Studies on Mexican Paleontology

The joint discussion of biotic and abiotic factors allows proposal of a hypothesis in which the dynamic nature of the geological processes and of the organisms facilitate the understanding of the biodiversity of Mexico. In this respect, we highlight that since the Cretaceous, Mexico had a configuration similar to that of a peninsula that increased in size through time, but that with the establishment of the Isthmus of Panama approximately 3.5 my ago, it no longer had such a configuration. During this peninsular configuration, the main mountain belts of Mexico formed in different episodes of time attaining variable sizes through dynamic and fluctuant processes until they reached their extant physiognomy. At the same time the retreat of the seas was significant in exposing positive zones of the continent, where life was established. Finally, after occupying their current positions, the Baja California Peninsula and the Yucatan Peninsula started providing cover to the central continental mass of Mexico. The fossil record suggests that while all of this occurred, the newly generated environments were occupied by plants that were formerly part of higher latitude communities of North America. That through the land bridges that connected North America with Europe and Asia, plants with different geographical affinities arrived in Mexico. Also with the establishment of the Chords Block as the northern part of Central America, at least part of the plants already growing on the Chortis Block arrived into the rest of Central America eventually expanding their distribution into South America. The interaction of the biological processes, as suggested by the fossil record with the environment, coupled with the geological setting, promoted the development and establishment of a distinctive biota among which, the endemic plants that characterize today the vegetation of Mexico stand out.

Some of the extant Mexican plants were already present in the Oligocene and possibly since the Eocene. Finally, the current landscape and the elements that conform it began their “modernization” some 3.5 million years ago, but they may have a longer history that started in the Cretaceous.

It is evident that the coniferous forest and oak communities inhabit the mountain ranges since the Eocene becoming more abundant in the Oligocene, but they became the dominant taxa in the pollen assemblages from the Neogene to Quaternary.

According with the Tertiary flora analyzed in this survey the actual vegetation present in Mexico is a response to climatic changes and active volcanism throughout the Tertiary. Finally, different types of vegetation began to be recognizable in the fossil assemblages since the Oligocene. The historical development of Mexico (tectonic, rifting, volcanism) explains the richness and endemism of Mexican flora.

The majority of the Mesozoic scleractinian corals reported from Mexico, 117 species or about 80 percent, are from Cretaceous strata. Comparatively little is known of the earlier Mesozoic corals. The Jurassic corals, a total of 17 species, have been described from five localities in Mexico, all in Upper Jurassic rocks. The 10 Triassic coral species described from Mexico are from the Upper Triassic Antimonio Formation of northwestern Sonora. The majority of the Jurassic and Triassic coral species reported from Mexico have been described previously from occurrences in Europe.

Mexico, as a country, is remarkable for its abundant Cretaceous fossil invertebrate fauna. During the Cretaceous, what now is Mexico, was almost entirely covered by large shallow seas, which record a number of rudist deposits. The distribution of land masses, seas, climate and low ocean water depth were very suitable for rudist growth. As time passed, the geography changed and rudist abundance, distribution and faunal associations declined. This chapter reviews the close relationship between rudist assemblages and Mexican paleogeography. The data offered herein, concerning the number of genera and species of rudists, is based on what is recorded in the literature. This is not a systematic review because we never take into account synonyms or particular opinions about the validity of one or another genera or species of rudists. Finally we will examine some selected areas with interesting fossil rudist faunas.

In recent years, the number of described species of fossil crustaceans from Mexico has increased noticeably. Numerous new occurrences and new taxa were described from lithostratigraphic units that range in age from Early Cretaceous to Miocene. These reports included not only the systematic description of the specimens, but also information on paleoenvironment, paleoecology and evolution. The present work is a review of the fossil record of fossil crustaceans from Mexico, including comments on new localities and fauna that are under study. Based on the ongoing research in this field, it is possible that the number of fossil species of brachyurans will reach twice as much as the ones reported nowadays in a few years.

