Dinosaurs in MexicoRené Hernández Rivera*

Mexico is a vast, colorful mosaic of rocksof different origins and ages, witness-es to the biological and geological

events that occurred as they were forming. Someof the most outstanding studies of the differenteras include the discovery of the fossils of main-land vertebrates, among them the dinosaurs of theMesozoic era, in particular the Jurassic and Cre -taceous periods. The importance of this type offossil lies in their diversity, abundance and oftenexcellent state of preservation, all of which makes

them key to reconstructing prehistoric environ-ments and possible lines of origin, evolution anddissemination of the different animal groups thatlived in our country in that era.1

Until the late 1980s, paleo-geographic recon -structions of the upper Cretaceous period inNorth America, particularly of the Campanian-Maestrichtian ages (from 85 million to 65 mil-lion years ago, quite an important span of time),only included the southern United States and/oralways depicted Mexico as covered by the sea.Thanks to discoveries and the studies done

since then in different states of Mexico, thisidea has changed. Today, we can say that the

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* Researcher in the Paleontology Department ofthe UNAM Institute of Geology.

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The first dinosaur remains found in Mexico were discovered in 1926.

Voices of Mexico • 55

southeastern part of what is now the state of Coa -huila was covered with broad expanses of shal-low, salt-water bays linked to the prehistoricSea of Tethys, and that dinosaurs, and theimpressive flora and fauna that accompaniedtheir existence on Earth, lived in the estuariesand islands in an environment that has nowbeen dubbed the Cretaceous Beaches.

LOCALES OF THE JURASSIC PERIOD

Until now, no discoveries of dinosaur fossilsfrom the Triassic period in Mexico have beenreported. Finds dating from the Jurassic peri-od, however, have been made in the HuizachalCanyon in Tamaulipas state: a few isolatedteeth from Ornithischia (an order of Archo -saurian reptiles made up of herbivorous di -no saurs, whose waist is similar to that of abird), tentatively re lated to the Hetero don to -saurids (considered the equivalent of pigs among

mammals).2

The remains of another ani -mal from the Sau ris -chia dinosaur group(with a reptilian-typewaist) have been clas -

sified as from a new spe cies

called Mexica num, of the Syn tarsus genus andCoe lo physoidea family. It is a small carnivorousdinosaur with a slim body, averaging about 3meters in length. Cranial matter from the Ce ra -tosaurus genus, Abelisauridae family, has alsobeen identified. These were carnivorous di no -saurs about 7 meters in length.The Huizachal Canyon area in Tamaulipas,

discovered by Dr. James M. Clark in 1982, isvery important for Mexico because it is therethat the country’s oldest remains of terrestrialvertebrates have been found, dating to approxi-mately 180 million years ago, placing them inthe latter part of the early Jurassic period.Among these are the Tritylodontids, repre -

sent ed by the Bocatherium mexicanum genus,of the group of proto-mammals or mammal-likereptiles. In addition, at least five different taxahave been identified, that is, new groups ofmammals.Other vertebrates found are the Croco dylo -

morphs and at least three distinct types ofSphenodontians related to the Rhyncho cepha -lians, two of them named Cynosphenodon huiza -chalensis and Zapatodon ejidoensis.Another animal unearthed has been named

the Tamaulipasaurus morenoi, which resembledtoday’s Amphisbaenians, digging reptiles withvery small eyes.

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Centrosaurus. Fragments of large bones have been collected.

Science, Art and Culture

An important find was a flying reptile deter-mined to be a new species of the genus Di mor pho -don, which has been given the name Weintraubi.Its state of preservation was surprising since itwas found in three dimensions, or “in bulk”,which is very rare since almost all Pterosaursfound have been crushed. Its anatomy, particu-larly the form of its foot, proves that flying rep-tiles or primitive Pterosaurs walked on four feetand not two as some authors suggest.Paleo-ecological reconstruction of the area

shows volcanic activity including eruptions,falling ash and mud slides, something rathersimilar to what happened a few years ago at theChichonal volcano in the state of Chiapas. Theanimals, trapped by lava flows or the fallingash, were buried and at the same time pre-served through a rather unusual process of fos-silization almost unique in the preservation ofvertebrates. This also explains why there are noexamples of medium- to large-size animals andonly small fossils have been located.In Puebla, in the “red layers” near the town of

San Felipe Ameyaltepec, fragments of large bonesidentified as from Sauropods have also been col-lected. The age of the rocks, however, is still amatter of debate. Also being researched are thequite fragmented dinosaur remains discovered inChiapas in rocks from the Cretaceous period.

