University of Heidelberg
Im Neuenheimer Feld 234
D-69120 Heidelberg
Germany
Acknowledgments
A
We thank James Farlow (Indiana University-Purdue University Ft. Wayne) and Bob Sloan (Indiana University Press) for their support in making this volume possible. Thanks also to Amy J. Schneider, copyeditor, for her hard work. Thanks also to the contributors for pushing themselves to complete their manuscripts.
Preface
H ctor E. Rivera-Sylva, Kenneth Carpenter, and Eberhard Frey
P
Dinosaurs and other Mesozoic reptiles of Mexico have long been underappreciated despite their first discovery more than 100 years ago. The first Mexican paleontological studies were mainly conducted on invertebrates and mammals. Other vertebrates did not draw research attention until recently. The discoveries and subsequent studies of the Mesozoic reptiles from Mexico were driven mainly by foreigners during the 1970s, and later during the 1980s, by Rene Hernandez. The discoveries provided encouragement for others to carry on the research. Many of those are contributors to the present volume.
The book summarizes several years of research from around the country. The first two chapters offer an introductive background on the history of Mexican paleontology and the geology of the country, while the other chapters cover different reptile taxa that have been found. Finally, the last chapter is an account of the extinction at the end of the Mesozoic from a perspective from Mexico. A geographic distribution of these chapters by state is shown on the map. Our intent in this edited volume, Dinosaurs and other Reptiles from the Mesozoic of M xico, is not to be the last word on the subject, but rather have it trigger more and expanded studies.


0.1. Map of Mexico showing the distribution of chapters by state. The greatest number of chapters include the state of Coahuila, which reflects the result of more field work, as well as fossil richness; the more fossils found, the greater the taxonomic richness.
Dinosaurs and Other Reptiles from the Mesozoic of Mexico
History of the Discoveries of Dinosaurs and Mesozoic Reptiles in Mexico
Jose Ruben Guzman-Gutierrez and H ctor E. Rivera-Sylva
1
The Earliest Records
The oldest records for the discovery of prehistoric gigantic bones in what is today Mexico begins with the mythology of pre-Hispanic Aztecs who believed in the existence of giants they called Quinametzin or Quinametin (Maldonado-K erdell, 1948; see also Mayor, 2005). Later, during the Spanish conquest, some documents chronicle the discovery of gigantic bones. One of these reports was by Antonio de Herrera y Tordesillas (1549-1626), who wrote in 1615 about the bones of giants from what are today the Mexican states of Tlaxcala and Yucat n. He also wrote that Hernando Cort s shipped some of the bones to the king of Spain during the first years of the Spanish occupation (Herrera y Tordesillas, 1730; published posthumously).
In 1754, Jesuit father Fray Joseph Torrubia (1698-1761) wrote in his work on the natural history of Spain and its colonies that fossils (petrifactions, as they were called) were organic in origin, which was contrary to the then-accepted explanation of them as games of Nature. He also dissents about what he calls Spanish Gigantology, which documented the existence of a race of giants in both the Old and New Worlds (Torrubia, 1754). These bones were recognized as belonging to fossil elephants as early as 1795. The remains of extinct proboscidians were commonly found during the excavations for buildings and wells. Although Pleistocene and other Late Tertiary mammals are common throughout Mexico, we have no record of dinosaur bones being found at this early date (Anonymous, 1799). Additional fossils from Mexico were mentioned by Don Andr s Manuel del R o (1764-1849) in his Elements of Orictognosy [mineralogy] (Del R o, 1795). Del Rio was a Spanish mineralogist who had studied with Abraham Gottlob Werner and Antoine Lavoisier.
The first report about dinosaurs in Mexico turned out to be an error. In 1840, Wilhelm Mahlmann briefly mentioned the discovery by Carl Degenhardt (a German mining engineer and friend of Baron Alexander von Humboldt) of footprints belonging to great birds at Oiva, which was erroneously stated as being in Mexico, instead of Colombia (Mahlmann, 1840). This error has subsequently confused many workers, who have perpetuated the mistake (e.g., Winkler, 1886; Kuhn, 1963; Thulborn, 1990) and was only recently corrected (Buffetaut, 2000). French geologists M. M. Dollfus and E. de Montserrat of the Commission Scientifique du Mexique made the first authentic report of dinosaurs from Mexico. Their report on the geological reconnaissance of the mining district of Sultepec, now a municipality in the southern state of Mexico, makes reference to some small saurian footprints : in the Barranca of Tisate, there is a whitish tuff, very fine, appearing to be perhaps silica. It is claimed that footprints of small saurians have been found in these tuffs, and we saw also the imprints of leaves and snails (Dollfus and Montserrat, 1867:479). This report essentially initiates the discipline of vertebrate paleontology in Mexico. About this time, Richard Owen, the creator of the Dinosauria, described the fossil horses from the Valley of Mexico and a fossil llama from Tequixquiac. However, he makes no mention of older vertebrate faunas.
In the mid-1880s, the American dinosaur hunter par excellence (along with his nemesis Charles Marsh) Edward Drinker Cope (1884, 1886) published on the extinct Mammalia of the Valley of Mexico and the Loup Fork Miocene in Mexico. He described Tertiary and Quaternary fossil mammals from the mining districts in the Valley of Mexico. However, he did not explore the Mesozoic strata that generally crop out in the northern part of the country, and so he missed the chance to find dinosaur remains.
In 1869, Mexican geologist Antonio del Castillo wrote his opus S ugethierreste aus der Quart r-Formation des Hochthales von Mexico which is a milestone in the history of Mexican vertebrate paleontology because it is the first paper written by a Mexican paleontologist (the previous publications were all written by European naturalists). In 1888, del Castillo was instrumental in founding the Geological Commission and the Instituto Geol gico de M xico (the National Geological Institute of Mexico). Construction of the Instituto Geol gico building was started on July 17, 1900. The building, which is now the Museum of Geology at the National Autonomous University of Mexico, was the first building in Mexico designed to house the collections and laboratories of the institute. The fa ade of the building shows interesting decorative elements comprising high and bas-reliefs of fossil marine fauna, such as ammonites and ichthyosaurs, and of pterosaurs, a fossil vertebrate group that was not discovered in Mexico until recently (see chapter 8 ). The architect Carlos Herrera L pez and the geologist Jos Guadalupe Aguilera Serrano (1857-1941), then the Director of the Instituto Geol gico, designed the fa ade. The building was dedicated on September 6, 1906, during the tenth International Geological Congress, held in Mexico City. Inside the magnificent building are ten large paintings by the great Mexican landscape artist and naturalist Jos Mar a Velasco (1840-1912), representing the evolution of life through the geological ages, from its origin in the primeval seas to the appearance of humankind. Unfortunately, neither dinosaurs nor marine reptiles appear in any of them. By the turn of the twentieth century, a group of mostly European scientists began publishing on the fossils of Mexico in the institute s two journals, the Instituto Geol gico de M xico Bolet n and Parergones del Instituto Geol gico de M xico. However, nothing appeared on dinosaur fossils.