This chapter offers an up to date list of Upper Cretaceous Mexican fossil echinoids. There are 149 known Cretaceous echinoids from 16 states of Mexico including: Baja California Sur, Chiapas, Chihuahua, Coahuila, Guerrero, Hidalgo, Jalisco, Michoacán, Nuevo León, Oaxaca, Puebla, San Luis Potosí, Sonora, Tamaulipas, Tlaxcala and Colima. Up until the first half of the XIX century reports on the fossil echinoids of Mexico were only vaguely mentioned within geological papers. After the XIX century date specific paleontological work on echinoids began to frequently appear. Recently, diverse paleontological studies are in progress, in the State of Chiapas which has generated a local list of nine species registered for the fist time in Mexico belonging to the following genera: Orthopsis sp., Salenia sp., Goniopigus sp., Neotlingaster sp., Gompechinus sp., Lanieria sp., Petalobrissus sp., Gitolampas, sp., and Heteraster sp. These species show great affinities with Caribbean, European and Asiatic faunas.

In Mexico there are two principal problems referring to the study of fossil echinoids: one is the lack of investigations focused on biogeographic, evolution and paleobiological aspects; and the second is the lack of a taxonomic revision of echinoid species reported in the literature.

Here we present an up to date study on fossil ophiuroids from Mexico. The specimens come from the following States within the Mexican Republic: Baja California Sur, Chiapas, Chihuahua, Coahuila, Oaxaca and Puebla. The total number of specimens is 20 and one ophiuroid bed that bears many specimens. The ophiuroids have been collected in rocks from upper Paleozoic age (Pennsylvanian) to Pliocene. Almost all specimens are well preserved and with the aid of several techniques such as X rays and detailed images taken from SEM, they are being described. We have recognized two families and four genera all belonging to the order Ophiurida. A new species will soon be published in a research paper.

Upper Paleozoic fossil invertebrates of Mexico represent communities of marine environments associated with continental margins. A high percentage of very diverse brachiopods and mollusks, accompanied by other lesser diverse groups such as echinoderms, arthropods, bryozoans and corals occur throughout the Upper Paleozoic sequence of Mexico. Outcrops of Carboniferous and Permian rocks are especially abundant and Mexican fossil faunas from those systems exhibit an affinity to similar age fossil faunas from the rest of North America; this is especially true in the youngest faunas. This similarity may be explained by the configuration and geographic position of the Late Paleozoic crustal portions that form modern day Mexico. During the Late Paleozoic Pangea was forming and the ocean that existed between Gondwana and the northern continents was narrowing: during the entire Carboniferous and Early-Middle Permian, that ocean covered large regions of North America and extended over the majority of what constitutes present-day Mexico.

This paper provides an updated review of the Mesozoic fossil Osteichthyan record of Mexico. This catalogue includes multiple entries of papers, theses, and diverse scientific meeting reports regarding Cretaceous Osteichthyan fishes of Mexico that reflect the present state of knowledge. In addition, it provides a review of the Mexican paleoichthyology history, including the most recent fossil Osteichthyan discoveries. Within this framework some systematic and paleobiogeographical implications generated from the enrichment of the Mexican Mesozoic Osteichthyan record are briefly discussed providing future perspectives to this young field of Mexican science.

