Trabajos Futuros

Archaeopteryx lithographica is the geologically oldest known bird discovered. The fossil, about 150 million years old, was found in a limestone quarry in Solnhofen, Bavaria. In 1861, a print of a fossil feather was found in the limestone. The feather was a flight feather, shaped with its central vein off-center, characteristic of today’s bird flight feathers. The following year, a skeleton of a fossil animal complete with feathers for flight was found in the same quarry. Since that time, seven more skeleton specimens have been found in the Solnhofen limestone. The skeletons of these eight fossils have been thoroughly studied by specialists who addressed the questions of habitat, biology, anatomical issues, the phylogenetic significance, and the evolution of flight of Archaeopteryx.

They have published a vast array of scientific articles on the subject. By no means do they agree on every topic. But they do recognize that Archaeopteryx is a bird (primitive) and that it could fly.

Let’s look at Archaeopteryx and

Topic: Evolution in the Science Classroom Go to: www.scilinks.org Code: AP012

A D V E N T U R E S I N P A L E O N T O L O G Y (Figure 6.1). We will compare the three animals as skeletons because that is the only form the remains of the 150-million-years-old Archaeopteryx and Compsognathus occur. Each skeleton is in a circle that includes the characteristics of the animal. Each smaller enclosed circle represents the additional characteristics that set that animal apart from its ancestor. Therefore Archaeopteryx has flight feathers in addition to three primary toes in the feet and a wishbone. Find other traits that distinguish Archaeopteryx from its dinosaur ancestor and the chicken from Archaeopteryx (There are more than those listed.).

The intermediate position of Archaeopteryx between the modern bird and the nonavian dinosaur is fairly obvious in the

characteristics it shares with each. Archaeopteryx is about as much a half dinosaur and a half bird as one could find. Yet it clearly represents a missing link between the bird and the dinosaur. As such it occupies the position in the evolutionary boundary between the thousands of species of living birds and extinct dinosaurs. The evolution of dinosaurs and birds can be viewed in Figure 6.2.

The discovery of one species of fossil can suggest a major shift in the thinking about the course evolution has taken. But the appearance of Archaeopteryx has raised far more questions about the origin of bird flight than it has answered. What was the origin of birds? What is the ancestry of Archaeopteryx? How did bird flight evolve? Could Archaeopteryx fly? And how? From the ground or from the tree? How well? Could it perch in a tree using its hallux? What was its diet?

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Evolution of dinosaur to birds

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Then there was the question concerning what Archaeopteryx looked like when it was alive.

Its restoration when shown with Compsognathus takes a bit of creative imagination, as in the illustration of the animal by the shore of the Tethys Sea about 150 million years ago (Figure 6.3). You can see that though their skeletons were quite similar, their bodies appeared to be different. How much of the difference was the addition of feathers to the body cover?

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Archaeopteryx and Compsognathus 150 mya at Solnhofen

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Homology

Topic: Vertebrate Evolution Go to: www.scilinks.org Code: AP011

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Bones in the forelimbs of vertebrate animals.

Ever since vertebrates crawled out of the sea, they had a similarity of body structure resulting from a commonality of ancestry known as homology. Such signs of evolution are called homologous structures. The bones of the forelimbs of vertebrates are constructed from the same skeletal elements in all vertebrates, from the coelacanth fish to modern man. The basic architecture is seen in the makeup of all vertebrates

Archaeopterx

Sauripterus

Eryops

Seymouria

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from emerging fish, amphibians, reptiles, sea lizards, crocodiles, pterosaurs, dinosaurs, birds, and mammals (Figure 6.4). The bones of the forelimbs are scapula, humerus, radius, ulna, carpals, metacarpals, and phalanges. Each animal has the same general architecture but the forelimbs are modified for different functions depending on the needs of the animal. These functions may include running, walking, climbing, flying, swimming, seizing food, and mating. Skeletal specializations lead to the survival of different species of animals.

All organs in the vertebrate body have homologous structures that have been modified by evolution, causing changes in the bodies to adapt to different environmental factors. The skin, kidneys, vascular system, and digestive system of all animals are constantly changing as the animals adapt to changing environments. We know that anatomical structures have changed dramatically from their earlier forms, but we have no clear evidence. The fossils of most species have not been preserved. We can infer changes by observing embryos, or

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The embryos of a chicken (left) and a human (right).

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Crests on the heads of duckbill dinosaurs.

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Deinonychus Struthiomimus Triceratops T. rex

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Forelimbs of Deinonychus, Struthiomimus, Triceratops, and Tyrannosaurus.

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A human and Brachiosaurus.

we can see organs that have become vestigial. Or we can infer what the kidney was like in a fish or in an amphibian, but we don’t have the clear fossil proof we get from the hard fossil bones that endure millions of years in the crust of Earth. Bones are the most durable and revealing fossil evidence. Nevertheless, embryos suggest much of the evolutionary history of vertebrates. Compare the embryo of the chicken with the embryo of the human (Figure 6.5).

In both the embryos, note gill pouches and the post anal tail. In each embryo, the homologous pouches will develop into structures other than gill pouches (though in fish and amphibians they will develop into gill pouches). The tails of fish and most vertebrates develop prominent external tails, but in most birds and apes, the tail is not an external structure, even though the embryo

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has a prominent tail. Homologous structures become modified or disappear during the embryological development of the animal.

Let’s take the dinosaurs for example. For over 200 million years they adapted to conditions from pole to pole, from mountains, to deserts, and to jungles. In that interval, every force in nature acted upon them to produce a diversity of structure.

For example, the crests of duckbill dinosaurs show a remarkable diversity that is probably related to display in order to attract mates (Figure 6.6)

The forelimbs of dinosaurs also became specialized for particular functions (Figure 6.7). The forelimbs of Deinonychus and Struthiomimus were slender and flexible for grasping, while those of Triceratops were thick and strong for supporting its great weight. There is some controversy surrounding the purpose of the forelimbs in Tyrannosaurus. Though they have claws and look like they might have been useful for grasping things, the arms were too short to reach the mouth! Can you think of what purpose they might have served?

Dinosaurs also varied tremendously in size. For example Brachiosaurus, the largest of the sauropod dinosaurs, weighed over 70 tons (Figure 6.8), while the skeleton of Mussasaurus, one of the smallest sauropods, fits in two human hands (Figure 6.9).

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Skeleton of juvenile Mussasaurus.

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