The Twentieth Century
The first dinosaur or marine reptile bone found in Mexico may have been found by Jos Guadalupe Aguilera Serrano, director of the Instituto Geol gico de M xico from 1865 to 1912. The report of the discovery was made by Yale paleontologist George Reber Wieland (1865-1953), who wrote that Serrano had previously found in Chihuahua a weathered vertebral centrum attributable to a reptile from the beds equivalent to the American Pierre Formation (Wieland, 1910:360). Unfortunately, Wieland does not specify whether this information is based on a publication or a personal communication from Serrano. It is impossible to track down this specimen to determine its identity. The Pierre Formation of the United States is a marine deposit, so the bone may well have belonged to some marine reptile instead of a dinosaur.
In 1921, the German geologist and paleontologist Wilhelm Freudenberg published his book about the geology of Mexico and provided the first record of dinosaur ichnites in Mexico. The locality was in the state of Chihuahua, near the city of Parral. As far as we are aware, historians of Mexican paleontology previously have always mentioned Werner Janensch s 1926 report as the first published record of dinosaurs from Mexico, rather than Freudenberg s report. Unfortunately, Freudenberg did not illustrate the specimen, nor can the slab with tracks be found at the Instituto Geol gico de M xico, where it was sent. All that remains is Freudenberg s (1921:122) description: in 1907 the footprint of a dinosaur in a red sandy shale was found in Parral and was sent to the Instituto Geol gico in M xico. Such tracks are common in the Laramie Formation of the Union.
The first record of a marine reptile from Mexico appeared in a brief work by Wieland (1910), who described a new taxon, Plesiosaurus (= Polyptychodon ) mexicanus, from Tlaxiaco, Oaxaca. The specimen consisted of a partial rostrum with teeth that Wieland recognized as the first plesiosaur from Mexico. The specimen was lost for many years but was located by Mar a del Carmen Perrilliat through the suggestion of H ctor Rivera-Sylva in the collections of the Colecci n Nacional de Paleontolog a, Instituto de Geolog a of the National Autonomous University of Mexico. The specimen was restudied by Marie-C line Buchy (2008), who showed that it was of a thalattosuchian crocodile instead of a plesiosaur.
The first discovery of dinosaur skeletal remains in Coahuila was made by Erich Haarmann ( Fig. 1.1 ), a German geologist traveling in northern Mexico in the early twentieth century. While making a geological reconnaissance of Coahuila state in September 1910 (in the midst of the Mexican revolutionary war), fossil bones ( Saurierreste ) were found in what he called Soledad-Schichten, which he correlates with the Laramie Formation of the western United States. The discovery was made northeast of Soledad, north of Hacienda de Movano, in a place known as Lomas de Buenavista, in the state of Coahuila. Haarmann reported: Fossils are rare, silicified wood and vertebrate remains were found only in one place in the huge conglomerate. I had to content myself with picking up the weathered pieces that were not usually pretty well preserved. Unfortunately, there were no teeth, which would facilitate a determination. Eventually, I sent the vertebrate bones to Mr. Henry Schroeder, who kindly took over their investigation and came to the conclusion that it is most probable that they belong to dinosaurs. For an exact identification, collecting more abundant and better material is of course required, but shortage of water in that area makes that difficult, and it is currently impossible because of the revolution. Nevertheless, I hope that one day it will be possible to excavate more material on a larger scale (Haarmann, 1913:26).


1.1. (Left) Erich Haarmann (1882-1945), German geologist who collected the first dinosaur bones in Mexico (from Geologische Rundschau, 1942). (Right) Werner Janensch (1878-1969), German paleontologist who published the first report on dinosaur fossils from Mexico. Courtesy of the Museum f r Naturkunde der Humboldt-Universit t zu Berlin Historische Bild- u. Schriftgutsammlungen (Sigel: MfN d. HUB, HBSB Bestand: Pal. Mus. Signatur: B I/69).
The fossils found by Haarmann were later transferred to the Geological and Palaeontological Institute and Museum of the University of Berlin and examined by Dr. Werner Janensch (1878-1969; Fig. 1.1 ) of Tendaguru fame. Janensch (1926) published a brief account of the discovery and described the few remains. The material consisted of (1) a large fragment of right squamosum, (2) a small caudal vertebral body, (3) a large section of a big limb bone (femur?), and (4) two indeterminate fragments. Janensch refers the material to a ceratopsian, probably to the genus Monoclonius (= Centrosaurus ) or Triceratops. Re-examination of the bones suggests that they are hadrosaurian instead of ceratopsian (see chapter 10 ).
During the late 1920s, the great Mexican naturalist Alfonso L. Herrera (1868-1943) was director of the National Museum of Natural History in Mexico City, an institution founded in 1914. Herrera read in an important Spanish encyclopedia (Espasa-Calpe) that the steel entrepreneur and philanthropist Andrew Carnegie had made several replicas of the skeleton Diplodocus carnegii and had donated them to the most important natural history museums in the Western world. Herrera asked for a replica for his museum in Mexico City ( Fig. 1.2 ) but mistakenly thought that the replicas were made of bronze, which he planned to position outdoors near the big lake of the botanical gardens of Chapultepec (Anonymous, 1928). His request was made through the ambassador of Mexico to the United States, Manuel T llez. The ambassador made the formal petition to Carnegie, who liked the idea and turned the request over to William J. Holland (1848-1932), director of the Carnegie Museum. However, before the installation of the cast in the museum could be completed, Herrera resigned when control of the museum was transferred to the National Autonomous University of Mexico (Rea, 2001).


1.2. Cast of Diplodocus carnegii at the Museo de Historia Natural de la Ciudad de M xico as seen during the 1960s. This specimen was the first dinosaur mounted in Mexico (picture by Carlos L pez Campos, through the courtesy of the Museo de Historia Natural de la Ciudad de M xico).
While in Mexico City during the installation of the Diplodocus replica, Holland recalled seeing a dinosaur bone on exhibition at the National Museum of Natural History. He wrote, I found in a corner of the museum among the paleontological material the femur of a small dinosaur found in the state of Chihuahua. There are Jurassic deposits in northern Mexico, and possibly elsewhere, and our Mexican friends may find on their own soil some of the huge dinosaurs of the Mesozoic age. Let us hope so (Holland, 1930:86). Holland s words have proven to be prophetic as seen by recent discoveries (see chapters 9 and 10 ).
The first known report of dinosaur fossil remains from Baja California was made in 1925. Bone fragments were collected in Cretaceous sediment outcrops some 160 miles south of the American-Mexican border. Unfortunately, it was not possible to identify them due to the fragmentary state of the bones (Morris, 1971).
