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The fossil record of North America reveals how fins became fingers

Posted in America, Animals, Dinosaurs, Prehistory, Wildlife on Wednesday, 18 December 2013

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This edited article about prehistoric animals first appeared in Look and Learn issue number 496 published on 17 July 1971.

Pteranodon, picture, image, illustration

Pteranodon (top) and Rhamphorhynchus (bottom) by Roger Payne

Many scientists today think that the continents of the world were once united into one or two super-continents and then, in the distant past, they broke apart to drift into their present positions.

As these movements took millions of years to accomplish, the changes would not be noticed; in fact, some scientists think that this drifting is still going on, with New York moving away from London at the rate of a fraction of an inch a year.

Over the vast ages of the earth’s history, the landmass which is now North America has been subjected to many changing patterns of sea and land. Great transgressions – or invasions of the sea – have from time to time flooded huge areas of the land, only to retreat – so that what is now dry land was once a wide deep sea.

North America has been the scene of some of the most important stages in the evolution of life, and the fossils of these stages have been excavated from the rocks of Greenland and the Red-beds of Texas.

The discovery in 1931, by Dr. Lauge Koch, of the remains of the Ichthyostegids, caused great interest in scientific circles, because here at last was the evidence proving beyond doubt that all land-living creatures had evolved from a group of fishes known as the “Lobe-fins,” which possessed fins on the way to becoming limbs or fingers. The Ichthyostegids had all the in-between features required for the missing-link that marked the transition from fish to mammal.

Thirty years before the discovery of the Ichthyostegids, a Dr. Watson uncovered the fossil fragments, and later the complete skeleton, of an amphibian-reptile. These were found in the Texan Red-beds near a town called Seymour, and so came to light the famous Seymouria, the linking form between the amphibians and the reptiles. Although Seymouria was too late in time to be the actual ancestor of the reptiles, it resembled the form that had been. For instance, it had the skull, teeth and parts of the backbone typically amphibian, while the limbs, shoulder girdle and jaw were those of a reptile.

While North America is famous as the area in which many transitional forms of life developed, it is probably best known for that group of incredible reptiles – the Great Dinosaurs.

The discovery of vast bone-beds in the Great Plains and Rocky Mountains during the 1870s, by Professors Marsh and Cope, make one of the most exciting stories in the history of palaeontology. Unfortunately these great scientific discoveries were marred by the jealous rivalry that sprang up between the two men, who abused each other in print, poached each other’s sites, which led to fierce gang fights between the opposing camps’ workmen.

In spite of this, the contribution made to our knowledge of the past by these fiery gentlemen was immense, for not only did they collect and describe the fossils of hundreds of reptiles, but also founded the modern methods of removing these bones from the sites to the laboratory.

The story of the Dinosaurs began about 200 million years ago, when the primitive reptiles stood up on their hind legs and ran!

At first they were mostly small, lightly-built creatures that left the three-toed prints of their bird-like feet in the rocks of the Connecticut Valley. They probably tracked flying insects and smaller reptiles among the cycads and tall conifers of the Triassic Period – trees, whose fossilized trunks may still be seen in the famous Petrified Forest of Arizona.

From such small beginnings sprang the dinosaurs, great reptiles whose names every schoolchild has heard, such as Brontosaurus and Brachiosaurus. Diplodocus was, at 87 feet, the longest animal ever to walk on land. Then there were flying reptiles; the Pterosauria, which were part of life’s first experiment in the conquest of the air. These leathery-winged reptiles culminated in the giant Pteranodon, a hammer-headed pterosaur with a wing span of 27 feet. Because it had a smallish body it was thought to have laid undersized eggs, with the result that the young, when hatched, would need care for some time from the parent. Reptiles as a whole have little parental instincts, so it may be that in Pteranodon we have an early example of the fostering care later shown by the birds and mammals.

But perhaps best known of all the past’s monsters is the great and terrible Tyrannosaurus Rex, the last and most awesome of an awesome breed.

This mighty reptile steps out of the Cretaceous Period to dwarf us with the vision of nature’s most gigantic killing machine.

Yet, strangely enough, at the height of their power, the Great Dinosaurs became extinct.

It must however be remembered that they had been the dominant animals on land for over a hundred million years – not a mean survival time, especially when one thinks that man has, at most, been on this planet for two million years and, if he wishes to compete with the dinosaurs, must inhabit the earth for at least ninety-nine million more!

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