DINOSAURS FROM THE CRETACEOUS PERIOD

The first dinosaur remains found in Mexicowere discovered in 1926. The remains of a Ce -ra topid (a dinosaur with horns on its face) wereunearthed in a dig at Soledad, near Múzquiz inthe state of Coahuila. Later, there were reportsof the discovery of Hadrosaurs (literally largereptiles, better known as picos de pato) in So no -ra (1942) and Baja California (1954) and Hadro -saurs and Ceratopids in Coahuila (1959).During the 1980s and part of the 1990s, dif-

ferent research projects were launched to inves-tigate the existence of dinosaurs from the Cre -taceous period in Coahuila and Sonora.3

Paleo-ecological studies suggest that inSonora the enormous herbivore dinosaurs, theHadrosaurs and Ceratopids, must have neededenormous quantities of vegetation to surviveand that the climate must have been hot, withabundant rainfall. The evidence from the kindof sedimentary rock formed by the Cabullonagroup reinforces this idea, given that they seemto have been deposited by vast lakes and riversthat covered the prehistoric landscape.In late 1987, the UNAM Institute of Geology

(IGLUNAM) formally began the study of dino -saurs of the Cretaceous period with a projectcalled “First Mounting of a Dinosaur Gathered,

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René Hernández participates in several paleontological digs. Hadrosaur of the Kritosaurus genus prepared and pieced together in the IGLUNAM laboratory.

Voices of Mexico • 55

Prepared and Pieced Together in Mexico.” Inspring 1988, 65 percent of a 7-meter-long di -nosaur was gathered in the Presa San Antoniocollective farm, or ejido, in Parras, Coahuila. Itwas then prepared and pieced together in thelaboratory of the UNAM’s Institute of Geologymuseum.The dinosaur skeleton is now on exhib-it at the museum’s Paleontology Room. Basedon an examination of its extremities, it has beenidentified as a Hadrosaur of the Kritosaurusgenus (a genus still under discussion becausesome paleontologists consider it identical to theGryptosaurus). Curiously, the central metatar-suses of its left hand are fused.In the same excavation in which the Krito -

saurus was discovered, parts of two other Lam -

beosaur-like dinosaurs (with crests on theirheads) were gathered. These were identifiedmainly by their ischia, bones from their pelvicregion.The IGLUNAM further supported the study of

Mexican dinosaurs and in collaboration withthe Coahuila state Education Ministry Paleon -tology Commission and the enthusiastic partic-ipation of renowned U.S. paleontologist Dr.James I. Kirkland, the year 1993 began with anambitious project called “Prospecting, Recoveryand Study of the Cretaceous Dinosaurs of Coa -huila.” The study’s objectives are to discover thefossil potential of the state’s Cretaceous sedi-ment, particularly for dinosaurs, in order todetermine what type they are and disseminatethe research results to the scientific communi-ty and the public in general through lectures,articles and books.During the first excavations in February

1993, several important finds were made. Un -doubtedly the best was the relocation of the fos-sil fields on the Rincón Colorado ejido.Over a 40-square-kilometer area, almost 200

deposits of dinosaur fossils have been found.The most important, both in terms of quantityand their state of preservation, have been thosespotted at the Hill of Our Lady, re-christenedthe Hill of the Dinosaurs. Some of the fossilseven preserve the impression of the animal’s skin,and in other cases, the animal seems to havebeen mummified and then permineralized.The largest group of dinosaurs found were

the Hadrosaurs, both with and without crests,followed by the Ceratopids. Several Terapods,carnivorous dinosaurs of the same family asthe Tyrranosaurs, Dromaeosaurs and Orni tho -mi mids, have also been found, however. Amongother the vertebrates gathered were fishes,particularly sharks and ray fish, crocodi les andtortoises.Among the indirect evidence is a great quan-

tity of fossilized excrement attributed to tortois-es, crocodiles and dinosaurs. Footprints of atleast two different groups, probably Tero podsand Hadrosaurs, have also been unearthed.

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Fossils found in different locations are important because of their diversity and excellent state of preservation.