M. G. Mehl, from the University of Missouri at Columbia, published on a new genus of mosasaur, Amphekepubis johnsoni. The specimen was discovered and collected by Carlyle D. Johnson from Upper Cretaceous concretionary clay cropping out from a locality about forty miles east and a little north of Monterey [sic], in the state of Nuevo Le n, Mexico (Mehl, 1930:384). The material consisted of nine presacral vertebrae, four of which were articulated; the right and left ilia; the left pubis and part of the right; the left ischium; the left femur; the right and left tibia; the left astragalus and part of the right; the left and right first metatarsal; and the base of the fourth digit and one complete phalanx, as well as parts of other small phalanges. Mehl mentions: It is clear from the condition of the remains that a careful search would reveal other pieces of the specimen (Mehl, 1930:384). Other mosasaur specimens were discovered by the German geologist and paleontologist Friedrich Karl Gustav M llerried (1891-1952)-known during his stay in Mexico as Federico K. G. M llerried. M llerried (1931) recorded the discovery of two partial mosasaurs from a Late Cretaceous locality named Ray n. The site lay north of the Yames River, 60 km northwest of Tampico, Tamaulipas. The specimens were deposited in the National Museum of Natural History in Mexico City, but they are presently lost (Reynoso, 2006).
The American geologist Nicholas Lloyd Taliaferro (1890-1961), in his survey of Upper Cretaceous sediments in northern Sonora published in 1933, described the discovery of dinosaur remains: in the upper part of the Snake Ridge Formation large dinosaur bones were found in greenish, sandy shales not more than 100 feet stratigraphically below the base of the Camas sandstone. Dinosaur bones and teeth were found in greatly baked, dark carbonaceous shales between two rhyolite sills in the gap between the Muste as and Magallanes Peak. These were submitted to Mr. Barnum Brown, of the American Museum of Natural History, who very kindly furnished the following information: I have compared these series with our various Trachodonts and are of the opinion that this is an undescribed species. The material, however, is not sufficient for one to find a reliable species. From the teeth I infer this horizon to be comparable in age to the Edmonton Formation of Alberta. It was a large duck-bill dinosaur, equal in size to Trachodon mirabilis. The specimens are especially interesting to me, because it is the most southerly extension of these dinosaurs that has been recorded (Taliaferro, 1933:28). Richard Lull and Nelda Wright (1942) in their review of the hadrosaurian records of North America referred to Taliaferro s discovery as the southernmost record for a hadrosaurian dinosaur.
William Langston and Millis H. Oakes (1954) reported hadrosaur pedal remains from Punta San Ysidro, Baja California, which had been deposited in the Museum of Paleontology at the University of California. The material consisted of two similar-sized specimens that they referred to the genus Kritosaurus.
During 1959, a team of university students under the direction of Clarence O. Durham and Grover E. Murray conducted field work in the Parras Basin in the southeast portion of the state of Coahuila. They reported the discovery of dinosaur remains associated with marine mollusks in the Difunta Group near the village of Hip lito, Coahuila. Subsequently, the locality was re-examined by Grover E. Murray, Donald R. Boyd, James A. Wolleben, and John A. Wilson of the University of Texas. They collected material of at least four different dinosaurs in the area. The specimens were identified by Wilson and Edwin H. Colbert (at that time curator of vertebrate paleontology at the American Museum of Natural History) as belonging to the ceratopsian genus Monoclonius and generically unidentifiable hadrosaurs (Murray et al., 1960).
Colbert later collected the first theropod dinosaur remains consisting of isolated teeth from Baja California in 1960. Other specimens included a hadrosaur mandibular fragment with teeth, a hadrosaur vertebral centrum, and the distal end of a hadrosaur femur (Hilton, 2003). Further collecting in Baja was conducted from 1964 to 1981 by William J. Morris, who led teams from the Natural History Museum of Los Angeles County. They collected specimens of Late Cretaceous dinosaurs at El Rosario and El Gallo, including the giant hadrosaur ? Lambeosaurus laticaudus and the enigmatic theropod Labocania anomala (Morris, 1967, 1972, 1973, 1976, 1981; Molnar, 1974; Hern ndez-Rivera, 1994), as well as additional fossil remains of tyrannosaurids, ankylosaurids, and dromaeosaurids.
In 1969, the Mexican vertebrate paleontologist Angel Silva-B rcenas, in his account of Mexican vertebrate fossil localities, published a brief note at the end of his work about the discovery of dinosaur remains. The specimens were found in the Olmos Formation by the coal geologist Jose I. Delgado at Ejido de la Cuchilla, 2 km. east of Pala , Coahuila (Silva-B rcenas, 1969). The remains were shown to John A. Wilson, who considered it to be a complete skeleton of a ceratopsian, probably of the genus Triceratops. We were not aware of any other mention of this find, and Silva-B rcenas does not state whether the specimen was collected and housed in an institution. However, Ruben Rodr guez de la Rosa gave a brief report at a meeting of the Sociedad Mexicana de Paleontolog a in 2011. He reported that remains of horned dinosaurs were discovered near Pala , Coahuila, in the 1960s from the coal zone of the Olmos Formation. Wann Langston tentatively referred the material to the ceratopsian genus Chasmosaurus in an unpublished report addressed to the municipal government of Pala . The identifiable elements consisted of dorsal vertebrae, ilium and ischium fragments, and right appendicular elements, including femur, tibia, fibula, humerus, radius, and ulna (Rodr guez de la Rosa, 2011).
Silva-B rcenas also reported the discovery of what he determined to be theropod fossil remains including some fragments of rib, vertebrae, and mandibular sections provided with strong conical teeth, the dimensions and morphological features of which correspond to a carnivorous dinosaur. This discovery is noteworthy and important due to the kind of deposits where the dinosaur fossil remains were found. The strata surrounding Esquiveles, Durango, are lithologically [similar] with the Tendaguru Formation of East Africa than with the Morrison Formation. Attention is drawn to the fact that these skeletal remains were found in an area extensively studied by several authors, such as Burckhardt (1906, 1912, 1919), B se (1923), and Imlay (1938, 1939). The latest work about the region corresponds to Pe a-Mu oz (1964), but neither refers to dinosaur fossils (Silva-B rcenas, 1973:290). We presume that the remains reported by Silva-B rcenas are those of a marine reptile instead of a theropod dinosaur, but because the whereabouts of the specimen is unknown, this cannot be verified pending its rediscovery and re-study.
Ismael Ferrusqu a-Villafranca, Shelton P. Applegate, and Luis Espinosa-Arrubarrena from the Geological Institute, Universidad Nacional Aut noma de M xico (UNAM), described in 1978 the Chuta dinosaur ichnofauna from Michoac n state. The tracks included seven morphotypes assignable to bipedal theropods and ornithopod dinosaurs and are probably Middle Jurassic-Early Cretaceous age (Ferrusqu a-Villafranca et al., 1978, 1980). In 1980, the men, joined by Victor Torres Rold n, prospected in the Cretaceous sediments in the state of Coahuila in search of Mesozoic mammals. In the city of Torre n they met an amateur fossil collector, Dr. Luis Maeda Villalobos, who had in his collection huge dinosaur bones that he misidentified as belonging to mammoths. The specimens were found in the Ejido Presa de San Antonio, in the municipality of Parras, Coahuila (Applegate, 1988; Espinosa-Arrubarrena et al., 1989). Later, in Saltillo, Coahuila, they met Jos Rojas, another amateur collector who in 1977 had collected dinosaurs in the Ejido Rinc n Colorado, in the municipality of General Cepeda, Coahuila. One of his specimens was an articulated skeleton of a hadrosaur with skin impressions, and another was a ceratopsian. Rojas donated the fossil to the Geological Institute, UNAM (Hern ndez-Rivera, 1994).
