fossils

Dinosaurs south of the border

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Vasika
Udurawane

Writer
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Julio
Lacerda

Staff Artist and Writer
For millions of years, dinosaurs dominated the conifer forests and plains of ancient Mexico. They were also on ground zero during the earth-shattering cataclysm that ended their 165-million-year reign.

Today, Mexico is filled with unique natural wonders. From bone-dry deserts in the north to sprawling rainforests in the south and mountains in between, it covers more than 1.9 million square kilometers of diverse country. Mexico is on top of biodiversity lists, along with places like Indonesia and Brazil. And just like the rest of North America, Mexico had its own dinosaur fauna.


We often associate the western United States and Canada with the best dinosaur remains but Mexico has been no slouch when it comes to unveiling its buried Mesozoic past. A whole range of beasts made their living here, including gigantic horned dinosaurs, fast-running birdlike creatures and some of the biggest duckbill dinosaurs ever. The country’s first dinosaur remains consisted of horned dinosaur bones and since then, fossils have been uncovered in several Mexican states.


The majority of these come from the Late Cretaceous period, ranging from 85.3 to 72 million years ago. Many of these fossils were quite fragmentary and could not be described scientifically while others are much more complete. Among the best dinosaur-bearing rock formations here are the La Bocana Roja and El Gallo of northern Baja California, and the Cerro del Pueblo Formation of the State of Coahuila.


The apex predators here, just like in the rest of the continent, were the famous tyrannosaurs. Here, unlike up north, there was a mix of both evolutionarily derived tyrant dinosaurs and those showing ancestral features. The best-known and most complete of these animals is the medium-sized Labocania. For a while, Labocania was very difficult to classify.


Labocania was named by Ralph Molnar in 1974 and the original fossil consisted of skull parts as well as teeth, leg bones and parts of the vertebrae. Molnar was, at the time, unable to properly classify the animal due to its fragmentary nature. Instead it was tyrannosaur worker Thomas Holtz who reassigned it to the basal tyrannosaurs in 2010. Besides this, not much is known about it. All we know is that Labocania was smaller than many other American tyrants, roughly 7 meters (23 feet) long and weighed 1.5 tons.


Labocania by Nathan E. Rogers


Another big tyrannosaur was described from the neighboring “El Gallo” locality in 2014. It is still unnamed and has been historically assigned to the well-known Canadian genus Albertosaurus, an evolutionarily derived tyrant. However this new tyrant is too fragmentary to be classified properly beyond the fact that it is in fact a tyrannosaur. The rest of the 73-million-year “El Gallo” fauna lies above the La Bocana Roja and is very well known. “El Gallo” preserves the remains of a floodplain environment close to the coast. The fauna here includes turtles, small mammals, lizards and even one of the biggest ever duckbill dinosaurs.


The mighty Magnapaulia was first described in 1981 as a species of the northern genus Lambeosaurus. It was only separated later in 2012 by a team consisting of Albert Prieto-Márquez and colleagues from the Natural History Museum of Los Angeles. At 12.5 meters (41 feet) long, and weighing close to 15 tons, it was bigger than its contemporary tyrannosaurs and is the biggest dinosaur found in Mexico. It is also among the best-represented, and even skin impressions have been found. The skull of Magnapaulia shows the base of a large, hollow, bony crest that was likely used as a vocal resonator and for display within the species.


Velafrons by Fabrizio De Rossi


Velafrons, a much smaller crested duckbill was its closest relative in Mexico. The well-known Velafrons hails from the Cerro del Pueblo Formation. It existed alongside yet another member of the family called Latirhinus. Latirhinus lacked any bony crests and thus was named for its arched nasal bones. The name Latirhinus means "broad nose." These two contemporary herbivores were of similar dimensions, yet lived in different ecological niches. The broad mouth of the crestless Latirhinus enabled it to take in large mouthfuls indiscriminately while Velafrons was much more selective.


Latirhinus by Franz Anthony


A rich coastal forest and plains fauna of lizards, turtles and smaller dinosaurs like dromaeosaurs or “raptors” existed in the Cerro del Pueblo. On top of the food chain were large tyrannosaurs while the smaller herbivores are represented by ornithomimids. They were about the size and shape of an ostrich and probably had a similar lifestyle. The Coahuila finds were only informally described as "Saltillomimus."


Saltillomimus by Franz Anthony


Among the formally described animals from the area is a horned dinosaur with the most formidable horns ever. Named Coahuilaceratops after the state itself, it sported a pair of immense horns above its eyes. These curving horns were not just thickly constructed but they were huge, reaching 1.2 meters (4 feet) in length. It was also very large in terms of overall size, weighing at least 5 tons, as much as an African elephant. Coahuilaceratops may have been among the most powerful animals in its time and place in the Coahuila fauna.


Dinosaurs were not the only large prehistoric animals to exist here. Flying reptiles or pterosaurs flew over their heads during the Late Cretaceous, with one in particular being very well documented. Called Muzquizopteryx, it is around 85 million years old. It flew over the Western Interior Sea that split the continent into two parts. Muzquizopteryx was smaller than most other pterosaurs of its time, with wings spanning just 2 meters (6.6 feet). It was comparable to some of the biggest birds living today and would have been an impressive sight nonetheless.


It belonged to a highly specialized family of flying reptiles called the nyctosaurids which were completely devoted to a life flying over the waves. It had lost most of its fingers, using only the fourth elongated finger to support its wing membranes. This meant that it spent less time on land than other pterosaurs did.


The location of the infamous end-Cretaceous asteroid impact is the Yucatan Peninsula, specifically the town of Chicxulub. Buried underneath the Gulf of Mexico is the third-largest impact crater on Earth, spanning 110 miles in diameter and 12 miles wide. The area was first confirmed to be the site of an impact in 1978, according to findings by geophysicists Antonio Camargo and Glen Penfield. The two men were working for the state-owned oil company Pemex, and the find was only publicized three years later. It was in 1980 that renowned geologists Walter and Luis Alvarez published the theory that a massive asteroid or similar object struck the planet 66 million years ago.


The Chicxulub impactor seemed the likely candidate. It is thought that the 6-mile wide asteroid hit the Yucatan with the combined force of a billion atomic bombs. The resulting impact sent an enormous mega-tsunami sweeping over the North American continent, and soon the entire planet would be covered in a thick cloud of ash, dust and debris. Although this cataclysm marked the end of the non-avian dinosaurs, it left the landscape free of most large-bodied terrestrial vertebrates allowing mammals, including our ancestors, to quickly replace them as masters of the land.

3986b46bb4f35aa1ff4d42167a12e0fc

Vasika
Udurawane

Writer


http://sulc.us/rzow6
http://www.eartharchives.org/articles/dinosaurs-south-of-the-border/