Science, Art and Culture

The most abundant invertebrates found, themollusks, belong to the genera Ethmo-cardi um sp.,Inoceramus vanuxemi, Turritella vertibroi des, Eu -treohoceras sp., and Spheno discus sp. The as -sociation of the Inoceramus vanuxemi and theSphenodiscus indicate that they date from the lateCampanian age, that is, that they lived 74 mil-lion years ago, a calculation based on the Geo -logical Society of America’s most recent timetable.A considerable quantity of fossilized fruit has

also been gathered, including at least seven dif-ferent types of fossilized plants related to palms,magnolias, birds of paradise, zingiberaceae andothers now being studied.The fossils have been discovered in what

geologists call the Cerro del Pueblo For mation.Its sediments are composed of alternating lay-ers of mudstone and sandstone of both dry-landand marine origin. They display plentiful marksshowing ancient waves and hurricanes.The environment in which the dinosaurs of

southeastern Coahuila lived seems to havebeen a river delta. Some of the river’s branchesled into a salt-water lagoon connected to thesea, which explains finding dinosaurs, croco-diles, tortoises, fruit, leaves and marine inverte-brates side by side. Sometimes the sea’s influencewas predominant, and sometimes, the land’s:this explains the alternating layers of mudstoneand sandstone.This location is considered enormously im -

portant for the late Cretaceous period becauseof its diversity, abundance and the excellentstate of preservation of both the plant and ani-mal fossils. It is probably unique in the worldand has quite justly been nicknamed the “Cre -taceous Beaches.” In an analogy with today’ssites, it would undoubtedly be similar to SanBlas and Tobara in Nayarit or Morro de la Man -cha or Coatzacoalcos in Veracruz.Currently, research is underway to establish

the distribution patterns of the Hadrosaur fam-ily during the upper Cretaceous in North Amer -ica, taking into account that the Hadro saurswere the most diverse and largest group of all the

dinosaurs: herbivorous Ornitho -pods inhabited the NorthernHemisphere during the upperCretaceous period, but theyhave also been documented inArgentina. These animals seem tohave represented 75 percent of thetotal biomass of the local fauna inmost cases, certainly of that of Coahuila. In Chihuahua, dinosaurs from the Ha d -

ro saur, Ceratopid, Ornithomimid and Tyrra no -saur families have been discovered, as well asother materials currently under study.The aim of this research project is to empha-

size the provincial aspects of Hadro saurs in amore detailed way, taking into account both the

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The study of dinosaur footprints renders information that supplements that supplied by their bones.

Voices of Mexico • 55

geographical and physical context, that is thecoastlines, climate, topographical and hydro-graphical barriers, biomass, etc., so as to be ableto establish correlations between the differenttaxa and one or more characteristics of the envi-ronment.

DINOSAUR FOOTPRINTS

The study of dinosaur footprints and other ves-tiges is undoubtedly very important because itrenders information that supplements that sup-plied by their bones, such as the speed at whichthey walked or ran; whether they were qua d -ru peds, bipeds, or combined both positions;whether they traveled in herds or alone; howthey hunted; and many other characteristics.The problem with footprints, as well as drop-pings, is that it is very difficult to determinewhich genus and species of dinosaur producedthem.Dinosaur footprints discovered in Oaxaca

have been attributed to Teropods and two fam-ilies of Sauropods from the middle Jurassicperiod. In Michoacán, two families of Teropodsand two of Ornithopods form the upper Jurassicperiod have been reported. And in Puebla, rep-resentatives of a family of Ornithopods and

another of Sauropods from the upper Creta -ceous period have been found.In Coahuila, footprints from the upper Cre -

taceous have been located in the Cerro del Pue -blo Formation, attributed to Hadrosaurs andTeropods. Particularly outstanding are thosespotted in February 1998 on the Rincón Colo -rado ejido, attributed to a Teropod.These recent studies of Mexican dinosaurs

have begun to reveal their diversity, the differentkinds that lived in our country during theMesozoic era; which were exclusive to Mexico,which came from elsewhere and how they weredistributed.

NOTES

1 For several years, the UNAM Institute of Geology (IGLU-NAM), together with other institutions and researchers,has been developing projects on Mesozoic continentalfauna, the results of which are given here, particularly thediscoveries about the early Jurassic in Tamaulipas andthe late Cretaceous in Coahuila.

2 For example, the “tanks” of the Cretaceous were theAnkylosaurus, while rhinos are the equivalent of theTriceratopids.

3 Dr. Carlos González León has reported from the UNAMInstitute of Geology IGLUNAM Northwest Regional Stationin Sonora evidence of pico de pato, Ceratopids, Tyrra no -saurs and Ornitho mimids. The fossils were unearthed southof Naco, in rocks of the Cabullona group, dating from thelate Cretaceous period (Santonian, early Maestrichtian),and are from 66 million to 74 million years old.

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Recent studies of Mexican dinosaurs have begun to reveal the different species that lived here during the Mesozoic era.