In 1982, at the Annual Meeting of the Society of Vertebrate Paleontology in Mexico City, Shelton Applegate mentioned to Gordon Edmund of the Royal Ontario Museum finding quantities of dinosaur bone on a recent survey in the state of Coahuila. Edmund and fellow paleontologist Chris McGowan conducted field work during May 1984 in the fossil fields of Coahuila. The work resulted in the discovery of large hadrosaurs, a ceratopsian, a probable ornithomimid, and the crocodile-like reptile Champsosaurus (Edmund, 1985). The following year, a new party from the Royal Ontario Museum made a collection of dinosaur fossils in the locality known as Ejido Presa de San Antonio, Coahuila. Christopher McGowan, Kevin Seymour, Andrew Leitch, and Brian Iwama discovered a partial skeleton of a hadrosaur and a less complete ceratopsian. Theropod material was also recovered (Hern ndez-Rivera, 1997).
During 1988, a field team led by the Mexican dinosaur paleontologist Rene Hern ndez-Rivera spent forty days in the Coahuila desert collecting the skeleton of a gryposaurine hadrosaur, previously discovered by Ram n L pez, a rancher from Ejido Presa de San Antonio. They recovered 60 percent of the skeleton, comprising 218 fossil bones and about 400 other bones belonging to hadrosaurian and ankylosaurian dinosaurs. In June 1992, a replica of the hadrosaur 3.5 m high and 7 m long was mounted at the Museum of the Institute of Geology, UNAM, in Mexico City (Hern ndez-Rivera, 1997). This skeleton was nicknamed Isauria by a group of schoolchildren who visited the exhibit in honor of one of them, named Isaura. Replicas of this skeleton can be seen elsewhere in museums throughout Mexico.
In 1985, German geologist Walter Haehnel, then on the earth sciences faculty at Nuevo Le n Autonomous University, described what he erroneously supposed to be remains of a terrestrial dinosaur found encased in two blocks of marine limestone from the La Casita Formation (Upper Jurassic), near the town of Aramberri, Nuevo Le n. One of the blocks contained seven vertebrae, each measuring 23 cm in diameter, and another block included part of the skull, with some coniform teeth that Haehnel interpreted as belonging to a carnivorous dinosaur.
In the late 1980s, Jack M. Callaway and Judy A. Massare, from the University of Rochester, New York, described shastasaurid ichthyosaur material collected in 1925 by C. L. Baker (chief geologist of the East Coast Oil Company of Mexico) and in 1926 by Charles L. Camp (University of California at Berkeley). The specimens came from the Upper Triassic outcrops of the marine Antimonio Formation, in the El Antimonio district in northwestern Sonora state. Named Shastasaurus altispinus, the material consists of the anterior part of the skull and lower jaw, as well as a block of matrix with vertebrae, ribs, neural arches and spines, and fore-paddle elements (Callaway and Massare, 1989). Possibly three different individuals are represented. These specimens represent the first known occurrence of Triassic ichthyosaurs in Mexico.
In the early 1980s, vertebrate paleontologist James M. Clark from George Washington University discovered fragmentary dinosaur remains in the Early Jurassic deposits of Huizachal Canyon, near Ciudad Victoria, in Tamaulipas state. The diverse vertebrate assemblage includes therapsids, tritylodontids, and a new pterosaur species named Dimorphodon weintraubi. Most of the dinosaurs were identified by isolated teeth and large bone fragments as belonging to ornithischian and sauropodomorph dinosaurs.
In 2006, Regina C. Munter described the skull and postcranial material of an adult theropod dinosaur, previously collected by Clark in 1994 (Munter and Clark, 2006). The specimen consisted of the right half of an articulated sacrum and pelvis, as well as the left posterior section of the braincase. The material had been referred to a coelophysoid close to Coelophysis and Megapnosaurus kayentakatae.
In the early 1990s, a new collaborative project started between the recently created Paleontological Commission of the SEP Coahuila and the Dinamation International Society. The project gave American scientists access to the dinosaur fossil fields of Coahuila and led to international promotion of the dinosaur localities.
The Twenty-First Century
In 2000, the inauguration of the Museo del Desierto in Saltillo, Coahuila, and the foundation of its department of paleontology broadened the possibilities of new discoveries and publications with the collaboration of German, Canadian, and American scientists. For example, a new species of lambeosaurine hadrosaurian dinosaur (the first in Coahuila and the first in Mexico in more than thirty years) was named Velafrons coahuilensis (Gates et al., 2007), and a chasmosaurine ceratopsian was named Coahuilaceratops magnacuerna (Loewen et al., 2010). The ceratopsian was collected by schoolteacher and amateur paleontologist Claudio de Le n D vila in the Ejido Porvenir de Jalpa, General Cepeda, Coahuila.
The French paleontologist Marie-C line Buchy (formerly at the Museo del Desierto) collected and studied Late Jurassic and Cretaceous marine reptiles from the states of Coahuila and Nuevo Le n. She reported the first occurrences for Mexico of the following taxa: the ichthyosaurs Ophthalmosaurus and Brachypterygius, the thalattosuchian crocodilyforms Geosaurus and Dakosaurus, giant pliosaurs (Pliosauridae indet.), and the basal mosasauroid Vallecillosaurus (Buchy et al., 2003, 2005; Smith and Buchy, 2008; Buchy, 2010).
Conclusion
The history of dinosaur discoveries in Mexico is almost eighty years old, and other branches of paleontology have existed since colonial times. However, the growth of paleontology in Mexico was hardly noticeable until recently. Since the beginning of the 1990s, there have been several projects to learn more about the dinosaurs and Mesozoic vertebrates of Mexico. The surge in the discovery of new taxa has produced much valuable and varied information about the diversity of dinosaurs in Mexico, as well as their paleobiological, paleoecological, and paleogeographic information. Interestingly, many of the taxa are new genera and so far are unique to the country. Previously, the diversity of the dinosaurs and reptiles from the Mesozoic of Mexico was underappreciated, despite being well documented. Indeed, with each new discovery, the diversity of Mesozoic vertebrates will continue to increase and will lead to a greater appreciation of Mexico s paleontological past.
Acknowledgments
We thank our colleagues who have helped us get hard-to-find literature for this chapter: Gerardo Carbot Chanona (Museo de Paleontolog a Eliseo Palacios Aguilera, Tuxtla Guti rrez, Chiapas, M xico); Denver Fowler (Museum of the Rockies, Bozeman, Montana); Jerry D. Harris (Director of Paleontology, Dixie State College, St. George, Utah); and many others.
