Throughout the last twenty years or so, palaeontologists studying the Late Cretaceous fauna of North America have found a fantastic selection of Ornithischian dinosaurs in strata laid down between 80 million and 70 million years ago. Several horned dinosaurs such as Vagaceratops, Utahceratops and Kosmoceratops as well as numerous new genera of Hadrosaurs (duck-billed dinosaurs) have already been described from western North America. Most palaeontologists have already been focused on mapping the faunal distribution and studying the myriad of new plant-eating dinosaur species that have been found, but numerous scientists are now turning to the mystery of why so many several types of dinosaur evolved in this area of the world over the last few million years of the Cretaceous.

Diversity Explanation Is based on the Geology

For just one team of researchers based at Ohio University, the explanation as to dinosaur diversity lies in the geology. The rise of the Rocky Mountain range and the appearance and then disappearance of a massive, inland seaway that split North America into some islands, could have been the catalysts for an explosion in megafauna diversity. The study team from the University’s Heritage College of Osteopathic Medicine experienced their paper published in the online scientific journal PloS One (public library of science).  what dinosaur has 500 teeth They declare that the rapid changing geology generated populations of animals being isolated which can explain the patterns of evolution, migration and rapid dinosaur diversification.

Terry Gates, the lead composer of the paper and a post-doctoral student at the University commented that within the last few decades palaeontologists have become increasingly alert to the huge array of several types of plant-eating dinosaur that roamed what was to become the United States and Canada. However, immediately, ahead of the Cretaceous mass extinction, there were only a few dominant dinosaur species across the entire continent. This phenonmenon has yet to be fully explained.

Examining the Geological Record of North America

The study team attempted to examine the geological record of what was to become the continent of North America, emphasizing the United States and Canada. During the Campanian faunal stage of the Cretaceous, a amount of time in the Earth’s history that roughly relates to 83 million years ago to 74 million years ago there is extensive plate tectonic activity that generated mountain ranges being pushed up and the sinking of much of the continental landmass under an inland sea (known while the Western Interior Seaway). At its most extensive, this seaway covered much of North America from Canada to the Gulf of Mexico.

In the later Maastrichtian faunal stage, that lasted from 74 million years ago up until the mass extinction event 65 million years ago, there is less extensive plate activity. This coincided with a decline in the number of genera of dinosaur known from the fossil record. Palaeontologists have interpreted this as evidence as a fall in the number of dinosaur species living in North America towards the very end of the Cretaceous – dinosaur genera became less diverse.

Mountain Building Isolating Populations

Geologists have calculated that throughout the Early Cretaceous there is a considerable quantity of geological activity in the western United States. Several processes involving subduction, the movement of ocean crust on to the Earth’s mantle occurred along what was to become the western coast of North America. These immense geological forces caused the western area of the Americas to be lifted up and this generated the synthesis of an enormous mountain range that extended from Alberta (Canada) in a south-western direction to as far south while the southern United States. The region to the east with this newly formed mountain range (the Sevier Mountains), flexed downwards and this coincided with a rise in global sea levels, flooding much of the continent and splitting what land remained above sea level into some large islands. This sea (Western Interior Seaway), teemed with life and the marine deposits left out in places as far apart as Alberta and Kansas have provided palaeontologists with an amazing selection of marine reptile fossils to review – Dolichorhynchops, Elasmosaurs and huge Mosasaurs such as Tylosaurus.

The Ohio based research team have focused on the dinosaur fossils that have been within association with the islands. At its most extensive, the Western Interior Seaway split the North American land mass into three large islands. These islands each had a considerable and diverse population of Ornithischian dinosaurs.

The Island of Laramidia

The absolute most western of the hawaiian islands, referred to as Laramidia contained land that was to make Alberta in the north with the American states of Dakota and Montana in the centre with the land that was to become Utah forming the southern area of the island. Formations laid down in the north with this island, the famous Dinosaur Provincial Park for instance, have provided palaeontologists with a massive array of horned and duck-billed, Ornithischian dinosaurs. Fossils within Utah, animals like the horned dinosaurs Kosmoceratops and Utahceratops from rocks of roughly exactly the same age, indicate that several types of plant-eating dinosaur evolved in the south. The Ohio University scientists have postulated that mountain building and the rising sea levels caused the available habitat for dinosaurs to shrink on Laramidia. Populations became isolated and this is further compounded by later plate tectonic movements that generated the nascent development of what was to become the North American Rockies.

New Species Every One Hundred Thousand Years

The team postulate a new species of large, Ornithischian dinosaur evolved every few hundred thousand years during the time that the mountain ranges and the Western Interior Seaway isolated populations. These geological processes generated a rapid burst of dinosaur evolution in these cut-off populations, in exactly the same way that the isolated populations of animals in the Galapagos archipelago rapidly diversified into new species.

However, this extensive speciation of mega-herbivores was brought to a conclusion with the continued rise of the embryonic Rock Mountains which eventually forced the Western Interior Seaway to contract. This opened a large, open territory for the Ornithischian dinosaurs to exploit. This reduced the turnover in species with new species evolving at a much slower rate. New species taking greater than a million years to evolve.

A Barrier to Migration

The study team warn that their work on the major, herbivorous dinosaur faunas of North America cannot be used as a template to explain the rise and then the decline in dinosaur diversity on a worldwide scale. However, the rapidly changing geology due to plate movements might have had an influence over the migration of dinosaurs from the Americas into Asia and into South America. The rise of the Rocky Mountains for instance, might have created a barrier that the dinosaurs could not cross. Only dinosaur species resident north with this barrier could have migrated into Asia and only those species living in the southern part of Laramidia might have had a migration route open in their mind to South America.