The amphibian and reptile fossil record in Mexico is scarce. There have been reported 255 unique taxon/locality/age records from 85 localities. Most fossils are fragmentary and their taxonomic status is difficult to assess; however, some are beautifully preserved, permitting even good cladistic and biogeographical analysis. Among amphibians there are records of caudates and anuras, restricted to the Neogene and Quaternary. Reptiles include turtles (which are the most abundant), crocodiles, ichthyosaurs, sauropterygians, a basal lepidosauromorph, sphenodontians, a basal squamate, true squamate lizards, and snakes, from the Late Triassic to the Holocene. Localities are distributed in 21 Mexican states from which Sonora bears most records. Nineteen species based on Mexican fossils have been erected. Three have been considered invalid, one has changed taxonomic status, and at least two more are questioned. Most of the type specimens designated are housed in Mexican institutions. There have been 99 main non-redundant published documents that mention, list, or describe fossil amphibians and reptiles, including scientific papers, books and book chapters of international and national circulation, meeting abstracts, guidebooks and newsletters. Of all papers published, only 51 are formal publications in either national or international press and 40 were published in international journals or editorials. Mexicans have published only 10 international papers. However, Mexican paleoherpetology has significantly increased in the last two decades, although most information is hidden in gray literature of difficult access to the international community. It is imperative that Mexican workers document their discoveries in formal publications.

Several dinosaur localities have been reported from Mexico. In 1926 Werner Janensch reported the first dinosaur bone remains as Monoclonius from La Soledad, western Coahuila. Since 1926, more dinosaur remains have been reported from the northern Mexican states of Baja California, Sonora, Chihuahua, Coahuila and Tamaulipas. Several dinosaur footprint localities have also been reported from the northern portion of Mexico. From the central and southern Mexican states of Michoacán, Puebla and Chiapas dinosaur remains have also been reported. Recently, fragments of Heterodontosaurus and Syntarsus were found in Tamaulipas. Re-evaluation of dinosaur remains from Puebla indicate that they belong to a tetrapod. In Baja California there are recent findings of Aublysodon teeth. In Chihuahua fragments of a member of Titanosauridae have been found. In Michoacán hadrosaur bones have been reported. The ichnological record has increased in recent years with the discovery of new localities in Coahuila and Puebla, where two localities include tracks of tetrapods, sauropods and ornitopods. Ichnites from one ornitopod family and from several tetrapod families have been reported from Michoacán. Since the discovery of dinosaur remains about 80 years ago, dinosaur discoveries in Mexico have been significant however the field is still in its infancy with much still to be learned.

Although the earliest report of a fossil mammal from Mexico dates from 1799, our knowledge of the group is still poor. The Mexican mammalian fossil record is biased towards the large-sized taxa and younger ages.

The mammalian record in Mexico ranges from the Jurassic to the Quaternary. Most of the Cenozoic epochs, except for the Paleocene, have mammal bearing deposits. There are exists gaps, such as late Eocene early Oligocene, early Miocene, and early-late Miocene, where no fossils have been found. The Mesozoic mammalian record is poorly represented by three localities in the northern part of Mexico. The early Tertiary is also represented by only a few localities widespread throughout Mexico. The late Tertiary (late Miocene-early Pliocene) is represented from well documented exposures in the central part of Mexico. Although the Quaternary record is very abundant and is present in all the states of Mexico, there are problems with detailed dating of these localities and little is known about small size mammals.

Due to its geological history and geographic position, Mexico bears a great diversity of environmental conditions, which is reflected in its high extant mammal biodiversity. Mexico played an important role in the evolution of several mammal groups during the Cenozoic. During the late Mesozoic and early Cenozoic, Mexico was the southernmost landmass of North America, where several mammal groups evolved, adapting to more tropical conditions. Later, during the Great American Faunal Interchange, Mexico was the path for the South American mammal immigrants to northern lands and it was the region where some immigrants remained. The role that Mexico played during the Ice Ages, whether if it was a refuge for northern forms or not has been questioned.

Additional work is needed in order to have a more comprehensive understanding of Mexican paleomammals.

The Tlayúa Quarry in Tepexi de Rodríguez, Puebla, Mexico, is one of the most important localities in the New World because of its uniquely rich biota. For the last twenty years, its exceptionally well-preserved fossil content and its taphonomical and paleoecological implications have caught the attention of many geologists and paleontologists since this Lagerstätte represents an important new asset in Mexican Paleontology. In this paper, we review all the recent investigations related to a development of the paleoecological models to explain the extraordinary conservation of the rich and diverse fossil assemblage of Tlayúa.