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Paleogeography and Paleoenvironment of Mexico during the Mesozoic
Wolfgang Stinnesbeck and Eberhard Frey
2
Introduction
We provide an overview on the geotectonic and paleogeographical development of northeastern Mexico during the Mesozoic, emphasizing the sediment record with findings of reptiles. We also briefly address other regions of Mexico with important coeval discoveries of reptilian remains, such as Michoac n, Puebla, and Baja California Norte.
To date, most discoveries of dinosaurs and other Mesozoic reptiles in Mexico are from the northeastern states of Chihuahua, Coahuila, Nuevo Le n, and Tamaulipas. The tectonic evolution of this region was dominated by passive-margin development associated with the opening of the Gulf of Mexico during the Middle to Late Jurassic, overprinted by effects of the non-igneous Laramide orogeny during the Upper Cretaceous to the Paleogene, and by the subduction of the Farallon plate along the Pacific margin since the Upper Cretaceous. The stratigraphic and environmental changes are principally dominated by eustasy (Salvador, 1991; Goldhammer and Johnson, 2001).
Triassic to Middle Jurassic: Mexico as the Western Margin of Pangaea
The convergence and collision of the North American and Gondwana continental plates at the end of the Paleozoic marked the origin of the Oachita-Marathon orogeny. Northeastern Mexico constituted the western margin of Pangaea for the next 100 million years, from the Triassic to Late Jurassic times. The Gulf of Mexico was not yet open and Mexico formed part of a single huge land mass with North and South America, as well as Africa ( Fig. 2.1 ). No marine sediments are known to exist from this time in northeastern Mexico (Salvador, 1991).
The northwestern part of South America was positioned where south and central Mexico are today. Terranes and blocks, which today are recognized in this region, must have been located some 700 km to the northwest. They moved into the region during the Jurassic along the Sonora-Mojave megashear fault zone, which crosses Mexico in a northwest-to-southeast direction (Pindell and Kennan, 2001, 2009).
The rupture of Pangaea began during the late Triassic to Middle Jurassic, along a barely defined zone between the continents Africa and North America of today. In the Gulf of Mexico region, as well as in north and central Mexico, the extension and thinning of continental crust, subsidence, and rifting was associated with the deposition of continental red beds in north-south-directed graben systems and with coeval volcanism (Salvador, 1991; Goldhammer and Johnson, 2001). In northeastern Mexico, the red bed sediments are known as the Huizachal and La Boca Formations ( Fig. 2.2 ; Imlay et al., 1948; Mixon et al., 1959; Michalzik, 1988, 1991; Barboza-Gudino et al., 1999). The La Boca Formation sensu Barboza-Gudino et al. (1999) is of Early to Middle Jurassic age and contains near the base the remains of dinosaurs, rhynchocephalians, crocodyliforms, synapsids, and one single pterosaur specimen ( Dimorphodon weintraubi ). This tetrapod assemblage was discovered in a canyon east of Ciudad Victoria in the state of Tamaulipas and is preserved in red siltstones and sandstones (Clark and Hopson, 1985; Clark et al., 1994, 1998).


2.1. The distribution of continents in the Early Jurassic period, prior to the breakup of the supercontinent Pangaea. The opening of the Gulf of Mexico during the Middle and Late Jurassic leads to a separation of North and South America and Africa.
Dinosaur trackways in the Mixteca region of Oaxaca were also discovered in continental red beds of Middle Jurassic age. This area was located on a small tectonic block, the Mixteca terrane, which was isolated during the disruption of Pangaea and then became an island. Sediments in the area were deposited under lagoonal environments (Ferrusqu a-Villafranca et al., 2007). Dinosaur bones were also discovered in southwest Puebla in late Jurassic (Tithonian) to early Cretaceous (Berriasian) sediments (Ortega-Guerrero, 1989; Rivera-Sylva, 2003). Late Jurassic dinosaur tracks near Playa Azul in Michoac n were assigned to theropods and ornithopods by Ferrusqu a-Villafranca et al. (1978).
Upper Jurassic to Lower Cretaceous: Opening of the Gulf of Mexico
The Gulf of Mexico formed during the Late Jurassic as a consequence of the breakup of Pangaea and the separation of North and South America. This divergent-margin development strongly influenced the Middle Jurassic through Upper Cretaceous tectonic development in the region. The Yucatan block, which was originally connected to present-day Texas, Louisiana and Florida, separated from North America, drifted southward and rotated in a counterclockwise direction (e.g., Pindell and Kennan, 2009).
The opening of the Gulf of Mexico was accompanied by widespread rifting and continental extension related to sea-floor spreading within the Gulf of Mexico. In northeastern Mexico, the Late Jurassic marks the onset of marine transgression during the late Callovian and Oxfordian. A coarse polymictic conglomerate, the La Joya Formation, grades into evaporates (gypsum, halite) and carbonates, which were deposited under marginal to shallow marine conditions. This unit, known as the Minas Viejas Formation, overlies the continental red beds of the La Boca Formation with an angular unconformity of 5 to 8 , related to the breakup of Pangaea ( Fig. 2.2 ). Deposition was locally accompanied by volcanism of significant volume and impact on the environment (Michalzik, 1988; G tte, 1990; Salvador, 1991; Goldhammer, 1999; Kr ger and Stinnesbeck, 2004).
During the late Oxfordian to early Kimmeridgian, sea level rise established more open marine conditions in the region. Limestones of the Zuloaga Formation overlie the evaporites and were deposited in shallow environments of a marine carbonate ramp deepening to the South ( Fig. 2.2 ; G tte and Michalzik, 1992; Goldhammer, 1999). The La Caja Formation is characterized by shale, siltstone, and phosphorite. This unit is of late early Kimmeridgian to early Berriasian age and conformably overlies the Zuloaga Formation (Goldhammer, 1999) ( Fig. 2.2 ). Lithologies and abundant and diverse invertebrate fossil record, principally ammonites, indicate deepening from shallow marine toward outer shelf environments and increased terrigenous influence. The laminated sediment and absence of benthic organisms, as well as a high degree of organic carbon and dispersed pyrite, indicate that oxygenation of the sea floor was poor.
A prominent paleo-high, the Coahuila Peninsula, persisted in the region northwest of Saltillo and shed enormous quantities of terrigenous sediment southward into the epicontinental sea ( Fig. 2.3 ). Proximal to this paleo-high, between Saltillo and Monterrey, the La Casita Formation is a shallow-water equivalent of the La Caja Formation. Its conglomerate, sandstone, and siltstone characterize deltaic fan and inner shelf environments (e.g., Fortunato and Ward, 1982; Michalzik, 1988; G tte, 1990; Adatte et al., 1994, 1996; Michalzik and Schumann, 1994; Goldhammer, 1999; Goldhammer and Johnson, 2001).
