Buried Treasures of El Rosario, Baja California

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By Marisol Montellano

During the 60’s and 70’s, American paleontologists spent several summers surveying the surroundings of El Rosario and discovered remains of dinosaurs, crocodiles, lizards and other vertebrates. Later, students and teachers from the Autonomous University of Baja California (UABC) made expeditions to the area.

Since 2004, staff members of the Institute of Geology of the National Autonomous University of Mexico, together with researchers from the United States, have resumed the study in the area. Our objective is to know the fauna that lived there during the Late Cretaceous (74 million years ago), to do an environmental reconstruction, and how this fauna is compared with those of other North American locations, among other questions. The emphasis has been on recovering the remains of small vertebrates (microvertebrates) such as amphibians and reptiles, to name a few.

Looking for microvertebrates in the sediments of El Gallo. Photo: René Hernández.

How do we do it?

We began prospecting, this is walking through the canyons, climbing the walls of the hills, and looking for evidence of fossil remains, like pieces of bones and teeth. When those are found, the area and its surroundings are examined more carefully, and sacks are filled with sediment, which are sent to Mexico City for sieving in the facilities of the Institute of Geology at the National Autonomous University of Mexico.

In the laboratory, the sediment is soaked, passed through sieves of different apertures sizes, dried and then observed under a microscope to separate the remains of microvertebrates for further study. It is a tedious but very exciting work when identifiable remains are found, since most of the material is fragmented and difficult to identify. This material is being studied by undergraduate, graduate and specialist students in different groups.

Remains of microfossils ready to be collected. Photo: René Hernández.

To give context to how and where Baja California was 70 million years ago: North America did not look like we see it now, it was bisected by a sea that divided it into two great continents Laurasia and Appalachia; Baja California was on the Pacific coast of Laurasia and was attached to the continent, the Sea of ​​Cortez did not exist, it was formed many millions of years later.

When we compare the faunas of the same age of El Rosario with those of the rest of North America (Canada and the United States) the results indicate a peculiar faunistic composition, so this site is important since it suggests that the fauna of continental vertebrates was not homogeneous throughout Laurasia.

What have we found?

Amphibians, different groups of lizards, crocodiles, a type of ray, fish, turtles and mammals have been identified. Duck-billed dinosaur teeth have also been collected, which are the most abundant, as well as small and large carnivorous dinosaurs.

Among the interesting and abundant fossils we found a lizard that was only known by a pair of teeth, now we know it for almost all its skeleton and it has been possible to determine how it replaced its teeth.

Dinosaur tooth of small carnivore. Photo: René Hernández.

The presence of dinosaur egg shell fragments and the remains of young dinosaurs is something exciting. Along with these studies, isotopic studies are also being carried out, which will help us determine the temperature and humidity.

Traces of petrified trees have been found in different places, as well as leaves, fruits and seeds. Unfortunately, in recent years the petrified trees have been looted, which is a cause for concern because, on the one hand, part of the area’s paleontological heritage is lost, and on the other, when they are removed without taking into account the geological context, valuable information is lost to reconstruct the paleoflora and paleoenvironment.

Infructescence. Photo: Marisol Montellano.

How do we visualize El Rosario 74 million years ago?

It was a fluvial environment with a great river that opened in many branches, there were episodes with a lot of energy that dragged everything, and there were emergent places that were stable and that developed soils where the plants could grow. Associated with this landscape were freshwater fish and some of them were brackish water species, such as amphibians, reptiles, turtles, crocodiles, mammals and small and large dinosaurs.

Finally, I would like to acknowledge the disinterested support of the inhabitants of El Rosario, without whom our work would not be possible year after year.

Fossil wood. Photo: Marisol Montellano.

This article was published in the #14 issue of the Mediterranews magazine.

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