Microfossils and invertebrates in both the La Caja and La Casita Formations show that the Gulf of Mexico was at least temporarily isolated from both the paleo-Pacific ( Fig. 2.3 ) and the eastern and central Tethys (Adatte et al., 1994, 1996). The Florida uplift formed a barrier between the central Tethys and the Gulf of Mexico (e.g., Goldhammer and Johnson, 2001, and references therein). Moreover, block tectonic related to rapid seafloor spreading in the Gulf of Mexico led to a sea floor with an extensive topography subdivided into basins with restrictive flow (e.g., Goldhammer and Johnson, 2001; Fig. 2.3 ).
The La Caja Formation is exceedingly rich in marine vertebrates, among them pliosaurs, plesiosaurs, ophthalmoid ichthyosaurs, and metriorhynchid crocodilians, but curiously, no turtles (Frey et al., 2002; Buchy et al., 2003, 2006a, 2006b, 2006c, 2006d, 2007a, 2007b; Buchy, 2007; Buchy and Lopez-Oliva, 2009). Most discoveries are from Nuevo Le n and southern Coahuila. Ichthyosaurs (e.g., Ophthalmosaurus icenicus ) are more abundant in the proximal shelf sites in the Saltillo and Monterrey area, but they are also reported from distal shelf sites further to the south of Nuevo Le n in the vicinity of a local uplift (Buchy et al., 2006a, 2006b, 2006c, 2006d; Buchy, 2007). Thalattosuchians also tend to become rare toward the south, while pliosaurs are more abundant in southern localities (Buchy et al., 2006a, 2006d; Buchy, 2007). A gigantic pliosaur was discovered in the La Caja Formation at Aramberri, in southern Nuevo Le n. The specimen popularly named monster of Aramberri is a 15-m-long sub-adult, which was bitten to death by a still larger animal, probably also a pliosaur (Buchy et al., 2003).


2.2. Stratigraphic column of Mesozoic sediments in northeastern Mexico, from Michalzik (1988), Goldhammer and Johnson (2001), and Ocampo-Diaz et al. (2008).


2.3. Late Jurassic (Tithonian) paleogeography of northern Mexico. Based on Goldhammer and Johnson (2001).
Near Gomez Far as, about 70 km south of Saltillo (Coahuila), remnants of marine reptiles of early Tithonian age are concentrated in a 1- to 2-m-thick coquina layer, which almost entirely consists of ammonites and ostreid bivalves ( Fig. 2.4 ). Pliosaurs, plesiosaurs, ichthyosaurs, thalattosuchian crocodilians, and fishes are represented by isolated bones, as well as partial skeletons (Buchy et al., 2006b). Most of them are endemic species. The concentration of vertebrates in this site is outstanding and may have resulted from upwelling conditions near the shelf edge, widespread eutrophication combined with anoxia in seafloor and bottom water, and winnowing effects and formation of phosphorites (Buchy et al., 2006b).
The Passive Margin during the Middle Cretaceous
During the early Cretaceous, the pre-Callovian basement is still recognized in northeastern and central Mexico by changes in facies and submarine topography and consequently diverging thicknesses of sediment units. These differences are gradually smoothened out as spreading ceases in the Gulf of Mexico, creating a tectonically inactive shelf (Goldhammer, 1999). The Coahuila land mass still emerged and continued to shed fine-grained terrigenous sediment into the proximal seas. Its influence waned by Barremian to Aptian times. In distal areas to the south, and in the Sabinas basin to the east, hemipelagic carbonate mud and marls of Berriasian to Barremian age are known as the Taraises and Lower Tamaulipas Formations (Goldhammer, 1999; Goldhammer and Johnson, 2001) ( Fig. 2.2 ). Their microfossil and invertebrate content indicates that from Berriasian times onward, an oceanic connection existed with the European Tethys (Adatte et al., 1994, 1996).


2.4. Outcrop of the La Caja Formation at Gomez Far as, southern Coahuila. (A) Coquina unit of approximately 1.5 m thickness, representing a condensed interval. The coquina consists almost entirely of flattened ammonite shells and aptychs, as well as ostreid bivalves, but also contains abundant remains of marine reptilians. (B) Detail of the coquina layer with articulated vertebrae of an ichthyosaur.
Some of the world s largest Cretaceous carbonate platforms displaying reef and lagoonal environments are located in Mexico. Among them are the isolated Yucatan, Valles-San Luis Potos and Golden Lane platforms in southern and east-central Mexico, and the Cupido-Aurora platform to the northeast ( Figs. 2.2 , 2.5 ). This latter carbonate buildup developed on topographic heights rimming the Coahuila peninsula and reached up to 1,000 m thickness in the Monterrey area. Today, its thick-bedded limestone units, containing rudist biostromes, form the high mountain peaks in the northern Sierra Madre Oriental. All carbonate platforms in eastern Mexico developed during the Early Cretaceous in response to subsidence around the margins of the gulf. Vertical growth of the platforms kept up with the overall transgression (Goldhammer et al., 1991; Wilson and Ward, 1993). Deeper off-shelf equivalents of the Aurora Formation are the Upper Tamaulipas and Cuesta del Cura Formations of Albian to Early Cenomanian age ( Figs. 2.2 , 2.5 ).
The shallow-water platform carbonates underlie a series of monotonous limestone and shale known as the Agua Nueva, Indidura, and Eagle Ford Formations. These formations blanketed northeastern Mexico during the Cenomanian and Turonian and reached as far as the Western Interior and the Gulf of Mexico Basin ( Fig. 2.6 ; Goldhammer and Johnson, 2001; Ifrim, 2006). These sediments indicate continued deepening and development on a moderately deep shelf, low in relief, which formed along the western coast of the Gulf of Mexico. Monotonous, organic-rich black limestone, marly limestone and marl, rich in planktonic foraminifers and calcispheres, were deposited in the distal areas of this shelf, uninfluenced by the input of coarse terrigenous material from places such as the Woodbine Uplift in Texas or the Coahuila High (Sohl et al., 1991).


2.5. Albian paleogeography of northern Mexico. Based on Goldhammer and Johnson (2001).
The Agua Nueva Formation is generally considered to be poor in macrofossils (Sohl et al., 1991), but it includes a unit of thin-bedded laminated limestone and marl (plattenkalk) with abundant and exquisitely preserved fossils of latest Cenomanian to early Turonian age. The fossils include a diverse assemblage of fishes (selachians, coelacantids, pycnodontoforms, aspidorhynchiforms, teleosts) and marine reptiles (aigialosaurs, mosasauroids, turtles, pliosaurs, and polycotylids). The paleontological wealth of the Vallecillo plattenkalk was discovered in the 1990s (Blanco et al., 2001; Buchy et al., 2005; Ifrim, 2006; Ifrim et al., 2008). Vallecillo is situated 100 km north of Monterrey in the state of Nuevo Le n ( Fig. 2.6 ), but similar coeval lithologies and fossil deposits were subsequently discovered in other areas in northeastern Mexico such as Monterrey, Arteaga, and Ciudad Victoria. The Vallecillo-type plattenkalk is thus widespread and covers an area of at least 10,000 square km. It formed in an open shelf environment under anoxic conditions caused by the global Oceanic Anoxic Event 2 (OAE 2 of Schlanger and Jenkyns, 1976) (Ifrim and Stinnesbeck, 2008; Ifrim et al., 2008, 2011).


2.6. Cenomanian-Turonian paleogeography of northern Mexico. Based on Goldhammer and Johnson (2001). Note that late Early Cretaceous carbonate platforms are mostly drowned. Position of early Late Cretaceous plattenkalk localities with well-preserved marine reptilians are marked by stars. Note that these plattenkalks were deposited in an open marine environment, several hundred kilometers from the coastline.
Plattenkalk units also occur in central and northern Coahuila (e.g., north of Muzquiz, Fig. 2.6 ) and likely form part of the Upper Cretaceous Austin Group. This unit of open marine marl and limestone of Late Turonian-Santonian and possibly early Campanian age (Goldhammer and Johnson, 2001) is widely distributed in northeastern Mexico and southern Texas (Sohl et al., 1991). Although generally poor in fossils, plattenkalk units occasionally yield well-preserved marine vertebrates similar to Vallecillo, including fishes, mosasaurs, and pterosaurs (Buchy et al., 2005; Stinnesbeck et al., 2005; Frey et al., 2006, 2012; Ifrim et al., 2008; Giersch et al., 2010). Similar to the Vallecillo deposit, these fossil deposits also may be causally related to global Oceanic Anoxic Events (e.g., OAE 3).
The Upper Cretaceous Difunta Foreland Basin
The subduction of the Farallon Plate at the western margin of the North American plate induced magmatic activity and crustal shortening in the western North American continent. In western Mexico, the active Alisitos Magmatic Arc formed a continuous barrier toward the Pacific ( Fig. 2.7 ). The region east of the Alisitos Arc thus formed a back arc basin with a variety of volcanoclastic facies. Gradual uplift of the Alisitos Arc during Late Cretaceous times is expressed by a regional regression, which gradually progressed from west to east.
As a result of the eastward-directed uplift and sea-level regression, as well as rapid subsidence related to the Late Cretaceous Laramide orogeny, a large foreland basin developed north of the rising Sierra Madre Oriental (SMO) in an extended area of the Mexican states of Coahuila and Nuevo Le n. Immediately north of the SMO mountain range between Saltillo and Monterrey, sediment accumulated in this foreland basin from Campanian to Paleogene times and is known as the Difunta Group ( Figs. 2.7 , 2.8 ; McBride et al., 1974, 1975; Vega-Vera et al., 1989; Soegaard et al., 2003; Vega et al., 2007).


2.7. Late Cretaceous (Campanian-Maastrichtian) paleogeography. Based on Goldhammer and Johnson (2001). Sediment transport is via rivers from the northeast feeding an extensive deltaic system in Coahuila and eastern Coahuila, known today as the Difunta Group.
The Difunta Group represents shallow coastal and deltaic sediments and displays coastlines more than 500 km long ( Fig. 2.8 ). These sediments reach more than 6000 m in thickness proximal to the SMO and gradually decrease to the north. In the Rio Grande area, about 300-400 km north of Saltillo, their total thickness reaches only 100-200 m, and the sediments are known as the Escondido Formation (Cooper, 1970, 1971). The Aguja Formation in eastern Chihuahua and northern Coahuila is also coeval with the Difunta Group and was deposited under deltaic conditions, including marsh, lagoonal, and eulittoral to shallow marine environments (Hopkins, 1965; Weidie et al., 1972; Lehman, 1982).
The Difunta Group delta was fed by a river system located in Chihuahua and western Coahuila that emptied into the ancient Gulf of Mexico. Progression of the delta front gradually moved the coastlines eastward. During the Campanian, they reached the area of Saltillo and, during the Maastrichtian, the coastlines expanded toward Monterrey ( Fig. 2.8 ; Weidie et al., 1972; McBride et al., 1975). Sediment deposition in these areas was cyclical and consisted of alternating prodelta, delta front, delta plain, and fluvial sediments.
In the La Popa Basin north of Saltillo and Monterrey ( Fig. 2.8 ), rising salt diapirs formed small islands in the delta plain environment. At their rims, carbonate sedimentation prevailed, and rudist and coral/rudist lithosomes thrived in near coastal settings. These carbonate lentils are Maastrichtian to Paleogene in age and reach thicknesses of more than 300 m (Lawton et al., 2001).


2.8. Paleogeographic map of northeastern Mexico during the Late Cretaceous (Campanian-Maastrichtian). Note that coastlines of the Difunta deltaic complex progradated from west to east. To the east, in Nuevo Le n and Tamaulipas, the Mendez Formation is coeval to the Difunta Group and was deposited under open marine conditions. North of Monclova, extensive paralic coal swamp systems developed in the Rosita region of northern Coahuila.
The entire area extending from eastern Chihuahua to northern Nuevo Le n is extremely rich in well-preserved tetrapod remains. The assemblages are diverse and include trionychid turtles; crocodilians, among them eusuchians and the massive alligatoroid Deinosuchus (Rivera-Sylva et al., 2011b); mosasaurs (Buchy et al., 2005); and abundant and diverse dinosaur remains (Kirkland et al., 2006; Rivera-Sylva et al., 2006, 2009, 2011a; Gates et al., 2007). Important dinosaur-bearing localities are known from the Parras Basin at Las Aguilas, near the hamlet of Porvenir de Jalpa, and at Rinc n Colorado. The deposits are late Campanian in age and display a rhythmical sequence of sandstones and siltstones deposited under brackish conditions, as indicated by oyster banks and by abundant non-ostrean bivalves and gastropods, but also by sharks, mosasauroids, dyrosaurid crocodilians, and the shallow water ammonite Sphenodiscus . Freshwater to brackish conditions are indicated by characean oogonids, vascular plants, shell fragments of trionychid turtles, goniopholid crocodilians, and dinosaurs (e.g., Eberth et al., 2004; Rodr guez-de la Rosa, 2007; Meyer et al., 2008). See Plate 1.
Toward the northwest, the Aguja Formation in northeastern Chihuahua is also known for coeval tetrapod assemblages. Dinosaurian remains include nodosaurids, ceratopsians, theropods, and titanosaurid sauropods. Trionychid turtles and fishes are also reported from the braided river system deposits (Ballesteros, 2003; Rivera-Sylva et al., 2006, 2011a). See Plate 2.
East and south of Monterrey, the Campanian to Maastrichtian M ndez Formation was deposited in a prodeltaic to deep marine shelf environment, with water depths of approximately 100 m near Los Ramones, 40 km northeast of Monterrey, and more than 400 m in the La Sierrita region, 40 km north of Linares (Stinnesbeck et al., 2001). The M ndez Formation is widely distributed in the Gulf Coast plain of Mexico ( Figs. 2.7 , 2.8 ). The unit is up to 1,000 m thick and consists of rhythmically bedded marl, shale, and minor sandstone. At present, the vertebrate content of this unit is restricted to rare mosasaurs (Buchy et al., 2007a, 2007b).
Tetrapods from the Late Cretaceous in Other Regions of Mexico
Morris (1967, 1973) reported on hadrosaurian dinosaurs from Campanian strata of the El Gallo Formation in northern Baja California, near the village of El Rosario. Molnar (1974) documented theropod dinosaur remains from the La Bocana Roja Formation from the same region. Remnants of other dinosaurs (e.g., ankylosaurids; Rivera-Sylva et al., 2011a), birds, squamates, mammals, and amphibians have been collected since then. The El Gallo Formation consists of siltstone, shale, cross-bedded sandstone, and conglomeratic lenses, which were deposited in near-shore lagoons and playas. The assemblage is the only extensive Late Cretaceous terrestrial fauna from the Pacific margin of North America and formed on the western margin of the Alisitos Magmatic Arc.
In northeastern Sonora, the Cabullona Group has been dated to the late Campanian to early Maastrichtian and was deposited in fluvial and lacustrine environments of the Alisitos forearc. In addition to dinosaurian remains (e.g., hadrosaurs, ceratopsians, and carnosaurs), the vertebrate assemblage includes fishes, turtles, crocodilians, and lacertoids (Lucas and Gonz lez-Le n, 1993, 1996; Rivera-Sylva et al., 2006). Benammi et al. (2005) reported on the occurrence of hadrosaurid dinosaurs in the region of Tiquicheo in the state of Michoac n, from a continental sedimentary unit of Late Cretaceous age previously unknown from the region and unrelated to the widespread Balsas Group of Cenozoic age. From southern Mexico, dinosaurian remains are also known to exist in the Ocozocoautla Formation (Maastrichtian) from Chiapas, but they are yet undescribed (Rivera-Sylva et al., 2006). They are currently the southernmost dinosaur remains from North America.
Timing of the Chicxulub Impact Crater
The extinction of non-avian dinosaurs, marine reptiles, and pterosaurs at the end of the Cretaceous is frequently attributed to the bolide impact at Chicxulub near the city of Merida in Yucatan. Cretaceous-Paleogene (K/Pg) boundary sections in the M ndez Formation between Monterrey and Ciudad Victoria are of critical importance for an evaluation of the timing and consequences of this impact. A complex sequence of sandstone, siltstone, and claystone is widely present in this area and includes an impact-derived spherule-rich deposit at its base and an iridium anomaly at its top. These siliciclastic deposits are intercalated between hemipelagic marls of the latest Maastrichtian and the early Paleocene ( Fig. 2.9 ; Smit et al., 1992, 1996; Stinnesbeck et al., 1993, 1996; Keller et al., 1997, 2002; Smit, 1999; Schulte et al., 2010).


2.9. Outcrop at Mimbral, Tamaulipas, presenting the Cretaceous-Paleogene (K-Pg) boundary. Note that the late Maastrichtian Mendez Formation is present at the base of the section and is disconformably overlain by a siliciclastic deposit, with a spherule-rich unit, a massive sandstone unit, and a unit of interlayered sandstone-siltstone at the top. The iridium anomaly known to exist globally at the K-Pg boundary was identified in the uppermost 0.1 m of the sandstone-siltstone unit.
During the past two decades, these northeastern Mexican K/Pg boundary sections have been controversial. Advocates of the Chicxulub impact theory suggest that the entire siliciclastic unit was deposited within hours or days after the impact as a result of a mega-tsunami backwash from the nearby coast (Smit et al., 1992, 1996; Smit, 1999, Schulte et al., 2010). An alternative long-term depositional scenario places the K/Pg boundary at the level of extinction of tropical and subtropical planktonic foraminifers at the top of the unit coincident with the global iridium anomaly (Stinnesbeck et al., 1993, 1996; Keller et al., 1997, 2002). These latter authors advocate that the unit of Chicxulub impact glass spherules, located below the siliciclastic deposit, is stratigraphically separated from the iridium anomaly above this unit and thus represents two different impact events. The two are separated by a period of 250-300 kyr, with burrows of endobenthonic organisms, erosion, and transportation of sediments. In this interpretation, the Chicxulub impact is considered to be a late Maastrichtian event which is not aligned with the great end-Cretaceous global extinction (see also chapter 12 ).
Laramide Uplift and Compression
The 1.5-km-thick Upper Jurassic to end-Cretaceous sediment sequence was uplifted and folded during the Late Cretaceous-Early Paleogene Laramide Orogeny, giving rise to the Sierra Madre Oriental (SMO). The orogeny was non-igneous. In northeastern Mexico, folding principally occurred along the Upper Jurassic Minas Viejas evaporite unit, using this horizon as a shear and slide zone. In this area of the SMO foldbelt, outcrops of Upper Jurassic sediments are thus found only in the center of anticlines and diapiric domes. Laramide foreland deformation in the Difunta Group was less intensive than in the SMO and also culminated through the early Paleogene. In other regions the SMO is a relatively complex fold and thrust belt with significant differences in the style of deformation (Eguiluz de Antu ano et al., 2000).
During the Paleogene, a change in subduction style at the western active continental margin of Mexico caused magmatism in or near the Gulf coastal plain east of and parallel to the Sierra Madre Oriental. Seven magmatic centers of the eastern Mexican Alkaline Province are recognized, among them the Candela-Monclova volcanic belt, the Sierra de Picachos, and the Sierra de San Carlos.
With respect to the Mesozoic record of dinosaurs and other reptiles, Mexico represents a rare, if not unique geological data set. A nearly undisturbed sequence of sedimentary regimes is available from the Middle Jurassic to the end of the Cretaceous that allows the reconstruction of paleoenvironment and climate changes during the second half of the Mesozoic. These changes influenced the evolution and distribution of dinosaurs and other reptiles worldwide. During the Mesozoic, the area of modern Mexico was the only connection between Lauramerica and Gondwana, but it also connected marine regimes of the Tethys with the paleo-Pacific and, during Late Cretaceous times, with the Western Interior Seaway and the paleo-Atlantic. However, Mexico was also characterized by geographical separations resulting in endemisms on land and in the sea, thus forming a unique window into the evolution of ecosystems.
Acknowledgments
We thank J. G. L pez-Oliva (Facultad de Ciencias de la Tierra, Universidad Aut noma de Nuevo Le n, Linares, Mexico), A. H. Gonz lez-Gonz lez, and J. M. Padilla Gutierrez (Museo del Desierto, Saltillo, Mexico) for their support in the field and Dante Mor n Zenteno (Universidad Nacional Aut noma de M xico) for helpful comments on this paper. During numerous years our research was supported by the German Science Foundation (DFG) (FR1314/10, STI128/9 and STI128/17) and the Volkswagen Foundation (I/78866).
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